advanced techniques for characterization of heterogeneous...

Advanced techniques for characterization Advanced techniques for characterization of heterogeneous catalystsof heterogeneous catalysts

Graduate School of Materials Research (GSMR)Graduate School in Chemical Engineering (GSCE)

Department of Chemical Engineering (ÅA)

(4 credits/sp/op/Bologna)

Main lecturer:Prof. Andrey Simakov

Centro de Nanociencias y NanotecnologiaUniversidad Nacional Autonoma de Mexico

Lecturers:Prof. Tapio Salmi, Åbo Akademi

Prof. Dmitry Murzin, Åbo Akademi

Transient analysis of catalytic reactions including Transient analysis of catalytic reactions including in in situsitu and and operando operando spectroscopic measurementsspectroscopic measurements

Advanced techniques for characterization of heterogeneous catalysts. Part 2

Transient analysis of catalytic reactions including in situ and operando spectroscopic measurements

In general “transient method” means that one or more parameters of the system are perturbed or varied and certain kinds of responses are measured