optimization of n2o decomposition rhox/ceria...
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Optimization of N2O decomposition RhOx/ceria catalysts and design of a high N2-selective
deNOx system for diesel vehicles
Verónica Rico Pérez
Optimization of N2O decomposition
RhOx/ceria catalysts and design of a high N2-selective deNOx system for diesel vehicles
PhD Thesis July 2013
Verónica Rico Pérez
UNIVERSIDAD DE ALICANTE
Departamento de Química Inorgánica Grupo de Materiales Carbonosos y Medio Ambiente (MCMA)
Optimization of N2O decomposition RhOx/ceria catalysts and design of a high N2-selective deNOx system for
diesel vehicles
Memoria presentada para aspirar al grado de Doctor
Verónica Rico Pérez
Directores del trabajo:
Agustín Bueno López Concepción Salinas Martínez de Lecea
Profesor Titular de Catedrática de Química Inorgánica Química Inorgánica
Alicante, Julio 2013
Index
i
SUMMARY OF CONTENTS 1
CHAPTER 1 7
Introduction
1.1 Environmental effects of N2O. 9
1.2 Sources of N2O. 10
1.3 Diesel versus Gasoline engines. 12
1.4 TWC for gasoline vehicles. 15
1.5 Gas pollution control in diesel vehicles. 16
1.5.1 Diesel Oxidation Catalysts (DOC). 17
1.5.2 Diesel Particulate Filters (DPF). 17
1.5.3 NOx emissions control. 21
1.5.3.1 Catalytic decomposition of NOx. 21
1.5.3.2 NOx Storage Reduction (NSR). 22
1.5.3.3 Selective Catalytic Reduction (SCR). 24
1.6 N2O abatement. 28
1.7 Background of the research group. 29
1.8 Objectives. 31
CHAPTER 2 45
Characterization techniques and catalytic tests
2.1 Characterization techniques. 47
2.1.1 N2 adsorption at -196 ºC. 47
2.1.2 X-Ray Diffraction (XRD). 50
2.1.3 Raman spectroscopy. 54
2.1.4 X-ray Photoelectron Spectroscopy (XPS). 57
2.1.5 Temperature Programmed Reduction with H2 (H2-TPR). 63
2.1.6 Transmission Electron Microscopy (TEM). 64
Index
ii
2.1.7 Scanning Electron Microscopy (SEM). 66
2.1.8 Thermobalance coupled to a Mass Spectrometer (TG-MS). 67
2.2 Catalytic tests at different scales. 68
2.2.1 Powder and small size honeycomb monolith catalysts. 69
2.2.2 Medium-size honeycomb monolith catalysts. 70
2.2.3 Full-size honeycomb monolith catalysts. 72
CHAPTER 3 79
Effect of the calcination conditions of the RhOx/CeO2
catalysts on N2O decomposition activity
3.1 Introduction. 81
3.2 Experimental. 82
3.2.1 Study of the metal precursors decomposition. 82
3.2.2 Catalysts preparation. 82
3.2.3 Catalysts characterization. 83
3.2.4 N2O decomposition tests. 84
3.2.5 CO oxidation tests. 84
3.3 Results and discussion. 84
3.3.1 Thermogravimetry - Mass Spectroscopy (TG-MS) study of
metal precursors decomposition. 84
3.3.2 XRD, Raman spectroscopy and N2 adsorption at -196 ºC
characterization. 86
3.3.3 N2O decomposition tests. 88
3.3.4 Characterization by XPS of fresh catalysts and after “in
situ” pre-treatments with N2O at 225 ºC. 90
3.3.5 H2-TPR characterization. 93
3.3.6 TEM characterization. 95
3.3.7 CO oxidation tests. 96
3.4 Conclusions. 97
Index
iii
CHAPTER 4 105
Preparation of RhOx/CeyPr1-yO2 N2O decomposition
catalysts by rhodium nitrate impregnation with different
solvents
4.1 Introduction. 107
4.2 Experimental. 108
4.2.1 Catalysts preparation. 108
4.2.2 Catalysts characterization. 109
4.2.3 N2O decomposition tests. 109
4.3 Results and discussion. 109
4.3.1 Catalysts temperature during rhodium nitrate thermal
decomposition. 109
4.3.2 N2O decomposition tests. 111
4.3.3 Catalysts characterization by N2 adsorption at -196 ºC,
XRD and Raman spectroscopy. 114
4.3.4 Catalysts characterization by TEM, XPS and H2-TPR. 120
4.4 Conclusions. 129
CHAPTER 5 135
Preparation, characterization and N2O decomposition
activity of honeycomb monolith-supported
RhOx/Ce0.9Pr0.1O2 catalysts
5.1 Introduction. 137
5.2 Experimental. 138
5.2.1 Catalysts preparation. 138
5.2.2 Catalysts characterization. 141
5.2.3 N2O decomposition tests. 141
5.3 Results and discussion. 141
5.3.1 SEM-EDS-chemical mapping characterization. 141
5.3.2 XRD and Raman spectroscopy characterization. 145
Index
iv
5.3.3 Characterization by N2 adsorption at -196 ºC. 147
5.3.4 H2-TPR characterization. 148
5.3.5 TEM characterization. 149
5.3.6 N2O decomposition tests. 151
5.4 Conclusions. 155
CHAPTER 6 161
NOx reduction to N2 with commercial fuel in a real diesel
engine exhaust using a dual bed which constists of
Pt/Beta zeolite and RhOx/ceria monolith catalysts
6.1 Introduction. 163
6.2 Experimental details. 165
6.2.1 Catalysts preparation. 165
6.2.1.1 Medium-size Pt/Beta zeolite/monolith catalysts
preparation. 165
6.2.1.2 Full-size Pt/Beta zeolite/monolith catalyst preparation. 168
6.2.1.3 Medium-size RhOx/Ce0.9Pr0.1O2/monolith catalyst
preparation. 170
6.2.2 SEM characterization. 171
6.2.3 Catalytic tests. 171
6.3 Results and discussion. 171
6.3.1 Study of the Beta zeolite suspensions viscosity. 171
6.3.2 Effect of the Beta zeolite suspension viscosity on medium-
size monoliths dip-coating. 174
6.3.3 Medium-size monolith coating with Beta zeolite in
consecutive dipping steps. 175
6.3.4 SEM characterization of medium-size monolith catalysts. 178
6.3.5 SCR experiments performed with commercial diesel fuel
and the medium-size Pt/Beta zeolite/monolith catalyst. 180
6.3.6 SCR experiments performed with commercial diesel fuel
and the full-size Pt/Beta zeolite/monolith catalyst. 182
Index
v
6.3.7 SCR experiments performed at 300 ºC with commercial
diesel fuel and a dual-bed which contains Pt/Beta zeolite
and RhOx/Ce0.9Pr0.1O2 medium-size monolith catalysts. 186
6.4 Conclusions. 188
CHAPTER 7 195
General Conclusions
RESUMEN 201
Optimización de catalizadores RhOx/ceria para la
descomposición de N2O y diseño de un sistema deNOx
altamente selectivo a N2 para vehículos diésel
ABBREVIATIONS 227
CURRICULUM VITAE 233
Summary of contents
Summary
3
Summary of contents
This thesis gathers a full research process ranging from basic or
fundamental investigation to actual implementation. This work deals with
rhodium catalysts supported on ceria-based materials preparation,
characterization and catalytic test towards N2O decomposition for a better
understanding of the effect of their physico-chemical properties on the
catalytic behavior.
This thesis is structured in seven chapters. Chapter 1 describes
the environmental problems caused by N2O and its main sources, focusing
on diesel engines. A review of technology and catalysts used for diesel
pollutants abatement is presented, including previous work performed in
our group and the main objectives of this PhD thesis.
Chapter 2 describes the characterization techniques employed to
obtain information about the physico-chemical properties of the materials
and the experimental systems and conditions used to carry out the
catalytic tests.
Chapters 3 to 6 present the discussion of experimental results. In
Chapter 3 the calcination conditions for catalysts preparation were
modified in order to improve the distribution of rhodium and the RhOx-ceria
interface of RhOx/CeO2 catalysts. Improved catalytic activity for N2O
decomposition and CO oxidation was obtained by flash calcination, which
consisted of introducing the ceria support-impregnated rhodium precursor
in a furnace which was pre-heated at 250 ºC. The speed at which water
evaporates from the ceria-based support seemed very important for the
catalyst properties, because it affects the size of the RhOx particles on the
final catalyst. The following publication describes the results obtained in
part of this study: V. Rico-Pérez, M.A. Velasco-Beltrán, Q. He, Q. Wang,
C. Salinas-Martínez de Lecea, A. Bueno-López. Preparation of
ceria-supported rhodium oxide sub-nanoparticles with improved
catalytic activity for CO oxidation. Catalysis Communications 33 (2013)
47.
In Chapter 4 was studied the effect of the solvent (water, ethanol
or acetone) used to impregnate CeyPr1-yO2 (y = 1, 0.9 or 0.5) supports with
Summary
4
rhodium nitrate, in order to prepare N2O decomposition catalysts. The
activity of the catalysts studied was related with the RhOx-support
interaction, and both the nature of the ceria support and of the solvent
used for rhodium impregnation affected such interaction. Ceria doping with
10 % praseodymium had a positive effect on RhOx-support interaction, but
the benefit on the catalytic activity was only obtained for water
impregnation because the temperature peaks created during calcination of
ethanol and acetone-impregnated catalysts promoted Ce0.9Pr0.1O2 and
RhOx sintering. The interaction between RhOx and Ce0.5Pr0.5O2 was not as
good as that with Ce0.9Pr0.1O2. The best catalyst was obtained by
impregnating Ce0.9Pr0.1O2 with a water solution of rhodium. However, if
acetone or ethanol must be used for any reason the bare ceria support is
more suitable (under the calcination conditions of this study; 250 to 500 ºC
at 10 ºC/min) because is more stable towards sintering during solvents
combustion.
In Chapter 5 RhOx/Ce0.9Pr0.1O2 active phases were loaded by
sequential impregnation into small-size (1 cm diameter) cordierite
honeycomb monoliths following the procedure (i) cerium and
praseodymium nitrates impregnation + calcinations and ii) rhodium nitrate
impregnation + calcination. The supported catalysts were characterized by
XRD, Raman spectroscopy, SEM-EDS, TEM-EDS and H2-TPR, and tested
for N2O decomposition. Rhodium oxide particles were selectively attached
to the Ce0.9Pr0.1O2 and not to the cordierite. The optimum content of
rhodium was 0.2 wt.% (in total weight). The calcination procedure
significantly affected the supported catalyst features. The best catalyst was
prepared by flash calcination yielding to smaller RhOx particles and
lowering the temperature for surface Rh-Ce-Pr entities reduction in
comparison to ramp calcination, improving both the distribution of active
phases on the cordierite substrate and the catalytic activity for N2O
decomposition. These results have been published in: V. Rico-Pérez,
S. Parres-Esclapez, M.J. Illán-Gómez, C. Salinas-Martínez de Lecea,
A. Bueno-López. Preparation, characterization and N2O decomposition
activity of honeycomb monolith-supported Rh/Ce0.9Pr0.1O2 catalysts.
Applied Catalysis B: Environmental 107 (2011) 18.
In Chapter 6 medium-size (2.3 cm diameter) and full-size (14 cm
diameter) Pt/Beta zeolite monolith catalysts were successfully prepared
and tested in SCR technology for NOx abatement in a real diesel engines
Summary
5
exhaust using commercial diesel fuel as reducing agent. Also a
medium-size dual bed catalytic system consisting of a
Pt/Beta zeolite/monolith SCR catalyst located upstream a N2O
decomposition RhOx/Ce0.9Pr0.1O2/monolith catalyst were tested in the
same reaction with the objective to improve N2 selectivity. The dip-coating
method was optimized for the Pt/Beta zeolite/monolith preparation. The
catalyst used for N2O decomposition, RhOx/Ce0.9Pr0.1O2/monolith, was
prepared by nitrate precursor decomposition. The production of N2O as
undesired NOx reduction product, which is a drawback of platinum SCR
catalysts, has been solved by using the dual bed configuration, where both
monolith catalysts operated at the same temperature, and 100 % N2
selectivity has been obtained.
Finally, Chapter 7 summarizes the most relevant conclusions
obtained in this thesis.
Since this thesis applies for the International Doctorate mention,
this is written in English and a summary in Spanish is presented at the end
of the thesis to fulfil the requirements of this mention.
CHAPTER 1
Introduction
This chapter describes the environmental problems caused by
N2O and its main sources, focusing on diesel engines. A review of
technology and catalysts used for diesel pollutans abatement is
presented, including previous work performed in our group, and the
main objectives of this PhD thesis.
Introduction
9
1.1 Environmental effects of N2O.
Nitrous oxide (N2O) is naturally produced by biological processes
occurring in soil and water and by a variety of anthropogenic activities
related with agriculture, energy, industry and waste management activities
[1, 2].
From an environmental point of view, Kapteijn et al. [3] reported
some years ago that N2O was considered as a relatively harmless species
and suffered from a lack of interest by scientists, engineers and politicians
for long time, due to the underestimation and unawareness of the potential
contribution of this species to environmental problems. From the middle
80’s a growing concern was noticed since N2O was identified as a
contributor to the destruction of the ozone in the stratosphere. Nowadays,
it is well known that N2O is a dangerous environmental pollutant because it
contributes to the destruction of the stratospheric ozone, being at the same
time a greenhouse gas (GHG).
The greenhouse effect is a phenomenon caused by strong
absorbance of infrared radiation in the atmosphere, increasing the
temperature of the Earth. Without a natural greenhouse effect the average
Earth temperature would be 33 ºC below the actual one and this would
hinder life on the planet. The problem occurs when the amount of
greenhouse gases increases, enhancing the natural greenhouse effect,
and as consequence, the average global temperature and the related
climate changes also increase.
N2O is the major source of NOx (NO + NO2) in the stratosphere,
and therefore, is an important natural regulator of the stratospheric ozone.
The overall influence of N2O on the ozone layer is complex and very
different from that of substances covered by the Montreal protocol in
September 1987. At this time, there were no doubts about the negative
effect of N2O on the ozone depletion layer [1]. Indeed, N2O emissions are
the largest of all the ODSs (Ozone Depleting Substances) and are
projected to remain the largest for the rest of the 21st century [4].
The environmental impact of this gas is increasing because the
atmospheric N2O concentration is increasing mainly by human activities
[1, 5].
Chapter 1
10
Although N2O is not the major contributor to global warming
(∼6 %), it is much more potent than either the other two most common
anthropogenic greenhouse gases, CO2 and CH4. Due to its long lifetime of
approximately 150 years in the atmosphere, N2O has around 310 times the
Global Warming Potential (GWP)1 of CO2. For this reason, a relatively
limited emission (compared to other greenhouse gases) is equivalent to
about 10 % of the CO2 emission [1, 2]. At the Third Conference of the
Parties (COP-3) of the United Nations Framework on Climate Change
(UNFCC), in Kyoto (Japan) in December 1997, legal binding targets were
set for reducing emissions of six greenhouse gases (CO2, CH4, N2O, HFC,
PFC, SF6) to be achieved in the period 2008–2012 [6]. In Doha, Qatar, on
8 December 2012, the "Doha Amendment to the Kyoto Protocol" was
adopted. The composition of Parties changed and the amendment
included a second commitment period, from 1 January 2013 to 31
December 2020, a revised list of greenhouse gases (GHG) and some
updates of several articles pertaining to the first commitment period.
During the second commitment period, Parties committed to reduce GHG
emissions by at least 18 % below 1990 levels in the eight-year period from
2013 to 2020 [6].
1.2 Sources of N2O.
As mentioned before N2O was produced by both natural and
anthropogenic sources, the main sources of atmospheric N2O being the
microbial action in soils, the manufacture of nylon and adipic acid, and
fossil fuel combustion in stationary and mobil sources [1-3, 7]. It was also
reported that chemical processes associated with the production and use
of nitric acid and fluidized bed combustion were N2O sources, and their
contribution to the total N2O emissions amount were about 20 %.
A new inventory of U.S. Greenhouse gas emissions and sinks:
1990–2011 has been published indicating that although the direct
greenhouse gases CO2, CH4, and N2O occur naturally in the atmosphere,
1 The Global Warming Potential (GWP) of a greenhouse gas is defined as the
ratio of the time-integrated radioactive forcing from the instantaneous release of 1 kilogram (kg) of a trace substance relative to that of 1 kg of a reference gas (IPCC 2001) [1].
Introduction
11
human activities have changed their atmospheric concentrations. From the
pre-industrial era (i.e., ending about 1750) to 2010, concentrations of these
greenhouse gases have increased globally by 39, 158 and 18 %,
respectively [1]. Figure 1.1 illustrates the relative contribution of the direct
greenhouse gases to the total U.S. emissions in 2011 and the main
anthropogenic sources of N2O emissions. As observed in Figure 1.1 N2O
represents 5.6 % of global greenhouse gases emission. Agricultural soil
management, stationary fuel combustion, mobile source fuel combustion,
manure management and nitric acid production were the major sources.
From 1990 to 2011, a 4 % N2O emissions (14.5 Tg CO2 eq) increase has
been detected [1].
2011 Sources of N2O Emissions
2011 Greenhouse Gas Emissions by Gas
(percentages based on Tg CO2 Eq)
Figure 1.1. 2011 Greenhouse Gas Emissions (percentages based on Tg CO2-eq.) and
Sources of N2O Emissions [1].
Emissions of N2O that can be reduced in the short term are
associated with chemical production and energy industry (∼35 % in the
EU). This emission is concentrated in a limited number of large facilities,
which holds promise for an economic and efficient reduction strategy to
fulfill the Kyoto commitment. In the particular case of adipic acid and nitric
acid production, technologies are commercially available, but their
application (extrapolation or adaptation) to other sources, e.g. light-duty
Chapter 1
12
vehicles, is not feasible due to the dissimilar characteristics of the
exhausts. Diluted N2O streams, relatively low temperature
(typically < 500 ºC), and presence of catalyst inhibitors (O2, H2O, NOx and
SO2) are features of some light-duty vehicles [1-3, 8].
Historically, medium-duty vehicles were not part of Corporate
Average Fuel Economy (CAFE) regulations. However, manufacturers
provide GHG emission data for carbon dioxide (CO2), methane (CH4), and
nitrous oxide (N2O) to the Environmental Protection Agency (EPA) since
2011 [9, 10]. The proposed emission standards for both N2O and CH4 from
medium-duty vehicles certified on a chassis or engine dynamometer are2
50 mg/mile and 50 mg/bhp-h (milligrams per brake horsepower-hour) and
must be reported as CO2 equivalent (CO2-eq) in g/mile, taking into account
the global warming potential of the other gases. Thus emissions at the
proposed standards equate to 14.9 gCO2-eq/mile and 14.9 gCO2-eq/bhp-h
for N2O, and 1.25 g CO2-eq/mile and 1.25 gCO2-eq/bhp-h for CH4 [9, 11].
As above reported, one of the main N2O sources is mobile
combustion. This thesis is focused on N2O abatement in diesel engines.
For a better understanding of the problem faced, a brief explanation of
gasoline and diesel engines operation and their exhaust emissions will be
carried out in the next section.
1.3 Diesel versus Gasoline engines.
Currently, automobile prevails across the globe as the most
popular and necessary mode of transportation in our daily lives. About
50 million cars are produced every year, and over 700 million cars are
used worldwide [12]. The increasing number of cars is seen as the major
source of pollutants causing the decrease in air quality, health problems,
and recirculation of polluted air by frequent ambient temperature inversion
and formation of “photochemical smog” in the major cities [13, 14].
2 “miles” are used instead of kilometers in this document when talking about
US regulations, in order to keep the original terminology (1 mile = 1.609 Km).
Introduction
13
Due to environmental concerns and the increasing price of
traditional fossil fuels, alternative fuels are now receiving more attention
than before in many countries. Also, many developed countries are
currently encouraged to find out alternative approaches to promote fuel
economy and reduce the environmental impact from internal combustion
diesel engines [15].
Diesel and gasoline engines have different modes of operation. In
diesel engines, the fuel auto-ignites as it is sprayed into the combustion
chamber at a high pressure. A spark is not required to ignite the diesel
fuel, which is a heavier, less volatile mixture of hydrocarbons than gasoline
and chemically more susceptible to auto-ignition. Relative to gasoline
engines, diesel engines have higher compression ratios, faster
combustion, lower throttling losses, operate leaner (i.e., at a greater
air–fuel ratio) and work with O2 excess. In gasoline engines (also known as
Otto engines), air and fuel are mixed before introduction into the cylinder.
The air-to-fuel ratio is constant and is often chosen to be stoichiometric,
i.e. about 14.6 on an air-to-fuel weight basis [14]. As a result, diesel
engines have an inherently greater thermodynamic efficiency than gasoline
engines. Diesel fuel also has approximately 12 % greater volumetric
energy content than gasoline, and hence, diesel engines operate with a
higher volumetric fuel economy (lower fuel consumption) than gasoline
engines [9, 15], which tipically leads to lower CO2 emission.
The use of catalysts for purifying exhaust gases is absolutely
necessary and indispensable in every vehicle taking into account current
regulations [12, 14]. The major pollutants emmited by a gasoline engine
are HC (hydrocarbon), CO (carbon monoxide), NOx (NO with only traces
of NO2) and PM (particulate matter) in very low concentration. Standards
of exhausts emissions were tightened in subsequent years which
conducted to a successful development and introduction of the three-way
catalysts (TWC). Nowadays, all new gasoline cars are equipped with a
catalytic convertor, reducing CO, HC and NOx emissions.
Currently, CO and HC emissions from diesel engines are about the
same than catalytically equipped gasoline engines. However, NOx and
certainly PM emissions from diesel engines are much higher. TWC does
not match in diesel engine mainly for two different reasons:
Chapter 1
14
High O2 concentration that hinders direct reduction of NOx.
The need of a specific system for soot abatement based on
a filter which should be regenerated.
Even though diesel engines were considered clean in comparison
with gasoline exhaust gases (when TWC were introduced, PM
concentration was not legislated), NOx and PM emissions from diesel
engines are much higher. Since 1982 PM standards for diesel engines
have also been tightened over the years [14].
Typical diesel exhaust gases are subdivided in three groups:
harmless compounds, regulated harmful compounds (harmful compounds
subjected to regulation) and unregulated harmful compounds. The first
group (O2, CO2, H2O and N2) is harmless in the sense that the compounds
have no direct adverse effect on health, despite CO2 does contribute to the
greenhouse effect. The second group of regulated harmful compounds
includes CO, HC, NOx, PM and SOx, and the unregulated harmful
compounds include aldehydes, ammonia, cyanide, benzene, toluene and
PAHs (polycyclic aromatic hydrocarbon) [14]. Since 1993 an increasing
restriction in emission standards over the regulated harmful compounds
group has been done [13].
Emission targets of different contaminants have become more
restrictive over the years and several techniques are promising to reduce
emissions from diesel engines. Modified or alternative fuels have been
studied, trying to correlate emission levels with certain fuel specifications in
order to optimize fuel composition towards low emissions. Also, some
engine modifications have been very effective in reducing diesel engine
emission levels and have been implemented. Finally, after-treatment
techniques have been studied extensively for NOx and also for PM
removal from diesel exhaust gases [14]. The after treatment techniques
will be the alternative to reduce diesel emissions used in this work. In the
next section, the different configuration commercialized or proposed in the
literature for CO and HC oxidation, NOx abatement and finally N2O
decomposition will be presented.
Introduction
15
1.4 TWC for gasoline vehicles.
In gasoline engines, CO and HCs oxidation catalysts eventually
chosen in the early catalytic converters, were based on platinum group
metals (PGMs). The catalytic oxidation of CO and HCs (HmCn) would
follow the reactions (1) and (2).
2CO + O2 → 2CO2 (1)
4HmCn + (m+4n)O2 → 2mH2O + 4nCO2 (2)
NOx has also to be controlled, and Pt/Rh catalyst formulations
were used for the selective reduction of NO to N2, under rich fuel
conditions, according to reactions (3) and (4).
(8n+2m)NO + 4HmCn → (4n+m)N2 + 2mH2O + 4nCO2 (3)
2NO + 2CO → N2 + 2CO2 (4)
Early TWC used two catalysts. The engine ran slightly rich fuel to
enable reduction of NOx over a Pt/Rh catalyst, and air was introduced
before a second catalyst to oxidize the CO and HCs excess. After some
time, some European car manufacturers, notably Volkswagen and Volvo,
used Pt/Rh oxidation catalysts and when operated around the air/fuel
stoichiometric point, it was found they could provide good NOx control and
oxidize HC and CO at the same time. The use of Pt/Rh catalysts to control
HC, CO and NOx simultaneously became the preferred system because
all three major pollutants were controlled by one catalyst, and the concept
was christened as the “three way catalyst” (TWC) [13].
To use a catalyst in real gas streams it is necessary to load the
active phases (powder) into an appropriate inert support. Diverse
substrates can be used, such as pellets or inorganic oxide particles,
honeycomb ceramic monoliths, ceramic foams, etc [16-20]. In the 1960s,
Johnson Matthey was involved with catalytic control of gaseous pollutants
such as NOx from nitric acid plants. When a plan to reduce emissions from
US cars by 90 % was announced in 1970, the company was ready to
develop catalytic technology for automotive emission control. Two types of
Chapter 1
16
pellet catalysts were first introduced, but Johnson Matthey concentrated on
the “monolithic” type that is used in all auto-catalytic systems today [13].
Among the potential catalyst supports, honeycomb monoliths
present some attractive properties. They provide a good contact between
the active phases of the catalyst and the treated gases and they present a
high dust tolerance. The pressure drop is minimized because the gas flow
is not significantly impeded through the catalytic bed and also they have no
degradation problems typically occurring, for instance, in particles friction
[16-19].
By far, the most successful substrate materials are based on
compositions which when extruded and fired at high temperature form
aligned cordierite, 2MgO·2Al2O3·5SiO2, and about 85 % of the substrates
on cars today are made of this material [13].
The monoliths had no catalytic activity, they are inert supports, and
a process was developed for coating the monoliths with both high surface
area alumina and platinum group metals (PGMs). Today, TWCs are based
on combinations of platinium and/or palladium and rhodium, alumina and
ceria (also known as cerium oxide or CeO2), together with a variety of
support stabilizers, activity promoters, and selectivity improvers. Elements
used include iron, nickel, manganese, calcium, strontium, barium,
lanthanum, neodymium, praseodymium and zirconium. While most
commercial catalysts contain one, and usually more, of these minor
elements, all of them contain PGMs, alumina and ceria in different
proportions.
1.5 Gas pollution control in diesel vehicles.
The different pollutants emmited by diesel engines require specific
post-combustion strategies, opposite to that described for TWC in gasoline
vehicles. CO and HC are ususally oxidized in a diesel oxidation catalyst
(DOC), soot particles are collected in filters and oxidized and NOx
reduction must be accomplished by adding a reductant in the presence of
a suitable catalyst. All these strategies are discussed in this section.
Introduction
17
1.5.1 Diesel Oxidation Catalysts (DOC).
Catalytic emission control from diesel vehicles began with the
fitting of platinum monolithic oxidation catalysts to Volkswagen diesel cars
in 1989 [13], before the demanding European legislation was in place. The
catalyst removed the characteristic odor of diesel exhaust and controlled
HC and CO emissions. With the introduction of the legislation in 1993
fitting of oxidation catalysts to all new European diesel cars became
necessary. However, the temperature of a diesel car exhaust is low
compared to that of a gasoline counterpart and this was a major challenge
for catalyst design [13].
The oxidation of HCs and CO in diesel exhausts is not
straightforward as temperatures are low, due to the fuel efficient nature of
the diesel engine, so higher amounts of HCs can absorb on active sites
and block them. Also poisons, such as sulphur oxides, are strongly
adsorbed. Traditionally platinum-based oxidation catalysts are used. When
the engine is started, the catalyst is insufficiently warm to oxidize HCs
initially present, but incorporating zeolites into the catalyst significantly
improved the performance during the so-called “cold start” period (when
the temperature is not high enough for the catalyst to work). The zeolite
adsorbs HCs preventing inhibition of the platinum active sites. This
improves low temperature CO oxidation and HC removal. At higher
temperature the HCs are desorbed and oxidized on the platinum sites [13].
1.5.2 Diesel Particulate Filters (DPF).
Several types of ceramic and sintered metal diesel particulate
filters (DPFs) have been developed for soot (PM) removal and the most
successful and commonly used are porous ceramic wall-flow filters.
Porous refractory materials used to make them include cordierite, silicon
carbide (SiC) and aluminum titanate (Al2O3TiO2) among others. The
preferred materials to manufacture DPF filters are cordierite and SiC due
to their adequate properties under regeneration conditions. Melting
temperatures (∼1400 and ∼2700 ºC, for cordierite and SiC, respectively)
and expansion coefficients (2.0 × 10−6
and 4.3 × 10−6
ºC-1
from 25 to
800 ºC, respectively) are important differences in the physical properties
between cordierite and SiC. Due to these differences, SiC is able to
Chapter 1
18
support higher temperatures reached during filters regeneration than
cordierite, but is more prone to suffer damages due to thermal shock
[21, 22].
A DPF filter is illustrated schematically in Figure 1.2. Alternate
channels are plugged on one side and opened on the other of the piece
and vice versa, so the exhaust gas is forced through the channel walls but
the PM does not and it is trapped in the filter. As PM accumulate in the
filter, the backpressure across increases. Before backpressure is too high
and the engine stops working the filter must be regenerated, so PM is
removed by oxidizing to CO2 and H2O. The arrows indicate the gas flow
through the walls [13].
Figure 1.2. Schematic representation of a ceramic diesel wall-flow filter.
The DPFs regeneration can be reached by different ways:
-The PSA (Peugeot-Citroën Societé d’Automobiles) system: a
cerium-fuel additive leads to the formation of ceria particles well embedded
into the PM structure, which lower its ignition temperature. Once a high
pressure drop is detected by a sensor, fuel is injected and its combustion
produces an increase of the exhaust gas temperature that promotes PM
ignition. Ceria catalyzed PM combustion diminishes the amount of fuel
required for trap regeneration. Recently, iron based catalysts are also
being used [21].
-Passive regeneration: known as Continuously Regenerating Trap
(CRT®), where, at speeds around 100 km/h, the temperature can be
Introduction
19
sufficiently high for NO to be oxidized over an upstream platinum oxidation
catalyst, producing NO2 and also oxidazing CO and HCs:
2NO + O2 → 2NO2 (5)
NO2, which is more oxidizing than NO and O2, rapidly reacts with
PM in the filter [13, 21, 23]. Another alternative additionally includes a
catalyst in the filter to re-oxidize NO to NO2 therefore improving the overall
efficiency of PM removal. This alternative is called Catalyzed Continuously
Regenerating Trap (CCRT) [24].
-Active regeneration: it is employed on cars that periodically
increase the exhaust gas temperature to burn PM in the filter with oxygen
(typically starting at 550 – 600 ºC) every 400 – 2000 km, depending on
actual driving conditions. Here, additional fuel from the engine is oxidized
over upstream platinum or platinum/palladium oxidation catalyst to provide
the high temperature to initiate PM burning that is then carefully controlled
by restricting the O2 available by throttling the engine.
According to the different technologies, three filter systems (named
as Generation 1, 2 and 3) have been developed and used commercially for
cars that use periodic active regeneration in which catalysis has key roles,
and these are illustrated in Figure 1.3.
Generation 1. This system employs one or two platinum based
oxidation catalysts upstream of a filter to control HC and CO emissions,
and to convert NO to NO2 for passive PM combustion when conditions
permit this to take place. It requires a fuel additive that is converted to
oxide in the engine which is retained in the filter and can lower the
temperature for PM combustion [13].
Generation 2. This system has the advantage of not using a fuel
additive, so it does not require a fuel additive tank and the associated
pump, etc. This system has one or two separate oxidation catalysts
upstream of a filter. The filter has platinum or platinum/palladium catalyst in
its channel walls to promote PM combustion, and today many cars use this
configuration [13].
Chapter 1
20
Generation 3. It does not require a fuel additive nor an upstream
catalyst, and it combines in a single filter (typically SiC, Al2O3TiO2, or
cordierite) with the oxidation catalyst to oxidize HC and CO during normal
driving, and to periodically oxidize extra partially burnt fuel to raise the
temperature to combust PM with O2 during active regenerations. The
catalyst also oxidizes NO to NO2 to provide some passive PM removal
during high-speed driving. This system is thermally the most efficient, and
during active regenerations only the filter must be heated, which is
mounted on the engine turbocharger to minimize heat losses. The
oxidation reactions used to boost the temperature take place actually in the
filter in the same location as the retained PM. In contrast, earlier systems
with a separate upstream catalyst lose considerable heat during
regenerations to the environment via the pipe between the turbocharger
and the filter [13].
Figure 1.3.Three filter systems used on European diesel cars [13], where DOC =
Diesel Oxidation Catalyst, DPF = Diesel Particulate Filter and CSF = Catalyst Soot
Filter.
During these PM combustion processes, few NOx is reduced to
N2, typically less than 15 – 20 %, and an additional NOx removal system
must be located downstream the PM filter, where the temperature is
usually lower than 400 ºC [25].
Introduction
21
1.5.3 NOx emissions control.
In this section the technologies that are already commercially
available for NOx abatement in vehicles, and also some proposals for
possible implementation are described. Some of these technologies are
not only suitable to remove NOx from the exhaust gases of diesel and
lean-burn engines, but also to remove NOx from the flue gases of large
combustion plants used for heating or power generation.
Three different processes have been proposed for NOx
abatement:
i) Catalytic decomposition of NOx.
ii) NOx Storage Reduction (NSR), also known as Lean NOx
Trap (LNT), NOx trapping or NOx absorbing catalysts
(NACs).
iii) Selective Catalytic Reduction (SCR).
1.5.3.1 Catalytic decomposition of NOx.
Decomposition of NOx is thermodynamically favorable at
temperatures below 900 ºC, but the activation energy required for this
reaction is too high. Therefore, a catalyst is necessary to lower the
activation energy thus facilitating the reaction. This approach is the
simplest and the most desirable because no reducing agent is required.
However, the relatively high activation energy of a direct NOx
decomposition limits the practical use of this approach [25].
Over different catalysts studied, Ishihara et al. [26] proposed
La0.7Ba0.3Mn0.6Cu0.2In0.2O3 as active catalyst for NO decomposition even in
the coexistence of H2O, O2, and SO2. For this type of catalyst, the NOx
species are strongly adsorbed, and removal of these adsorbed species
and/or surface oxygen plays an important role in the sequence of reaction
steps. It seems likely that NO decomposition proceeds on the vacant
adsorption site, which is formed by removing adsorbed NOx and/or surface
oxygen, and N2O may be the intermediate species. The reaction
Chapter 1
22
temperature, which is too high, makes this catalyst difficult to apply under
actual exhaust conditions [27].
1.5.3.2 NOx Storage Reduction (NSR).
The first NSR catalyst was developed and launched by Toyota in
1994 [23]. Reactions (5 - 9) take place during lean and rich cyclic periods.
During lean operation (excess air) NO is oxidized to NO2 over a platinum
based component in the presence of excess O2, forming a stable nitrate of
an alkaline catalyst component. During the rich or stoichiometric operation,
the exhaust lowers the O2 partial pressure to a point where the nitrate
phase is not stable at the normal operating temperature and it
decomposes, that effectively is the reverse of the storage process, and the
NOx formed is reduced to N2 usually over rhodium based catalysts
[13, 24, 27-29].
2NO + O2 → 2NO2 (5)
Lean
NO2 + MCO3 → MNO3 + CO2 (6)
2MNO3 → MO + 2NO + O2 (7)
2MO + CO2 → MCO3 (8) Rich
2NO + 2CO → N2 + 2CO2 (9)
In the reactions (5) to (9), M typically represents an alkaline earth
metal cation. NOx-trapping catalysts commonly have two layers with the
bottom one containing platinum for oxidation of NO to NO2 together with
the storage component that can be mainly a barium or strontium
compound. Rhodium is incorporated into the top layer. During the periodic
enrichments of the exhaust gas the NOx released from the bottom has to
pass through the rhodium-containing layer, in the presence of a reductant,
being reduced to N2 as shown in Figure 1.4 [24, 29, 30].
During the lean phase, NOx is removed from the exhaust gas by
adsorption onto platinum sites where it is oxidized to NO2 and then
Introduction
23
converted into a solid nitrate phase. During rich phase, the NSR capacity is
recovered by releasing NOx that is reduced to N2 over rhodium catalyst
sites [31].
Figure 1.4. Schematic operation of a NOx Storage Reduction (NSR) catalyst: (a) lean
phase; b) rich phase.
Although a large number of investigations have been conducted,
some problems still remain unsolved. The main problem was the catalysts
deactivation by sulphur, as barium component forms very stable sulfates
from traces of SOx derived from fuel sulphur compounds [27, 32, 33].
Another problem is the thermal deterioration due to reaction of the NOx
storage material with compounds within the wash-coat and particle growth
of both precious metals and NOx storage material. Also contact between
the platinum particles and metal alkaline earth oxide (BaO) reduces the
platinum oxidation activity by an electronic effect. This causes a lowering
of the activity at low temperature (below 250 ºC), a critical problem
especially for emissions from light-duty diesel engines, since a large part of
the testing cycle is characterized by low-temperature emissions, typically in
the range 120–200 ºC [28, 33].
Chapter 1
24
Most NSR catalysts are based on the utilization of BaO as the NOx
storage component (about 15 wt.%), but new generation catalysts also
contain alkali metal oxides to improve high-temperature behavior, TiO2
nanoparticles to enhance regeneration and Rh/ZrO2 to promote in-situ
generation of H2 during periodic high-temperature (about 650 ºC)
regeneration [28].
1.5.3.3 Selective Catalytic Reduction (SCR).
The SCR is one of the technologies proposed for NOx emissions
control in diesel-engine exhausts to convert NOx to N2 in an O2-rich
environment [24, 27, 34, 35]. In the SCR process, NOx reduction
successfully competes with the reduction of O2, even though the latter is
present in a large excess. Mainly based on laboratory experiments, several
reductants have been proposed to accomplish the SCR of NOx in O2-rich
gas streams (lean fuel conditions), including H2, CO, different HCs,
ammonia (NH3), urea (CO(NH2)2) or even diesel fuel among others
[24, 27, 34, 35].
Some of the SCR technology for mobile sources is based on the
well-established technology used in power plants, where NH3 or urea is
injected in the post-combustion stream for NOx reduction to N2 [35]. Based
on that technology, it has been proved that with an appropriate catalyst
NH3 can be used as a very selective NOx reductant in NH3-SCR system
for mobile sources. Among all the catalysts tested, platinum catalysts can
work at relatively low temperatures, and vanadium-based catalysts are
commonly used at temperatures typical of heavy-duty diesel engine
exhaust gas. Traditionally, V2O5 supported on TiO2 has been used
because its resistance to sulphur poisoning. Also there has been much
work reported on metal exchanged zeolite systems; especially those
containing iron, cobalt, copper, and cerium [24]. Fe/zeolite catalysts have
been commercialized and used in several NH3-SCR applications,
achieving high NOx conversions, but oxidation of NH3 affords NO at high
temperatures so the apparent conversion of NO decreases as increasing
amounts of NO are formed from NH3 [24].
Transportation of NH3 (being a corrosive compound) hinders this
technology implementation, becoming a promising alternative the use of
Introduction
25
urea. NH3 is then derived from an aqueous urea solution (via urea
hydrolysis, reaction (10)) that is injected into the hot exhaust upstream of
the SCR catalyst [13].
(NH2)2CO + H2O → 2NH3 + CO2 (10)
Koebel et al. [35] reported that NOx in diesel exhaust is usually
composed of > 90 % NO and therefore, once NH3 is generated, the main
reaction of SCR with NH3 will be:
4NH3 + 4NO + O2 → 4N2 + 6H2O (11)
This reaction implies a 1:1 stoichiometry for NH3 and NO and the
consumption of some O2. The non O2 consuming reaction (12) is much
slower and is therefore not relevant in lean combustion gases:
4NH3 + 6NO → 5N2 + 6H2O (12)
On the other hand, the reaction rate with equimolar amounts of NO
and NO2 (reaction (13)) is much faster than that of the main reaction (11)
4NH3 + 2NO + 2NO2 → 4N2 + 6H2O (13)
At high temperatures (>400 ºC) the commonly used catalysts
based on TiO2–WO3–V2O5 tend to form N2O (reaction (14))
4NH3 + 4NO + 3O2 → 4N2O + 6H2O (14)
At still higher temperatures, NH3 may be oxidized to NO (reaction
(15), thus limiting the maximum NOx conversion:
4NH3 + 5O2 → 4NO + 6H2O (15)
In spite of many research efforts have been focussed on the
utilization of NH3 or urea as reductants, this technology has not been
successfully developed for light-duty vehicles, and one of the reasons is
the weight penalty of the additional urea/NH3 tank. In addition, a network of
urea/NH3 suppliers should be available to fill up the tanks.
Chapter 1
26
SCR of NOx with HC has been proposed as an alternative to the
urea/NH3 technology. The SCR of NOx with HC has been studied in detail
for HC of different nature (propene [36-39], octane [40], methanol [39],
ethanol [39], acetaldehyde [41] and decane [38]). However, as far as we
know, direct reduction of NOx with commercial diesel fuel has not been
reported, and one of the goals of this study is to do so. The main
advantage of diesel fuel with regard to some other reductants is that it is
already on board, and additional tanks and filling facilities are not required.
Using HC as a reductant, the reduction of NOx successfully
competes with the reduction of O2, even though the latter is present in a
large excess [24].
The reactivity of HCs in lean-NOx conversion depends on their
nature, the catalyst and the temperature [30]. At higher temperatures
competitive oxidation of HC becomes increasingly important, and then
most of the HC reductant is oxidized giving little opportunity for NOx
reduction. A feature of many lean-NOx reduction reactions is that there is
insufficient reduction capability on the surface to reduce NOx completely to
N2, and a significant amount of N2O is released as reaction by-product
[24].
The relative importance of N2O emission depends on the nature of
the catalyst surface concerned, the nature and concentration of reductant,
and the temperature as well as exhaust gas flow rates, etc [24].
The proposed catalysts for HC-SCR include PGMs [42-45], copper
[46], iridium [47], and silver [44] among others. Among noble metals,
platinum is very active for HC-SCR at low temperature (T < 300 ºC), and is
not significantly affected by the presence of H2O in the exhaust stream
[27, 48-50]. It has also been reported that platinum catalysts are resistant
to SO2 at sulphur levels similar to those in vehicle exhaust gas
(25 – 50 ppm) [51].
Bearing in mind the sensitivity of the HC-SCR reaction in noble
metals to the reaction conditions it is not surprising that different
mechanisms have been proposed [30]. In the case of platinum catalysts,
two alternative mechanistic schemes have been developed to explain the
Introduction
27
kinetics of the HC-SCR reaction. The first reaction scheme proposed was
the following, which is described for propene as model hydrocarbon [30]:
C3H6 → CxHy(ads) (16)
CxHy(ads) + O2→ I1(ads) (17)
I1(ads) + NO → CN(ads) (18)
CxHy(ads) + NO → CN(ads) (19)
I1(ads) + O2 → CO2 + H2O (20)
CN(ads) + NO/NO2 → N2O + N2 + CO2 (21)
CN(ads) + O2 → N2 + CO2 (22)
where CxHy (ads) represents adsorbed HCs and I1 represents some form
of an activated HC intermediate created after partial O2 oxidation.
The second mechanism for the HC-SCR reaction on platinum
catalysts involves the dissociation of NO and subsequent removal of
adsorbed oxygen by the reductant. This model was first presented by
Burch [30] and the main features of this mechanism are summarized
below:
NO ↔ NO(ads) (23)
NO(ads) ↔ N(ads) + O(ads) (24)
N(ads) + N(ads) ↔ N2(ads) (25)
NO(ads) + N(ads) ↔ N2O (26)
O2(g) ↔ 2O(ads) (27)
CxHy ↔ CxHy(ads) (28)
CxHy(ads) + (2x + y/2)O(ads) ↔ xCO2 + y/2H2O (29)
Chapter 1
28
In addition, at low temperatures the following reactions could
occur:
NO(ads) + N(ads) ↔ N2(ads) + O(ads) (30)
NO(ads) + NO(ads) ↔ N2O(ads) + O(ads) (31)
Zeolite-containing catalyst formulations can provide enhanced NOx
reduction due to their ability of maintaining a high concentration of HC in
the catalyst [30].
As described above N2O is one of the non-desired by-products
from SRC of NOx with HC. The main challenge of this thesis is to develop
and optimize a catalyst for N2O decomposition to be located after a SCR
catalyst used for NOx reduction in diesel exhausts, by using diesel fuel as
reductant.
1.6 N2O abatement.
The development of N2O abatement systems during nitric acid
production has been broadly studied by industry, research institutions and
universities. According to the process, different locations and catalysts
have been tested which operate at different temperatures. To apply that
knowledge at the end pipe exhaust treatment in diesel engines of light-duty
vehicles, the low temperature options will fit better.
From studies made for N2O abatement in nitric acid plants it can
be said that the low-temperature catalytic decomposition of N2O is
definitely more beneficial than SCR, because additional reducing agents
are not needed in this case. This reduces the cost of the reductant and
avoids the emissions involved (slip or undesired combustion products)
[2, 28]. However, the use of a catalyst in the low temperature zone is
connected with the requirement of high activity and resistance to inhibitors
always present in the exhaust gases, such as O2, H2O and residual NOx.
Catalytic decomposition of N2O was a subject of numerous studies
involving simple oxides [2, 52], perovskites [53], spinels [54-57], zeolites
[58], hydrotalcites [59, 60], mesoporous silicas [61] and supported
catalysts [62-64]. However, many of the catalysts proposed in the literature
do not show a good activity and stability in N2O conversion under realistic
Introduction
29
conditions of feed composition and space velocities. Laboratory results
often deviate from what is normally met within industrial practice and in
most of the cases severe stability test under realistic conditions have not
been taken into consideration.
Among all the catalyst reported in the literature working in the
range of low temperatures (200-600 ºC) in a batch reactor [3], including
pure oxides (CaO, Fe2O3, CuO, Rh2O3, IrO2, CoO, La2O3, pure and
doped CeO2), solid solutions (CoO in MgO), spinels (MCo2O4 where
M = Co, Cu, Ni, Zn, Ni+Cu), hydrotalcites (M-Al-CO3-HT where M=Co, Ni,
Cu), zeolites (Fe-ZSM-5), supported systems over alumina (Mn2O3, Rh),
and silica (Cr, Co, Ni, Fe)), we will focus on rhodium supported on pure or
doped ceria catalysts. Based on previous work, this system presents high
activity at laboratory and nitric acid pilot plant scale [65, 66] and also
shows resistance to H2O, NO and O2 atmospheres [67]. For all these
reasons, it was assumed that the RhOx/ceria formulations could be also
suitable catalysts for N2O decomposition in diesel exhausts, after the SCR
catalyst for NOx reduction with HCs.
1.7 Background of the research group.
This research work has been developed at the Carbon Materials
and Environment Research Group (MCMA) in the Inorganic Chemistry
Department that belongs to the University of Alicante. This group has wide
experience in environmental catalysis; it may be high-lighted several thesis
and numerous papers related with research on the pollutants abatement in
diesel engine exhausts, especially in NOx abatement and N2O
decomposition over different materials.
Regarding ceria and doped ceria, they were studied as catalysts
for soot combustion in two doctoral theses [68, 69], which have greatly
contributed to the knowledge and understanding of the physico-chemical
properties of ceria based supports used in the present thesis [70-80].
With respect to NOx abatement, another thesis [81] was developed
working with platinum, palladium and rhodium among other metals
supported over activated carbon and different zeolites for HC-SCR in NOx
abatement. Ptatinum-based catalysts were the most active at
low-temperature (200 - 250 ºC), showing thermal stability and resistance to
Chapter 1
30
poisons. The main drawback of ptatinum catalysts was the low selectivity
to N2 which was not improved with other metals [82, 83]. Later on it was
demonstrated that Pt/Beta zeolite achieved higher NOx conversions and
slightly higher selectivity to N2 due to the acidic properties of the support,
relatively open structure and thermostability [84, 85].
As a solution for the low selectivity to N2 of platinum-based
catalysts in C3H6-SCR, a system based on a double catalytic bed for
deNOx and deN2O was developed. The problem of that system was that,
under optimal conditions, the first bed operated at 200 ºC and the second
bed at 425 ºC [82]. Then, the gas heating would be required between both
beds and for a practical application it would result in unacceptable extra
fuel consumption. For this reason the desired conditions would be a dual
catalytic bed system that would operate in a similar temperature window.
To accomplish this requirement is another objective of the present thesis.
Focusing on N2O abatement, N2O decomposition was carried out
in our group with Fe-ZMS-5 catalysts using different reductants [86-88] and
also by direct N2O decomposition with rhodium supported on pure and
doped ceria [65]; proving the high activity for N2O decomposition related
with the redox properties of the support and concluding that the support
nature is crucial. A further work ended up with another thesis [66] focused
on the study of the N2O decomposition mechanism on rhodium/ceria
catalysts [89] and its application in nitric acid plants [90]. Rhodium,
palladium and platinum supported on alumina, bare and doped ceria with
lanthanum or praseodymium were also studied concluding that the best
catalytic composition was rhodium over praseodymium doped ceria [63]. In
addition the use of different gases (O2, H2O and NOx) in the gas stream
and their effect on N2O decomposition was also studied [90].
All these previous results allowed us to select RhOx/CeyPr1-yO2 as
a promising catalyst for N2O removal in diesel exhaust engines.
Introduction
31
1.8 Objectives.
Bearing in mind the above described background, the general
objective of the present thesis is to develop and optimize an effective
catalyst for N2O abatement in diesel engine exhausts, to be located after a
Pt/Beta zeolite SCR catalyst for NOx reduction with diesel fuel. To achive
this general goal, the specific objectives to be achieved are:
To study the effect of rhodium/ceria catalysts calcination conditions
in the catalyst physico-chemical properties and N2O decomposition
activity.
To study the effect of using different solvents (water, ethanol and
acetone) for rhodium loading over ceria-based supports in the
catalysts physico-chemical properties and N2O decomposition
activity.
To prepare, characterize and test for N2O decomposition at
laboratory honeycomb monolith-supported RhOx/Ce0.9Pr0.1O2
catalysts.
To prepare and test in a real diesel engine exhaust honeycomb
monolith-supported Pt/Beta zeolite catalysts for NOx reduction with
commercial fuel.
To test a dual bed consisting of a honeycomb monolith-supported
Pt/Beta zeolite catalyst followed by a honeycomb monolith-
supported RhOx/ceria catalyst in order to reduce NOx to N2 with
commercial diesel fuel. Diesel fuel will be introduced before the first
bed and both beds must operate at the same temperature.
Chapter 1
32
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Introduction
35
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Introduction
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Chapter 1
40
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Introduction
41
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Chapter 1
42
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Introduction
43
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Journal of Greenhouse Gas Control 11 (2012) 251.
CHAPTER 2
Characterization techniques and
catalytic tests
This chapter describes the characterization techniques used to
obtain information about the physico-chemical properties of the
catalysts. These techniques are N2 adsorption at -196 ºC, X-Ray
Diffraction (XRD), Raman spectroscopy, X-ray Photoelectron
Spectroscopy (XPS), Temperature Programmed Reduction with H2
(H2-TPR), Transmission Electron Microscopy (TEM), Scanning
Electron Microscopy (SEM), and Thermogravimetry-Mass
Spectroscopy (TG-MS). The experimental set-ups and reaction
conditions used in the catalytic tests are also described in this chapter.
Characterization and experimental techniques
47
This chapter describes the characterization techniques and
experimental methods used in this thesis to obtain information about the
physico-chemical properties of the catalysts together with the experimental
details about the set-ups used to perform catalytic tests. The details about
the preparation of the different catalysts studied throughout this thesis
have been described in the experimental section of each chapter devoted
to the presentation of experimental results, because most details of these
preparation procedures are specific for each particular chapter.
2.1 Characterization techniques.
Catalysts are defined by composition, structure, surface and
textural properties that are related with their catalytic activities. In order to
understand and determine their physico-chemical properties several
characterization techniques were used in this work.
N2 adsorption at -196 ºC was used to determine
(Brunauer-Emmett-Teller) BET surface areas. Crystalline properties were
studied by X-Ray Diffraction (XRD) and Raman spectroscopy. The atomic
surface composition and metal oxidation states were determined by X-ray
Photoelectron Spectroscopy (XPS). Temperature Programmed Reduction
with H2 (H2-TPR) was employed to study the reducibility of the samples.
The metal particle size was determined by Transmission Electron
Microscopy (TEM). Finally, on supported catalysts, the active phase
distribution on the solid substrate was observed by Scanning Electron
Microscopy (SEM). Thermogravimetry-Mass Spectroscopy (TG-MS) was
used to study the decomposition of the catalysts precursors.
A brief description of each technique, the experimental equipment
used and the operation conditions is presented below.
2.1.1 N2 adsorption at -196 ºC.
Many of the most popular methods for determining the surface
area of powders and porous materials depend on the measurement of gas
adsorption. The appearance of Langmuir's comprehensive review of the
nature of adsorption [1] stimulated several investigators to consider the
possibility of using gas adsorption for surface area determination.
Chapter 2
48
Nowadays the physical adsorption (or physisorption) of gases is one of the
most used techniques to study the porous texture of solids. The first
significant advances were made by Brunauer and Emmett [2, 3] and their
work prepared the way for the development of the Brunauer-Emmett-Teller
(BET) theory in 1938 [4].
An adsoption isotherm is a successive measure of the adsorbed
quantities of a certain gas (N2 in this case) by a solid depending on the
equilibrium pressure when the solid and gas are in contact in a closed
volume at a constant temperature. The amount of adsorbed gas by the
solid and the pressure at constant temperature is related to the porosity of
the solid, allowing the calculation of the solid surface area. The adsorbate
must meet certain properties being chemically inert, having a saturation
pressure relatively high at work temperature and presenting a spherical
shape to minimize uncertainty in calculating the cross section of the
molecule. Then the following molecules can be used for this approach: N2,
CO2, Ar, He, CH4 and H2O, but the most common absorbate which has
been also used in this work is N2, working at -196 ºC.
Over the past 50 years the BET method has become an extremely
popular method for determining the surface area of adsorbents, catalysts
and various other finely divided and porous materials.
Two stages are involved in the evaluation of the surface area by
the BET method from physisorption isotherm data. First, it is necessary to
construct the BET plot and from it to derive the value of the monolayer
capacity, nm. The second stage is the calculation of the specific surface
area, a(BET), from nm, and this requires a knowledge of the average area,
am, occupied by each molecule in the completed monolayer (i.e. the
molecular cross-sectional area) [5].
The BET equation is conveniently expressed in the linear form:
0mm
0
0
P
P
Cn
1C
Cn
1
P
P1 n
P
P
(1)
Characterization and experimental techniques
49
where P is the pressure, P0 is the saturation pressure, n is the number of
moles adsorbed, nm is the number of moles adsorbed in the monolayer
and C is a parameter related to the heat of adsorption.
In the original work of Brunauer, Emmett and Teller [4] and
subsequent studies [6, 7] it was found that Type II nitrogen isotherms on
various adsorbents gave linear BET plots over the approximate range
P/Po = 0.05-0.30.
The BET plot of
0
0
P
P1 n
P
P
versus 0P
P should be a straight line,
from which the values of C and nm can be calculated by resolving the
simultaneous equations of the slope, s:
mn
1Cs
(2)
and intercept, i:
Cn
1i
m
(3)
From the value of nm the specific area of the material is determined
by the following equation:
21amm 10NanS (4)
where S is the apparent surface area of the material (m2/g), am is the area
occupied by one adsorbate molecule (nm2/molecule; in the case of N2 at
-196 ºC is 0.162 nm2) and Na is the Avogadro's number
(6.0231023
molecules/mol).
In this thesis the surface areas of the catalysts studied have been
determinated applying the BET method (equation 1) to the data obtained
from N2 adsorption isotherms at -196 ºC, using a volumetric equipment
Chapter 2
50
Quantachrome (modele Autosorb-6B) which is shown in Figure 2.1 and is
available in the laboratories of the MCMA group at the University of
Alicante. This system includes two units, a degassing unit and an
adsorption unit. The degassing unit consists of a vacuum system and six
independent furnaces. Before the N2 adsorption isotherms were measured,
the samples were degassed for 4 hours at 150 ºC. The degassed samples
were moved to the physical adsorption unit, where all subsequent
experimental steps take place. The measurement process is controlled by
a computer. The steps include a vacuum step and a filling step and
different pressures of adsorbate (0 < P/P0 < 1 for N2 at -196 °C) are
supplied. The system is able to calculate the adsorbed gas volume at each
relative pressure.
Figure 2.1. Degassification (left) and adsorption (right, Quantachrome Autosorb-6B)
units used to obtain N2 adsorption isotherms at -196 ºC.
2.1.2 X-Ray Diffraction (XRD).
A first approximation of a crystal can be considered as an ordered
and periodic atoms aggrupation. X-rays are electromagnetic radiation with
wavelengths in the range 0.5 - 2.5 Å. Since this is of the same order of
magnitude as the interatomic distances in solids, X-rays are frequently
used to study the internal (crystalline) structure of materials [8]. When a
solid is reached by an X-ray beam, the X-rays are reflected, and in most
Characterization and experimental techniques
51
cases the reflected waves interfere one to each other being annulled.
However if the solid presents crystalline order, in certain directions the
waves are reinforced to form a new wave front. This constructive
interference is known as diffraction. These preferential directions are
directly related with the lattice geometry, while the intensity of the diffracted
beams depends on the type and form of atoms aggrupation in the crystal.
Therefore, the study of the geometry and intensity of the diffraction allow
the determination of the crystal structure [9].
XRD is based on the optical interferences produced when a
monochromatic radiation crosses a split of similar thickness to the
wavelength of the radiation. The X-ray wavelength is around few
Angstroms (Å), that is the same order than interatomic distances of the
crystal structures. When the solid to be analyzed is irradiated with X-rays,
these are diffracted with angles that depend of the interatomic distances
according to the Bragg’s law (see Figure 2.2):
send2n hkl (5)
where dhkl is the distance between planes, is the radiation wavelength
(nm) used to obtain the diffractogram, θ is the angle of the diffracted X-ray
beam with the surface normal (rad) and n is an integer number [10].
Figure 2.2. Diffraction (i.e. constructive interference of the scattered X-rays) will occur if
the Bragg’s equation (equation. 5) is fulfilled.
Chapter 2
52
The representation of the radiation intensity after interaction with
the sample as a function of incidence angle (usually 2θ) is called
diffractogram and is characteristic of each crystal. XRD provides both
qualitative and quantitative information about crystalline solids and
indicates the structure and position of atoms within the structure [9].
From the position of the reflections collected in the X-ray
diffractogram and Bragg’s equation (5), the interatomic distance dhkl can be
determined. The parameter dhkl depends on the crystalline system and it is
related with the lattice parameters through diverse equations. In the case
of ceria, which crystallizes in a cubic fluorite structure, the diffraction peak
corresponding to the plane (111) is the most intense reflection and is used
to calculate the lattice parameter. For this approach firstly the d111 is
calculated by the following equation:
111
111sen2
d
(6)
From d111 value the lattice parameter (a) can be determined by the
following crystallographic relation [9]:
2
222
2
hkl a
)lkh(
d
1 (7)
where (a) is the lattice parameter (nm) and h,k,l are the Miller’s indices of a
specific crystallographic plane ((111) in this case).
The crystal size can also be calculated with this technique from the
full-width at half maximum (FWHM) of the (111) peak. The Scherrer’s
equation (equation 8) can be used to determine the average size of the
crystals (D) for a crystalline solid [9]:
cos
KD (8)
where K is the Scherrer’s constant or shape factor (0.94 has been
proposed for ceria and praseodymium doped ceria [11]), is the radiation
wavelength used to obtain the diffractogram (in our particular case
Characterization and experimental techniques
53
= 0.15406 nm), β is the FWHM expressed in radians and θ is the
diffraction angle.
Nevertheless, Scherrer’s equation, despite being widespread, does
not take into account factors as crystalline perfection, crystal curvature,
tensions, etc. that also affects FWHM [9, 11]. When these factors affect
FWHM, Williamson-Hall’s method is used to obtain a more accurate crystal
size value using the following expression:
cosθd
sinθΔd4
cosθD
0.9βββ StrainSizeTotal
(9)
where βTotal is the FWHM, is the wavelength of the incident radiation, D is
the crystal size, θ is the diffraction angle, and ∆d is the difference of the d
spacing corresponding to a typical peak. The Williamson-Hall’s method
separates the contribution of the crystal size and strain (caused by atoms
that are not in their ideal positions in a non-uniform crystal) in the FWHM
[11]. When βTotalcosθ is plotted versus 4sinθ the crystal size can be
calculated from the intercept value and the tension value from the slope of
the straight line [12].
This technique has been used in this study to elucidate the
crystalline structure of both bare and doped ceria catalysts, to determine
their lattice parameter and to calculate the average size of the crystals,
using the above mentioned Scherrer’s and Williamsom-Hall’s equations,
(8) and (9) respectively.
The equipment used is a Bruker D-8 Advance with Göebel mirror
(for no flat samples), with a high temperature chamber (up to 900 ºC), and
an X-ray generator Kristalloflex K 760-80 F (Power: 3 KW, Tension:
20 - 60 KV and Current: 5 - 80 mA). All the measure conditions are
controlled by the computer system. This diffractometer is available in the
scientific instrumentation area of the University of Alicante [10].
X-ray diffractograms were recorded using CuKα radiation
( = 1.5406 Å), between 10º and 80º (2θ) with steps of 0.02º and a step
time of 3 sec. The working conditions of the diffractometer were a power of
1600 KW, a tension of 40 KV and a current of 40 mA.
Chapter 2
54
Figure 2.3. X-ray diffractometer Bruker D-8 Advance.
2.1.3 Raman spectroscopy.
This technique is a tool to study solid materials, especially with
crystalline structure. The information obtained by Raman spectroscopy
complements that obtained by X-ray diffraction and both techniques are
non-destructive for the material analyzed.
Raman spectroscopy analysis is carried out by the interaction of a
monochromatic light beam with a material. The oscillating electric field of
the incident radiation (with frequency ν0) causes an oscillation in the
electronic density of the molecule, originating the Rayleigh dispersion (with
the same frequency as the incident radiation, elastic photon-molecule
collision) and the Raman dispersion (with a frequency change, inelastic
photon-molecule collision).
Raman dispersion is a small fraction of all dispersed radiation
which provides information about the sample analyzed, where in the
Raman dispersion, the photon is dispersed with a different frequency than
the incident ν0 due to the energy transfer between the molecule and the
photon. Raman dispersion is the result of electron excitement momentarily
in a prohibited quantum levels, but never achieving an excited electronic
level, being the relaxation from the prohibited quantum level to permitted
quantum levels very fast (see Figure 2.4). The difference between the new
Characterization and experimental techniques
55
frequencies formed (Raman bands) and the original radiation frequency is
characteristic of the irradiated molecule and is numerically equal to some
of its vibration or rotation frequencies [13].
ν ν
ν0 - νk ν0 + νk
Figure 2.4. Energy diagram showing energetic transitions leading to IR absortion,
Rayleigh and Raman dispersion.
Raman dispersion can be Stokes or anti-Stokes. Stokes Raman
dispersion is produced when the dispersed photon has lower frecuency
than indicent one (ν0), the frequency of the dispersed photon being
(ν0 – νk). On the contrary, anti-Stokes Raman dispersion is produced when
the dispersed photon has higher energy than the incident frequency, being
(ν0 + νk). The new frequencies (–νk and +νk) are the Raman frequencies
and each material has a set of values νk characteristic of each structure.
Figure 2.4 shows an energetic diagram that compiles the energetic
transitions above commented.
In regular Raman spectroscopy, anti-Stokes lines are much
weaker than Stokes lines and usually they are not used, because both
types of lines give the same information. Stokes lines are usually placed in
the positive part of the Raman spectra plot, and the Reyleigh band will be
the beginning of the axis (see Figure 2.5).
Chapter 2
56
Figure 2.5. Position of Stokes lines in the Raman shift axis.
As mentioned, Raman dispersion is an extremely weak
phenomenon, so this technique is not adequate for trace analysis. Another
inconvenience is the potential fluorescence of the samples. This
phenomenon, even in weakly fluorescent samples, masks the Raman
signal and hinders the analysis of the Raman spectrum [13].
In this thesis Raman spectroscopy has been used to obtain
information about the structure of bare and doped ceria catalysts. Two
different equipments available in the scientific instrumentation area of
University of Alicante have been used.
In Chapters 3 and 4, Raman spectra were recorded in a Jobin
Yvon Horiba Raman dispersive spectrometer with a variable-power He-Ne
laser source (632.8 nm) and using a confocal microscope. The detector is
a CCD Peltier-cooled detector (Figure 2.6).
Characterization and experimental techniques
57
Figure 2.6. LabRam (Jobin-Ivon) Raman dispersive spectrometer, with a confocal
microscope.
On the other hand, a Bruker RFS 100/S Fourier transform Raman
spectrometer with a variable-power Nd:YAG laser source (1064 nm) was
used in Chapter 5 (Figure 2.7), which is particularly suitable for the
analysis of fluorescent samples. This device uses a liquid nitrogen cooled
Ge detector.
Figure 2.7. FT-Raman (Bruker RFS/100) spectrometer with coupled microscope.
2.1.4 X-ray Photoelectron Spectroscopy (XPS).
X-ray Photoelectron Spectroscopy is known as XPS and also as
Electron Spectroscopy for Chemical Analysis (ESCA). This technique is
Chapter 2
58
able to obtain the chemical composition of a solid surface, analyzing up to
few nanometers depth, and providing information about the oxidation state
and environment of the different elements. Most elements can be detected
by XPS, except hydrogen [14].
In the material analyzed each electron has unique identification
information depending of the surrounding environment. The goal of this
technique is to find out from which atom they are coming. To free the
electron from the nucleus attractive force it is necessary to excite them by
exposure to X-ray bombardment. Magnesium and aluminium are the
normal X-ray sources that emit at 1253.6 and 1486.6 eV respectively,
bringing enough energy to free the electron from the nucleus.This process
is known as photoemission principle, and is shown in Figure 2.8. Once an
electron is removed, another electron from a more external layer jump
down to cover the gap, releasing the energy corresponding to the
difference between the initial and final orbitals. This energy is characteristic
of each element.
Figure 2.8. Photoemission principle. Process of electron excitation by an X-ray beam
and the subsequent release of the photoelectron.
According to the principle of energy conservation the energy of the
incident photon must be the sum of the binding and kinetic energies of the
removed electron (equation 10):
Characterization and experimental techniques
59
Bindingkinetic E E hv (10)
The XPS analyses are carried out under high vacuum and the
electrons emmited are drived in these conditions to an electron analyzer to
determine the Binding Energy (BE). As mentioned before, the BE is
characteristic for an element and the number of photoelectrons emmited is
proportional to the concentration of each element in a surface. XPS
spectrum obtained by plotting Intensity (which depends of the amount of
electrons collected) versus their BE (in eV) provides information about the
atomic surface composition and concentration [14, 15].
In the current study, the rhodium, cerium and praseodymium
oxidation states have been estimated for the different catalysts, together
with the Ce/Pr surface ratios. The Rh 3d photoelectron spectra of the
catalysts show two peaks, corresponding to the 3d5/2 and 3d3/2 transitions
(around 309 and 313 eV, respectively). Both peaks provide similar
information about the oxidation state of rhodium. As reported in the
literature, the Rh 3d5/2 peak appears at 307.0–307.5 eV for Rh0, at about
308 eV for Rh+, and from 308.3 to 310.5 eV for Rh
3+ [12, 14]. The
percentage of each rhodium species was determinated by peak
deconvolution analysis (Figure 2.9).
300 305 310 315 320
Inte
nsity (
a.u
.)
Binding Energy (eV)
Rh0
Rh3+
Figure 2.9. Rh 3d5/2 spectrum of a representative sample RhOx/CeyPry-1O2
The Ce3+
percentage (with regard to total surface cerium) was
estimated following the method proposed elsewhere [12, 16, 17] and the
Chapter 2
60
Pr3+
percentage (with regard to total surface praseodymium) by using the
semi-quantitative method proposed by Borchert et al. [18].
875 885 895 905 915 925
Inte
nsity (
a.u
.)
Binding Energy (eV)
v0
v
v'
v''
v'''
u0
u
u'
u''
v'''
Ce 3d
Figure 2.10. Ce 3d spectrum of a representative sample RhOx/CeyPry-1O2
The proportion of Ce3+
cations with regard to the total cerium was
calculated as the ratio of the sum of the intensities of the u0, u’, v0, and v’
bands to the sum of the intensities of all the bands (see in Figure 2.10
spectra of Ce 3d) [16].
uv area
'uu'vv area100%
CeCe
Ce 0043
3
(11)
In comparison to cerium oxide, less literature is available on XPS
analysis of praseodymium oxide. Pr 3d spectra have a rather similar shape
like the Ce 3d spectra of Ce3+
/Ce4+
compounds (Figure 2.11) and also
allow studying redox processes. However, the precise analysis of the
oxidation states by deconvolution of the spectra into components has been
less elaborated until now. Three spin-orbit split doublets labeled as a/b,
a’/b’, and a’’/b’’ were found. The 3d3/2 sub-level presents an additional
feature labeled “t”, which can be explained by a multiplet effect.
Characterization and experimental techniques
61
920 930 940 950 960 970
Inte
nsity (
a.u
.)
Binding Energy (eV)
a
a'
a''
b
b'
tb''
Figure 2.11. Pr 3d spectrum from representative sample RhOx/CeyPr1-yO2
It has not been reported the direct possibility to determine the
amounts of praseodymium ions in the different oxidation states from the
relative peak areas. Doublet a’’/b’’ is absent in the spectra of clean Pr2O3
and can therefore be assigned to Pr4+
ions. However, doublets a/b and
a’/b’ are present in Pr2O as well as in PrO2 and cannot be assigned to a
specific oxidation state. Semi-quantitative estimation of the amount of Pr3+
ions was done using the following expression reported by Borchert et al.
[18].
'a area
''a area
28.0
11100%
PrPr
Pr43
3
(12)
XPS spectra were recorded with two different equipments available
in the scientific instrumentation area of University of Alicante. XPS spectra
of Chapter 3 were obtained with a VG‐Microtech Multilab 3000
spectrometer (Figure 2.12) equipped with a hemispherical electron
analyzer with 9 channeltrons (with energy of 2 ‐ 200 eV step) and Mg and
Al X‐ray radiation sources. In our case, Mg Kα radiation (1253.6 eV) was
used, with the detector in constant energy mode and a pass energy of
50 eV.
Chapter 2
62
The pressure in the analysis chamber was maintained at
5∙10‐10 mBar. The BE scales and Auger kinetic energy (KE) were
established using the C1s transition at 284.6 eV as reference [15].
A high temperature-pressure cell with controlled atmosphere is
available in this device for in situ pre-treatments. This cell has been used
to pre-treat selected catalysts with a 1000 ppm N2O/He flow at 225 ºC
(typical N2O decomposition conditions in laboratory experiments) before
the XPS analysis.
Figure 2.12. X-ray Photoelectron spectrometer VG-Microtech Multilab 3000.
A fully automated K-Alpha spectrometer from Thermo-Scientific
(Figure 2.13) was used in Chapter 4. It has a source of electrons and ions
for automated charge balancing and an argon ion source for high-precision
etching with a beam size of less than 300 μm. The high performance
colour optical system allows precise alignment of analysis position [15]. All
spectra were collected using Al-Kα radiation (1486.6 eV),
monochromatized by a twin crystal monochromator, yielding a focused
X-ray spot with a diameter of 400 μm, at 3 mA × 12 kV. The alpha
hemispherical analyzer was operated in the constant energy mode and
pass energy of 50 eV. Charge compensation was achieved with the
system flood gun that provides low energy electrons and low energy argon
ions from a single source.
Characterization and experimental techniques
63
Figure 2.13. Fully automated X-ray Photoelectron spectrometer K-Alpha from
Thermo-Scientific
2.1.5 Temperature Programmed Reduction with H2 (H2-TPR).
Temperature Programmed Reduction by H2 is a technique to study
the reduction of solids. To carry out the experiments a H2 flow was used
and the reduction was carried out in temperature programmed conditions,
i.e., increasing the temperature using a constant heating rate. The
equipment registers thermic conductivity changes of the gas stream that
has interacted with the sample. In this study, this technique allows to
determine the reducibility of the catalysts, which will be related to their
catalytic performance.
These tests have allowed the analysis of the catalysts prepared
(RhOx/CeyPr1-yO2) and to observe how praseodymium doping ceria, and
the synthesis conditions affect the redox properties of the ceria catalysts.
H2-TPR experiments were carried out in a Micromeritics Pulse ChemiSorb
2705 device (Figure 2.14), consisting of a tubular quartz reactor (inner
diameter 5 mm) coupled to a Thermal Conductivity Detector (TCD)
analyzer to monitor H2 consumption. A cold trap was placed before the
TCD, consisting of a mixture of isopropyl alcohol and liquid nitrogen
(temperature −89 ºC). This equipment was available in the MCMA group
laboratories at the University of Alicante.
Chapter 2
64
Most experiments were carried out with 20 mg of fresh catalyst,
which were pre-treated in situ at 500 ºC for 1 hour in a 50 mL/min flow of
5 vol.% O2 in He. Once cold, the flow gas was switched to 40 mL/min of
5 vol.% H2 in Ar and the temperature was increased at 10 °C/min up to
1050 °C. Taking into account that the active phases used in chapter 5 are
diluted by the cordierite support, 60 mg of fresh catalysts were used in this
case.
Figure 2.14. Micrometitics Pulse Chemisorb 2705 equipment.
2.1.6 Transmission Electron Microscopy (TEM).
In a TEM microscope a thin sample (maximum thickness 100 nm,
to be transparent to the electrons) is irradiated with a high energy electron
beam (200 keV in our case). As shown in Figure 2.15, part of these
electrons are transmitted, other are scattered and some of them result in
interactions that produce different phenomena such as light emission,
secondary and Auger electrons, X rays, etc. The electrons passing through
the sample may scatter due to interaction with the sample (elastic or
inelastic) or may not experience any trajectory change. Elastically
scattered electrons can be used to record diffraction images and no
scattered electrons project direct images of the dispersed material.
Characterization and experimental techniques
65
Inelastically scattered electrons are responsible for the background noise
in electron microscopy.
Figure 2.15. Scheme of the electron beam interaction with a sample.
The TEM microscope uses the transmission/scattering of the
electrons to form images, the diffraction of the electrons to obtain
information about the crystal structure and the emission of characteristic
X-rays to determine the elemental composition of the sample [19]. It is
possible to determine the morphology of the sample (size and position of
the microcrystals or the particles observed) and also the crystallography
(position of the crystal planes, study of the defects, and etc; as well as
chemical composition of the material).
TEM characterization was performed using a JEOL (JEM-2010)
microscope, equipped with a detector (Oxford, model INCA Energy TEM
100) for microanalysis Energy-Dispersive X-ray Spectroscopy (EDS). A
few droplets of an ultrasonically dispersed suspension of the catalyst in
ethanol were placed in a copper grid with lacey carbon film and dried at
ambient conditions. This microscope (Figure 2.16) is available in the
scientific instrumentation area of University of Alicante.
Chapter 2
66
Figure 2.16. JEOL (JEM-2010) microscope.
In this work, this technique was used to observe the catalysts at
nanometer scale and enable analyzing the crystal size and morphology of
the samples. The samples composition was measured by EDS.
2.1.7 Scanning Electron Microscopy (SEM).
SEM provides morphological and topographical information of the
solids surface. To obtain an image of the surface, this is scanned with a
very thin electron beam with high energy that gives several types of signals
as retrodispersed electrons, secondary electrons, etc [20].
Catalysts were characterized by SEM in a Hitachi S-3000N
microscope with a secondary electrons detector, a retrodispersed
electrons detector and X-ray detector (XFlash 3001 de Bruker) for
microanalysis (EDS) and chemical mapping, this equipment is available in
in the scientific instrumentation area of University of Alicante (Figure 2.17).
Characterization and experimental techniques
67
Figura 2.17. SEM Hitachi S-3000N microscope.
2.1.8 Thermobalance coupled to a Mass Spectrometer (TG-MS).
Thermogravimetry (TG) is a technique based in the weight change
(gain or loss) during a thermal treatment performed in a controlled
atmosphere [21]. In addition, the detection and analysis of emitted gases
provides complementary information about the process occurring during
the thermal treatment, and a mass spectrometer has been used in this
study for this purpose.
The TG-MS system (Figure 2.18) used in this study is available in
the scientific instrumentation area of University of Alicante and consists of
a thermobalance TG–DTA (Mettler Toledo model TGA/SDTA
851e/LF/1600) coupled to a mass spectrometer (Pfeiffer Vacuum model
Thermostar GSD301T). This combined thechnique has been used to study
the decomposition of 10 mg of either Ce(NO3)3·6H2O or Ceria-impregnated
Rh(NO3)3 under 100 mL/min flow of synthetic air, with a heating rate of
50 °C/min.
Figura 2.18. TG-MS system.
Chapter 2
68
2.2 Catalytic tests at different scales.
As it has been previously mentioned, the details about the
preparation of the different catalysts studied in this thesis have been
described in the experimental section of each chapter devoted to the
presentation of experimental results. These catalysts include both powder
and cordierite honeycomb monolith-supported catalysts. Honeycomb
monoliths of different size have been used and are shown in Figure 2.19,
including cylindrical substrates of 1, 2.3 and 14 cm of diameter, which are
referred to as small, medium and full-size monoliths, respectively. In order
to study the N2O decomposition over catalysts of such different shape and
size specific experimental set-ups have been required, and these set-ups
are described in this section.
Figure 2.19. Different size cordierite honeycomb monoliths used in this thesis.
Characterization and experimental techniques
69
2.2.1 Powder and small size honeycomb monolith catalysts.
N2O decomposition experiments were performed at laboratory
scale with powder and small size honeycomb monolith catalysts. These
experiments consisted of point-by-point isothermal reactions, increasing
the temperature with intervals of 25 ºC, which were extended until the
steady state was reached. The gas composition was analyzed by a
HP 6890 gas chromatograph equipped with a TCD and two columns
(Porapak Q, for N2O and CO2, and Molecular Sieve 13X, for O2 and N2 and
CO). In the particular case of Chapter 5, a chemiluminescence analyzer
(Signal 400VM) was additionally used for NO and NO2 monitoring. This
system is available in the laboratories of the MCMA group at the University
of Alicante (see Figure 2.20).
N2O decomposition tests of Chapters 3 and 4 were performed with
powder catalysts in a U-shaped fix-bed quartz reactor, located in a vertical
furnace at atmospheric pressure, with a 100 mL/min flow
(GHSV = 2000 h−1
) of 1000 ppm N2O in He, using 100 mg of catalyst
(Figure 2.20).
In Chapter 3 CO oxidation experiments were also performed in the
same set-up using a 100 mL/min flow of 1000 ppm CO/5 % O2/He and
100 mg of powder catalyst (GHSV = 42000 h−1
).
Most N2O decomposition tests of Chapter 5 were performed with
catalysts supported in small size honeycomb monoliths, in a cylindrical
reactor (cylindrical shape with 1 cm inner diameter) at atmospheric
pressure. The total flow rate was 500 mL/min (GHSV = 27000 h−1
). N2O
decomposition tests were carried out under different gas streams:
1000 ppm N2O/He or 1000 ppm N2O/1000 ppm NO/5 % O2/0.6 % H2O/He
(Figure 2.20).
Particular experiments performed in Chapter 5 with a powder
catalyst were carried out in the same reactor to that used to test the
monoliths, and the amount of powder catalyst introduced in the reactor is
similar to the amount of active phase present in a small-size monolith
catalyst. The powder catalyst was diluted with SiC for the experiments to
prepare a catalytic bed with the volume of a monolith. This facilitates the
comparison of results obtained with the powder active phase and with the
Chapter 2
70
supported catalysts, since N2O conversion results can be directly
compared.
Analysis Unit:
Cromatograph HP6890
Quimiluminiscence analyzer
Oven
Temperarture controller
Quartz reactor
Feed and mix gases unit
N2O
NO
He
H2OO2
Figura 2.20. Scheme and picture of the set-up used to perform the catalytic tests with
powder and small-size monolith catalysts.
2.2.2 Medium-size honeycomb monolith catalysts.
NOx reduction (SCR) and N2O decomposition experiments were
carried out with medium-size honeycomb monoliths catalysts in the
power-bench facilities belonging to the MCMA group of the University of
Alicante (see Figure 2.21).The system contains a Turbo Diesel 2.0 engine
running with commercially available diesel fuel at 880 rpm (idle conditions).
Characterization and experimental techniques
71
At this running conditions, the exhaust flow was 800 L/min, and the gas
composition was 17.2 % O2, 2.4 % CO2, 10 % H2O, 100 ppmV NO,
35 ppmV NO2, 120 ppmV CO and 10 ppmV THC (THC = total
hydrocarbons, expressed as CH4). The gas exhaust temperature ranged
from 90 to 50 ºC along the exhaust pipe.
Stream to
analyzers
Figure 2.21. Power bench for catalytic test of medium-size monolith catalysts
A gas stream of 10 L/min was continuously pumped out from the
main stream and was used to perform the tests. This gas flow passed
throughout two consecutive furnaces with independent control of
temperature, the first one containing a crucible with commercial diesel fuel
and the second one with the monolith catalysts.
Experiments in single or dual-bed configurations were performed.
Only the SCR catalyst (Pt/Beta zeolite/monolith) was placed in the second
furnace for single-bed experiments, and additionally, the deN2O catalyst
(RhOx/Ce0.9Pr0.1O2/monolith) was located in the same furnace,
downstream the SCR catalyst, for dual-bed experiments. The amount of
Chapter 2
72
diesel fuel evaporated in the first furnace, and used in the SCR process in
the second furnace, was controlled by fixing the temperature of the first
furnace. In these experiments, the space velocity (GHSV) in one monolith
was 19000 h-1
.
The gas composition was continuously monitored during the
experiments by specific gas analyzers from Signal Instruments for THC
(model 3000HM), CO, CO2 and N2O (model 7000FM) NO and NO2 (model
4000VM), and O2 (model 8000 M).
The following parameters were determined:
in
2in
out2
outin2
in
][NO[NO]
][NO-[NO]- ][NO [NO] 100· % removalNOx
(13)
out2
outin2
in
out2
2][NO-[NO]- ][NO [NO]
O][N 1/2 -1100· %y selectivit N (14)
in
outin
[THC]
[THC]- [THC] 100· % removal THC (15)
where the superscripts “in” and “out” refer to inlet and outlet
concentrations, respectively.
2.2.3 Full-size honeycomb monolith catalysts.
SCR experiments were also performed in a engine test bench
Horiba Titan S190 (Figure 2.22), with the full-size monolith loaded with
Pt/Beta zeolite and a 1.6 HDI diesel engine running with commercial diesel
fuel at 1100 rpm and different loading (torque between 45 and 83 N·m).
These experiments were performed in the facilities of the Galician
Automotive Technology Centre (Centro Tecnológico de Automoción de
Galicia, CTAG).
Characterization and experimental techniques
73
Diesel injector
gas to opacimeter
Gas flow Fuel injector
Gas to analyzers
Fuel pump
Pressure sensor
Pressure sensor
Thermocouple
Thermocouple
Gas to opacimeter
Figure 2.22. Photograph of the engine test bench and detail of the metal holder used
for the full-size SCR monolith catalyst, which is fitted in the exhaust pipe.
Depending on the engine loading, the temperature at the entrance
of the SCR catalyst and the gas composition ranged as indicated in Table
2.1
Table 2.1. Gas temperature and composition measured at the entrance of the monolith
catalyst (without fuel injection) for different engine loading.
Exhaust
gas flow
(kg/h)
Torque
(N·m)
GHSV
(h-1
)
Gas
temp
(ºC)
O2
(%)
CO2
(%)
CO
(ppm)
THC
(ppm)
NO
(ppm)
NO2
(ppm)
17 45 9460 220 8 11 534 310 91 6
17 60 10035 250 7 12 320 375 218 0
18 83 11640 300 5 13 455 143 411 0
Chapter 2
74
In these experiments the whole exhaust stream was treated.
Diesel fuel was used as reducing agent, being fed to the exhaust stream
by a fuel injector located at the entrance of the metal holder used to fit the
catalyst to the exhaust pipe (see Figure 2.22.). The catalyst holder was
designed and fabricated for this study. The holder was provided with five
connections at the inlet part of the monolith catalyst, in order to connect a
thermocouple, a pressure sensor, the fuel injection system and the two gas
conductions connected to the gas analyzers and gas opacimeter,
respectively. At the monolith catalyst exit, the holder was provided with
four connections for a thermocouple, a pressure sensor and also two gas
conductions connected to the gas analyzers and the gas opacimeter,
respectively. Horiba Mexa 7170D analyzers were used to follow gas
composition (NOx, CO, CO2, THC, and O2).
The fuel injection system consisted of a fuel pump, a
programmable touch screen, a programmable logic controller (PLC), and a
low-pressure injector. Both the frequency and amount of fuel pulsed could
be selected in order to ensure a precise control of the THC concentration
in the inlet gas stream.
SCR experiments were performed by feeding different amounts of
fuel in order to reach THC (total hydrocarbons) concentrations in the
exhaust (measured at the entrance of the catalyst holder) in the range
1000 - 5000 ppm. The SCR experiments were extended until all engine
parameters and gas compositions were at steady state.
Characterization and experimental techniques
75
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Chapter 2
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Stabilization of active Rh2O3 species for catalytic decomposition of
N2O on La-, Pr-doped CeO2. Journal of Catalysis 244 (2006) 102.
[13] http://sstti.ua.es/en/scientific-instrumentation/unit-single-crystal-x-
ray-and-vibrational-spectroscopy-and-optical/raman-
spectroscopy.html. Access May 2013
[14] http://www.lasurface.com/xps/index.php. Acces May 2013
[15] http://sstti.ua.es/en/scientific-instrumentation/x-ray-unit/x-ray-
photoelectron-spectroscopy.html. Acces May 2013
[16] N. Guillén-Hurtado, I. Atribak, A. Bueno-López, A. García-García,
Influence of the cerium precursor on the physico-chemical features
and NO to NO2 oxidation activity of ceria and ceria–zirconia
catalysts. Journal of Molecular Catalysis A: Chemical 323 (2010)
52.
[17] S. Parres-Esclapez, I. Such-Basáñez, M.J. Illán-Gómez,
C. Salinas-Martínez de Lecea, A. Bueno-López. Study by isotopic
gases and in situ spectroscopies (DRIFTS, XPS and Raman) of the
N2O decomposition mechanism on Rh/CeO2 and Rh/ɣ-Al2O3
catalysts. Journal of Catalysis 276 (2010) 390.
[18] H. Borchert, Y.V. Frolova, V.V. Kaichev, I.P. Prosvirin,
G.M. Alikina, A.I. Lukashevich, V.I. Zaikovskii, E.M. Moroz,
S.N. Trukhan, V.P. Ivanov, E.A. Paukshtis, V.I. Bukhtiyarov,
V.A. Sadykov. Electronic and chemical properties of
nanostructured cerium dioxide doped with praseodymium. Journal
of Physical Chemistry B 109 (2005) 5728.
[19] http://sstti.ua.es/en/scientific-instrumentation/microscopy-
unit/transmission-electron-microscopy.html. Access May 2013.
[20] R.F. Egerton. Physical principles of electron microscopy: an
introduction of TEM, SEM and AEM. Springer (2008).
Characterization and experimental techniques
77
[21] http://sstti.ua.es/en/scientific-instrumentation/thermal-analysis-
unit/thermogravimetry-tg-atd-tg-mass-spectrometry.html. Access
May 2013.
CHAPTER 3
Effect of the calcination conditions of
the RhOx/CeO2 catalysts on N2O
decomposition activity
In this chapter the effect of the calcination conditions (ramp
and flash calcinations, starting calcinations at 25 or 250/350 ºC,
respectively) of 2.5%RhOx/CeO2 catalysts on the catalytic activity for
N2O decomposition has been studied. The calcination conditions of
cerium nitrate have neither effect on the physicochemical properties of
ceria, observed by XRD, Raman spectroscopy and N2 adsorption, nor
on the catalysts performance for N2O decomposition. On the contrary,
flash calcination of rhodium nitrate improved the catalytic activity for
N2O decomposition. This is attributed to the small size of RhOx
nanoparticles obtained which allows a higher rhodium oxide-ceria
interface, favouring the reducibility of the ceria surface and stabilizing
the RhOx species under reaction conditions.
Effect of the RhOx/CeO2 calcination conditions
81
3.1 Introduction.
In catalytic applications, ceria is used either as metal support or as
catalyst itself [1-3]. The use of ceria as noble metal support has attracted
intense interest due to their vast applications in heterogeneous catalysis.
Noble metal/ceria catalysts are widely used in important applications such
as TWC for gas pollution control in gasoline vehicles [4], fluid catalytic
cracking (FCC) [1], PM combustion [5], selective catalytic reduction of NOx
[6], electrocatalyst in cathodes for phosphoric acid fuel cells [7, 8], partial
oxidation of methane [9], combustion of volatile organic compounds [10]
and N2O decomposition [11] among others. Many factors, including the
size and distribution of the noble metal particles, the surface morphology
and defects on the oxides, affect the performance of noble metal/ceria
catalysts. Indeed, effectively controlling the size of noble metal particles is
crucial for maintaining high catalytic activity. The behavior of the noble
metal-ceria interface is of critical importance in this regard [1, 3] and new
ways for optimizing the noble metal-ceria interactions, in order to improve
the catalytic performance, are being investigated [12].
The promoting effect of ceria-based materials in the catalytic
activity of rhodium, and other platinum group metals, is very well known for
several chemical reactions [4, 12-22]. The best example is found in TWC
used in gasoline vehicles for the simultaneous removal of NOx, HC and
CO. Also, rhodium, supported on different materials (ZnO, ceria, ZSM-5),
was investigated for N2O decomposition, Rh/ceria being the most active
[23].
In a previous work performed in our group, noble metals (rhodium,
palladium and platinum) supported on -Al2O3, pure ceria and lanthanum
and praseodymium-doped ceria were studied as N2O decomposition
catalysts [11]. It was concluded that rhodium is the most active noble metal
among those tested, and that the redox properties of the ceria support
affect the catalytic activity of the supported noble metals. For rhodium
catalysts, those with a ceria-based support present higher activity than
Rh/-Al2O3 demonstrating the participation of ceria, either pure or doped, in
the N2O decomposition process. The ceria support interacts strongly with
rhodium partially stabilizing cationic species of the noble metal during N2O
decomposition [24]. For this reason, the formulation adopted for these
Chapter 3
82
catalysts is RhOx/CeO2, evidencing the cationic nature of the metal. The
low-temperature activity of RhOx/CeO2 for N2O decomposition was
attributed to electron excess sites at the micro-interfaces between the
dispersed RhOx particles and the ceria support [24, 25].
The RhOx particles size and their interaction with the ceria support
could be affected by the preparation variables, such as the drying and
calcination conditions used during the powder catalysts synthesis. Such
drying and calcination conditions are also known to affect the distribution of
active phases along the honeycomb monoliths channels [26], which are
the real configuration for many practical applications. The preparation of
honeycomb monolith-supported catalysts is the objective of further
chapters of this thesis, and knowing the effect of the drying and calcination
conditions on the RhOx/CeO2 active phase performance will be useful to
design a suitable preparation procedure for the honeycomb
monoliths-supported catalysts used in further chapters.
Bearing this in mind, the goal of the current chapter is to get further
insights into the effect of the drying and calcination conditions on the
properties of N2O decomposition RhOx/CeO2 catalysts
3.2 Experimental.
3.2.1 Study of the metal precursors decomposition.
Decomposition of 10 mg of either Ce(NO3)36H2O or
ceria-impregnated with rhodium nitrate (Rh(NO3)3/Ce25 sample, see
section 3.2.2 for nomenclature details) was performed in air in a TG-MS
set up in order to select the adequate starting heating temperatures for
calcination of catalysts precursors. The experimental details of these tests
were described in Chapter 2.
3.2.2 Catalysts preparation.
Different RhOx/CeO2 catalysts have been synthetised for this
study. Powder CeO2 was prepared by Ce(NO3)3·6H2O (Alfa-Aesar,
99.5 wt.%) calcination in two different ways:
Effect of the RhOx/CeO2 calcination conditions
83
Ramp calcination: consisted of heating the cerium precursor in a
muffle furnace (static air) from 25 to 600 ºC at 10 ºC/min,
maintaining the maximum temperature for 90 min. This ceria
support is referred to as Ce25.
Flash calcination: consisted of heating the empty muffle furnace at
250 ºC. Then, the cerium precursor was introduced and the
temperature was raised at 10 ºC/min up to 600 ºC, maintaining the
maximum temperature for 90 min. This ceria support is referred to
as Ce250.
Rhodium was loaded on both ceria powders by incipient wetness
impregnation with an aqueous solution of Rh(NO3)3·xH2O (Sigma-Aldrich,
~36 wt.% as Rh) of the appropriate concentration to obtain 2.5 wt.% of
rhodium on the catalysts. The impregnated supports were placed in the
furnace immediately after impregnation. The rhodium precursor
decomposition was also performed using different calcination conditions
(ramp and flash). The muffle furnace temperature was stabilized at 25 ºC
(ramp calcination) or at 250 or 350 ºC (flash calcinations) before the ceria-
impregnated with rhodium nitrate was introduced (denoted by Rh25,
Rh250 and Rh350, respectively). In all cases the heating rate was
10 ºC/min and the final temperature was 500 ºC, maintaining this
temperature for 30 min. Following the described procedures, four
RhOx/CeO2 catalysts have been prepared, which are referred to as
Rh25Ce25, Rh250Ce25, Rh350Ce25 and Rh350Ce250.
3.2.3 Catalysts characterization.
Catalysts crystalline properties were studied by XRD and Raman
spectroscopy. BET surface area was determined from the N2 adsorption
isotherms at -196 ºC. XPS was used to determine the oxidation states of
the elements. In addition, samples reducibility was examined by
experiments of H2-TPR. Finally, particle size distribution of rhodium oxide
was determined by TEM. These experimental techniques and the
conditions used for each measurement were previously described in detail
in Chapter 2.
Chapter 3
84
3.2.4 N2O decomposition tests.
N2O decomposition tests were performed at laboratory scale using
a 100 mL/min flow (GHSV = 42000 h−1
) of 1000 ppm N2O in He, and
100 mg of catalyst. Details of these experiments were described in
Chapter 2.
3.2.5 CO oxidation tests.
CO oxidation has been selected as an additional test reaction, in
order to analyze if the conclusions achieved about the effect of the thermal
treatment conditions on the catalytic decomposition of N2O can be
extended to some other reactions. CO oxidation experiments were
performed at atmospheric pressure in the same experimental set-up used
for N2O decomposition (see details in Chapter 2). In this case 100 mL/min
flow of 1000 ppm CO/5 % O2/He and 100 mg of catalyst
(GHSV = 42000 h−1
) were used.
3.3 Results and discussion.
3.3.1 Thermogravimetry - Mass Spectroscopy (TG-MS) study of metal
precursors decomposition.
The thermograms corresponding to Ce(NO3)3·6H2O and
Rh(NO3)3/Ce25 decomposition are plotted in Figure 3.1 together with
representative MS signals. In both experiments, most water molecules
(m/z 18) evolved below 250 ºC with the corresponding weight loss. During
Ce(NO3)3·6H2O decomposition (Figure 3.1.a), three well-defined m/z 18
peaks were observed assigned to coordination water release. In the
Rh(NO3)3/Ce25 decomposition experiment (Figure 3.1.b) a broad m/z 18
band with two maxima appeared. It must be outlined the difference in the
Y-axis scales due to the low Rh(NO3)3 percentage in the sample studied.
Effect of the RhOx/CeO2 calcination conditions
85
0
10
20
30
40
50
60
70
80
90
100
8.00E-11
1.80E-10
2.80E-10
3.80E-10
4.80E-10
5.80E-10
6.80E-10
0 100 200 300 400 500 600 700 800
Ma
ss (%
)
MS
sig
nal (a
.u.)
Temperature (ºC)
(a)
MS 18 (H2O)
MS 30 + MS46 (NOx)
94
95
96
97
98
99
100
0 100 200 300 400 500 600 700 800
Ma
ss (%
)
MS
sig
nal (a
.u.)
Temperature (ºC)
(b)
MS 18 (H2O) MS 30 + MS46 (NOx)
Figure 3.1. Metal precursors decomposition followed by TG-MS. (a) Ce(NO3)3·6H2O
and (b) Rh(NO3)3/Ce25.
Cerium nitrate (Figure 3.1.a) decomposed in a relatively narrow
range of temperature (240-320 °C) with NO (m/z 30), NO2 (m/z 46) and O2
(m/z 32, which is not shown for clarity) release. This result lead us to select
25 and 250 ºC for the preparation of the ceria supports in ramp (slow water
release and slow nitrate decomposition) and flash (rapid water release and
slow nitrate decomposition) calcination conditions, respectively.
Ceria impregnated with rhodium nitrate (Figure 3.1.b) decomposed
in the range 200-550 ºC, and decomposition in such wide range of
temperature is an evidence of the interaction of the noble metal species
Chapter 3
86
with the ceria support (otherwise a narrower range of decomposition
temperatures would be expected). Taking the results of Figure 3.1.b into
account, three temperatures were selected to start the thermal treatments
for catalysts preparation: 25, 250 and 350 ºC.
In the calcination from 25 ºC, water release will be slow and the
impregnated rhodium salt will be allowed to move on the ceria particles
surface forced by the concentration gradients created during drying. Once
dry, rhodium nitrate is expected to be distributed on the ceria surface more
heterogeneously than if water is released very rapidly (as in calcinations
starting at 250 or 350 ºC), and this heterogeneity is expected to affect the
final RhOx particle size, as it will be demonstrated afterwards. It is also
expected that this kind of slow treatment starting at 25 ºC will also lead to a
poor distribution of active phase into a cordierite monolith when the
catalyst is scaled up in Chapters 5 and 6.
On the contrary, in the calcination starting at 250 ºC most water
release will be very rapid but not rhodium nitrate decomposition, while
starting at 350 ºC both water release and rhodium nitrate decomposition
will be fast.
3.3.2 XRD, Raman spectroscopy and N2 adsorption at -196 ºC
characterization.
All catalysts were characterized by XRD, Raman spectroscopy and
N2 adsorption at -196 ºC. These three techniques have provided
information mainly about the ceria support properties. XRD and Raman
spectroscopy are complementary, and provide information about the
structure of the ceria supports. XRD is sensitive to the cations position in
the ceria lattice, while Raman spectra of cerium oxides are caused by
oxide anions vibration.
All XRD diffractograms (Figure 3.2) show fluorite structure
characteristic reflections, corresponding to the (111), (200), (220), (311),
(222) and (400) planes. No other peaks but those of fluorite were observed
in the diffractograms [27] indicating that rhodium species should be highly
dispersed and/or should present low crystallinity.
Effect of the RhOx/CeO2 calcination conditions
87
10 20 30 40 50 60 70 80
Inte
nsity (
a.u
.)
2θ (º)
Rh25Ce25
Rh250Ce25
Rh350Ce25
Rh350Ce250
(111)
(200)
(220)(311)
Figure 3.2. XRD patterns of fresh catalysts.
The lattice parameter values were determined from XRD patterns,
and the values obtained are presented in Table 3.1. The differences are
within the experimental error of the determination. The average crystallite
sizes of the ceria particles were determined with the Scherrer’s and
Williamson-Hall’s equations [28]. The values obtained are included in
Table 3.1 as well as the BET surface areas. Similar crystallite sizes and
BET surface areas have been found for all catalysts.
Table 3.1. XRD and N2 adsorption characterization results.
*The lattice parameter experimental error is estimated to be ± 0.0014 nm in our
experimental conditions.
Raman spectra included in Figure 3.3 are also similar for all
catalysts. All of them show the main band at about 465 cm-1
assigned to
the F2g mode of the fluorite-type structure of cerium oxides, based on the
Sample
Lattice
parameter
(nm)*
Crystal Size
by Scherrer
(nm)
Crystal Size by
Williamson-Hall
(nm)
BET
surface
area
(m2/g)
Rh25/Ce25 0.5401 12 14 64
Rh250/Ce25 0.5412 12 13 66
Rh350/Ce25 0.5401 12 13 66
Rh350/Ce250 0.5424 12 13 69
Chapter 3
88
face-centered cubic cell [29,30], and a small peak at ca. 230 cm-1
assigned
to RhOx species [31-33].
150 250 350 450 550 650 750
Inte
nsity (
a.u
.)
Raman Shift (cm-1)
Rh25Ce25
Rh250Ce25
Rh350Ce25
Rh350Ce250
Figure 3.3. Raman spectra of fresh catalysts.
In conclusion, the XRD, Raman spectroscopy and N2 adsorption
characterization reveal that all the ceria supports seems to be very similar,
that is, the heating conditions used in the calcinations steps do not
significantly affect ceria properties.
3.3.3 N2O decomposition tests.
N2O decomposition experiments were performed with a 1000 ppm
N2O/He stream in the temperature range 175-275 ºC. The N2O
decomposition profiles in steady-state are plotted in Figure 3.4. As
expected, the N2O decomposition percentage increases with temperature,
reaching total conversion at 275 ºC for all the catalysts. According to these
results, and in agreement with previous XRD, Raman spectroscopy and N2
adsorption characterization, the decomposition conditions of cerium nitrate
has no significant effect on the catalytic performance (profiles for
Rh350Ce25 and Rh350Ce250 are almost equal) while rhodium nitrate
decomposition affects the activity. The catalyst where rhodium nitrate was
decomposed in ramp (starting heating at 25 ºC (Rh25Ce25)), which is the
most conventional calcination procedure, presents lower activity than
catalysts calcined by a flash procedure (Rh250Ce25 and Rh350Ce25
Effect of the RhOx/CeO2 calcination conditions
89
catalysts). This can be tentatively attributed to the improved interaction
between RhOx and ceria particles obtained by flash calcinations, and this
interaction is studied in detail in the coming sections.
0
20
40
60
80
100
175 200 225 250 275
N2O
decom
positio
n
(%)
Temperature (ºC)
Rh25Ce25
Rh250Ce25
Rh350Ce25
Rh350Ce250
Figure 3.4. N2O decomposition in steady state as a function of temperature.
Additional information about the catalyst performance is obtained
from the N2O decomposition profiles as a function of time for the different
temperatures, that is, from the behavior of the N2O decomposition profiles
before the steady-state is achieved. The steady state was reached after
20-30 minutes of reaction for all catalysts at temperatures below 225 ºC. At
225 ºC longer times were required to reach a constant N2O decomposition
level. On the other hand, at temperatures above 225 ºC the steady state
was again reached in 20-30 minutes. The curves corresponding to 225 ºC
are included in Figure 3.5 for selected catalysts (Rh25Ce25 and
Rh250Ce25; rhodium nitrate decomposed by ramp and flash calcinations,
respectively). From a qualitative point of view both profiles are similar, with
a first step of 35-40 minutes where the N2O decomposition level increased
until a pseudo steady-state was reached. The conversions then increased
again for 20-25 additional minutes before the real steady-state level was
stabilied. Despite both profiles on Figure 3.5 are qualitatively similar, the
sample Rh250Ce25 needed more time to reach the final steady-state than
Rh25Ce25, the former also being more active than the latter. The
transformations suffered by these two catalysts at 225 ºC before the
steady state was achieved were studied by XPS.
Chapter 3
90
35
40
45
50
55
60
0 20 40 60 80
N2O
decom
positio
n (
%)
Time (min)
Rh25Ce25
Rh250Ce25
Figure 3.5. N2O decomposition at 225 ºC as a function of time.
3.3.4 Characterization by XPS of fresh catalysts and after “in situ”
pre-treatments with N2O at 225 ºC.
XPS spectra were recorded with the fresh Rh250Ce25 and
Rh25Ce25 catalysts and with these catalysts pre-treated in situ with
1000 ppm N2O/He at 225 ºC for different periods of time. The profiles
obtained for the Rh 3d (see Figure 3.6 as an example) and Ce 3d
transitions were qualitatively similar to those previously obtained with some
other RhOx/ceria catalysts [24, 34, 35]. The percentages of Rh0 (with
regard to total rhodium) and Ce3+
(with regard to total cerium) were
calculated following the methods reported elsewhere [24, 34, 35] and
described in Chapter 2. Rh0
and Ce3+
percentages are compiled in Table
3.2.
Effect of the RhOx/CeO2 calcination conditions
91
303 308 313 318
Inte
nsity (
a.u
.)
Binding Energy (eV)
t=0 min
t=5 min
t=15 min
t=60 min
Figure 3.6. Rh 3d photoelectron spectra of the Rh25Ce25 catalyst as an example.
As described in Chapter 2, the peaks assigned to Rh0 appear at
307.0–307.5 eV and the one assigned to Rh3+
appear from 308.3 to
310.5 eV. Therefore, rhodium appeared fully oxidized on both fresh
catalysts (t = 0 min), which was expected since the catalysts were calcined
at 500 ºC, and the Ce3+
percentages (38-40 %) were in accordance with
values typically obtained for similar synthesis conditions [24, 34, 36].
As observed in Figure 3.6 and Table 3.2, rhodium was
progressively reduced and only Rh0 was identified in both catalysts after
the 15 min pretreatment, while after 60 minutes both Rh3+
and Rh0
were
observed again. The behavior of the cerium oxidation state (Table 3.2) was
qualitatively similar to that of rhodium, Ce4+
being first slightly reduced to
Ce3+
and reoxidized afterwards. It has to be mentioned that only the trend
of the metals oxidation state must be considered, but not the absolute
values, since the reducing environment of the XPS measurements (high
vacuum and an electron beam) could affect such absolute oxidation state
values.
These XPS results (Table 3.2), together with the N2O
decomposition profiles included in Figure 3.5, evidence that the
RhOx/CeO2 catalysts suffer an activation process at 225 ºC, and that the
reoxidation of the catalysts (both for rhodium and cerium) is stronger for
the most active catalyst (Rh250Ce25; see Table 3.2 for time = 60 min),
suggesting a deeper transformation during the activation period.
Chapter 3
92
Table 3.2. Rh0 and Ce
3+ percentages determined by XPS after in situ thermal
treatments with 1000 ppm N2O at 225 ºC for different times.
This activation process is consistent with the reaction mechanism
previously proposed for the RhOx/CeO2-catalyzed N2O decomposition
[24, 36]:
Rh–O + N2O → Rh* + N2 + O2 (1)
Rh* + N2O → Rh*–O + N2 (2)
2Rh*–O ↔ 2Rh* + O2 (3)
Rh* + Ce*–O → Rh*–O + Ce* (4)
Ce* + N2O → Ce*–O + N2 (5)
2Ce*–O ↔ 2Ce* + O2 (6)
Rh*–O + Ce*–O ↔ Rh* + Ce* + O2 (7)
Ce–OH + N2O → Ce* + N2 + H2O (8)
Ce* + Ce–O + H2O → 2Ce–OH (9)
Sample
Pre-treatment
Time
(min)
Ce3+
(%)
Rh0
(%)
Rh25Ce25 0 38 0
5 40 34
15 42 100
60 39 75
Rh250Ce25 0 40 0
5 43 28
15 44 100
60 38 60
Effect of the RhOx/CeO2 calcination conditions
93
where Rh*–O and Rh* represent oxidized and reduced rhodium sites,
respectively, and Ce*–O and Ce* are oxidized and reduced ceria sites.
Before the steady state is achieved, the Rh3+
and Ce4+
reduction
observed by XPS evidence that the reducing steps (1), (3), (6), (7) and (8)
prevail against those where N2O oxidizes catalysts sites (steps (2) and
(5)), which makes sense taking into account that the fresh catalysts is
highly oxidized. After some time, once rhodium is reduced, N2O and also
Ce4+
oxide partially re-oxidizes the rhodium sites via steps (2), (4) and (5),
reaching a net null balance between the oxidizing and reducing steps
rates, that is, achieving the real steady state.
Coming back to the fact that this behavior was neither observed
below 225 ºC nor above this temperature, it is postulated that the
activation process does not occurs at low temperatures because the steps
involving reduction and re-oxidation of cerium are not occurring in a great
extent. Secondly, since the experiments are performed sequentially, the
effect is not observed at 250 ºC and higher temperatures because the
catalysts have already been activated at 225 ºC.
3.3.5 H2-TPR characterization.
Additional information about the catalysts has been obtained by
H2-TPR. The TCD profiles obtained in these experiments are included in
Figure 3.7. Three peaks are observed for all catalysts. The
lowest-temperature peak at around 100 ºC is attributed to both RhOx
reduction and noble metal-catalyzed surface reduction of ceria due to the
hydrogen spillover on the surface [14, 35, 37-39]. The TCD signal
appearing between 200 and 400 ºC can be attributed to different events:
the reduction of surface cerium oxide which is not in close contact with
rhodium, the decomposition of surface carbonates (occluded within the
ceria structure) and/or the reduction of hydroxyl and peroxide/superoxide
surface groups [15,40-42]. The H2 consumed above 700 ºC is assigned to
bulk ceria reduction, that is, to the reduction of Ce4+
cations placed within
the bulk oxide particles.
A detailed analysis of the lowest temperature reduction peak
shows a slightly lower reducibility of the catalyst prepared by ramp
calcination (Rh25Ce25, see Figure 3.7) in comparison to the flash calcined
Chapter 3
94
catalysts. A larger ceria surface reducibility must be a consequence of a
better noble metal-support interaction since the process occurs through the
catalytic action of the noble metal.
0 150 300 450 600 750 900 1050
TD
C s
ignal (a
.u.)
Temperature (ºC)
Rh25Ce25
Rh250Ce25
Rh350Ce25
Rh350Ce250
25 50 75 100 125 150 175 200
TC
D s
ign
al (a
.u.)
Temperature (ºC)
Figure 3.7. H2-TPR profiles of fresh catalysts after an in situ pretreatment with
5 % O2/He at 500 ºC.
The catalysts with larger reducibility at low temperature also exhibit
the higher catalytic activity for N2O decomposition (Figure 3.4). This
observation is in agreement with the conclusions of previous studies,
where a relationship between ceria surface reducibility and N2O
decomposition capacity was obtained for a set of RhOx/CeO2 catalysts
prepared with different ceria carriers (either pure and doped with
lanthanum or praseodymium) [11]. Moreover, the rate limiting step of the
RhOx/CeO2 catalyzed N2O decomposition mechanism, once the steady
state is achieved, was reported to be the reduction of the catalyst sites by
N2O [24]. The redox properties of the support are decisive for RhOx
stabilization under reaction conditions, the larger the amount of reducible
CeO2 at the surface, the better the catalytic activity. These results suggest
that flash calcination of rhodium nitrate (starting heating either at 250 or
350 ºC) allows obtaining a much better noble metal–support interaction
than the conventional ramp calcinations (starting heating at 25 ºC)
[24, 35, 43], which was confirmed by TEM analysis.
Effect of the RhOx/CeO2 calcination conditions
95
3.3.6 TEM characterization.
Catalysts Rh25Ce25 and Rh250Ce25 were selected for TEM
characterization. Figure 3.8 shows, as an example, a representative
micrograph of each catalyst. The typical crystalline planes of the fluorite
structure of ceria are clearly observed in both micrographs as well as some
RhOx particles.
In the catalyst prepared by conventional ramp calcination
(Rh25Ce25), RhOx particles smaller than 3 nm are distinguished. In this
case, the number of RhOx particles which are clearly distinguished from
the ceria support is enough to make a RhOx particle size distribution (inset
in Figure 3.8.a). Most RhOx particles size is in the range 0.5-1 nm.
In the catalyst prepared by flash calcination of rhodium nitrate
(Rh250Ce25; Figure 3.8.b), RhOx particles have been hardly observed,
assuming that most of them are much smaller than 0.5 nm, and evidencing
a better dispersion of RhOx on this catalyst. This difference in RhOx size is
attributed to the different water release rate during drying. Water
evaporates much slower during conventional ramp calcination than during
a flash heating, creating concentration gradients along the ceria surface.
Consequently, rhodium nitrate is more heterogeneously distributed than if
water is released very rapidly (as in flash calcination) affecting the final
RhOx particle size.
As a summary, smaller RhOx particles are supported on ceria by
flash calcinations of impregnated rhodium nitrate in comparison to
conventional ramp calcinations. Catalysts prepared by this procedure are
more active than the counterpart prepared in ramp, since the RhOx-ceria
interface is improved.
Chapter 3
96
0.0 0.5 1.0 1.5 2.0 2.5 3.00
10
20
30
40
50
60
Per
centa
ge
(%)
Particle size (nm)
(a) (b)
Figure 3.8. TEM micrographs of catalysts (a) Rh25Ce25 and (b) Rh250Ce25.
3.3.7 CO oxidation tests.
As discussed, the flash calcination of ceria-impregnated rhodium
nitrate improves the catalytic activity for N2O decomposition with regard to
conventional ramp heating. In order to analise if this is a particular
conclusion that only applies to the N2O decomposition reaction, or on the
contrary, it can be extended to some other catalyzed reactions, CO
oxidation was selected as catalytic reaction test with practical relevance.
Figure 3.9 plots CO conversions obtained in steady state at
temperatures from 25 ºC to 90 ºC.
The activity of the catalyst prepared by flash calcination is
considerably higher than that calcined in ramp, showing significant activity
even at room temperature. For instance, the amount of CO oxidized at
45 ºC by Rh250Ce25 doubles that of the catalyst calcined by conventional
ramp. The CO oxidation rates achieved by this novel catalyst with RhOx
sub-nanoparticles are also much higher than those typically reported in
literature for rhodium catalyst [44].
Effect of the RhOx/CeO2 calcination conditions
97
0
20
40
60
80
100
0 25 50 75 100
CO
oxid
ation
(%
)
Temperature (ºC)
Rh25Ce25
Rh250Ce25
Figure 3.9. CO conversion in oxidation reactions as a function of temperature for
Rh250Ce25 and Rh25Ce25.
In accordance with the previous discussion about the N2O
decomposition results and about the catalysts characterization, the
enhanced activity for CO oxidation of the catalyst prepared by flash
calcination is attributed to the larger noble metal oxide-support interface
due to the smaller RhOx particles obtained by flash calcination. Highly
active catalytic sites are located at the RhOx-ceria interface, as reported in
previous studies for CO oxidation and also for some other catalytic
reactions. For instance, it has been reported that the oxidation of CO
occurs at the interface between the rhodium particles and the ceria support
[45], and also that the active sites for low temperature N2O dissociation are
located at the RhOx-CeO2 interface [24].
3.4 Conclusions.
The conclusions of this study can be summarized as follows:
The calcinations method (ramp or flash) of cerium nitrate to obtain
the ceria support has no effect neither on the ceria properties
(those observed by XRD, Raman spectroscopy and N2 adsorption)
Chapter 3
98
nor on the RhOx/CeO2 catalyst performance for N2O
decomposition.
Flash calcination of rhodium nitrate impregnated on ceria improves
the catalytic activity for N2O decomposition and CO oxidation of
RhOx/CeO2 catalysts in comparison to that of similar catalysts
calcined in ramp.
The improved catalytic activity for N2O decomposition and CO
oxidation of catalysts with rhodium nitrate decomposed by flash
calcinations is attributed to the smaller size of RhOx nanoparticles
(smaller than 0.5 nm), which allow a larger noble metal
oxide-support interface.
The improved noble metal oxide-support interface favors the
reducibility of the ceria surface and stabilizes the RhOx species
under N2O decomposition conditions.
Fresh RhOx/CeO2 catalysts suffer a transformation process at
225 ºC during the N2O decomposition reaction, during which both
rhodium and cerium are reduced and re-oxidized. This process is
consistent with the mechanism proposed for N2O decomposition on
RhOx/CeO2.
Effect of the RhOx/CeO2 calcination conditions
99
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(
b)
CHAPTER 4
Preparation of RhOx/CeyPr1-yO2 N2O
decomposition catalysts by rhodium
nitrate impregnation with different
solvents
The effect of the solvent (water, ethanol or acetone) used to
impregnate CeyPr1-yO2 (y = 1, 0.9 or 0.5) supports with rhodium nitrate
in order to prepare N2O decomposition catalysts has been studied. The
activity for N2O decomposition of the catalysts studied was related with
the RhOx-support interaction, and both the nature of the ceria support
and of the solvent used for rhodium impregnation affected such
interaction. Ceria doping with 10 % praseodymium had a positive effect
on RhOx-support interaction, but the benefit on the catalytic activity
was only obtained for water impregnation. The interaction between
RhOx and Ce0.5Pr0.5O2 was not as good as that with Ce0.9Pr0.1O2. The
best catalyst was obtained by impregnating Ce0.9Pr0.1O2 with a water
solution of rhodium.
Effects of the solvents
107
4.1 Introduction.
In addition to the reasons mentioned in Chapters 1 and 3, ceria
based materials are of interest in catalysis because of their oxygen storage
capacity (OSC) and lattice oxygen mobility. These properties are
dependent of the crystal size and defects as well as could be varied by
ceria doping. Regarding ceria-based oxides used as a metal catalyst
support, they are materials affecting the catalytic behavior of the metals
loaded. In this sense, praseodymium-doped ceria has showed enhanced
performance with regard to pure ceria as rhodium oxide support for
catalytic decomposition of N2O [1]. Incorporation of praseodymium to ceria
modifies the lattice oxygen properties and catalytic performance, due to
the lower metal-oxygen binding energy in the mixed oxide, and the
increase in the number of defects (oxygen vacancies); improving the N2O
decomposition activity of rhodium oxide when combined with the
praseodymium doped ceria [2, 3].
In the previous chapter, the calcination conditions were modified in
order to improve the distribution of rhodium and to enlarge the
rhodium-ceria interface of RhOx/CeO2 catalysts. Improved catalytic activity
for N2O decomposition and CO oxidation was obtained by flash
calcination, which consisted of introducing the ceria support-impregnated
rhodium precursor in a furnace which was pre-heated at 250 ºC. The
solvent used for rhodium precursor impregnation was water, as usually,
and the speed at which water evaporates from the ceria-based support
seemed very important for the catalyst properties, because it affects the
size of the RhOx particles on the final catalyst [4, 5]. This lead us to look for
different procedures to accelerate the evaporation of the rhodium
precursor solvent, and one of the options was to change water by some
other more volatile solvent.
In addition, the features of the solvent used in the impregnation
step not only potentially affects the distribution of the impregnated metal
precursor, due to the evaporation rate, but could also had some other
important roles. The optimum conditions for a successful infiltration of the
support pores mainly depend on the surface polarity, the polarity of the
solvent, and the solubility of the precursor in the solvent. For a surface with
a high density of polar functions a polar solvent will be suitable, in order to
Chapter 4
108
obtain a high degree of wettability and convenient diffusion through the
pores [6]. The polarity and density of the solvent used to impregnate a
metal precursor are also important parameters to be taken into account in
the impregnation of metal precursors on coated substrates, such us
honeycomb monoliths previously coated with a mixture of oxides (alumina,
ceria-based oxides, etc), because the solution must enter into the
substrate channels [7]. Therefore, the conclusions of this study will be also
useful for the preparation of honeycomb monoliths-supported catalysts in
further chapters.
As far as we know, the effect of the solvent used to impregnate
praseodymium doped ceria supports (samples that will be referred as
doped cerias from now on) with a rhodium salt on the properties of the
obtained catalysts has not been reported, and the goal of the current study
is to compare three solvents for such purpose: water, ethanol and acetone.
4.2 Experimental.
4.2.1 Catalysts preparation.
Nine catalysts, labelled as RhOx(solvent)/CeyPr1-yO2, were
prepared, the solvent could be water, ethanol or acetone and y takes
values of 1, 0.9 or 0.5. Cerium and praseodymium nitrate precursors
(Ce(NO3)3·6H2O (Aldrich, 99.99 wt.%) and (Pr(NO3)3·6H2O (Aldrich,
99.9 wt.%)) were mixed in an agate mortar to obtain CeO2, Ce0.9Pr0.1O2
and Ce0.5Pr0.5O2 after calcination at 600 ºC for 90 min (heating rate
10 ºC/min). Rhodium was loaded on these oxides by incipient wetness
impregnation with the proper amount of Rh(NO3)3·xH2O (Sigma-Aldrich,
~36 wt.% as Rh) dissolved in water, ethanol or acetone in order to obtain
1 wt.% rhodium on the final catalysts. The catalysts were calcined in flash
conditions, that is, the impregnated supports were introduced in a muffle
furnace that was pre-heated at 250 ºC, and then the temperature was
increased at 10 ºC/min up to 500 ºC (the maximum temperature was
maintained for 30 min).
Additionally, three portions of bare ceria were impregnated with
rhodium nitrate solutions using water, ethanol and acetone, respectively,
and were placed in test tubes with a thermocouple located inside the
Effects of the solvents
109
solids. The tubes were introduced in a vertical furnace that was previously
heated at 250 ºC, and the temperature was registered as a function of
time.
4.2.2 Catalysts characterization.
The techniques used for catalysts characterization have been XRD
and Raman spectroscopy to study the crystalline properties of the
catalysts, XPS to determine oxidation states and surface composition,
H2-TPR to know the catalysts reducibility, N2 adsorption isotherms to
determine the BET surface area and TEM to observe RhOx particle size.
The details of the experimental systems and procedures were described in
Chapter 2.
4.2.3 N2O decomposition tests.
N2O decomposition tests were performed at laboratory scale using
a 100 mL/min flow (GHSV = 42000 h−1
) of 1000 ppm N2O in He, and
100 mg of catalyst. Details of these experiments were described in
Chapter 2.
4.3 Results and discussion.
4.3.1 Catalysts temperature during rhodium nitrate thermal
decomposition.
The effect of the solvent used for rhodium precursor impregnation
on the temperature profile during the further calcination step was studied
as described in section 4.2.1 of the present chapter. The same experiment
was performed with an empty test tube. The temperature profiles
registered are plotted on Figure 4.1.
Chapter 4
110
0
50
100
150
200
250
300
350
0 5 10 15 20 25 30
Tem
pera
ture
(ºC
)
Time (min)
empty test tube
RhOx(acetone)/CeO2
RhOx(H2O)/CeO2
RhOx(ethanol)/CeO2
Figure 4.1. Temperature profiles during the thermal treatment of ceria-impregnated
rhodium precursor (using water, ethanol or acetone as solvent) in test tubes placed in a
vertical furnace pre-heated at 250 ºC.
The temperature profile of the empty test tube shows a sharp
increase during approximately 5 minutes followed by a smooth increase
reaching a constant value around 220 ºC (slightly lower than the set-point
temperature = 250 ºC) after 10 minutes. The temperature profiles of the
impregnated ceria samples are different. In the case of the sample
impregnated with the water solution, the temperature increased until
100 ºC, and reached a plateau at this temperature that can be attributed to
water evaporation. A second increase of temperature occurs afterwards,
reaching the same temperature than the empty tube in 15 min. The
samples impregnated with the ethanol or acetone solutions exhibit a first
step of solvent evaporation (boiling temperature = 78 ºC and 56 ºC for
ethanol and acetone, respectively) but considerable shorter than in the
case of water, since a sharp increase in temperature is observed reaching
a temperature considerably higher than the furnace temperature
(as observed for the empty tube). This behavior is attributed to the
exothermal combustion of the solvent. As it will be appealed for several
times throughout the chapter, the temperature increase ocurred during the
thermal treatment affects the final features of the catalysts impregnated
with acetone or ethanol rhodium solutions.
Effects of the solvents
111
4.3.2 N2O decomposition tests.
N2O decomposition catalytic tests were performed with the nine
catalysts prepared, and the conversion curves obtained are compiled in
Figure 4.2.
The nature of the ceria-based support and the solvent used for
rhodium precursor impregnation affect the final activity of the catalysts. For
bare ceria, the type of solvent has no effect on the behavior of the
catalysts (Figure 4.2.a). The three RhOx(solvent)/CeO2 catalysts
decompose N2O from 200 ºC approximately and achieve total
decomposition at 375 ºC following the same decomposition profile. On the
contrary, for the catalysts prepared with doped ceria supports the solvent
used for rhodium precursor impregnation strongly modifies their behavior
(Figures 4.2.b and 4.2.c). Regardless the molar fraction of praseodymium
on doped-ceria, the best results were obtained with catalysts impregnated
with the water solution of rhodium nitrate. Both ethanol and acetone
impregnations lead to a significant decrease on the catalytic activity with
regard to the counterpart catalysts impregnated with water. The N2O
decomposition curves obtained for praseodymium-containing catalysts
impregnated with acetone or ethanol rhodium solution were delayed by
50-75 ºC with regard to curves of the catalysts impregnated with the water
solution of rhodium. The effect of the solvent on the physicochemical
properties of the catalysts, and at the end on their catalytic performance, is
analyzed in detail in the coming sections.
Chapter 4
112
0
20
40
60
80
100
200 225 250 275 300 325 350 375 400 425
N2O
de
co
mp
ositio
n(%
)
Temperature (ºC)
RhOx(H2O)/CeO2
RhOx(ethanol)/CeO2
RhOx(acetone)/CeO2
(a)
0
20
40
60
80
100
200 225 250 275 300 325 350 375 400 425
N2O
decom
positio
n(%
)
Temperature (ºC)
RhOx(H2O)/Ce0.9Pr0.1O2
RhOx(ethanol)/Ce0.9Pr0.1O2
RhOx(acetone)/Ce0.9Pr0.1O2
(b)
0
20
40
60
80
100
200 225 250 275 300 325 350 375 400 425
N2O
decom
positio
n(%
)
Temperature (ºC)
RhOx(H2O)/Ce0.5Pr0.5O2
(c)
RhOx(ethanol)/Ce0.5Pr0.5O2
RhOx(acetone)/Ce0.5Pr0.5O2
(c)
Figure 4.2. N2O decomposition as a function of temperature for catalysts supported on:
(a) CeO2, (b) Ce0.9Pr0.1O2 and (c) Ce0.5Pr0.5O2.
Effects of the solvents
113
The temperatures required to decompose 50 % of N2O (T50) in
these catalytic tests have been compiled in Table 4.1.
Table 4.1. Temperature required to decompose 50 % of N2O (T50) in the catalytic tests.
Catalyst T50 (ºC)
RhOx(H2O)/CeO2 252
RhOx(ethanol)/CeO2 252
RhOx(acetone)/CeO2 252
RhOx(H2O)/Ce0.9Pr0.1O2 242
RhOx(ethanol)/Ce0.9Pr0.1O2 287
RhOx(acetone)/Ce0.9Pr0.1O2 301
RhOx(H2O)/Ce0.5Pr0.5O2 252
RhOx(ethanol)/Ce0.5Pr0.5O2 326
RhOx(acetone)/Ce0.5Pr0.5O2 319
Comparing the T50 values, it can be concluded that the
impregnation with ethanol or acetone rhodium solutions has a negative
effect on catalysts supported on doped ceria with regard to catalysts
supported on bare ceria. However, using water as the solvent, superior
performance of RhOx(H2O)/Ce0.9Pr0.1O2 is observed with regard to
catalysts with bare and 50 % praseodymium doped ceria supports. The
positive effect of 10 % ceria doping with praseodymium is in agreement
with previous publications [2, 8]. The N2O decomposition capacity of
catalysts prepared by water impregnation of rhodium follows the trend:
RhOx/Ce0.9Pr0.1O2 > RhOx/CeO2 ≈ RhOx/Ce0.5Pr0.5O2
According to this trend, ceria doping with 10 % praseodymium had
a positive effect on the catalytic activity, as already observed [2, 8], while
50 % praseodymium doping had no effect. As it will be discussed
afterwards, the amount of praseodymium not only affects ceria properties
but also the RhOx-ceria interaction and this can explain the observed
trend. For a future work, it will be desirable to perform an optimization
study of the praseodymium amount on the RhOx/CeyPr1-yO2 catalysts
Chapter 4
114
(by using water impregnation of rhodium precursor), but this is out of the
scope of the current study.
4.3.3 Catalysts characterization by N2 adsorption at -196 ºC, XRD and
Raman spectroscopy.
To understand the effect of the solvent used for rhodium precursor
impregnation on the final activity of the catalysts, N2 adsorption at -196 ºC,
XRD and Raman spectroscopy techniques were used to analyze the
physicochemical properties of the materials prepared. The methods used
were described in Chapter 2. These techniques provide (not only but
mainly) information about the properties of the ceria-based supports. The
characterization results obtained are presented in Table 4.2, including the
BET surface area of the supports and catalysts; the ceria supports crystal
size and the lattice parameters of the ceria-based supports determined by
XRD analysis.
Table 4.2. Results of the N2 adsorption and XRD characterization.
Sample
BET
surface
area (m2/g)
Crystal
size
(nm)
Lattice
parameter
(nm)
CeO2 61 - -
RhOx(H2O)/CeO2 60 14 0.5413
RhOx(ethanol)/CeO2 60 14 0.5412
RhOx(acetone)/CeO2 56 15 0.5412
Ce0.9Pr0.1O2 50 - -
RhOx(H2O)/Ce0.9Pr0.1O2 50 18 0.5417
RhOx(ethanol)/Ce0.9Pr0.1O2 31 21 0.5416
RhOx(acetone)/Ce0.9Pr0.1O2 33 21 0.5415
Ce0.5Pr0.5O2 18 - -
RhOx(H2O)/Ce0.5Pr0.5O2 20 24 0.5412
RhOx(ethanol)/Ce0.5Pr0.5O2 17 21 0.5420
RhOx(acetone)/Ce0.5Pr0.5O2 18 21 0.5420
Effects of the solvents
115
The BET surface areas of all catalysts prepared with the un-doped
ceria support are almost equal (56-60 m2/g), regardless the solvent used
for rhodium impregnation, and are also similar to that of the ceria support
(61 m2/g). In accordance with the BET values, the ceria crystal sizes and
ceria lattice parameters corresponding to these three catalysts are also
similar to each other. These results allow concluding that the nature of the
solvent used for rhodium impregnation do not affect the particle size/area
(both parameters are related to each other in this type of oxides) of the
bare ceria support, which is in agreement with the same catalytic activity
obtained with the three praseodymium-free catalysts (see Figure 4.2.a).
The BET surface area of the Ce0.9Pr0.1O2 support is 50 m2/g,
slightly lower than that of the pure ceria. The catalysts with Ce0.9Pr0.1O2
support impregnated with ethanol or acetone solutions present a
considerably lower BET area (31-33 m2/g) than the support, while the
catalyst impregnated with the water solution of rhodium
(RhOx(H2O)/Ce0.9Pr0.1O2) keeps the same BET area than the support
(50 m2/g). These results must be related to the N2O decomposition results
obtained with these three Ce0.9Pr0.1O2-supported catalysts (see
Figure 4.2.b), that is, the highest activity was obtained with the
water-impregnated catalyst (also with the highest BET surface area among
catalysts of this series) and the worse catalytic results were obtained with
ethanol/acetone-impregnated catalysts. As it was previously demonstrated
(see Figure 4.1), temperature gradients are created during the calcination
of catalysts that are impregnated with ethanol or acetone rhodium
solutions (while not with water). These gradients created due to the
exothermic combustion of the solvents should favor Ce0.9Pr0.1O2 sintering
and decrease the activity of the resulting catalysts. The BET surface area
of a Ce0.9Pr0.1O2 sample impregnated with acetone (but without rhodium)
and calcined under the same conditions than the catalysts was 50 m2/g
(the same than that of fresh Ce0.9Pr0.1O2). This is an evidence of the
rhodium role catalyzing the solvents combustion and support sintering.
Finally, the BET area is low and very similar for all
Ce0.5Pr0.5O2-containg catalysts (17-20 m2/g), being also similar to that of
the Ce0.5Pr0.5O2 support. This means that rhodium impregnation and
further calcination do not affects the area of this Ce0.5Pr0.5O2 support,
which is already much lower to that of CeO2 synthesized in equal
conditions (61 m2/g). In this case, a relationship between catalytic activity
Chapter 4
116
and catalyst sintering during calcination is not found, since the BET areas
of all Ce0.5Pr0.5O2-containg catalysts are similar while important differences
on activity were observed (see Figure 4.2.c). As it will be demonstrated by
XPS, TEM and H2-TPR characterization afterwards, the RhOx-CeyPr1-yO2
interaction also plays a key role on the activity of these catalysts, and the
nature of the solvent used for rhodium impregnation affects such
interaction.
As a summary, the BET surface area of CeO2 (61 m2/g) and
Ce0.5Pr0.5O2 (18 m2/g) do not change significantly upon rhodium
impregnation and calcination, regardless the solvent used for rhodium
impregnation, while the area of Ce0.9Pr0.1O2 (50 m2/g) drops (to
31-33 m2/g) upon rhodium impregnation with ethanol or acetone solutions
and further calcination. On the contrary, impregnation with the water
solution of rhodium precursor does not affect the surface area of
Ce0.9Pr0.1O2.
Additional information about the features of the ceria-based
supports was obtained from the XRD diffractograms (Figure 4.3) and
Raman spectra (Figure 4.4) of the catalysts. All X-ray diffractograms only
contain the main reflections typical of a fluorite-structured material with a
face centered cubic unit cell, corresponding to the (111), (200), (220),
(311), (222) and (400) planes. Evidences of segregated phases are not
obvious on Figure 4.3. However, the presence of segregated PrOx species
is difficult to be detected by XRD, because the XRD patterns of such PrOx
species are quite similar to that of ceria [9]. However, in some doped ceria
samples, asymmetric XRD peaks could suggest the presence of
segregated CeO2-rich and PrOx-rich separated phases, but this is not the
case of the diffractograms on Figure 4.3.
Effects of the solvents
117
10 20 30 40 50 60 70 80
Inte
nsity (
a.u
.)
2θ(°)
(a)
(111)
(200)
(220)(311)
(222) (400)
3
1
2
27.5 28 28.5 29 29.5
Inte
nsity (
a.u
.)
2θ(°)
28.54⁰
10 20 30 40 50 60 70 80
Inte
nsity (
a.u
.)
2θ(°)
(b)(111)
(200)(220)
(311)
(222) (400)
6
4
5
27.5 28.5 29.5
Inte
nsity (
a.u
.)
2θ(°)
28.52⁰
10 20 30 40 50 60 70 80
Inte
nsity (
a.u
.)
2θ(°)
9
7
8
(c)(111)
(200)(220)
(311)
(222) (400)
27.5 28 28.5 29 29.5
Inte
nsity (
a.u
.)
2θ(°)
28.50⁰
28.54⁰
Figure 4.3. X-ray diffractograms of catalysts supported on (a) CeO2, (b) Ce0.9Pr0.1O2
and (c) Ce0.5Pr0.5O2. Diffractograms 1, 4, 7 correspond to samples impregnated with
ethanol; 2, 5, 8 to samples impregnated with acetone and 3, 6, 9 to samples
impregnate with water.
Chapter 4
118
The position and shape of the diffraction peaks is quite similar for
all catalysts. See, for instance, the zoom of the (111) peaks inset on
Figure 4.3. As a result, the lattice parameter of the ceria-based supports is
also quite similar for all catalysts (see data on Table 4.2). The expansion
and contraction of the crystal lattice is expected to occur due to ceria
doping with large or small cations, respectively [10]. However, the sizes of
the Ce3+/4+
cations (0.114 nm/0.097 nm) are quite similar to those of the
Pr3+/4+
cations (0.113 nm/0.096 nm), and therefore, the partial substitution
of cerium by praseodymium cations has a minor effect of the lattice
constant of doped ceria. The slightly higher lattice constant values
obtained with some doped ceria catalysts, with regard to values of
catalysts with bare ceria, must be attributed to the presence of more
+3 cations, which are larger than +4 cations, mainly Pr3+
because Pr4+
is
reduced more easily than Ce4+
[11].With regards to crystal size the data
are in accordance to the changes observed in BET surface area.
Raman spectroscopy characterization is consistent with XRD
conclusions. Evidences of praseodymium incorporation into the ceria
framework have been observed by this technique. As a general behavior,
four Raman bands are detected on the spectra included on Figure 4.4, but
these four bands are not clear in all spectra.
The band at 444-463 cm−1
is ascribed to the Raman active F2g
mode of fluorite ceria. This can be viewed as a symmetric breathing mode
of the oxygen atoms surrounding each cation. The intensity of this peak is
highest for catalysts with the bare ceria support (Figure 4.4.a). An slight
deformation of ceria structure can be elucidated because of the
introduction of praseodymium into the ceria structure (in agreement with
the lattice parameter determinated by XRD, Table 4.2), due to the
presence of Pr3+
cations that are bigger than Ce4+
, affecting the oxygen
breathing mode and therefore F2g signal intensity. In addition, fluorescence
produced by praseodymium also diminishes the intensity of the main peak.
Moreover, it is also important to highlight the position of F2g peak, which
undergoes a shift towards lower Raman Shifts when increasing the
praseodymium content, being an evidence of praseodymium introduction
within the fluorite lattice of ceria (Figure 4.4). These results are in
agreement with other reported work [9].
Effects of the solvents
119
150 350 550 750 950 1150 1350
Inte
nsity (
a.u
.)
Raman Shift (cm-1)
RhOx(H2O)/CeO2
RhOx(ethanol)/CeO2
RhOx(acetone)/CeO2
(a) 463 cm-1
150 350 550 750 950 1150 1350
Inte
nsity (
a.u
.)
Raman Shift (cm-1)
RhOx(H2O)/Ce0.9Pr0.1O2
RhOx(ethanol)/Ce0.9Pr0.1O2
RhOx(acetone)/Ce0.9Pr0.1O2
(b)
458 cm-1
150 350 550 750 950 1150 1350
Inte
nsity (
a.u
.)
Raman Shift (cm-1)
RhOx(H2O)/Ce0.5Pr0.5O2
RhOx(ethanol)/Ce0.5Pr0.5O2
RhOx(acetone)/Ce0.5Pr0.5O2
(c)
444 cm-1
Figure 4.4. Raman spectra of catalysts supported on (a) CeO2, (b) Ce0.9Pr0.1O2 and (c)
Ce0.5Pr0.5O2 (c).
Chapter 4
120
The weak peak at 1170 cm-1
, which is not observed in all catalysts,
has been related to surface oxygen groups [13]. Several interpretations
have been proposed for peaks around 200 cm-1
and 570 cm-1
[2, 8, 10, 12-16 ]. Some authors have assigned peaks at ca. 195 and 570
cm-1
to RhOx species [8, 14-16] and others attributed these bands to the
formation of CeyPr1-yO2 solid solutions, because a physical mixture of the
pure cerium and praseodymium oxides did not show these features [13].
Going into more detail, these bands at 195 and 570 cm−1
have been
assigned to lattice defects, which results in the creation of oxygen
vacancies that is, to the asymmetric oxygen vibration caused by the
formation of vacancies [10, 13].
The creation of vacant sites on ceria would explain the important
increase of the relative intensity of the 570 cm-1
band; with regard to the
intensity of the main F2g mode at 444-463 cm−1
by increasing the
praseodymium content in our catalysts (see Figure 4.4.b and 4.4.c). The
creation of vacant sites on ceria by praseodymium doping is an evidence
of solid solution formation [13].
4.3.4 Catalysts characterization by TEM, XPS and H2-TPR.
The results obtained by TEM, XPS and H2-TPR, presented and
discussed in this section, provide (not only but mainly for the purposes of
the current study) information about the RhOx particles and their interaction
with the ceria-based supports.
TEM images of selected catalysts are included on Figure 4.5. All
the micrographs show the crystals corresponding to the ceria-based
supports. The ceria crystalline planes are even identified in some of them.
The size of the ceria-based crystals seems consistent with the BET areas
of the catalysts included on Table 4.2, that is, the size of the ceria-based
crystals observed by TEM for RhOx(H2O)/CeO2 and
RhOx(H2O)/Ce0.9Pr0.1O2 (60 an 50 m2/g, respectively) are smaller than
those of RhOx(H2O)/Ce0.5Pr0.5O2, RhOx(acetone)/Ce0.9Pr0.1O2 and
RhOx(acetone)/Ce0.5Pr0.5O2 (20, 33 and 18 m2/g, respectively). The ceria
particles size observed by TEM also correlates with the crystal sizes
obtained by XRD (see Table 4.2).
Effects of the solvents
121
Small dark spots (marked with red circles) corresponding to RhOx
nanoparticles are observed in all TEM images as well. The size of the
RhOx particles observed in Figure 4.5 is smaller than 2 nm in most cases.
However, a deeper TEM analysis of these catalysts, and of some others of
similar composition previously studied [5], confirmed the presence of very
small RhOx particles (even smaller than 1 nm) that can be hardly observed
with the magnification used to take the images on Figure 4.5.
The EDS local analysis of the area of the TEM images and the
global analysis of the catalysts by FRX confirmed that the amount of
rhodium is similar in all catalysts, and therefore, the amount of RhOx
particles of ca. 2 nm observed in the TEM images of Figure 4.5 can be
related with RhOx dispersion. If only few RhOx particles are observed in a
certain TEM image is because most RhOx particles are much smaller than
the detection limit, that is, RhOx is highly dispersed. On the contrary, if
there are a lot of RhOx spots in a TEM image of Figure 4.5 is because
RhOx is less dispersed.
Comparing the three TEM images (Figure 4.5) of catalysts
prepared by water impregnation of rhodium nitrate (a) (RhOx(H2O)/CeO2,
(b) RhOx(H2O)/Ce0.9Pr0.1O2 and (c) RhOx(H2O)/Ce0.5Pr0.5O2) it is observed
that there are much more RhOx particles on the catalyst with the
Ce0.5Pr0.5O2 support than on those with Ce0.9Pr0.1O2 or CeO2, and this is an
evidence of the worst RhOx dispersion over Ce0.5Pr0.5O2. This lower RhOx
dispersion on RhOx(H2O)/Ce0.5Pr0.5O2 is coincident with the lowest BET
area of this catalyst (see data on Table 4.2). On the other hand, comparing
the TEM images of the catalysts RhOx(H2O)/Ce0.9Pr0.1O2 and
RhOx(acetone)/Ce0.9Pr0.1O2, more RhOx particles are observed on the
second catalysts, that could be related with the BET area values (60 and
33 m2/g, respectively).
Chapter 4
122
(a)
(b)
(c)(d)
(e)
Figure 4.5. TEM images of catalysts (a) RhOx(H2O)/CeO2, (b) RhOx(H2O)/Ce0.9Pr0.1O2,
(c) RhOx(H2O)/Ce0.5Pr0.5O2, (d) RhOx(acetone)/Ce0.9Pr0.1O2, (e)
RhOx(acetone)/Ce0.5Pr0.5O2. RhOx particles have been circled.
Effects of the solvents
123
The conclusion of this TEM characterization is that both the nature
of the ceria-based support and of the solvent used for rhodium
impregnation affect RhOx dispersion. As a general trend, RhOx dispersion
on CeO2 and Ce0.9Pr0.1O2 is better than on Ce0.5Pr0.5O2, and RhOx
dispersion is better for water-impregnated catalysts than for ethanol or
acetone-impregnated catalysts. Some of these conclusions are supported
by the XPS characterization results.
Figure 4.6 shows the Rh 3d photoelectron spectra of all catalysts.
Two peaks are observed in all spectra, corresponding to the 3d5/2 and 3d3/2
transitions (around 309 and 313 eV, respectively). Both peaks provide
similar information about the oxidation state of rhodium. In all spectra, the
position of these peaks evidence that rhodium is as Rh3+
. The position of
the Rh 3d5/2 peaks is 309.0–310.6 eV for all catalysts tested. As reported in
the literature, the Rh 3d5/2 peak appears at 307.0–307.5 eV for Rh0, at
about 308 eV for Rh+, and from 308.3 to 310.5 eV for Rh
3+ [2, 17-19].
There are subtle differences in the position of the Rh 3d peaks on
Figure 4.6 that deserve a detailed analysis. As explained in Chapter 2, the
binding energy, and therefore the position of each element peaks, depend
on their surroundings. In RhOx-ceria catalysts, there is a negative charge
density transfer from the noble metal to the ceria support, and the extent of
such transfer affects the position of the Rh 3d peaks.
Chapter 4
124
300 305 310 315 320
Inte
nsity (
a.u
.)
Binding Energy (eV)
RhOx(H2O)/CeO2
RhOx(ethanol)/CeO2
RhOx(acetone)/CeO2
(a)
309.4
309.5
309.5
300 305 310 315 320
Inte
nsity (
a.u
.)
Binding Energy (eV)
RhOx(H2O)/Ce0.9Pr0.1O2
RhOx(ethanol)/Ce0.9Pr0.1O2
RhOx(acetone)/Ce0.9Pr0.1O2
(b)
309.9
309.8
310.0
300 305 310 315
Inte
nsity (
a.u
.)
Binding Energy (eV)
RhOx(H2O)/Ce0.5Pr0.5O2
RhOx(ethanol)/Ce0.5Pr0.5O2
RhOx(acetone)/Ce0.5Pr0.5O2
(c)
309.0
309.0
309.0
Figure 4.6. Rh 3d XPS spectra of catalysts supported on (a) CeO2, (b) Ce0.9Pr0.1O2 and
(c) Ce0.5Pr0.5O2.
Effects of the solvents
125
Regardless the solvent used for rhodium impregnation, the position
of the 3d5/2 peak is 309.5 eV for all RhOx/CeO2 catalysts (Figure 4.6.a),
while it is delayed to slightly higher values for RhOx/Ce0.9Pr0.1O2 catalysts
(Figures 4.6.b) and to lower values for RhOx/Ce0.5Pr0.5O2 (Figures 4.6.c).
This means that the RhOx-support interaction strongly depends on the
support nature, and ceria doping with 10 % praseodymium favors the
RhOx-support interaction while 50 % praseodymium doping hinders the
interaction. This is consistent with the lowest RhOx dispersion (see TEM
images on Figure 4.5 and previous discussion) obtained with the
Ce0.5Pr0.5O2 support and with the lowest BET area of this support
(see Table 4.2).
Table 4.3. Ce3+
and Pr3+
percentages (with regard to total Ce and Pr surface contents,
respectively) and Ce/Pr atomic ratio determined by XPS.
Ce3+
(%) Pr3+
(%) Ce/Pr
RhOx(H2O)/CeO2 37 - -
RhOx(ethanol)/CeO2 35 - -
RhOx(acetone)/CeO2 34 - -
RhOx(H2O)/Ce0.9Pr0.1O2 28 58 4.0
RhOx(ethanol)/Ce0.9Pr0.1O2 30 72 3.4
RhOx(acetone)/Ce0.9Pr0.1O2 28 66 3.7
RhOx(H2O)/Ce0.5Pr0.5O2 31 51 0.6
RhOx(ethanol)/Ce0.5Pr0.5O2 30 66 0.7
RhOx(acetone)/Ce0.55Pr0.5O2 30 50 0.7
The cerium and praseodymium oxidation states have been
estimated by XPS, and the values obtained are compiled in Table 4.3
together with the Ce/Pr surface ratios. The Ce3+
percentage (with regard to
total surface cerium) was estimated following the method proposed
elsewhere [2, 20] and the Pr3+
percentage (with regard to total surface
praseodymium) by using the semi-quantitative method proposed by
Borchert et al. [21], this procedure was described in Chapter 2. The
percentage of Ce3+
is similar for all catalysts prepared with the bare ceria
support (34-37 %) and these percentages slightly decrease upon
Chapter 4
126
praseodymium doping, evidencing the insertion of the dopant into the ceria
lattice. The Pr3+
percentages are much higher than those of Ce3+
due to
the easier reducibility of Pr4+
with regard to Ce4+
. Moreover, the presence
of Pr3+
cations, in somehow, partially decreases the reduction of Ce4+
.
Regarding the Ce/Pr ratios, they are well below the expected
nominal ratios deduced from the stoichiometric formula of the mixed oxides
(9 for Ce0.9Pr0.1O2 and 1 for Ce0.5Pr0.5O2). Rodríguez-Luque et al. [22]
reported the rhodium nanocrystallites decoration by patches of support in
Rhodium/CeyPr1-yO2 catalysts. It was argued that, during rhodium
impregnation, the acid character of the rhodium solution promotes Pr3+
leaching, and after catalyst drying and calcination, such species are
accumulated on the particles surface. This would explain the preferential
accumulation of praseodymium on the surface of our catalysts.
As a summary, the XPS analysis leads to conclude that
praseodymium is partially inserted into the ceria lattice for catalysts with
doped ceria supports, but with a preferential enrichment of praseodymium
on the particles surface. Such praseodymium doping affects the
RhOx-support interaction, being improved for 10 % praseodymium doping
but hindered for 50 % doping.
The RhOx-support interaction is known to affect the RhOx/ceria
catalysts reducibility, which is closely related to the catalytic activity for
N2O decomposition. In order to study such reducibility, H2-TPR
experiments were performed and the profiles obtained are compiled in
Figure 4.7.
Three peaks are shown in most H2-reduction profiles, as expected
[1, 2, 23]. As already mentioned in Chapter 3, the lowest-temperature peak
can be attributed to the reduction of RhOx, and in some cases, also to the
rhodium-catalyzed ceria-based support surface reduction. The
intermediate-temperature peak is attributed by some authors to surface
ceria reduction as well, but not catalyzed by the noble metal, whereas
other authors relate this peak to surface and/or bulk carbonates
decomposition [24] and/or to surface hydroxyls, peroxides or superoxides
reduction. Finally, the peak at highest temperature is attributed to bulk
ceria-support reduction.
Effects of the solvents
127
0 150 300 450 600 750 900 1050
Inte
nsity (
a.u
.)
Temperature (ºC)
RhOx(H2O)/CeO2
RhOx(ethanol)/CeO2
RhOx(acetone)/CeO2
(a)
0 150 300 450 600 750 900 1050
Inte
nsity (
a.u
.)
Temperature (ºC)
RhOx(H2O)/Ce0.9Pr0.1O2
RhOx(ethanol)/Ce0.9Pr0.1O2
RhOx(acetone)/Ce0.9Pr0.1O2
(b)
0 150 300 450 600 750 900 1050
Inte
nsity (
a.u
.)
Temperature (ºC)
RhOx(H2O)/Ce0.5Pr0.5O2
RhOx(ethanol)/Ce0.5Pr0.5O2
RhOx(acetone)/Ce0.5Pr0.5O2
(c)
Figure 4.7. H2-TPR profiles of catalysts supported on (a) CeO2, (b) Ce0.9Pr0.1O2 and
(c) Ce0.5Pr0.5O2.
Chapter 4
128
Special attention must be paid to the lowest temperature reduction
peak [2], mainly taking into account the symmetry of the peak. The
catalysts with high activity for N2O decomposition, those prepared with the
bare-ceria support and those impregnated with a water solution of rhodium
(see Figure 4.2), present single low-temperature H2 reduction peaks, while
two overlapped peaks are evident for catalysts with lower activity (those
impregnated with acetone or ethanol solutions in doped ceria supports)
[2, 4, 9, 25].
The presence of double-peaks or pronounced shoulders on the low
temperature H2 reduction peaks occur because Rh3+
, Pr4+
and Ce4+
are
reduced sequentially, while symmetric peaks are obtained if such
reductions occur simultaneously [4]. Therefore, the shape of this peak is
related with the RhOx-support interaction (and with the formation of doped
ceria solid solutions). The catalysts with good RhOx-support interaction
present high catalytic activity [1, 8]. It has been reported that the most
active sites for N2O decomposition are located at the RhOx-ceria interfaces
[26]. Also, a very effective N2O decomposition mechanism has been
demonstrated to occur on RhOx/ceria catalysts, where a synergy between
rhodium and ceria sites is proposed (see discussion on Chapter 3). This
effective mechanism needs a good RhOx-ceria interaction [12].
Regarding the peak at high temperature attributed to bulk
reduction, this band disappeared by increasing the amount of
praseodymium, evidencing improved reducibility with regard to pure ceria
due to increasing oxygen mobility into the catalyst [9].
In conclusion, the activity for N2O decomposition of the catalysts
studied is related with the RhOx-support interaction. Both the nature of the
ceria support and the solvent used for rhodium precursor impregnation
affect such interaction. Ceria doping with 10 % praseodymium has a
positive effect on such interaction (see XPS results; Figure 4.6) and on the
catalytic activity (see Figure 4.2 and Table 4.1), but the benefit on the
catalytic activity is only obtained using water for impregnation. On the
contrary, when Ce0.9Pr01O2 is impregnated with ethanol or acetone
solutions of rhodium, the temperature gradients created during calcination
(Figure 4.1) promote support (see BET areas on Table 4.2) and RhOx
sintering (see RhOx sizes on TEM images; Figure 4.5) hindering the
RhOx-Ce0.9Pr01O2 interaction. The interaction between RhOx and
Effects of the solvents
129
Ce0.5Pr0.5O2 is not as good as that with Ce0.9Pr0.1O2 (see larger RhOx
particles in TEM images (Figure 4.5) and the XPS binding energies of Rh3+
(Figure 4.6), but it seems to be enough to keep a high catalytic activity if
the impregnation is carried out with a rhodium precursor water solution. In
fact, the H2-TPR peak at low temperature (Figure 4.7.c) demonstrates a
considerable RhOx-support interaction. However, acetone or ethanol
impregnation also leads to an important decrease in activity, and this can
be only attributed to RhOx sintering, since Ce0.5Pr0.5O2 with a considerably
lower BET surface area does not sinter with regard to water impregnation
(the same BET areas were obtained for all Ce0.5Pr0.5O2 catalysts; see
Table 4.2). Finally, pure CeO2 is more stable towards sintering than doped
ceria under the calcination conditions of this study (contrarily to the
phenomenon observed at high-temperature calcination), and does not
sinter regardless the solvent used for rhodium impregnation, and for this
reason all the RhOx/CeO2 catalysts kept the same activity.
According to this study, the best catalyst is obtained by
impregnating Ce0.9Pr01O2 with a water solution of rhodium precursor.
However, if acetone or ethanol must be used for any reason (to improve
wettability of a honeycomb monolith channels, for instance) the bare ceria
support is more suitable.
4.4 Conclusions.
The effect of the solvent (water, ethanol or acetone) used to
impregnate CeyPr1-yO2 (y = 1, 0.9 or 0.5) supports with rhodium nitrate, in
order to prepare N2O decomposition catalysts, have been studied and the
following main conclusions can be summarized:
Both the nature of the ceria support and the solvent used for
rhodium precursor impregnation affect RhOx-support interaction,
modifing the activity for N2O decomposition of the catalysts
studied.
The use of ethanol or acetone as solvent has a very negative effect
on Ce0.9Pr0.1O2 and Ce0.5Pr0.5O2-containing catalysts, due to the
sintering of both the support and RhOx particles. This negatively
affects the RhOx-support interaction which directly hinders the
Chapter 4
130
catalytic activity for N2O decomposition. This negative effect is due
to the solvent combustion catalyzed by rhodium.
Ceria doping with 10 % praseodymium has a positive effect on the
RhOx-support interaction, observed as a negative charge density
transfer from the noble metal to the ceria support (only obtained
using water for impregnation) that improves the catalytic activity.
The interaction between RhOx and Ce0.5Pr0.5O2 is not as good as
that with Ce0.9Pr0.1O2, but it seems to be enough to keep a high
catalytic activity if rhodium is impregnated with a water solution.
However, acetone or ethanol impregnation leads to an important
decrease in activity, and this must be attributed to RhOx sintering
because Ce0.5Pr0.5O2 does not sinter additionally with regard to
water impregnation.
Effects of the solvents
131
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Catalysis 101 (1996) 681.
CHAPTER 5
Preparation, characterization and N2O
decomposition activity of honeycomb
monolith-supported RhOx/Ce0.9Pr0.1O2
catalysts
RhOx/Ce0.9Pr0.1O2 active phases have been loaded by
sequential impregnation on cordierite honeycomb monoliths following
the procedure: (i) cerium and praseodymium nitrates impregnation +
calcinations and (ii) rhodium nitrate impregnation + calcination. The
supported catalysts have been characterized by XRD, Raman
spectroscopy, SEM-EDS, TEM-EDS and H2-TPR, and tested for N2O
decomposition. Rhodium oxide particles are selectively attached to
Ce0.9Pr0.1O2 and not to cordierite.The optimum content of rhodium has
been 0.2 % in total weight. The calcination procedure significantly
affects the supported catalyst features. The best catalyst was prepared
by flash calcination, yielding better distribution of doped ceria in the
cordierite monolith, smaller RhOx particles and lowering the
temperature for surface Rh-Ce-Pr entities reduction in comparison to
ramp calcination. This improves both the distribution of active phases
on the cordierite substrate and the catalytic activity for N2O
decomposition.
Honeycomb monolith-supported RhOx/Ce0.9Pr0.1O2 catalysts
137
5.1 Introduction.
As described in the previous chapters, the RhOx/Ce0.9Pr0.1O2
catalyst is a promising candidate for low temperature N2O decomposition.
In Chapter 3, the calcination conditions were modified in order to improve
the distribution of rhodium and the RhOx-ceria interface of RhOx/CeO2
catalysts. Improved catalytic activity for N2O decomposition and CO
oxidation [1] was obtained by flash calcination, which consisted of
introducing the ceria support impregnated with the rhodium precursor in a
furnace which was pre-heated at 250 ºC. The speed at which water
evaporates from the ceria-based support seemed very important for the
catalyst properties, because it affects the size of the RhOx particles on the
final catalyst. This led us to look for a procedure to accelerate the
evaporation of the solvent used for rhodium impregnation and water was
compared with more volatile solvents (acetone and ethanol) in Chapter 4.
This study was carried out with RhOx catalysts with CeO2, Ce0.9Pr0.1O2 and
Ce0.5Pr0.5O2 supports. The main conclusion obtained was that the nature of
the solvent used for rhodium nitrate impregnation affects the activity of the
catalysts not only due to the evaporation rate but also due to the
exothermic combustion of acetone and ethanol during calcination. This had
a very negative effect on Ce0.9Pr0.1O2 and Ce0.5Pr0.5O2-containing catalysts
due to the induced sintering of these praseodymium-doped ceria and also
of the RhOx particles.
According to these previous studies, the best powder catalyst
prepared until now is RhOx/Ce0.9Pr0.1O2, with rhodium loaded with a water
solution of the nitrate precursor and with further calcination performed in
the so-called flash conditions.
One of the challenges in N2O emission control is to develop a
catalyst able to work in real gas streams. To use a catalyst in real gas
streams it is necessary to load the previously optimized active phase
(powder) into an appropriate inert support. Diverse substrates can be used
for this purpose, such as pellets or inorganic oxide particles, honeycomb
ceramic monoliths, ceramic foams, etc. [2-6]. Among those supports,
honeycomb monoliths present some attractive properties. They provide a
good contact between the active phases of the catalyst and the treated
gases, they present a high dust tolerance, the pressure drop is minimized
Chapter 5
138
because the gas flow is not significantly impeded through the catalytic bed
and also they have no degradation problems typically occurring, for
instance, by particles friction [2, 3, 4, 5].
The most common material used to manufacture honeycomb
monoliths is cordierite (2MgO:5SiO2:2Al2O3). Cordierite presents high
mechanical strength and low thermal expansion coefficient [2, 3, 4].
Incorporation of catalytically active phases into monoliths is not trivial, and
optimization studies of the loading process are required. Some studies of
the decomposition of N2O used a series of monolithic (ceria-alumina
washcoated cordierite) supported transition metal and noble metal oxide
catalysts [7]. Rh/ɣ-Al2O3–sepiolite monolithic catalysts have been studied
[8] and the importance of the drying process in the active phase
distribution into the monolith channels has been reported [2].
The goal of the current chapter is to give a step forward towards
scaling up the utilization of N2O decomposition RhOx/Ce0.9Pr0.1O2 catalysts
in real gas streams. Active phases with composition RhOx/Ce0.9Pr0.1O2
have been loaded into cordierite honeycomb monoliths following a simple
method, consisting of impregnation of the monoliths with the metal
precursors and calcination. The effect of the calcination conditions has
been studied, and the noble metal oxide amount has been also optimized.
The coated monoliths have been characterized and tested for N2O
decomposition in a N2O/He stream. The best supported catalyst has been
also evaluated for N2O decomposition in the presence of the main
inhibitors found in a diesel exhaust stream (O2 + NO + H2O).
5.2 Experimental.
5.2.1 Catalysts preparation.
Supported catalysts were prepared by impregnation of cordierite
honeycomb monoliths (by Corning; 1 cm diameter; 1.4 cm length, ~0.6 g
weight) with an aqueous solution of Ce(NO3)3·6H2O (Aldrich, 99.99 wt.%)
and Pr(NO3)3·6H2O (Aldrich, 99.9 wt.%), with the appropriate
concentrations to obtain a Ce:Pr molar ratio of 9:1. After calcination at
600 ºC, the monoliths contain Ce0.9Pr0.1O2 loading amounts of
12.5 ± 0.5 wt.%. Then they were impregnated with an aqueous solution of
Honeycomb monolith-supported RhOx/Ce0.9Pr0.1O2 catalysts
139
Rh(NO3)3·xH2O (Sigma-Aldrich, ~36 wt.% as Rh) to obtain the desired
rhodium loading in total base. After that, the catalysts were calcined in
static air at 500 ºC.
Two critical variables have been evaluated in this study: the
rhodium loading and the calcination conditions.Six samples with different
rhodium loading were prepared, denoted by x%RhOx/Ce0.9Pr0.1O2/M
(x = wt.% of rhodium in total basis from 0.001 to 0.9 % and
M = honeycomb monolith). These samples were prepared following the
so-called ramp-calcination process. After impregnation of the cerium and
praseodymium salts, these monoliths were dried at 80 ºC for 120 min, and
then the temperature was raised up to 600 ºC at 10 ºC/min, maintaining
the maximum temperature for 90 min. The calcination of the Ce0.9Pr0.1O2-
containing monoliths after impregnation of the rhodium salt was carried out
by heating the monoliths from room temperature to 230 ºC at 10 ºC/min
and from 230 to 500 ºC at 1ºC/min.
A sample labeled as 0.2%RhOx/Ce0.9Pr0.1O2/M (flash) was
prepared following a similar procedure to that described in Chapter 3. In
this case, after impregnation of the monolith with the cerium and
praseodymium precursors, the calcination was carried out by introducing
the impregnated monolith in a previously heated furnace at 250 ºC,
keeping the monolith at this temperature for 60 min, and increasing the
temperature up to 600 ºC afterwards (heating rate, 10 ºC/min). The
maximum temperature was maintained for 90 min. After cooling, the
monolith was impregnated with a water solution of the rhodium precursor,
and the impregnated monolith was introduced in a furnace previously
heated at 250 ºC. After 60 min at 250 ºC, the temperature was increased
up to 500 ºC (heating rate, 1 ºC/min). This thermal treatment has been
referred to as “flash calcination”.
In order to ensure mechanical resistance of the coated phases in
the supported catalysts, the poorly bound solids were blown out with a
pressurized air stream after the last calcinations step.
A powder sample, denoted by RhOx/Ce0.9Pr0.1O2 (flash), was also
prepared. This sample consisted of a powder active phase with similar
composition to that on the 0.2%RhOx/Ce0.9Pr0.1O2/M (flash) monolith
catalyst. The Ce0.9Pr0.1O2 powder was prepared with the required amounts
Chapter 5
140
of the cerium and praseodymium precursors, which were mixed in a mortar
and calcined in static air. Rhodium was loaded afterwards by incipient
wetness impregnation. The calcination steps were carried out by following
the flash calcination procedure. Table 5.1 compiles general information
about all the catalysts prepared in this study.
Table 5.1. Nomenclature, properties and preparation conditions of the catalysts.
Nomenclature Rh
(%)a
Active phase and
conformation
Calcination
conditions
Ce0.9Pr0.1O2/M 0
Ce0.9Pr0.1O2 active
phase supported into
the monolith
ramp
calcination
0.001%RhOx/Ce0.9Pr0.1O2/M 0.001
RhOx/Ce0.9Pr0.1O2 active
phase supported into
the monolith
ramp
calcination
0.1%RhOx/Ce0.9Pr0.1O2/M 0.1
RhOx/Ce0.9Pr0.1O2 active
phase supported into
the monolith
ramp
calcination
0.2%RhOx/Ce0.9Pr0.1O2/M 0.2
RhOx/Ce0.9Pr0.1O2 active
phase supported into
the monolith
ramp
calcination
0.6%RhOx/Ce0.9Pr0.1O2/M 0.6
RhOx/Ce0.9Pr0.1O2 active
phase supported into
the monolith
ramp
calcination
0.9%RhOx/Ce0.9Pr0.1O2/M 0.9
RhOx/Ce0.9Pr0.1O2 active
phase supported into
the monolith
ramp
calcination
RhOx/Ce0.9Pr0.1O2 (flash) 1b
Unsupported
RhOx/Ce0.9Pr0.1O2 active
phase (powder)
flash
calcination
0.2%RhOx/Ce0.9Pr0.1O2 /M (flash) 0.2
RhOx/Ce0.9Pr0.1O2 active
phase supported into
the monolith
flash
calcination
a Rh content expressed in % of total weight.
b Powder sample; the ratio rhodium to Ce0.9Pr0.1O2 is equivalent to that present in the
supported samples 0.2%RhOx/Ce0.9Pr0.1O2/M (flash) and 0.2%RhOx/Ce0.9Pr0.1O2/M.
Honeycomb monolith-supported RhOx/Ce0.9Pr0.1O2 catalysts
141
5.2.2 Catalysts characterization.
Catalysts were characterized by Raman spectroscopy and XRD to
determine crystalline structure, N2 adsorption at −196 ºC to calculate BET
surface area, TEM to mesure RhOx particle size, SEM to observe the
active phase distribution on the monolith and H2-TPR to determine the
reducibility of the samples.
The supported catalysts were milled and the powder obtained was
used in all characterization techniques, except in SEM-EDS-chemical
mapping characterization that was used to observe the distribution of the
active phase on the monolith. Detailed information about these
characterization techniques was reported in Chapter 2
5.2.3 N2O decomposition tests.
N2O decomposition tests were performed in a cylindrical reactor at
atmospheric pressure. The total flow rate was 500 mL/min
(GHSV = 27000 h−1
). N2O decomposition tests were carried out under
different gas streams: 1000 ppm N2O/He or 1000 ppm N2O/1000 ppm
NO/5 % O2/0.6 % H2O/He. For more details see Chapter 2.
5.3 Results and discussion.
5.3.1 SEM-EDS-chemical mapping characterization.
Figure 5.1 shows the micrographs obtained for two monolithic
catalysts prepared by ramp calcination (Figure 5.1.a) and flash calcination
(Figure 5.1.b); samples denoted 0.2%RhOx/Ce0.9Pr0.1O2/M and
0.2%RhOx/Ce0.9Pr0.1O2/M (flash), respectively. In both pictures the active
phase is observed (light grey), coating the monolith walls (dark grey).
Chapter 5
142
(a)
(b)
Figure 5.1. SEM microphotographs of 0.2%RhOx/Ce0.9Pr0.1O2 loaded into honeycomb
monoliths (a) ramp calcination; (b) flash calcination.
As revealed by SEM micrographs, flash calcination procedure
(Figure 5.1.b) allows a more homogeneous active phase coating of the
monolith than ramp calcination. This is attributed to the migration of the
precursor salts solution along the monolith channels during ramp
calcinations, due to the temperature gradients generated. This effect is
minimized during flash calcination. This heterogeneous distribution of the
active phases upon conventional ramp calcination has been also reported
by some other authors for monoliths loaded with different compounds, as
for instance nickel salts [2].
In order to study in detail the effect of calcination conditions in the
coating homogeneity, chemical composition of the monolith surface of
samples 0.2%RhOx/Ce0.9Pr0.1O2/M and 0.2%RhOx/Ce0.9Pr0.1O2/M (flash)
has been determined by EDS analysis. Figure 5.2 shows cerium
concentration (%), which has been selected as representative element of
Honeycomb monolith-supported RhOx/Ce0.9Pr0.1O2 catalysts
143
the coating for different channels. The analysis was done in different
positions along the channels and also in different channels of the monolith,
as indicated in the inset scheme of Figure 5. 2.
0
20
40
60
80
100
1 2 3 4 5
Ce (
%)
Channel
(a)Channel
1 8
Position (mm)0 7
0
20
40
60
80
100
1 2 3 4 5
Ce
(%
)
Channel
0 2 4 6 7
(b)
Position (mm)
Figure 5.2. EDS analysis of cerium distribution in the monoliths
0.2%RhOx/Ce0.9Pr0.1O2: (a) ramp calcination; (b) flash calcination.
The EDS analysis in Figure 5.2 also shows a more homogeneous
distribution of cerium using flash calcination (Figure 5.2.b) in comparison to
ramp calcination (Figure 5.2.a). In both samples the amount of cerium
along the central channels (channels 3 and 5) show not relevant
Chapter 5
144
differences in cerium concentration for the different positions. On the
contrary, outer channels of the monoliths (channel 1) have a higher
amount of active phase than inner (central) channels. Cerium
concentration in outer channels oscillates depending on the position
analyzed, and these changes are much more important in the sample
prepared by ramp calcination (Figure 5.2.a).
Chemical mapping was also used to analyze the distribution of the
different components of the coating into the monolith. Figure 5.3.a exhibits
a SEM micrograph of the catalyst 0.2%RhOx/Ce0.9Pr0.1O2 /M (flash) and
the distributions of rhodium (Figure 5.3.b; red), silicon (Figure 5.3.c; blue),
and cerium (Figure 5.3.d; green) were selected as elements representative
of RhOx particles, cordierite and Ce0.9Pr0.1O2, respectively. The active
phase is observed at the top of the pictures, forming a layer of several
micrometers over the cordierite substrate. This analysis points out that
rhodium oxide particles are preferably attached to Ce0.9Pr0.1O2, since the
areas where rhodium appears (Figure 5.3.b) preferably are those where
cerium is also detected (Figure 5.3.d). For a proper interpretation of these
analyses, it must be taken into account that the intensity of rhodium is
lower than that of the other elements, because the concentration of
rhodium is very low and the noise signal for this element is high. This type
of selective RhOx-ceria interaction, and the noble metals dispersion
capacity of ceria, is something very well known that has been deeply
studied in the context of the TWC used in gasoline vehicles for gas
pollution control in this type of vehicles [9].
Previous studies of N2O decomposition carried out with powder
RhOx/ceria catalysts [10-12] showed that rhodium catalytic activity
depends on the ceria promoter effect, and that is why it is important to
ensure a good contact between RhOx and the Ce0.9Pr0.1O2 mixed oxide in
supported catalysts.
Honeycomb monolith-supported RhOx/Ce0.9Pr0.1O2 catalysts
145
(a) (b)
(c) (d)
Figure 5.3. SEM-chemical mapping analysis of the sample
0.2%RhOx/Ce0.9Pr0.1O2/M (flash): (a) SEM picture, (b) silicon analysis (blue), (c) cerium
analysis (green), and (d) rhodium analysis (red).
5.3.2 XRD and Raman spectroscopy characterization.
The structure of doped ceria was studied by XRD and Raman
spectroscopy, both techniques providing complementary information. XRD
is sensitive to the position of the cations in the lattice, while Raman spectra
of ceria are caused by oxide anions vibration.
XRD patterns of selected samples are compiled in Figure 5.4,
including the bare cordierite monolith (without active phase), the powder
active phase RhOx/Ce0.9Pr0.1O2 (flash) and the powders obtained upon
milling of the monoliths coated with the active phases
0.2%RhOx/Ce0.9Pr0.1O2 prepared by ramp or flash calcinations.
The powder active phase without cordierite (sample
RhOx/Ce0.9Pr0.1O2 (flash)) presents reflections of the fluorite structure of
Chapter 5
146
ceria, corresponding to the planes (111), (200), (220) and (311). No other
peaks but those of fluorite were observed in the diffractograms, and
evidences of the segregation of praseodymium phases were not obtained
[10-14].
10 20 30 40 50 60 70 80
Inte
nsity (
a.u
.)
2 θ (º)
bare cordierite
0.2%RhOx/Ce0.9Pr0.1O2/M
0.2%RhOx/Ce0.9Pr0.1O2/M (flash)
RhOx/Ce0.9Pr0.1O2 (flash)(111) (200) (220)(311)
Figure 5. 4. XRD patterns of selected samples.
The active phases supported on monoliths,
0.2%RhOx/Ce0.9Pr0.1O2/M and 0.2%RhOx/Ce0.9Pr0.1O2/M (flash), also
present the characteristic peaks of the fluorite structure of ceria, confirming
the incorporation of the active phase into the monolith. These
characteristic peaks of doped ceria appear together with cordierite peaks.
However, overlapping of many of fluorite and cordierite peaks hinders a
detailed analysis of the structure of the supported doped ceria, since the
fluorite peaks broadening cannot be accurately measured. Changes of
relative intensities of peaks at 2 28 º and 30 º in samples
0.2%RhOx/Ce0.9Pr0.1O2/M and 0.2%RhOx/Ce0.9Pr0.1O2/M (flash) with
respect to the bare cordierite correspond to the superposition of (111)
reflexion from doped ceria with that of cordierite at 28 º.
The selected samples have been also characterized by Raman
spectroscopy and the spectra obtained are included in Figure 5.5. The
fluorite structure of ceria, based on the face-centered cubic cell, shows a
characteristic Raman band at ca. 460 cm-1
assigned to the F2g mode [11].
This band is identified in all the Raman spectra included in Figure 5.5,
except in that of the bare monolith. The position of the F2g bands in
Honeycomb monolith-supported RhOx/Ce0.9Pr0.1O2 catalysts
147
Figure 5.5 (at ca. 460 cm-1
) is slightly lower to the reference pure ceria
value (465 cm-1
), and this shift in the position is an evidence of the
introduction of praseodymium cations into the ceria lattice, as explained in
Chapter 4. It is worth noting that the position of the F2g peak is exactly the
same both for the pure active phase (RhOx/Ce0.9Pr0.1O2 (flash)) and for the
(0.2%RhOx/Ce0.9Pr0.1O2/M and 0.2%RhOx/Ce0.9Pr0.1O2/M (flash)).
supported samples. This suggests that the introduction of praseodymium
cations into the ceria lattice in coated monoliths is very similar to that
occurring when the active phase is prepared in powder.
200 250 300 350 400 450 500 550 600
Inte
nsity (
a.u
.)
Raman shift (cm-1)
bare monolithF2g mode of ceria (fluorite structure)
0.2%RhOx/Ce0.9Pr0.1O2/M
0.2%RhOx/Ce0.9Pr0.1O2/M (flash)
RhOx/Ce0.9Pr0.1O2 (flash)
Figure 5.5. Raman Spectra of selected samples.
5.3.3 Characterization by N2 adsorption at -196 ºC.
The BET surface areas of the catalysts are compiled in Table 5.2.
The bare cordierite has a very low surface area (1 m2/g) and the powder
catalyst presents an area of 57 m2/g. The BET area of the supported
catalysts ranges around 4-8 m2/g, and these were the expected values
considering the area of the powder catalyst and the active phase loading
(12.5 ± 0.5 wt.%). These results suggest that the size of the Ce0.9Pr0.1O2
particles (responsible of the BET area) [14] is similar in the powder and in
the supported active phases.
Chapter 5
148
Table 5.2. BET areas of the samples.
Sample BET surface area
(m2/g)
Bare monolith 1
RhOx/Ce0.9Pr0.1O2 (flash) 57
0%RhOx/Ce0.9Pr0.1O2/M 8
0.001%RhOx/Ce0.9Pr0.1O2/M 7
0.1%RhOx/Ce0.9Pr0.1O2/M 6
0.2%RhOx/Ce0.9Pr0.1O2/M 4
0.6%RhOx/Ce0.9Pr0.1O2/M 8
0.9%RhOx/Ce0.9Pr0.1O2/M 6
0.2%RhOx/Ce0.9Pr0.1O2/M (flash) 6
5.3.4 H2-TPR characterization.
Figure 5.6 includes the H2-TPR profiles of catalysts
0.2%RhOx/Ce0.9Pr0.1O2/M and 0.2%RhOx/Ce0.9Pr0.1O2/M (flash). The
species on the catalysts that can be reduced by H2 are the Rh3+
, Pr4+
and
Ce4+
. The phenomena occurring in H2-TPR were described in the previous
chapter. As a brief reminder, first peak at low temperature is attibuted to
RhOx reduction and to the noble metal-catalyzed surface reduction of the
doped ceria supports [2, 13]. Different events can contribute to the peaks
appearing between 200 and 600 ºC as reduction of surface doped ceria
which is not in close contact with RhOx [10], decomposition of surface
carbonates or carbonates occluded within the CeO2 structure [15, 16] and
the reduction of hydroxyls groups or some other surface oxygen groups on
ceria [17, 18].
As mentioned in previous chapters, N2O decomposition capacity of
the catalysts is related with the shape and position of the low temperature
H2 reduction peak [11]. The supported catalyst prepared by flash
calcination (0.2%RhOx/Ce0.9Pr0.1O2/M (flash)) shows a quite symmetric
peak, pointing out a simultaneous reduction of the different cations
(Rh3+
, Ce4+
and Pr4+
) due to a more homogeneity of this catalyst. On the
contrary, the sample prepared by ramp calcination
(0.2%RhOx/Ce0.9Pr0.1O2/M) presents an asymmetric peak with a shoulder
Honeycomb monolith-supported RhOx/Ce0.9Pr0.1O2 catalysts
149
at low temperature revealing the successive reduction of the different
cations. It is reasonable to think that Rh3+
reduction takes place first,
followed by the Pr4+
and Ce4+
reduction [11, 19, 20]. These results suggest
that the flash calcination procedure allows obtaining a much better noble
metal-support interaction than the conventional ramp calcinations, which is
consistent with the conclusions of Chapter 3 performed with powder
RhOx/CeO2 catalysts.
0 200 400 600 800
TC
D S
ignal (a
.u.)
Temperature (ºC)
0.2%RhOx/Ce0.9Pr0.1O2/M
0.2%RhOx/Ce0.9Pr0.1O2/M (flash)
50 100 150 200
TC
D S
ignal (a
.u.)
Temperature (ºC)
Figure 5.6. H2-TPR profiles of selected samples.
5.3.5 TEM characterization.
The supported catalysts 0.2%RhOx/Ce0.9Pr0.1O2/M and
0.2%RhOx/Ce0.9Pr0.1O2/M (flash) were studied by TEM microscopy and
Figure 5.7 shows, as an example, a picture of each catalyst.
Chapter 5
150
5 nm
(a) (b)
5 nm
Figure 5.7. TEM photograph of catalysts (a) 0.2%RhOx/Ce0.9Pr0.1O2/M and
(b) 0.2%RhOx/Ce0.9Pr0.1O2 /M (flash). Red circles show RhOx particles.
In the catalyst prepared by ramp calcination (Figure 5.7.a) RhOx
particles of ca. 2 nm (see red circles) are clearly distinguished from those
of Ce0.9Pr0.1O2, which exhibit a well-defined crystalline structure that is
observed in some areas. On the contrary, in the catalyst prepared by flash
calcination (Figure 5.7.b) RhOx particles have been hardly observed,
suggesting a much smaller particle size, and therefore a better dispersion
of rhodium than for the catalyst prepared by ramp calcination. A similar
conclusion was achieved in Chapter 3, where smaller RhOx particles were
observed by TEM on powder RhOx/CeO2 catalysts prepared by flash
calcination with regard to those on a similar catalyst calcined in ramp.
In order to confirm that the absence of RhOx particles in Figure
5.7.b can be really attributed to the small size of the particles and to rule
out the possibility that RhOx is not observed because is heterogeneously
distributed on the sample EDS analyses of both samples have been
performed. The EDS measurements, included in Table 5.3, confirm the
presence of similar amounts of rhodium in both samples (2.1 and
1.7 wt.%).
The small particle size of RhOx in the sample prepared by flash
calcinations (0.2%RhOx/Ce0.9Pr0.1O2/M (flash)) is consistent with the
conclusions of the H2-TPR experiments (Figure 5.6), that is, the small size
Honeycomb monolith-supported RhOx/Ce0.9Pr0.1O2 catalysts
151
of the RhOx particles allows a good contact with the support and improves
the reduction of the Ce4+
and Pr4+
surrounding the noble metal particles.
Table 5.3. EDS chemical analysis of photographs shown in Figure 5.7. The
percentages are expressed by weight.
Sample Rh (%) Ce (%) Pr (%) O (%)
0.2%RhOx/Ce0.9Pr0.1O2/M 2.1 71.5 7.1 19.3
0.2%RhOx/Ce0.9Pr0.1O2/M (flash) 1.7 72.4 8.9 17.0
5.3.6 N2O decomposition tests.
The effect of the noble metal loading on the catalytic
decomposition of N2O has been studied with the set of catalysts prepared
by ramp calcination using a N2O/He stream. The N2O decomposition
profiles obtained in these experiments are compiled in Figure 5.8, where
N2O decomposition (%) is plotted versus the rhodium content of the
catalysts for different reaction temperatures. In agreement with previous
studies carried out with powder active phases [10, 11], the supported
sample without rhodium (Ce0.9Pr0.1O2/M) has no catalytic activity in the
range of temperature studied, while all the catalysts with rhodium are able
to decompose N2O, even the catalyst with very low noble metal loading
(0.001%RhOx/Ce0.9Pr0.1O2/M). As expected, N2O decomposition increases
with temperature, almost reaching the complete conversion at 350 ºC for
all the catalysts. According to these results, the best formulation is
0.2%RhOx/Ce0.9Pr0.1O2/M because the addition of higher amount of
rhodium does not improve the conversion attained at each temperature.
This means that the specific activity of rhodium decreases by increasing
the rhodium loading above 0.2 wt.%.
The best formulation has been selected to study the effect of the
calcination conditions in the catalytic performance of the coated monoliths.
In Figure 5.9 the profiles obtained with the supported catalysts
0.2%RhOx/Ce0.9Pr0.1O2/M and 0.2%RhOx/Ce0.9Pr0.1O2/M (flash), and also
the results obtained with the powder catalyst RhOx/Ce0.9Pr0.1O2 (flash) are
compiled.
Chapter 5
152
0
25
50
75
100
0.0 0.2 0.4 0.6 0.8 1.0
N2O
decom
positio
n (
%)
Rh (%)
350 ºC
325 ºC
300 ºC
275 ºC
250 ºC225 ºC
Figure 5.8. N2O decomposition (%) in a 1000 ppm N2O/He stream versus rhodium
content at different temperatures for x%RhOx/Ce0.9Pr0.1O2/M catalysts where
0 ≤ x ≤ 0.9.
Comparing the N2O decomposition profiles obtained with the two
catalysts (Figure 5.9) prepared by flash calcinations (powder and
supported); it is observed that the N2O decomposition onset temperature is
the same. This means that the procedure followed to load the active
phases into the monoliths is able to mimic the properties of the powder
active phase. However, after certain level of decomposition (ca. 15 %), the
supported catalyst (0.2%RhOx/Ce0.9Pr0.1O2 /M (flash)) outperforms the
conversions of the powder active phase (RhOx/Ce0.9Pr0.1O2 (flash)),
successfully achieving the first objective of this chapter.
The shape of the N2O decomposition curve obtained with the
powder sample evidences an important contribution of the diffusion
phenomena to the N2O decomposition rate, while this behavior is not
obvious for the supported catalysts. This is not surprising since monolith
catalysts are characterized by their good mass-transfer in comparison to
randomly packed beds [2]. This observation could have practical
relevance, and honeycomb monoliths could be more suitable supports to
be used in real applications than randomly packed beds, like those of
particles or pellets. These diffusion phenomena is observed in this chapter
but not in previous ones due to the much lower ratio between mass of
active phase and flow of gas used in the experiments (current chapter;
Honeycomb monolith-supported RhOx/Ce0.9Pr0.1O2 catalysts
153
75 mg of catalyst active phase and 500 mL/min of gas; previous chapters:
100 mg of catalyst active phase and 100 mL/min of gas).
Regarding the calcination treatments, the supported catalyst
prepared by ramp calcination is much less effective for N2O decomposition
than the counterpart prepared by flash calcination, with 100 ºC of delay in
the N2O decomposition temperature at 50 % conversion. This can be
attributed to the improved interaction between RhOx and Ce0.9Pr0.1O2
particles obtained by flash calcinations, as deduced from H2-TPR and
TEM-EDS characterization. It is interesting that the flash calcination is not
only advantageous in preparing powder catalysts, as discussed in
Chapter 3, but also in preparing monolith supported catalysts.
0
25
50
75
100
200 250 300 350 400
N2O
deco
mp
ositio
n
(%)
Temperature (ºC)
0.2%RhOx/Ce0.9Pr0.1O2/M (flash)circles:1st runstars: 2nd run
0.2%RhOx/Ce0.9Pr0.1O2/M
RhOx/Ce0.9Pr0.1O2 (flash)
Figure 5.9. N2O decomposition (%) as a function of temperature (in a 1000 ppm
N2O/He stream) for catalysts with similar amount of rhodium and different calcination
procedure and shape.
In order to evaluate the stability of the supported catalysts two
consecutive cycles of N2O decomposition experiments were conducted
with the 0.2%RhOx/Ce0.9Pr0.1O2/M (flash) catalyst. The results of these
experiments, also included in Figure 5.9, were equal, proving that the
heating and cooling cycles do not modify the catalyst performance.
Finally, the best catalyst (0.2%RhOx/Ce0.9Pr0.1O2/M (flash)) among
all those prepared in this study has been tested using a complex gas
mixture which includes the main inhibitors typically present in a diesel
Chapter 5
154
exhaust stream (NOx, O2, and H2O). Null NOx removal was observed in
these experiments once the steady-state conditions were achieved, and
only N2O decomposition was detected. The N2O decomposition results
obtained are compiled in Figure 5.10. As expected, the presence of
inhibitors in the gas mixture shifts the N2O decomposition curve to higher
temperatures in comparison with the pure N2O/He stream. For the complex
gas mixture evaluated, the N2O decomposition started at 350 ºC and total
conversion was reached at 525 ºC. In spite of the important delay in the
N2O decomposition curve due to the presence of catalyst inhibitors
observed in Figure 5.10, the range of temperature obtained could be
suitable for N2O removal in a diesel engine exhaust with the N2O
decomposition catalyst located after a NOx reduction SCR catalyst. This
hypothesis will be confirmed in the next chapter.
0
25
50
75
100
100 150 200 250 300 350 400 450 500 550
N2O
decom
positio
n (
%)
Temperature (ºC)
1000 ppm N2O1000 ppm N2O1000 ppm NO5% O2
0.6% H2O
Figure 5.10. Effect of reactive gas atmosphere in the catalytic activity for N2O
decomposition. Tests conducted with 1000 ppm N2O/He or with 1000 ppm N2O/
1000 ppm NO/5 % O2/0.6 % H2O/He using the catalyst 0.2%RhOx/Ce0.9Pr0.1O2/M
(flash).
The inhibiting effect of NOx, O2 and H2O on the catalytic activity for
N2O decomposition of 0.2%RhOx/Ce0.9Pr0.1O2/M (flash) observed in
Figure 5.10 is consistent with a previous DRIFTS study performed with ɣ-
Al2O3 particles-supported Rh/Ce0.9Pr0.1O2 [21]. Among these three
inhibitors, the strongest effect was attributed to NOx while the weakest to
O2. The inhibiting effect of O2 was attributed to its reversible chemisorption
on catalyst sites, while the effect of H2O and NOx was mainly related with
Honeycomb monolith-supported RhOx/Ce0.9Pr0.1O2 catalysts
155
the irreversible chemisorption on the catalysts sites. The inhibiting effect of
H2O is not as high as that of NOx because the product of H2O
chemisorption (Ce–OH surface groups) is suitable for N2O chemisorption
and decomposition, while the surface nitrogen species created upon NOx
chemisorption are not [21].
5.4 Conclusions.
RhOx/Ce0.9Pr0.1O2 active phases have been loaded into cordierite
honeycomb monoliths, and the effects of the rhodium amount and of the
calcination conditions have been studied. The characterization and
catalytic test for the N2O decomposition of the supported catalysts allow
the following conclusions:
The nature of Ce0.9Pr0.1O2 in powder and supported catalysts
seems to be similar.
In honeycomb monoliths-supported catalysts, RhOx is preferentially
attached to Ce0.9Pr0.1O2, and not to the cordierite substrate.
Monolith supported catalyst has similar N2O decomposition
capacity than powder active phases of the same composition at
low N2O conversions (below ~15 % for the conditions of the current
study), but they outperform the conversion of powder catalysts due
to the improved mass transfer of the supported catalysts.
For the experimental conditions used in this study, the optimum
content of rhodium in total weight base is 0.2 wt.%. Increasing the
amount above this loading does not increase the N2O
decomposition capacity.
The distribution of the supported active phases on the cordierite
substrate depends on the calcinations conditions. More
homogeneous coatings are obtained by flash calcination in
comparison to conventional ramp calcinations.
The calcination procedure also affects the Rh-Ce-Pr interactions.
Flash calcination yields smaller RhOx particles and improves the
low temperature reduction of surface Rh-Ce-Pr entities in
Chapter 5
156
comparison to ramp calcination, which in turn improves the
catalytic activity for N2O decomposition.
The catalyst denoted by 0.2%RhOx/Ce0.9Pr0.1O2/M (flash) has
proved to be able to decompose N2O in presence of NO, O2 and
H2O.
The main goal of this work has been suscesfully accomplished and
N2O decomposition RhOx/Ce0.9Pr0.1O2 catalysts have been
successfully supported on honeycomb monoliths and have been
tested in simulated gas streams in the presence of typical
inhibitors.
Honeycomb monolith-supported RhOx/Ce0.9Pr0.1O2 catalysts
157
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CHAPTER 6
NOx reduction to N2 with commercial
fuel in a real diesel engine exhaust
using a dual bed which constists of
Pt/Beta zeolite and RhOx/ceria
monolith catalysts
In this chapter medium-size and full-size Pt/Beta zeolite
monolith catalysts have been prepared and tested in SCR technology
for NOx abatement in real diesel engines exhausts. A medium-size
dual bed catalytic system consisting of a Pt/Beta zeolite/monolith SCR
catalyst located upstream a N2O decomposition
RhOx/Ce0.9Pr0.1O2/honeycomb monolith catalyst has been also
successfully prepared and tested for NOx reduction to N2. The
dip-coating method was optimized for the Pt/Beta zeolite/monolith
preparation. The catalyst RhOx/Ce0.9Pr0.1O2/monolith was prepared by
nitrate precursor decomposition. The production of N2O as undesired
NOx reduction product, which is a drawback of platinum SCR
catalysts, has been solved by using the dual bed configuration, where
both monolith catalysts operated at the same temperature, and 100 %
N2 selectivity has been obtained.
Medium and full-size catalysts for deNOx and deN2O
163
6.1 Introduction.
As reported in Chapter 1, NOx removal in diesel-engine exhausts
can be mainly accomplished by two technologies: SCR (Selective Catalytic
Reduction) and NOx Storage Reduction (NSR) 1-3.
The main drawback of NOx removal on diesel vehicles is the
oxygen excess in the exhaust. For this reason, in both technologies (SCR
and NSR) a reducing agent must be fed into the exhaust stream, which
must react with NOx in the presence of excess O2. Different reducing
agents have been studied, most of them at laboratory scale, including H2,
CO, different HCs, NH3, urea, etc. 1-9].
In the NSR process 1-3, the NOx removal is carried out in cycles
of storage and reduction steps. During the storage steps, which occur in
normal driving conditions, NOx is chemisorbed on a basic oxide present on
the catalyst. During the reduction steps, the reducing gas is fed and reacts
with the nitrogen compounds previously stored (and also with O2).
In the SCR process [2-9], the reducing agent is continuously fed
into the gas stream and a selective catalyst is used to promote the reaction
of the reducing gas with NOx with respect to the unproductive direct O2
combustion.
During the last years, many research efforts have been focussed
on the utilization of NH3 or urea as reductants, in somehow inspired by the
well-established technology used in power plants, and some commercial
systems are now available for stationary diesel engines and heavy-duty
diesel vehicles [2, 6-9]. However, this technology has not been
successfully developed for light-duty vehicles, and one of the reasons is
the weight penalty of the additional urea/NH3 tank. In addition, a network of
urea/NH3 suppliers should be available to fill up the tanks.
An alternative option that overcomes these limitations of the
urea/NH3 technology would be the utilization of diesel fuel itself as reducing
agent. Diesel fuel is already on board of diesel vehicles, and therefore,
additional tanks and filling facilities are not required. The SCR of NOx has
been studied in detail with several HCs of different nature (propene
[10-12], octane [13], methanol [12], ethanol [12], acetaldehyde [14] and
Chapter 6
164
decane [11]. Nevertheless, most of these studies were performed under
laboratory conditions, but for real diesel exhausts the number of scientific
articles describing NOx removal studies is quite limited 2, 15, 16].
Recently, Cho et al. [17] have successfully tested in a real diesel
exhaust a NOx reduction system consisting of a plasma reactor, a diesel
fuel reformer and a dual-bed catalytic reactor. In this system the diesel fuel
reformer produces highly reactive oxygenated HCs for NOx reduction.
However, as far as we know, direct reduction of NOx with commercial
diesel fuel has not been reported.
In previous studies performed in our group, small-size (1 cm
diameter) cordierite honeycomb monoliths were coated with Pt/Beta zeolite
active phase and this supported catalyst was successfully tested for SCR
of NOx with propene at laboratory conditions [18, 19]. The activity of such
coated monolith was quite promising (NOx conversions above 80 % were
achieved), and the main drawback was that N2O was the main NOx
reduction product (while N2 is the desired one). In the study reported in the
previous chapter, small-size cordierite honeycomb monoliths were coated
with RhOx/Ce0.9Pr0.1O2 and successfully tested for low temperature N2O
decomposition at laboratory as well [20]. These results lead us to think that
a dual bed of Pt/Beta zeolite/monolith and RhOx/Ce0.9Pr0.1O2/monolith
could be a suitable option for NOx conversion into N2, with both catalytic
beds operation at the same temperature and in a range of temperatures
suitable for a diesel engine exhaust (around 200-250 ºC for propene as
reductant, and at higher temperatures for heavier HCs, as it will be later
demonstrated in this chapter). The dual bed configuration was previously
tested but using two catalytic systems working at different temperatures
[21].
One of the goals of the current study is to coat larger cordierite
honeycomb monoliths (medium-size monoliths; diameter 2.3 cm) with
Pt/Beta zeolite and to test the SCR of NOx in a real diesel engine exhaust
by using commercial diesel fuel as reducing agent. This system will be
scaled up into a full-size Pt/Beta zeolite monolith catalyst in order to
demonstrate and report, for the first time, that the SCR of NOx can be
performed in a real diesel exhaust stream by commercial diesel fuel using
this catalyst.
Medium and full-size catalysts for deNOx and deN2O
165
Another goal is to prepare a medium-size (diameter 2.3 cm)
RhOx/Ce0.9Pr0.1O2/monolith catalyst and to test the proposed dual bed
configuration (Pt/Beta zeolite/monolith and RhOx/Ce0.9Pr0.1O2/monolith) in
a real diesel exhaust to ensure that N2O is not released as undesired NOx
reduction by-product.
6.2 Experimental details.
6.2.1 Catalysts preparation.
Cordierite honeycomb monoliths of different size have been used
in this study, which were shown in Figure 2.18 (Chapter 2).
The small-size monolith was used in Chapter 5 and previous
studies [18, 19, 20], and the medium and full-size monoliths have been
used in the current chapter.
The medium-size monoliths were supplied by CTI (Céramiques
Techniques et Industrielles; France) and the dimensions are 2.3 cm
diameter and 7.5 cm length, with 400 cpsi3. These monoliths were used
both as Pt/Beta zeolite support and as RhOx/Ce0.9Pr0.1O2 support.
The full-size monoliths were supplied by Corning and the
dimensions are 14.4 cm diameter and 14 cm length, with 400 cpsi. One of
these monoliths was used as Pt/Beta zeolite support.
6.2.1.1 Medium-size Pt/Beta zeolite/monolith catalysts preparation.
Beta zeolite was loaded into the medium-size honeycomb
monoliths by dip-coating [22-24], with a water suspension of commercial
powder ammonia Beta zeolite (provided by Zeolyst International),
surfactant (Teepol) and binder (a 40 wt.% suspension of colloidal silica in
water; Ludox AS-40). A detailed study of the dip-coating process was
performed and different variables were evaluated. Suspensions with
different proportions of zeolite, surfactant and binder were studied.
3 cpsi: cell pieces per square inch (1 inch = 2.54 cm).
Chapter 6
166
To obtain homogeneously dispersed mixtures, the slurries were
stirred with a high-shear mixer (UltraTurrax T50 from IKA Labortechnik) for
15 minutes at 8000 rpm. The effect of the zeolite, surfactant and binder
amounts on the slurry viscosity was studied with a viscometer SV10 from
A&D.
The monoliths were dipped into the slurries with a home-made
dip-coater (Figure 6.1) at a rate of 1.5 cm/min, both for introducing the
monoliths into the slurries and to take them out. Monoliths coating either in
a single step or in several consecutive dipping steps were studied.
Figure 6.1. Home-made dip-coater with accessories for different size of monoliths.
In all cases, the coated monoliths were first dried overnight at room
temperature while rotating in horizontal position on the device shown in
Figure 6.2, and afterwards, at 90 ºC for 1 hour in static horizontal position.
Finally, selected monoliths were heat-treated in air in a furnace by heating
from room temperature to 200 ºC at 1 ºC/min and from 200 to 500 ºC at
10 ºC/min, keeping the maximum temperature for 4 hours.
Medium and full-size catalysts for deNOx and deN2O
167
Figure 6.2. Home-made rotating dryer.
A Beta zeolite-coated monolith (with 3.3 wt.% of zeolite coated in
two steps, 0.04 gzeolite/cm3monolith, by using fresh Beta zeolite slurry of
composition: 9 wt.% Beta zeolite, 0.24 wt.% binder and 0.36 wt.%
surfactant; viscosity 1.3 mPas) was selected for platinum loading. This
monolith was prepared with the optimal conditions obtained in section
6.3.3 of the current chapter.
[Pt(NH3)4](NO3)2 (Alfa-Aesar, 99.99 wt.%) was used as platinum
precursor, and the nominal platinum loading was 1 wt.% on zeolite basis.
The amount of water used to dissolve the platinum precursor and to
impregnate the monolith was the minimum amount that ensured no water
dropping upon impregnation. This amount was determined experimentally,
being 4.5 gwater/gzeolite. 50 % of the platinum solution was homogeneously
dropped with a pipette to one of the end sides of the Beta coated
substrate, and afterwards, the remaining 50 % was dropped to the other
one. The impregnated monolith was dried at 90 ºC in vertical position for
1 hour, changing the position of the monolith up-side-down every
10 minutes. After this step the monolith was apparently dry and was kept
overnight at the same temperature in horizontal position. Finally, the
monolith was calcined in air at 500 ºC (heating rate 5 ºC/min) keeping the
maximum temperature for 2 hours. Figure 6.3 shows the medium-size
Pt/Beta zeolite/monolith tested in the SCR experiments with commercial
diesel fuel.
Chapter 6
168
Figure 6.3. Photograph of the medium-size Pt/Beta zeolite/monolith tested in the SCR
experiments with commercial diesel fuel.
6.2.1.2 Full-size Pt/Beta zeolite/monolith catalyst preparation.
Beta zeolite was also loaded into a full-size honeycomb monolith
by dip-coating 22, 23, 24 with a water suspension of commercial powder
ammonia Beta zeolite (provided by Zeolyst International), using Teepol as
surfactant and a 40 wt.% suspension of colloidal silica in water; Ludox
AS-40 as binder. The optimized suspension was obtained from the study
described in section 6.3.3.
To obtain a homogeneously dispersed mixture, the slurry was
stirred with the mixer UltraTurrax T50 for 15 minutes at 1500 rpm. After
5 minutes stabilization, the slurry viscosity was 1.3 mPa·s. Using this
viscosity, blowing the suspension excess with compressed air was not
necessary.
The monolith was dipped into the slurry with a home-made
dip-coater (Figure 6.1) at a rate of 1.5 cm/min, both for introduction of the
monolith into the slurry and to take it out. Four consecutive dipping steps
were performed.
The coated monolith was dried overnight at room temperature
while rotating in horizontal position. Afterwards, it was heat-treated in air in
a furnace by heating from room temperature to 200 ºC at 1 ºC/min and
Medium and full-size catalysts for deNOx and deN2O
169
from 200 to 500 ºC at 10 ºC/min, keeping the maximum temperature for
4 hours. The Beta zeolite loading was 10.5 wt.% after calcination
(0.05 gzeolite/cm3monolith).
Finally, the Beta zeolite-coated monolith was impregnated with a
water solution of [Pt(NH3)4](NO3)2 (Alfa-Aesar 99.99 wt.%) to obtain 1 wt.%
platinum loading on zeolite basis. The amount of water used to dissolve
the platinum precursor and to impregnate the monolith was also the
minimum amount that ensured no dropping upon impregnation
(4.5 gwater/gzeolite). 50 % of the platinum solution was homogeneously
dropped with a pipette to one of the end sides of the Beta zeolite-coated
substrate, and afterwards, the remaining 50 % was dropped to the other
one. The impregnated monolith was dried at 90 ºC in vertical position,
changing the position of the monolith up-side-down every 30 minutes for
4 hours. After this time the monolith was apparently dried and was kept
overnight at 90 ºC in horizontal position. Finally, the monolith was calcined
in air at 500 ºC (heating rate 5 ºC/min) keeping the maximum temperature
for 2 hours. Figure 6.4 shows a picture of the full-size honeycomb monolith
loaded with Pt/Beta zeolite.
Figure 6.4. Photograph of the full-size Pt/Beta zeolite/monolith.
Chapter 6
170
6.2.1.3 Medium-size RhOx/Ce0.9Pr0.1O2/monolith catalyst preparation.
The active phase RhOx/Ce0.9Pr0.1O2 was loaded on a medium-size
honeycomb monolith by using Ce(NO3)3·6H2O (Alfa-Aesar, 99.5 wt.%),
Pr(NO3)3·6H2O (Alfa-Aesar, 99.9 wt.%) and Rh(NO3)3·xH2O
(Sigma–Aldrich, ∼36 wt.% as rhodium). Figure 6.5 shows a picture of the
honeycomb monolith loaded with RhOx/Ce0.9Pr0.1O2.
Figure 6.5. Medium-size RhOx/Ce0.9Pr0.1O2/monolith.
The preparation conditions were similar to those described as flash
calcination for a small-size (1 cm diameter) monolith, in Chapter 5,
corresponding to the optimum catalyst. The impregnated monolith, with
cerium and praseodymium precursors water solution, was introduced in
the furnace at 250 ºC for 1 hour and then was flash calcined (at 600 ºC for
90 min, using a heating rate of 10 ºC/min).The concentration of the
solution used for the impregnation was fixed to obtain 3 wt.% of
Ce0.9Pr0.1O2 loading (0.04 gCe0.9Pr0.1O2/cm3monolith). This weight increase
was confirmed gravimetrically.
Afterwards, the Ce0.9Pr0.1O2-containing monolith was impregnated
with a water solution of the rhodium precursor, with the proper
concentration to obtain 1 wt.% rhodium loading on Ce0.9Pr0.1O2 basis,
following the procedure previously described for platinum impregnation on
Medium and full-size catalysts for deNOx and deN2O
171
Beta zeolite-containing monoliths. After rhodium impregnation the monolith
was introduced in a furnace which was previously heated at 250 ºC, and
after 1 hour at this temperature, the temperature was increased at
1 ºC/min until 500 ºC [25].
6.2.2 SEM characterization.
SEM pictures of selected coated monoliths were taken. This
characterization technique and the equipment used were described in
Chapter 2.
6.2.3 Catalytic tests.
SCR experiments were performed with the medium and full-size
Pt/Beta zeolite/monolith catalysts. Dual-bed experiments for simultaneous
deNOx and deN2O were also performed with the medium-size
RhOx/Ce0.9Pr0.1O2/monolith catalyst located downstream the medium-size
Pt/Beta zeolite/monolith catalysts. Experiments with medium-size monolith
catalysts were performed at the University of Alicante and SCR
experiments with the full-size monolith catalyst were carried out at the
facilities of the Galician Automotive Technology Centre (Centro
Tecnológico de Automoción de Galicia, CTAG) as described in Chapter 2.
6.3 Results and discussion.
6.3.1 Study of the Beta zeolite suspensions viscosity.
Figure 6.6 shows the change of viscosity with time for several Beta
zeolite water suspensions prepared with different Beta zeolite and
surfactant concentrations. As a general trend, viscosity decreased with
time until a constant value was achieved. Both the zeolite and surfactant
concentrations affected the change of viscosity with time and the stable
viscosity value achieved. The viscosity decrease with time was attributed
to zeolite sedimentation, which in most cases occurred during few minutes
after the mixer was stopped.
The surfactant concentration significantly affected viscosity
changes with time. As expected, this effect was more important for the
Chapter 6
172
zeolite suspension with the highest zeolite loading (18 and 24 wt.% are
compared in Figure 6.6). As a general trend, the higher the surfactant
concentration the faster the stable viscosity level was achieved.
0
20
40
60
80
0 10 20 30 40 50 60
Vis
cosity (
mP
a.s
)
Time (min)
24 wt. % Beta zeolita + 0.25 wt. % binder
18 wt. % Beta zeolita + 0.25 wt. % binder
Surfactant (%)
0 0.40.8 1.6
Surfactant (%)
0 0.40.8 1.6
Figure 6.6. Change of viscosity with time of several Beta zeolite water suspensions of
different composition. (Time = 0 min when the mixer was stopped).
The practical conclusion from results on Figure 6.6 was that, for
dip-coating of monoliths on this type of suspensions, a certain stabilization
time was necessary before the monolith can be dipped, and this time
ranged from few minutes (3 at least) until 1 hour, depending on the zeolite
and surfactant concentrations.
As deduced from Figure 6.6, the zeolite concentration had an
important effect on the suspension viscosity, the higher the zeolite
concentration, the higher the suspension viscosity. This important variable
was studied in more detail. Figure 6.7 compiles viscosity values for several
Beta zeolite water suspensions of different composition, with zeolite
concentrations between 18 and 27 wt.%. The viscosity data were
measured after the time required in each case to obtain stable values. The
effect of surfactant and binder concentrations was evaluated as well. The
results on Figure 6.7 allowed concluding that the zeolite amount had the
strongest effect on viscosity. For instance, viscosity values ranged from 13
to 110 mPa·s for zeolite concentrations between 18 and 27 wt.%
respectively. However, for a particular zeolite concentration, the change of
the surfactant concentration between 0 and 1.6 wt.% only induced a
Medium and full-size catalysts for deNOx and deN2O
173
viscosity change of ± 5 mPa·s and the change of binder concentration
between 0.24 and 0.46 wt.% only modified viscosity in ± 2 mPa·s.
0
20
40
60
80
100
120
0 0.5 1 1.5
Vis
co
sity (
mP
a.s
)
Surfactant (wt.%)
27 wt.% beta zeolite + 0.24 wt.% binder
24 wt.% beta zeolite + 0.24 wt.% binder
22 wt.% beta zeolite + 0.24 wt.% binder
18 wt.% beta zeolite + 0.24 wt.% binder
20 wt.% beta zeolite + 0.24 wt.% binder
20 wt.% beta zeolite + 0.46 wt.% binder
Figure 6.7. Viscosity of Beta zeolite water suspensions with different compositions. (All
data were measured after the time required in each case to obtain stable viscosity
values).
The relationship between zeolite concentration and suspension
viscosity is plotted on Figure 6.8 for suspensions with similar surfactant
and binder concentrations. A wider range of zeolite concentrations to that
explored on Figure 6.7 was plotted on Figure 6.8, where an exponential
relationship is observed. The increase of viscosity was small for zeolite
concentrations between 5 and 13 wt.% (from 1 to 3 mPa·s respectively)
while viscosity increased drastically above 18 wt.% zeolite concentration.
Chapter 6
174
0
20
40
60
80
100
120
0 5 10 15 20 25 30
Vis
cosity (
mP
a.s
)
Beta zeolite (wt.%)
0.24 wt.% binder0.36 wt.% surfactant
Figure 6.8. Effect of the Beta zeolite concentration on suspension viscosity, (all data
were measured after the time required in each case to obtain stable viscosity values).
6.3.2 Effect of the Beta zeolite suspension viscosity on medium-size
monoliths dip-coating.
Once the effect of the Beta zeolite concentration on suspensions
viscosity was studied, honeycomb monoliths were dipped on Beta zeolite
suspensions of different composition and the monolith weight increase was
determined gravimetrically after drying the coated monoliths, calcination at
500 ºC and blowing the loosely bound zeolite with pressurised air. Figure
6.9 shows the effect of the Beta zeolite concentration (Figure 6.9.a) and
viscosity of the suspension (Figure 6.9.b) on the monolith weight increase
after a single dip-coating step.
A linear relationship between zeolite concentration and weight
increase was obtained for Beta zeolite suspensions up to 20 wt.%
concentration, corresponding to viscosities ≤ 23 mPa·s. Above this
viscosity the zeolite suspension was not able to penetrate into the monolith
channels and the weight increase dropped to zero. In a review article [26]
was reported that the length of honeycomb monoliths that can be
washcoated is mainly controlled by the viscosity of the washcoating
solution, being possible to washcoat monolith bodies up to 25 cm long with
solutions of viscosity below 30 mPa·s. However, our current results
evidence that this general rule do not apply to Beta zeolite suspensions,
since a threshold at 23 mPa·s was found for honeycomb monoliths of
Medium and full-size catalysts for deNOx and deN2O
175
7.5 cm length. In this conditions the maximum zeolite loading obtained was
1.5 wt.%.
0.0
0.3
0.6
0.9
1.2
1.5
1.8
0 5 10 15 20 25 30
Weig
ht
incre
ase (
%)
Beta zeolite (wt.%)
(a)0.24 wt.% binder0.36 wt.% surfactant
0.0
0.3
0.6
0.9
1.2
1.5
1.8
0 20 40 60 80 100 120
We
igh
t in
cre
ase
(%
)
Viscosity (mPa·s)
(b)
0.24 wt.% binder0.36 wt.% surfactant
Figure 6.9. Effect of the Beta zeolita concentration (a) and viscosity of the suspension
(b) on the weight increase of the monolith after one dip-coating step (after drying,
calcinations at 500 ºC, and blowing the loosely bound zeolite with pressurised air).
6.3.3 Medium-size monolith coating with Beta zeolite in consecutive
dipping steps.
Since the maximum weight increase achieved in the dipping
conditions used for Figure 6.9 experiments was 1.5 wt.%, different
Chapter 6
176
strategies were explored in order to increase the amount of Beta zeolite
loaded on the monolith.
In the experiments reported in section 6.3.2 it was observed that
an important proportion of the Beta zeolite remained on the water
suspension after dipping the monolith. In order to analyze the potential
reutilization of the same Beta zeolite suspension, several consecutive
coating steps were carried out with the same slurry. Figure 6.10 compiles
the monolith weight increase achieved in consecutive coating steps
performed with the same monolith and with the same Beta zeolite
suspension (circles). These data were obtained after drying the monolith
and blowing the loosely bound zeolite with pressurised air (this procedure
was repeated after every dipping step).
0
1
2
3
4
5
6
7
8
9
0 1 2 3 4 5 6 7 8 9
Weig
ht
incre
ase (
%)
Number of dip-coating steps
9 wt.% Beta zeolite
0.24 wt.% binder0.36 wt.% surfactant
Figure 6.10. Monoliths weight increase after several dip-coating steps. Circles: using
the same Beta zeolite suspension in all steps. The monolith was dried and the loosely
bound zeolite was blown with pressurised air after each dip-coating step; Squares:
Fresh Beta zeolite suspension was used in each dip-coating step. (Solid squares:
weight increase after dip-coating and drying; Open squares: weight after drying,
calcination at 500 ºC and air blowing).
The amount of zeolite loaded on the monolith increased
progressively with the number of dip-coating steps until a maximum
loading of 2.8 wt.% was achieved after eight steps. As observed, the
weight of the monolith did not increase linearly. This was attributed to the
progressive depletion of the zeolite on the water suspension. In
conclusion, the Beta zeolite suspension could be used in consecutive
Medium and full-size catalysts for deNOx and deN2O
177
dipping steps but the yield of the process would be lower to that obtained
by using fresh suspension on each step.
In order to increase the zeolite loading, four consecutive dipping
steps were performed with a new monolith, using fresh Beta zeolite
suspension on each step. The effect of calcination was also analyzed, and
the monolith was calcined at 500 ºC after the second and fourth steps. The
monolith weight increase achieved on these conditions is also included on
Figure 6.10 (squares).
The monolith weight increased progressively in consecutive
coating steps, and as expected, the zeolite loading was higher
(Figure 6.10; squares) to that obtained using the same suspension for
several times (Figure 6.10; circles). The maximum zeolite loading achieved
using fresh Beta zeolite suspension on each step was about 7.6 wt.%
(after four dipping steps), while only 2.8 wt.% was achieved with the same
zeolite suspension. Note that the zeolite loading increase almost linearly
during the first three dipping steps performed with fresh Beta zeolite
suspension (Figure 6.10, squares) but in the fourth one the loading
decrease from that tendency. This is because the monolith channels
become narrower upon zeolite accumulation, and this hinders further
zeolite loading. Actually, after the fourth dip-coating step part of the
monolith channels were closed, as it will be shown below.
Special attention must be paid to the weight increase in the first
step. The different weight increase observed in both series of Figure 6.10
must be attributed to the fact that, in one case (solid squares) the monolith
was not blown with pressurised air after drying, while it was in the other
series (circles). According to this comparison, a significant amount of
zeolite (the loading decreased from 2 to 0.6 %) was removed upon blowing
with pressurised air, and this amount of zeolite removed is wasted.
However, the proportion of zeolite removed by air blowing after calcination
at 500 ºC was much lower (see open squares), and this procedure is
therefore more convenient. These results evidenced that the blowing step
performed after drying is not really necessary if the monolith channels are
not blocked by zeolite, and blowing the loosely bound zeolite could be
done after calcination.
Chapter 6
178
Opening up the monolith channels with pressurised air is a habitual
procedure which is typically done after dip-coating and before calcination
[22-24, 26-29]. This is done in this way because the channels usually
become blocked after dip-coating, since high viscosity suspensions are
preferred in order to achieve high washcoat loading. Opening blocked
channels after calcination, once the binder has sintered, is almost
impossible for long channels and that is why air blowing is usually done
before calcination. According to our optimization study, an alternative
procedure consists of avoiding channels blockage by using low-viscosity
suspensions, and therefore the cleaning step can be done after
calcination, once the binder has sintered and the zeolite layer has been
stabilized. In this case, several consecutive dipping steps are required in
order to increase the active phase loading, and the goal of air blowing is
not to open closed channels but to remove loosely bound zeolite.
6.3.4 SEM characterization of medium-size monolith catalysts.
Figure 6.11 shows SEM images of selected coated monoliths.
Figures 6.11.a and 6.11.b show general views of several channels of the
Beta zeolite coated monoliths with 3.3 and 7.6 wt.% coating, respectively,
corresponding to data represented by open squares on Figure 6.10
(calcined samples). A detail of the few micrometers coating layer is also
shown in the inset of Figure 6.11.a. It is observed that the channels of the
monolith with lower coating content (3.3. wt.%; Figure 6.11.a) remain open
while those of the monolith with higher coating loading (7.6 wt.%; Figure
6.11.b) are partially or totally blocked.
Taking into account the optimization study, the dip-coating
conditions selected to prepare the medium and full-size
Pt/Beta zeolite/monolith catalysts were those corresponding to the sample
with 3.3 wt.% of zeolite (one of the open squares of Figure 6.10, coated in
two steps by using fresh Beta zeolite on each step).
Medium and full-size catalysts for deNOx and deN2O
179
Figure 6.11 SEM images of selected coated monoliths. (a) Beta zeolite coated
monolith with 3.3 wt.% coating, (b) Beta zeolite coated monolith with 7.6 wt.% coating
and (c) RhOx/Ce0.9Pr0.1O2/monolith catalyst. Figures 6.11.a and 6.11.b correspond to
open squares on Figure 6.10; Figures 6.11.a and 6.11.c are the samples used in the
catalytic tests sections 6.3.5 and 6.3.7.
Chapter 6
180
Figure 6.11.c shows pictures of the RhOx/Ce0.9Pr0.1O2/monolith
catalyst, where it is clearly observed that channels are also open in this
case. This is in accordance with our previous study performed to optimize
the preparation of this kind of supporter catalysts (Chapter 5 and reference
[20]).
6.3.5 SCR experiments performed with commercial diesel fuel and
the medium-size Pt/Beta zeolite/monolith catalyst.
Figure 6.12 shows results of the SCR experiments performed with
different diesel fuel concentrations and the medium-size Pt/Beta
zeolite/monolith catalyst at reaction temperatures between 300 and
400 ºC. NOx removal curves (Figure 6.12.a) present the typical
volcano-shape profile and the THC consumption (Figure 6.12.c) increases
with temperature [3, 4, 9]. A maximum in the curves of NOx removal level
versus temperature is typically reported in SCR NOx experiments
performed with model HCs. The temperature of the maximum and the
maximum NOx removal level achieved depend both on the nature of the
catalysts and the nature and concentration of the HC. Below the maximum
temperature the NOx-HC reaction takes place and the process is selective
in a certain extent, while above the maximum the selectivity decreases and
the HCs are consumed by undesired O2 combustion with less contribution
of the NOx-HC reaction pathway. The optimum temperature for platinum
catalysts is typically lower than that of some other transition metal catalysts
such us cupper or cobalt catalyst [4].
In our experiments, the optimum temperature for maximum NOx
removal was 350 ºC for all THC concentrations tested, and the maximum
NOx removal level achieved in these experimental conditions was 50 % for
3400-4500 ppm THC inlet concentration. The temperature for maximum
NOx removal is higher to that typically obtained with propene as reducing
agent and Pt/Beta zeolite catalysts tested at laboratory under simulated
gas streams (210-230 ºC) [18, 30]. This can be attributed to several
experimental differences between our previous laboratory experiments
[18, 30] and the current pilot plant tests: (i) catalyst active phase to gas
flow ratio, (ii) nature of the reducing agent and (iii) gas stream composition
(laboratory streams only included NOx and 5 % O2 in an inert carrier while
the real stream also includes H2O, CO2 and much more O2 (17.2 %)).
Medium and full-size catalysts for deNOx and deN2O
181
0
20
40
60
80
275 300 325 350 375 400 425
NO
x r
em
oval (%
)
Catalyst temperature (ºC)
45003400220013006001500
THC inlet concentration (ppm)
(a)
0
20
40
60
80
100
275 300 325 350 375 400 425
N2
sele
ctivity (
%)
Catalyst temperature (ºC)
4500
3400
2200
1300
600
THC inlet concentration (ppm)
(b)
0
20
40
60
80
100
275 300 325 350 375 400 425
TH
C r
em
oval (%
)
Catalyst temperature (ºC)
45003400220013006001500
THC inlet concentration (ppm)
(c)
Figure 6.12. SCR experiments performed with commercial diesel fuel and a
Pt/Beta zeolite/monolith catalyst (a) NOx removal, (b) N2 selectivity and (c) THC
removal.
Chapter 6
182
These results demonstrate that the SCR of NOx with diesel fuel is
feasible in a real exhaust by using a medium-size Pt/Beta zeolite/monolith
catalyst. After this proof of concept, next step is to scale up this catalyst
into a full-size catalyst, as described in section 6.2.1.2, and to test it.
One of the weak points of this technology is the reduction of part of
the NOx to N2O instead of to N2, which is the innocuous and desired
reaction product. This has been reported by several authors working at
laboratory [18, 30-33]. The selectivity results presented on Figure 6.12.b
show that, in our experimental conditions, N2 is always the main NOx
reduction product (N2 selectivity ≥ 67 %) but N2O is detected at 300 ºC for
all THC inlet concentrations tested and at 350 ºC for the low THC inlet
concentrations (600 and 1300 ppm THC inlet concentration) which are
desirable from the economic point of view and also to obtain the lower HC
emission as observed in Figure 6.12.c. The reason for the high N2
selectivity achieved in the current power bench experiments must be the
higher operation temperature compared to other laboratory experiments
where platinum-catalysts work below 250 ºC [18, 30]. However, in order to
avoid N2O emission dual-bed experiments have been performed and are
described afterwards.
6.3.6 SCR experiments performed with commercial diesel fuel and
the full-size Pt/Beta zeolite/monolith catalyst.
Figure 6.13 compiles the results obtained in SCR experiments
performed with the full-size Pt/Beta zeolite/monolith catalyst. THC and CO
removal increased with temperature (Figures 6.13.a and 6.13.b,
respectively) and NOx removal (Figure 6.13.c) followed a typical
volcano-shape profile, with maxima at 250 ºC for all THC inlet
concentrations studied. This behavior is similar to that typically observed in
laboratory experiments performed with model HCs 30-32.
Medium and full-size catalysts for deNOx and deN2O
183
0
20
40
60
80
100
200 225 250 275 300 325
TH
C r
em
ova
l (%
)
Temperature at catalyst inlet (ºC)
1000
2000
3000
4000
5000
THC inlet concentration (ppm)
(a)
0
20
40
60
80
100
200 225 250 275 300 325
CO
re
mo
va
l (%
)
Temperature at catalyst inlet (ºC)
1000
2000
3000
4000
5000
THC inlet concentration (ppm)
(b)
0
5
10
15
20
25
30
35
200 225 250 275 300 325
NO
x r
em
oval (%
)
Temperature at catalyst inlet (ºC)
1000
2000
3000
4000
5000
THC inlet concentration (ppm)
(c)
Figure 6.13. Catalytic results obtained at steady state with a full-size
Pt/Beta zeolite/monolith catalyst upon injection of different amounts of commercial
diesel fuel versus temperature at catalyst inlet.
Chapter 6
184
As expected, the profiles of NOx removal (volcano shape) in the
full size catalyst behaves in similar way than medium-size catalyst
(section 6.3.5). However, in the experiments performed with the full-size
monolith catalyst, it must be also taken into account that the inlet NOx
concentration increases with the exhaust temperature (see data on
Table 2.1), and this also affects the shape of the NOx removal profiles on
Figure 6.13.c.
The temperature for maximum NOx removal (250 ºC) was only
slightly higher than values previously obtained in laboratory experiments
with 1 cm diameter Pt/Beta zeolite/honeycomb monolith by using propene
as model hydrocarbon (210-230 ºC) 18, 19, confirming that the Pt/Beta
zeolite monolith catalyst is a good candidate for low temperature NOx
reduction with commercial diesel fuel. Maximum temperature for NOx
removal is lower in this section than in the previous one, although as
explained before this temperature depends on several factors.
0
5
10
15
20
25
30
35
0 1000 2000 3000 4000 5000 6000
NO
x r
em
ova
l (%
)
THC inlet concentration (ppm)
Temperature = 300 ºC411 ppm NOx
Temperature = 220 ºC97 ppm NOx
Temperature = 250 ºC 218 ppm NOx
at catalyst inlet :
Figure 6.14. Effect of the amount of fuel injected on NOx removal.
For a better observation of the effect of the amount of fuel injected
in the NOx removal, the NOx conversion has been represented versus
THC inlet concentration, for several inlet temperatures, in Figure 6.14. An
optimum concentration at 3000 ppm THC inlet concentration is observed.
Only considering the Le Chatelier's principle one would expect a
continuous increase of NOx removal with THC inlet concentration, but this
Medium and full-size catalysts for deNOx and deN2O
185
only occurred below 3000 ppm THC. Two arguments have been proposed
to explain the NOx removal decrease at high THC concentration 16. In
one hand, for the HC-NOx reactions to occur on a catalyst surface the
reactants must be chemisorbed on the catalyst, and a huge excess of one
of the reactants (THC in this case) hinders the chemisorption of the
remaining gases, inducing a certain poisoning effect. On the other hand,
the HC combustion reactions are highly exothermic, and the increase of
temperature within the catalyst promotes the HC-O2 reaction, decreasing
the selectivity of the process. This effect is evidenced in Figure 6.15,
where the temperature at the catalyst outlet is plotted versus the THC inlet
concentration. In most experimental conditions, the gas temperature
decreases within the catalytic bed, since most outlet temperatures are
below the corresponding inlet temperatures. However, an increasing trend
was obtained with the THC concentration for experiments performed at the
same inlet temperature due to the exothermic character of the THC
combustion reaction, therefore affecting the SCR selectivity.
200
250
300
1500 2000 2500 3000 3500 4000 4500
Te
mp
era
ture
at
ca
taly
st o
utle
t (º
C)
THC inlet concentration (ppm)
Temperature at catalyst inlet = 300 ºC
Temperature at catalyst inlet = 250 ºC
Temperature at catalyst inlet = 220 ºC
Figure 6.15. Effect of the amount of fuel injected on catalyst temperature.
The results obtained in this study confirm that the SCR of NOx can
be successfully achieved by commercial diesel fuel in real light-duty
vehicles with a Pt/Beta zeolite/monolith catalyst, therefore avoiding
additional tanks on board which are required for some other reductants like
urea or NH3. After this proof of concept, next step for further studies would
be to optimize the experimental variables in order to achieve NOx removal
levels as high as possible, for instance, optimization of the catalyst
Chapter 6
186
preparation, platinum loading, fuel injection, catalyst temperature control,
utilization of several consecutive small catalysts instead of in a single bed
with fuel injection before each catalyst, etc.
6.3.7 SCR experiments performed at 300 ºC with commercial diesel
fuel and a dual-bed which contains Pt/Beta zeolite and
RhOx/Ce0.9Pr0.1O2 medium-size monolith catalysts.
In spite of the good results presented in section 6.3.5, in order to
achieve 100 % N2 selectivity, dual bed experiments were performed with
the medium-size Pt/Beta zeolite/monolith catalyst (located upstream) and
the RhOx/Ce0.9Pr0.1O2/monolith catalyst (located downstream). Both
medium-size monoliths were placed on the same reactor and operate at
300 ºC. This temperature was selected because was the temperature
where more N2O was released in the single-bed SCR experiments (see
Figure 6.12.b).
Figure 6.16 compiles the NOx removal (Figure 6.16.a) and N2
selectivity (Figure 6.16.b) values obtained for different THC inlet
concentrations, where single and dual-bed configuration experiments are
compared. The N2 selectivity results obtained with the dual-bed
configuration (Figure 6.16.b) confirm that the rhodium catalyst avoids N2O
emission as undesired NOx reduction product, achieving 100 % N2
selectivity for all THC inlet concentrations tested. As far as we know, it is
the first time that this achievement is reported. A dual-bed configuration
was already proposed by Pérez-Ramírez et al. [21] in order to improve the
low N2 selectivity of SCR platinum catalysts, and several N2O
decomposition catalysts were evaluated in that study (Co-Rh and Co-Pd
mixed oxides derived from hydrotalcite-like compounds and ion-exchanged
Fe-ZSM-5 and Pd-ZSM-5 zeolites). That study was performed under
laboratory conditions by using propene as model HC. In that case the
optimum temperatures for the first and second beds were 200 and 425 ºC
respectively, and this ruled out the practical utilization of the mentioned
N2O decomposition catalysts in vehicles because the gas stream should
be heated up between the deNOx and deN2O catalysts. In that study [21],
it was noted that a much more active catalysts was required for practical
application of the dual-bed system to a realistic car exhaust situation, and
the results reported in the current study suggest that the
Medium and full-size catalysts for deNOx and deN2O
187
RhOx/Ce0.9Pr0.1O2/monolith catalysts is a suitable candidate to accomplish
this task.
0
10
20
30
40
50
0 1000 2000 3000 4000
NO
x r
em
oval (%
)
THC inlet concentration (ppm)
Pt/beta/honeycomb monolith +RhOx/Ce0.9Pr0.1O2/honeycomb monolith
Pt/beta/honeycomb monolith
Pt/beta zeolite/monolith + RhOx/Ce0.9Pr0.1O2/monolith
Pt/beta zeolite/monolith
(a)
0
20
40
60
80
100
0 1000 2000 3000 4000
N2
se
lectivity (
%)
THC inlet concentration (ppm)
Pt/beta+Rh/CePr
Pt/beta
Pt/beta zeolite/monolith + RhOx/Ce0.9Pr0.1O2/monolith
Pt/beta zeolite/monolith
(b)
Figure 6.16. SCR experiments performed at 300 ºC with commercial diesel fuel and a
dual bed of medium-size Pt/Beta zeolite/monolith and RhOx/Ce0.9Pr0.1O2/monolith
catalysts (a) NOx removal and (b) N2 selectivity.
The dual-bed configuration studied in the current study has a
certain benefit on NOx removal. For low THC inlet concentrations (THC
inlet concentration ≤ 2200 ppm) the NOx removal levels achieved with the
single and dual-bed configuration are quite similar, increasing with the
THC inlet concentration. This means that the RhOx/Ce0.9Pr0.1O2/monolith
catalyst do not contributes to NOx removal in this range of THC
concentrations. On the contrary, the NOx removal level remained more or
Chapter 6
188
less constant in the experiments performed with the single bed for THC
inlet concentration > 2200 ppm, because the THC excess poisons the
platinum catalyst, while increases monotonically in the dual bed
configuration experiments. Therefore, for high THC excess the
RhOx/Ce0.9Pr0.1O2/monolith catalyst contributes to NOx removal, which is
an additional benefit of the dual bed configuration. These results are in
agreement with the conclusions reported by Kotsifa et al. [34], who
compared the SCR of NO by propene over supported platinum and
rhodium catalysts. It was reported [34] that rhodium catalysts are active in
NOx SCR and selective for N2 production at low oxygen partial pressures,
but under severe oxidizing environments they lose their activity while
maintain N2 selectivity. This explains why our rhodium catalyst only
showed SCR activity at high THC inlet concentration, but improved N2
selectivity regardless the THC inlet concentration. On the contrary, Kotsifa
et al. [34] concluded that platinum catalysts are not able to catalyse NO
reduction at low oxygen partial pressures due to the accumulation of
hydrocarbonaceous intermediates on the metal surface, which hinder
adsorption and dissociation of NO. The reaction is promoted by higher O2
concentrations, where activation of HC enables efficient regeneration of
the catalytically active sites, in agreement with the proposed mechanism
[35]. The decrease of N2 selectivity of platinum catalysts with increasing O2
concentration was related with the higher coverage of adsorbed atomic
oxygen.
6.4 Conclusions.
The main conclusions of the current chapter can be summarized
as follows:
The preparation of the Pt/Beta zeolite/monolith has been optimized
by adjusting the viscosity of surfactant and binder-containing
Beta zeolite suspensions (viscosities ≤ 23 mPa·s). The effect of
zeolite concentration on slurry viscosity was much more relevant
than those of surfactant and binder.
A certain stabilization time was necessary before the monolith can
be dipped on Beta zeolite slurries, and this time ranged from few
Medium and full-size catalysts for deNOx and deN2O
189
minutes (3 at least) until 1 hour, depending on the zeolite and
surfactant concentrations.
The blowing step performed after drying is not really necessary if
the monolith channels are not blocked by zeolite, and blowing the
loosely bound zeolite could be done after calcination, but several
consecutive dipping steps are required in order to obtain the Beta
zeolite loading suitable for a practical application (3.3 wt.% in the
current study).
SCR experiments performed in real diesel exhausts with medium
and full-size Pt/Beta zeolite/monolith catalysts demonstrated that
NOx reduction is feasible with commercial diesel fuel.
The SCR behaviors observed in the real exhausts with commercial
diesel fuel were similar to that typically reported in laboratory
experiments performed with model HC.
The N2 selectivity of the medium-size Pt/Beta zeolite/monolith
catalyst in the studied conditions is significantly higher than
previous laboratory results, but the 100 % N2 selectivity is only
obtained by using RhOx/Ce0.9Pr0.1O2/monolith catalyst located
downstream the Pt/Beta zeolite/monolith catalyst, with both
monolith catalysts (deNOx and deN2O) operating at the same
temperature.
Chapter 6
190
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Chapter 6
194
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CHAPTER 7
General Conclusions
General conclusions
197
In this PhD thesis the optimization of the preparation conditions for
RhOx/CeyPr1-yO2 catalysts, in powder and supported in several cordierite
honeycomb monolith sizes, has been developed for further scaling up the
catalytic N2O decomposition process. The optimized conditions include
calcination process, use of different solvents (water, ethanol and acetone)
for rhodium loading, and amounts of rhodium, cerium and praseodymium
in the catalyst. A major understanding of the physico-chemical properties
and catalytic role of these materials in N2O decomposition process has
been facilitated by several characterization techniques and catalytic tests
carried out at different scales.
The main conclusions that are drawn from this research work are
as follows:
The calcination methods (ramp or flash) of cerium nitrate to obtain
the ceria support, has neither effect on the ceria properties (those
observed by XRD, Raman spectroscopy and N2 adsorption) nor on
the RhOx/CeO2 catalyst performance for N2O decomposition. On
the contrary, flash calcination of rhodium nitrate impregnated on
ceria support improves the catalytic activity for N2O decomposition
and CO oxidation of RhOx/CeO2 catalysts in comparison to that of
similar catalysts calcined in ramp.
The improved catalytic activity for N2O decomposition and CO
oxidation of catalysts with rhodium nitrate decomposed by flash
calcinations is attributed to the smaller size of RhOx nanoparticles
(smaller than 0.5 nm), allowing a larger noble metal oxide-support
interface favoring the reducibility of the ceria surface and stabilizing
the RhOx species under reaction conditions.
Fresh RhOx/CeO2 catalysts suffer a transformation process at
225 ºC during the N2O decomposition reaction, during which both
rhodium and cerium species are reduced and re-oxidized. This
process is consistent with the mechanism proposed for N2O
decomposition on RhOx/CeO2.
Both the nature of pure or praseodymium-doped ceria support and
the solvent used for rhodium precursor impregnation (with water,
Chapter 7
198
ethanol or acetone) affect RhOx-support interaction that determines
the activity for N2O decomposition of the catalysts studied.
The use of ethanol or acetone as solvent has a very negative effect
on Ce0.9Pr0.1O2 and Ce0.5Pr0.5O2-containing catalysts, regarding the
sintering of both supports and RhOx particles, affecting negatively
the RhOx-support interaction which directly hinders the catalytic
activity for N2O decomposition. This negative effect is due to the
solvent combustion catalyzed by the rhodium salt.
Ceria doping with 10 % praseodymium has a positive effect on the
RhOx-support interaction observed as a negative charge density
transfer from the noble metal to the ceria support (only obtained
using water for impregnation) that improves the catalytic activity.
The interaction between RhOx-Ce0.5Pr0.5O2 is not as good as that
with Ce0.9Pr0.1O2, but it seems to be enough to keep a high
catalytic activity if rhodium is impregnated with a water solution.
However, acetone or ethanol impregnation also leads to an
important decrease in activity, which is attributed to RhOx sintering.
RhOx/Ce0.9Pr0.1O2 active phases have been successfully loaded
into cordierite honeycomb monoliths. The nature of Ce0.9Pr0.1O2 in
powder and supported catalysts seems to be similar.
In honeycomb monoliths-supported catalysts, RhOx is preferentially
attached to Ce0.9Pr0.1O2, and not to the cordierite substrate.
For the experimental conditions studied in Chapter 5, the optimum
content of rhodium, in total weight-base of the monolith is 0.2 wt.%.
Increasing the amount above this loading does not increase the
N2O decomposition capacity.
The distribution of the supported active phases on the cordierite
substrate depends on the calcinations conditions. More
homogeneous coatings are obtained by flash calcination in
comparison to conventional ramp calcinations, affecting the
Rh-Ce-Pr interactions.
General conclusions
199
Supported catalysts have superior N2O decomposition capacity
than similar powder active phases due to its better mass-transfer in
comparison to randomly packed beds, avoiding gas diffusion
limitations in N2O decomposition rate.
The catalyst denoted by 0.2%RhOx/Ce0.9Pr0.1O2/M (flash) has
proved to be able to decompose N2O in presence of NO, O2 and
H2O accomplishing one of the main goals of this work which was scaling up RhOx/Ce0.9Pr0.1O2 catalysts for N2O decomposition in
real gas streams.
Diesel-SCR catalysts for NOx removal where also optimized and
prepared in order to build an efficient dual-bed system for
simultaneous deNOx and deN2O abatement.
The preparation of the Pt/Beta zeolite/monolith has been optimized
by adjusting the viscosity of surfactant and binder-containing Beta
zeolite suspensions (viscosities ≤ 23 mPa·s). The effect of zeolite
concentration on slurry viscosity was much more relevant than
those of surfactant and binder.
A certain stabilization time was necessary before the monolith can
be dipped on Beta zeolite slurries, and this time ranged from few
minutes (3 at least) until 1 hour, depending on the zeolite and
surfactant concentrations.
Several consecutive dipping steps are required in order to obtain
the Beta zeolite loading suitable for a practical application
(3.3 wt.% in the current study). The blowing step performed after
drying is not really necessary if the monolith channels are not
blocked by zeolite, and blowing the loosely bound zeolite could be
done after calcination.
SCR experiments performed in real diesel exhausts with medium
and full-size Pt/Beta zeolite/monolith catalysts demonstrated that
NOx reduction is feasible with commercial diesel fuel.
Chapter 7
200
The SCR behaviors observed in the real exhausts with commercial
diesel fuel were similar to that typically reported in laboratory
experiments performed with model HC.
The N2 selectivity of the medium-size Pt/Beta zeolite/monolith
catalyst in the studied conditions is significantly higher than
previous laboratory results, but the 100 % N2 selectivity is only
obtained by using RhOx/Ce0.9Pr0.1O2/monolith catalyst located
downstream the Pt/Beta zeolite/monolith catalyst, with both
monolith catalysts (deNOx and deN2O) operating at the same
temperature.
RESUMEN
Optimización de catalizadores
RhOx/ceria para la descomposición de
N2O y diseño de un sistema deNOx
altamente selectivo a N2 para
vehículos diésel
Catalizadores RhOx/ceria para la descomposición de N2O
203
1. Introducción General.
1.1. Efectos medioambientales del N2O.
El óxido nitroso se produce de forma natural en procesos
biológicos que se dan en suelos y aguas y también son emitidos por varias
fuentes antropogénicas relacionadas con la agricultura, energía, industria
y gestión de residuos [1, 2]
Desde el punto de vista medioambiental, en 1996 Kaptein y col. [3]
señalaron que con anterioridad el N2O había sido considerado un gas
inofensivo y no había presentado gran interés para los científicos,
ingenieros y políticos durante largo tiempo debido a la infravaloración y
desconocimiento de su potencial en la contribución a los problemas
medioambientales. Desde mitad de los años 80 se notó una creciente
preocupación al descubrir que el N2O contribuye a la destrucción de la
capa de ozono. Actualmente se sabe que el N2O es un contaminante muy
peligroso por lo comentado anteriormente y por ser un gas de efecto
invernadero.
El impacto medioambiental del N2O se ve incrementado como
consecuencia del aumento de las concentraciones atmosféricas a
consecuencia de las actividades humanas mencionadas anteriormente
[1, 4]. Aunque no es el gas que más contribuye al calentamiento global,
contribuyendo más el CO2 y el CH4. Su potencial de calentamiento es 310
veces superior al del CO2. Esto hace que emisiones relativamente
pequeñas (comparadas con otros gases de efecto invernadero) sean
equivalentes a un 10 % de las emisiones de CO2 [1, 2].
Por estos motivos, en la 3ª conferencia de la Convención Marco
de las Naciones Unidas sobre el Cambio Climático (UNFCC) en Kyoto en
1997, se legislaron límites de emisiones de seis gases de efecto
invernadero (CO2, CH4, N2O, HFC, PFC and SF6) para llevarse a cabo en
el periodo comprendido entre 2008-2012 [5]. En Doha, Qatar, a finales del
2012 se adoptaron nuevas enmiendas a incluir en el Protocolo de Kyoto,
incluyendo nuevos compromisos con una mayor ambición para reducir las
emisiones de gases de efecto invernadero, en un 18 % respecto al nivel
de emisiones de 1990 en el periodo comprendido entre 2013 y 2020 [5].
Resumen de la tesis doctoral
204
1.2. Fuentes de emisión de N2O.
Como se ha mencionado, las principales fuentes antropogénicas
de N2O son las derivadas de actividades agrarias y de las industrias de
producción de ácido adípico, ácido nítrico y nylon, así como la combustión
de combustibles fósiles tanto en fuentes estacionarias como en fuentes
móviles [1, 2].
Las emisiones de N2O que se pueden reducir a corto plazo están
asociadas a la producción de compuestos químicos e industria energética.
En concreto para la producción tanto de ácido adípico como ácido nítrico
ya existen tecnologías comercialmente disponibles para la reducción de
emisiones de N2O. Sin embargo su aplicación (extrapolación o
adaptación) a otras fuentes de emisión, como por ejemplo, en vehículos
ligeros no es factible debido a las diferentes características en la
composición de los gases de escape. En los vehículos ligeros, el N2O sale
a concentraciones y temperaturas relativamente bajas (T < 500 ºC)
acompañado de gases (O2, H2O, NOx, and SO2) que inhiben la actividad
de los catalizadores [2, 3, 6].
Como se ha mencionado anteriormente, una de las principales
fuentes de emisión de N2O son los vehículos y esta tesis se centra en la
eliminación de N2O procedente de motores diésel.
1.3. Motores diésel frente a motores gasolina.
Actualmente, los automóviles son el modo de transporte más
popular y necesario en nuestra vida cotidiana y en todo el mundo el
parque automovilístico asciende a más de 700 millones de vehículos [7].
En las últimas décadas, el incremento en el número de automóviles se
considera una de las principales fuente de contaminantes que disminuye
la calidad del aire, produciendo problemas de salud y dando lugar al
“smog fotoquímico” en las ciudades grandes [8].
Los motores diésel y gasolina funcionan de modo diferente. En los
motores diésel, el combustible en forma de spray se auto-inflama por la
alta presión alcanzada en la cámara de combustión, no siendo necesaria
la chispa inicie la combustión. El diésel es más pesado que la gasolina
debido a la mezcla de HC menos volátiles, siendo más susceptible a la
Catalizadores RhOx/ceria para la descomposición de N2O
205
auto-ignición. En comparación con los motores gasolina, los motores
diésel trabajan con mayores ratios de compresión, combustionan más
rápido, operan en condiciones pobres de combustible (mayor relación
aire-combustible) y trabajan en exceso de O2. En los motores de gasolina
(también conocidos como motores Otto), el aire y el combustible se
mezclan antes de introducirlos en el cilindro, manteniendo constante el
ratio aire-combustible, normalmente estequiométrico, que es 14.6 en peso
[9]. Como resultado, los motores diésel tienen mayor eficiencia
termodinámica que los motores gasolina. Con combustible diésel se
consigue alrededor de un 12 % de energía en base volumétrica más que
la gasolina, favoreciendo que los motores diésel operen con una mayor
economía en combustible (menor consumo) que los motores gasolina
[10, 11].
El uso de catalizadores para purificar los gases de escape es
absolutamente necesario e indispensable en todos los vehículos [9, 12].
Los principales contaminantes procedentes de motores gasolina son HC,
CO, NOx (mezcla de NO y NO2) y material particulado (PM), este en muy
bajas concentraciones. La legislación relativa a estos contaminantes se ha
ido restringiendo a lo largo de los años, favoreciendo así el desarrollo e
introducción de los catalizadores de tres vías (TWC). Actualmente, en
occidente, prácticamente todos los coches nuevos con motor de gasolina
están equipados con estos catalizadores para reducir las emisiones de
CO, HC y NOx.
En los automóviles diésel, las emisiones de CO e HC son del
mismo orden de magnitud que las emitidas por vehículos a gasolina
equipados con TWC. Sin embargo las emisiones de NOx y PM en los
motores diésel son muy superiores. Los catalizadores TWC no se pueden
usar en motores diésel principalmente por dos razones:
La alta concentración de O2, pues dificulta la reducción de los
NOx.
La necesidad de un sistema específico para la eliminación de PM
basado en un filtro que retiene el PM y que periódicamente tiene
que ser regenerado.
Resumen de la tesis doctoral
206
Al igual que ha pasado con las emisiones procedentes de los
motores de gasolina, la legislación para las emisiones en motores diésel
también se ha vuelto más restrictiva con los años y se han desarrollado
varias técnicas para reducir dichas emisiones. De entre todas, las técnicas
de post-tratamiento se han estudiado extensamente para la eliminación de
NOx y PM procedentes de motores diésel [9]. En este trabajo se aborda el
estudio de la eliminación de NOx procedentes de los motores diésel.
1.4. Control de contaminantes procedentes de vehículos diésel.
Los distintos contaminantes emitidos por motores diésel requieren
estrategias específicas de post-combustión, contrariamente a lo que pasa
en los TWC para vehículos de gasolina. El CO y los HC son normalmente
oxidados en un catalizador de oxidación (DOC), el PM es recogido en un
filtro de partículas (DPF) y la reducción de NOx se realiza añadiendo un
reductor en presencia de un catalizador adecuado. Se han propuesto tres
procesos diferentes para la eliminación de NOx:
Descomposición catalítica de NOx.
Almacenamiento y reducción de NOx (NSR).
Reducción catalítica selectiva (SCR).
De estos tres procesos propuestos este trabajo está centrado en
la estrategia SCR para convertir NOx en N2 en una atmosfera rica en O2
[13-16]. En el proceso SCR, la reducción de NOx compite con éxito con la
reducción de O2, aunque el último esté en gran exceso. Estudios basados
principalmente en experimentos en el laboratorio proponen varios
reductores incluyendo H2, CO, distintos HCs, NH3 y CO(NH2)2 (urea), o
incluso el mismo combustible diésel [14-17].
Basándose en la tecnología bien implantada en plantas de
energía, donde el NH3 o la urea se inyectan a la corriente de
post-combustión para la reducción de NOx a N2 [14], con un catalizador
apropiado; el NH3 se está utilizando como reductor selectivo de NOx en
fuentes móviles. Los catalizadores basados en pentóxido de vanadio se
utilizan en vehículos diésel pesados (autobuses, camiones, etc). Por otro
lado, catalizadores basados en Fe/zeolita se han comercializado y usado
Catalizadores RhOx/ceria para la descomposición de N2O
207
en varias aplicaciones SCR con NH3 como reductor, alcanzando
conversiones de NOx altas [13].
Sin embargo, el transporte de NH3 (que es un compuesto
corrosivo) dificulta la implementación de esta tecnología, convirtiendo la
urea en una alternativa, que por hidrolisis a alta temperatura da lugar al
NH3. Sin embargo, esta tecnología no ha sido implementada para
vehículos ligeros porque el tanque de urea/NH3 tiene el inconveniente de
añadir peso al automóvil, además de que sería necesario instaurar una
red de suministradores de estos reductores para llenar los tanques.
Los sistemas SCR de NOx con hidrocarburos (HC) han sido
propuestos como una alternativa a la tecnología con urea/NH3. Se han
probado HC de distinta naturaleza (propeno [18-21], octano [22], metanol
[21], etanol [21], acetaldehído [23] y decano [20]). Sin embargo, hasta
donde sabemos, la reducción directa de NOx con diésel comercial no ha
sido publicada, y uno de los objetivos de este trabajo es hacerlo. La
principal ventaja del combustible diésel con respecto a otros reductores es
que ya se encuentra en el vehículo y, por tanto, no son necesarios
tanques ni suministros adicionales.
La reactividad de los HCs en la conversión de NOx depende de su
naturaleza, del catalizador utilizado y de la temperatura, y por tanto
distintos HCs se comportan de forma diferente [24]. A altas temperaturas
la reacción de HC con O2 toma cada vez más relevancia y la mayoría de
los reductores (HC) se oxidan dificultando así la reducción de NOx. Una
característica de la reducción de NOx en exceso de O2 es la insuficiente
capacidad de reducción completa de los NOx a N2, dando lugar a una
considerable cantidad de N2O emitido [13]. La generación de N2O
depende de la naturaleza del catalizador, la naturaleza y concentración del
agente reductor, la temperatura y el flujo de salida de los gases de
escape, entre otros factores [13].
De entre todos los catalizadores propuestos para HC-SCR parece
que el platino es muy activo a bajas temperaturas (T < 300 ºC), y no se ve
significativamente afectado por la presencia de H2O en la corriente de
escape [14, 25-27]. Además, este es resistente al SO2 en los niveles de
azufre presentes en el escape del vehículo (25-50 ppm) [28].
Resumen de la tesis doctoral
208
1.5. Eliminación de N2O.
La descomposición catalítica de N2O ha sido estudiada con
catalizadores de distinta naturaleza. Las principales propiedades que debe
tener un catalizador para esta aplicación en escapes de vehículos es que
sea altamente activo a bajas temperaturas y resistente a los inhibidores
presentes en los gases de escape tales como O2, H2O y NOx residual.
Entre los ejemplos de catalizadores encontrados en la bibliografía se
pueden señalar los óxidos simples [2], perovskitas [29], spinelas [30-33],
zeolitas [34], hidrotalcitas [35], silicas mesoporosas [36] y catalizadores
soportados [37-39]. Sin embargo, muchos de los catalizadores propuestos
en la literatura no muestran una buena actividad ni estabilidad para la
conversión de N2O en condiciones reales.
Basándose en la experiencia de estudios previos en el grupo, de
entre todos los catalizadores propuestos, este trabajo se centra en
catalizadores de rodio soportado sobre ceria pura (CeO2) o dopada con
praseodimio (CeyPr1-yO2). Este tipo de catalizadores presenta gran
actividad en los experimentos realizados tanto en el laboratorio como en
una planta piloto de producción de ácido nítrico [40, 41] presentando gran
resistencia a H2O, NO y O2 [42]. Por ello, la hipótesis de partida es que los
catalizadores RhOx/ceria pueden ser adecuados para la descomposición
de N2O en vehículos diésel, situándolo detrás del catalizador SCR para la
reducción de NOx.
1.6. Objetivos.
Teniendo en cuenta lo descrito anteriormente, el objetivo general
de esta tesis es desarrollar y optimizar un catalizador efectivo para la
eliminación de N2O en las corrientes de escape de los vehículos diésel.
Éste irá situado después del catalizador SCR cuya composición fue
optimizada en trabajos anteriores y corresponde a Pt/zeolita Beta
soportado en un monolito de cordierita. Para alcanzar este objetivo
general, se deben lograr unos objetivos específicos, que se describen a
continuación:
Estudiar el efecto del tratamiento de calcinación de los
catalizadores rodio/ceria en las propiedades físico-químicas de los
mismos y en la actividad para la descomposición de N2O.
Catalizadores RhOx/ceria para la descomposición de N2O
209
Estudiar el uso de distintos disolventes (agua, etanol y acetona) al
impregnar los soportes basados en ceria con los precursores de
rodio y ver cómo afectan a las propiedades físico-químicas de los
catalizadores y su actividad para la descomposición de N2O.
Preparar, caracterizar y probar en el laboratorio catalizadores
RhOx/Ce0.9Pr0.1O2 soportados en monolitos tipo panal de abeja de
cordierita.
Preparar un catalizador Pt/zeolita Beta soportado en un monolito
tipo panal de abeja de cordierita de tamaño real y probarlo en una
corriente real de escape de un motor diésel para la reducción de
NOx con diésel comercial.
Probar un doble lecho catalítico basado en los catalizadores
Pt/zeolita Beta y RhOx/Ce0.9Pr0.1O2 soportados en monolitos de
cordierita que operen a la misma temperatura para la reducción de
NOx a N2 con diésel comercial, que se inyecta antes del primer
catalizador.
2. Técnicas de caracterización.
Los catalizadores se definen por su composición, estructura, y por
sus propiedades estructurales y texturales que están relacionadas con su
actividad catalítica. Para entender y determinar las propiedades
físico-químicas de los catalizadores en este trabajo se han utilizado
diversas técnicas de caracterización.
La termogavimetría acoplada a un espectrómetro de masas
(TG-MS) se utilizó para seleccionar la temperatura adecuada a la que se
empieza a descomponer los precursores de los catalizadores. La
adsorción de N2 a -196 ºC se utilizó para determinar el área superficial
BET. Las propiedades estructurales fueron estudiadas mediante difracción
de rayos X (DRX) y espectroscopia Raman. La composición atómica
superficial y los estados de oxidación de los metales fueron determinados
mediante espectroscopia fotoelectrónica de rayos X (XPS). La reducción a
temperatura programada con H2 se empleó para estudiar la reducibilidad
de las muestras. El tamaño de partícula se determinó mediante DRX y
Resumen de la tesis doctoral
210
microscopia electrónica de transmisión (TEM). Finalmente, la distribución
de la fase activa fue observada mediante microscopía electrónica de
barrido (SEM).
3. Efecto de las condiciones de calcinación de los catalizadores
RhOx/CeO2 en la descomposición de N2O.
En aplicaciones catalíticas, la ceria se usa como soporte de otras
especies o como catalizador en si misma [43-45]. Este tipo de catalizador
se usa principalmente para el control de contaminantes procedente de los
vehículos de gasolina (TWC) [46], combustión de PM [47], reducción
selectiva catalítica de NOx [48], oxidación parcial de metano [49],
combustión de compuestos orgánicos volátiles [50] y descomposición de
N2O [38] entre otros. En esta última aplicación, el tamaño y la distribución
de partículas de metal noble, así como la morfología superficial y los
defectos de los óxidos afectan el funcionamiento del catalizador Rh/ceria,
siendo crucial el comportamiento de la interfase metal-soporte con
relación a la actividad catalítica [40, 51-53]. En este sentido en este
trabajo se investiga la optimización de estas interacciones para mejorar el
funcionamiento del catalizador.
Esta parte del trabajo corresponde al estudio del efecto de las
condiciones de calcinación en catalizadores cuya composición es
2.5%RhOx/CeO2. Se estudian dos formas de calcinación. Cuando la
calcinación empieza a 25 ºC se denomina calcinación en rampa mientras
que si empieza a 250 o 350 ºC se denomina calcinación flash. Este
estudio se realiza a escala de laboratorio en un sistema compuesto por
una unidad de alimentación y mezcla de gases, un reactor, un horno y un
controlador de temperatura y por último la unidad de análisis compuesta
por un cromatógrafo de gases para analizar N2O, O2 y N2.
Los ensayos catalíticos de descomposición de N2O se realizaron
en un reactor de cuarzo en forma de U que se introduce en un horno
vertical a presión atmosférica usando un flujo de 100mL/min
(GHSV = 42000 h−1
) de 1000 ppm N2O en He, utilizando 100 mg de
catalizador.
Catalizadores RhOx/ceria para la descomposición de N2O
211
Los ensayos de oxidación de CO se realizaron en el mismo
sistema que los ensayos catalíticos anteriores usando 100 mg de
catalizador y un flujo de 100 mL/min (GHSV=42000 h−1
) de
1000 ppm CO/5 % O2/He y 100 mg de catalizador.
Este estudio deja patente que el método de calcinación (rampa o
flash) del nitrato de cerio utilizado para obtener el soporte de ceria no
afecta ni las propiedades de la ceria (observado mediante isotermas de
adsorción de N2, DRX y espectroscopia Raman) ni la actividad del
catalizador RhOx/CeO2. Por el contrario, la calcinación flash del nitrato de
rodio usado para impregnar la ceria mejora la actividad catalítica tanto
para la descomposición de N2O como para la oxidación de CO (reacción
de gran relevancia que se usó para confirmar si las mejoras del
catalizador sólo eran válidas para la descomposición de N2O o se podía
extrapolar a otras reacciones).
Estas mejoras en la actividad catalítica se atribuyen a un menor
tamaño de partícula RhOx que permite mejorar la interfase metal-soporte
cuando se hace una calcinación flash en lugar de calcinación en rampa,
favoreciendo a su vez la reducibilidad de la superficie de la ceria y
estabilizando las especies RhOx en condiciones de reacción.
4. Preparación de catalizadores RhOx/CeyPr1-yO2 para la
descomposición de N2O con distintos disolventes.
Además de las razones mencionadas anteriormente, los
materiales basados en ceria son de interés por su capacidad de
almacenamiento de oxígeno (OSC) y la movilidad de éste en la estructura.
Estas propiedades dependen del tamaño de cristal, de los defectos y de la
presencia de dopantes (en este estudio el praseodimio). La incorporación
del praseodimio en la estructura de la ceria modifica la movilidad del
oxígeno en la celdilla unidad y su comportamiento catalítico, ya que
disminuye la (BE) energía del enlace M-O en la ceria dopada, aumentando
el número de defectos en la red y por tanto las vacantes de oxígeno.
Como consecuencia se mejora la eficiencia en la descomposición de N2O
[40, 54].
Resumen de la tesis doctoral
212
En el apartado anterior se concluyó que la velocidad de
evaporación del disolvente que impregna la ceria es un factor
determinante para las propiedades del material, afectando al tamaño de
las partículas de RhOx en el catalizador final [55, 56]. Por ello en esta
segunda parte se presenta el estudio realizado sobre el efecto del
disolvente (agua, etanol o acetona) usado en la impregnación de los
soportes CeyPr1-yO2 (donde y es 1, 0,9 o 0,5) con nitrato de rodio para la
preparación de los catalizadores. Este estudio se realiza a escala de
laboratorio en el mismo sistema y condiciones descritos en el apartado
anterior para los ensayos de descomposición de N2O.
Las conclusiones obtenidas fueron que tanto la naturaleza del
soporte de ceria como del disolvente usado afectan la interacción
RhOx-soporte, la cual determina la actividad catalítica de los catalizadores
estudiados.
Tanto el etanol como la acetona afectan muy negativamente a los
catalizadores que contienen Ce0.9Pr0.1O2 y Ce0.5Pr0.5O2 en lo referente a la
sinterización del soporte y de las partículas de RhOx. Este hecho está
relacionado con la propia combustión de los disolventes catalizada por el
nitrato de rodio, afectando negativamente la interacción RhOx-soporte lo
que conduce a catalizadores menos efectivos para la descomposición de
N2O.
Mediante XPS se observó que el dopado de la ceria con un 10 %
de praseodimio tiene un efecto positivo en la interacción RhOx-soporte
observada como una transferencia de carga negativa desde el metal noble
al soporte (sólo obtenida con el uso de agua para la impregnación) y esto
mejora la eficiencia del catalizador.
En cuanto a la interacción RhOx-Ce0.5Pr0.5O2 no es tan buena
como con Ce0.9Pr0.1O2 pero parece ser suficiente para mantener una alta
actividad catalítica, si la impregnación con rodio se hace utilizando agua.
Pues cuando se utiliza acetona o etanol se produce un considerable
descenso en la actividad y esto solo puede atribuirse a la sinterización de
las partículas de RhOx puesto que Ce0.5Pr0.5O2 con considerable baja área
superficial BET no sinteriza con respecto a la impregnación con agua.
Catalizadores RhOx/ceria para la descomposición de N2O
213
5. Preparación, caracterización y actividad catalítica para la
descomposición de N2O de catalizadores monolíticos
RhOx/Ce0.9Pr0.1O2.
De acuerdo con los resultados anteriores, el mejor catalizador en
polvo preparado hasta el momento es RhOx/Ce0.9Pr0.1O2,donde el rodio ha
sido soportado sobre Ce0.9Pr0.1O2 con un disolución acuosa del nitrato
precursor y una posterior calcinación en condiciones flash.
Uno de los retos en el control de las emisiones de N2O es
desarrollar un catalizador capaz de trabajar en corrientes de gases reales.
Para emplear un catalizador en estas condiciones es necesario incorporar
la fase activa (RhOx/Ce0.9Pr0.1O2) previamente optimizada en polvo, a un
soporte inerte apropiado. Se pueden diversos sustratos con este
propósito, por ejemplo, pellets o partículas de óxidos inorgánicos,
monolitos cerámicos tipo panal de abeja, espumas cerámicas, etc [57-61].
De entre los soportes mencionados, los monolitos cerámicos tipo panal de
abeja presentan algunas propiedades atractivas, proporcionando un buen
contacto entre las fases activas del catalizador y los gases a tratar.
También tienen alta tolerancia al polvo y la caída de presión se minimiza
porque el flujo de gas no se ve obstaculizado de manera significativa a
través del lecho catalítico. Además no padecen problemas de degradación
como los que ocurren típicamente por ejemplo por la fricción de partículas
[57-60]
La cordierita (2MgO:5SiO2:2Al2O3) es el material más
comúnmente utilizado en la fabricación de monolitos tipo panal de abeja.
La cordierita presenta alta resistencia mecánica y bajo coeficiente de
expansión térmica [57-60]. La incorporación de una fase catalíticamente
activa en el monolito no es trivial, requiriéndose estudios de optimización
del proceso de carga de las fases activas.
El objetivo de esta tercera etapa del estudio fue preparar un
catalizador en forma de monolito (panel de abejas) y utilizarlo para la
descomposición de N2O en corrientes de gases reales. La fase activa de
composición RhOx/Ce0.9Pr0.1O2 se incorpora al interior del monolito
mediante un método sencillo consistente en la impregnación de los
monolitos con una disolución acuosa que contiene los precursores
metálicos y una posterior calcinación. En este caso, nuevamente, se ha
Resumen de la tesis doctoral
214
estudiado el efecto de las condiciones de calcinación. Además se ha
optimizado la cantidad de rodio que debe contener el catalizador. Los
monolitos recubiertos en su interior por la fase activa RhOx/Ce0.9Pr0.1O2
han sido caracterizados y probados para la descomposición de N2O en
una corriente gaseosa de N2O/He. El mejor catalizador soportado ha sido
evaluado en presencia de los principales inhibidores (O2 + NOx + H2O)
encontrados en el escape de un vehículos diésel.
Los ensayos catalíticos se realizaron a escala de laboratorio en el
sistema experimental antes mencionado incluyendo un analizador de
quimiluminiscencia para monitorizar NO y NO2. En esta ocasión se utilizó
un reactor cilíndrico de cuarzo y un horno horizontal. El flujo utilizado fue
de 500 mL/min (GHSV = 27000 h−1
) utilizando distintas atmosferas
1000 ppm N2O/He o 1000 ppm N2O/1000 ppm NO/5 % O2/0.6 % H2O/He y
un catalizador soportado. Para el catalizador en polvo se usó una cantidad
similar a la fase activa contenida en el soporte monolítico y se diluyo con
SiC has obtener un volumen de lecho similar al ocupado por el monolito.
Las resultados obtenidos en este apartado llevaron a la conclusión
de que la composición y estructura del Ce0.9Pr0.1O2 en polvo y soportado
en la cordierita son similares a bajas temperaturas pero a partir de una
cierta temperatura el catalizador soportado supera la conversión
conseguida por el catalizador en polvo. Esto se atribuye a la buena
transferencia de masa de los catalizadores soportados.
Para las condiciones experimentales aplicadas en este estudio el
contenido óptimo en rodio es de 0.2 wt.% en base a peso total.
Aumentando este valor no se consigue mejorar la capacidad de
descomposición de los catalizadores.
En los catalizadores soportados en el monolito se observa que las
partículas de RhOx se adhieren preferentemente al Ce0.9Pr0.1O2 y no al
sustrato de cordierita. Además, la distribución de la fase activa soportada
en el sustrato monolítico depende de las condiciones de calcinación,
obteniéndose un recubrimiento más homogéneo por calcinación flash en
comparación con la calcinación en rampa.
También se observó que el proceso de calcinación afecta a las
interacciones Rh-Ce-Pr. La calcinación flash da lugar a menores tamaños
Catalizadores RhOx/ceria para la descomposición de N2O
215
de partículas RhOx mejorando la reducibilidad de las entidades
superficiales Rh-Ce-Pr del catalizador a bajas temperaturas en
comparación con la calcinación en rampa. De esta forma se mejora la
actividad catalítica.
Finalmente, el catalizador denominado 0.2%RhOx/Ce0.9Pr0.1O2/M
(flash) ha demostrado ser capaz de descomponer N2O en presencia de
NOx, O2 y H2O. Por tanto, el objetivo principal de este apartado, consiste
en la preparación de un monolito con la fase activa RhOx/Ce0.9Pr0.1O2 para
su utilización en corrientes de gases reales, ha sido completado con éxito.
6. Reducción de NOx a N2 con combustible comercial en la corriente
de escape de un motor diésel utilizando un doble lecho de los
catalizadores monolíticos Pt/zeolita Beta y RhOx/Ce0.9Pr0.1O2.
La reducción de NOx (en exceso de O2) procedente del escape de
los motores diésel se puede conseguir principalmente mediante dos
tecnologías: Reducción catalítica selectiva (SCR) y el almacenamiento y
posterior reducción de NOx (NSR) [62-64] y en ambos casos es necesario
añadir un reductor.
Se han propuesto varios reductores incluyendo H2, CO, diferentes
HCs, NH3, urea, etc [62-70] en la tecnología SCR. La utilización comercial
de NH3 o urea como reductor para el sistema SCR ya está disponible para
camiones y autobuses, pero en el caso de coches, esta tecnología no se
ha implementado debido al inconveniente del sobrepeso que supone un
depósito adicional de NH3/urea. Como nueva alternativa proponemos el
combustible diésel como agente reductor y un catalizador Pt/zeolita Beta,
basándonos en la experiencia de trabajos previos [71, 72].
Esta última fase del estudio consiste en la preparación de
catalizadores monolíticos de tamaño medio y real con Pt/zeolita Beta
como fase activa para utilizarlos en la tecnología SCR para la eliminación
de NOx en el escape de un motor diésel real utilizando combustible
comercial como agente reductor, tanto en un lecho simple SCR (a escala
media y escala real) como en un doble lecho catalítico de tamaño medio el
cual consiste en el catalizador SCR Pt/zeolita Beta/monolito localizado
delante del catalizador de descomposición de N2O
Resumen de la tesis doctoral
216
RhOx/Ce0.9Pr0.1O2/monolito. Los ensayos catalíticos a escala media fueron
realizados en la Universidad de Alicante y a escala real en el Centro
Tecnológico de Automoción de Galicia (CTAG).
Los ensayos con catalizadores de tamaño medio se realizaron con
un motor Turbo Diesel 2.0 a 880 rpm (en ralentí). En estas condiciones de
funcionamiento, el flujo de escape era de 800 L/min, y la composición fue
17.2 % O2, 2.4 % CO2, 10 % H2O, 100 ppmV NO, 35 ppmV NO2,
120 ppmV CO y 10 ppmV THC (THC = hidrocarburos totales, expresados
como CH4). La temperatura del gas a lo largo del tubo de escape estaba
comprendida entre 90 y 50 ºC. Se extrajo una corriente de gas de
10 L/min continuamente de la corriente principal y se usó para realizar los
ensayos. Este flujo pasó a través de dos hornos consecutivos con un
control independiente de temperatura, el primero conteniendo un crisol
con diésel comercial y el segundo los catalizadores monolíticos.
Los ensayos catalíticos de reducción de NOx con catalizadores a
escala real se llevó a cabo en un banco de pruebas Horiba Titan S190 en
un motor diésel 1.6 HDI a 1100 rpm y cargas distintas (torque entre 45 y
83 N.m) usando diésel como reductor. La temperatura y la composición
del gas a la entrada del catalizador SCR dependen de la carga del motor.
En cuanto a la preparación de los catalizadores, se ha optimizado
el método de recubrimiento por inmersión (conocido como dip-coating)
para la preparación del catalizador Pt/zeolita Beta/monolito. El catalizador
(RhOx/Ce0.9Pr0.1O2/monolito) de descomposición de N2O, se ha preparado
por descomposición de nitratos en forma similar al método óptimo descrito
en [55]. La producción de N2O como producto no deseado generado en la
reducción de NOx, es el mayor inconveniente de los catalizadores de
platino para SCR, este problema se ha resuelto con la configuración de
doble lecho catalítico donde los dos catalizadores monolíticos operan a la
misma temperatura consiguiendo un 100 % de selectividad hacia el N2. El
objetivo de escalar el catalizador SCR a escala real o completa es
demostrar por primera vez que el sistema SCR puede ser implementado
en el escape de un vehículo diésel usando diésel comercial como
reductor.
De este estudio se concluye que la preparación del catalizador
Pt/zeolita Beta/monolito ha sido optimizada ajustando la viscosidad de la
Catalizadores RhOx/ceria para la descomposición de N2O
217
suspensión de zeolita Beta con surfactante y aglomerante a valores
≤ 23 mPa.s. El efecto de la concentración de zeolita es mucho más
relevante que el del surfactante o aglomerante en la viscosidad de la
suspensión.
Antes de que el monolito sea sumergido en la suspensión de
zeolita es necesario un cierto tiempo de estabilización de la suspensión.
Este tiempo de estabilización tiene un intervalo de entre unos pocos
minutos (3 por lo menos) y una hora, dependiendo de las concentraciones
de zeolita y surfactante.
La etapa de soplado después del secado no es realmente
necesaria en el proceso de preparación, si los canales del monolito no
están bloqueados por la zeolita. Por tanto la eliminación por soplado de la
zeolita débilmente unida al monolito podría hacerse después de la
calcinación. Sin embargo, son necesarias varias etapas de inmersión del
monolito en la suspensión de zeolita para incrementar la carga necesaria
para una aplicación práctica.
Los experimentos SCR realizados en el escape de un vehículo
diésel a escala media y real con catalizadores Pt/ zeolita Beta/monolito
han demostrado que la reducción de NOx es posible con diésel comercial
como reductor. El comportamiento de estos catalizadores, en cuanto a la
reducción de NOx se refiere, es similar a los realizados previamente a
escala de laboratorio con HC modelo.
La selectividad hacia N2 del catalizador Pt/zeolita Beta/monolito en
las condiciones estudiadas es significativamente mayor que en resultados
previos de laboratorio, pero para alcanzar un 100 % de selectividad es
necesario utilizar el doble lecho catalítico situando primero el catalizador
Pt/zeolita Beta/monolito y luego el RhOx/Ce0.9Pr0.1O2/monolito, ambos
catalizadores operando a la misma temperatura.
Resumen de la tesis doctoral
218
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Resumen de la tesis doctoral
226
[69] M.V. Twigg. Progress and future challenges in controlling
automotive exhaust gas emissions. Applied Catalysis B:
Environmental 70 (2007) 2.
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technology. Catalysis Reviews – Science and Engineering 48
(2006) 43.
[71] A. Bueno-López, D. Lozano-Castelló, I. Such-Basáñez,
J.M. García-Cortés, M.J. Illán-Gómez, C. Salinas-Martínez de
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Effect of NOx and C3H6 partial pressures on the activity of
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Abbreviations
Abbreviations
229
Abbreviations
BE Binding Energy
BET Brunauer-Emmett-Teller
ca. Latin word “circa” which means “around”
CAFE Corporate Average Fuel Economy
CCRT Catalyzed Continuously Regenerating Trap
CeO2 Ceria, cerium oxide
CeyPr1-yO2 Doped ceria, cerium praseodymium mixed oxide
CRT® Continuously Regenerating Trap
deNOx NOx abatement
deN2O N2O abatement
DPFs Diesel Particulate Filters
EDS Energy-Dispersive X-ray Spectroscopy
EPA U.S. Environmental Protection Agency
ESCA Electron Spectroscopy for Chemical Analysis
FCC Fluid Catalytic Cracking
FWHM Full-Width at Half Maximum
GHG Greenhouse Gas
GWP Global Warming Potential
H2-TPR Temperature Programmed Reduction with H2
HC Hidrocarbons
Abbreviations
230
i.e. Latin phrase "id est", meaning "that is"
IPCC Intergovernmental Panel on Climate Change
LNT Lean NOx Trap
MCMA Carbon Materials and Environment Research Group
from Spanish “Materiales Carbonosos y Medio
Ambiente”
mg/bhp-h Milligrams per brake horsepower-hour
NACs NOx absorbing catalysts
NOx (NO+NO2) Mononitrogen oxides (nitric oxide and nitrogen
dioxide).
NSR NOx Storage Reduction
ODSs Ozone Depleting Substances
OSC Oxygen Storage Capacity
PAHs Polycyclic Aromatic Hydrocarbon
PGMs Platinum Group Metals
PM Particulate Matter or soot
SEM Scanning Electron Microscopy
SCR Selective Catalytic Reduction
TCD Thermic Conductivity Detector
TEM Transmission Electron Microscopy
Tg CO2-eq. Teragrams (or million metric tons) of CO2 equivalent
TG-MS Thermogravimetry-Mass Spectroscopy
Abbreviations
231
TWC Three Way Catalysts
UNFCCC United Nations Framework Convention on Climate
Change
XRD X-Ray Diffraction
XPS X-ray Photoelectron Spectroscopy
Curriculum Vitae
Verónica Rico Pérez
235
Curriculum Vitae
Verónica Rico Pérez was born on the 11th of January of 1981 in
Alicante, Spain. She earned her Chemistry degree from University of
Valencia in 2008. After that, she spent one year in Germany doing
internship in the company Sartorious Biotech GmbH in Goettingen. In 2009
she started a master in Materials Science at the University of Alicante and
followed her PhD studies in the Inorganic Chemistry Department of this
University. This doctoral thesis describes the most important findings of the
research she performed in N2O decomposition and diesel-SCR for NOx
abatement from 2009 to 2013.
List of publications
1. G. Maniak, P. Stelmachowski, A. Kotarba, Z. Sojka, V. Rico-Pérez, A. Bueno-López. Rationales for the selection of the best precursor for potassium doping of cobalt spinel based deN2O catalyst. Applied Catalysis B: Environmental 136–137 (2013) 302.
2. M. Valencia, E. López, S. Andrade, Iris M.L., N. Guillén-Hurtado, V. Rico-Pérez, A. García-García, C. Salinas-Martínez de Lecea, A. Bueno-López. Evidences of the cerium oxide-catalyzed DPF regeneration in a real diesel engine exhaust. Topics in Catalysis 56 (2013) 452.
3. V. Rico-Pérez, M.A. Velasco-Beltrán, H. Qinggang, W. Qi, C. Salinas-Martínez de Lecea, A Bueno-López. Preparation of ceria-supported rhodium oxide sub-nanoparticles with improved catalytic activity for CO oxidation. Catalysis Communications 33 (2013) 47.
4. N. Guillén-Hurtado, V. Rico-Pérez, A. García-García, D. Lozano-Castelló, A. Bueno-López. Three-way catalysts: past, present and future. Dyna, año 79, Edición Especial, pp. 114. Medellín, Octubre 2012. ISSN 0012-73533.
5. V. Rico-Pérez, S. Parres-Esclapez, M.J. Illán-Gómez, C. Salinas- Martínez de Lecea, A. Bueno-López, Preparation, characterization and N2O decomposition activity of honeycomb monolith-supported Rh/Ce0.9Pr0.1O2 catalysts. Applied Catalysis B: Environmental 107 (2011) 18.
Curriculum Vitae
236
List of contributions to international conferences
1. V. Rico-Pérez, C. Salinas-Martínez de Lecea, A. Bueno-López.
Effect of calcination method in the N2O decomposition RhOx/CeO2
catalysts. 7th International Conference on Environmental Catalysis
(ICEC). Lyon, France. 2012.-Poster
2. M. Valencia, E. López, S. Andrade, M.L. Iris, N. Guillén-Hurtado,
V. Rico-Pérez, A. García-García, C. Salinas-Martínez de Lecea,
A. Bueno-López. Power-bench demonstration of the Ceria-
Praseodymia-catalyzed DPF regeneration. 7th International
Conference on Environmental Catalysis (ICEC). Lyon, France.
2012.-Poster
3. V. Rico-Pérez, C. Salinas-Martínez de Lecea, A. Bueno-López. Efecto del método de calcinación en la interacción metal-soporte en catalizadores RhOx/CeO2. XII Congreso Nacional de Materiales/ XII Congreso Iberoamericano de Materiales. pp. 40. Alicante, Spain. 2012.-Oral. ISBN 978-84-695-3316-1.
4. M. Valencia, E. López, S. Andrade, Iris M.L., N. Guillén-Hurtado, V. Rico-Pérez, A. García-García,C. Salinas-Martínez de Lecea, A. Bueno López. Evidences of the cerium oxide-catalyzed DPF regeneration in a real diesel engine exhaust. The ninth international Congress on Catalysis and Automotive Pollution Control (CAPoC9). Brussels, Belgium. 2012.-Oral.
5. G. Maniak, P. Stelmachowski, V. Rico-Pérez, A. Bueno-López A. Kotarba, Z. Sojka. Supported Co3O4 catalyst for deN2O reaction: preparation, characterization and activity. XLIV Polish annual conference on catalysis. Cracow, Poland. 2012.-Poster
6. V. Rico-Pérez, S. Parres-Esclapez, M.J. Illán-Gómez, C. Salinas-Martínez de Lecea, A. Bueno-López. Rh/Ce0.9Pr0.1O2/monolith preparation, characterization and catalytic performance for N2O decomposition. EUROPACAT X. Glasgow, United Kingdom. 2011.-Poster.
Verónica Rico Pérez
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7. V. Rico-Pérez, S. Parres-Esclapez, A. Bueno-López, M.J. Illán-Gómez, C. Salinas-Martínez de Lecea. Preparation, characterization and catalytic performance for N2O decomposition of Rh/Ce0.9Pr0.1O2/monolith. 2
nd International Symposium on Air
Pollution Abatement Catalysis. APAC. Cracow, Poland. 2010.- Poster. ISBN 978-83-926523-3-5
List of contributions to national conferences
1. V. Rico, S. Parres, M.J. Illán, C. Salinas, A. Bueno. Catalizadores Rh/Ce0,9Pr0,1O2 soportados en monolitos de cordierita. Preparación, caracterización y ensayos catalíticos de descomposición de N2O. La catálisis ante la crisis energética y ambiental (Reunión de la Sociedad Española de Catálisis SECAT). pp. 169. Zaragoza, Spain. 2011.-Oral. ISBN 978-84-939090-0-0.
2. M. Valencia, E. López, S. Andrade, M.L. Iris, N. Guillén, V. Rico, A. García, C. Salinas, A. Bueno. Demostración en condiciones reales de la regeneración catalítica de filtros de partículas diesel utilizando óxidos de cerio. La catálisis ante la crisis energética y ambiental (Reunión de la Sociedad Española de Catálisis SECAT). pp.199. Zaragoza, Spain. 2011.-Poster. ISBN 978-84-939090-0-0.
3. V. Rico-Pérez, S. Parres-Esclapez, M.J. Illán-Gómez, C. Salinas-Martínez de Lecea, A. Bueno-López. Catalizadores Rh/Ce0.9Pr0.1O2 soportados en monolitos de cordierita. Preparación, caracterización y ensayos catalíticos de descomposición de N2O. XIV Reunión Científica Plenaria de Química Inorgánica y VIII Reunión Científica. Plenaria de Química de Estado Sólido (QIES). Cartagena, Spain. 2010.-Oral. ISBN 978-84-693-3704-2.
Verónica Rico Pérez
Carbon Materials and Environment Research Group
Inorganic Chemistry Department, University of
Alicante,
Apto. 99, San Vicente del Raspeig
E-03080 Alicante, Spain
E-mail: [email protected]
This thesis gathers a full research process ranging from basic or
fundamental investigation to actual implementation. The
environmental problems caused by N2O and its main sources,
focusing on diesel engines, are described. This work deals with
the preparation conditions for RhOx/CeyPr1-yO2 catalysts, in
powder and supported in several cordierite honeycomb monolith
sizes, for further scaling up the catalytic N2O decomposition
process. The optimized conditions include calcination process,
use of different solvents (water, ethanol and acetone) for
rhodium loading, and amounts of rhodium, cerium and
praseodymium in the catalyst. A major understanding of the
physico-chemical properties and catalytic role of these materials
towards N2O decomposition process has been facilitated by
several characterization techniques and catalytic tests carried
out at different scales, including the monolithic materials
currently employed by the automotive industry.
The production of N2O as undesired NOx reduction product in
diesel engine emissions, which is a drawback of platinum SCR
catalysts, has been solved by using the dual bed configuration,
where both monolith catalysts operated at the same
temperature, and 100 % N2 selectivity has been obtained.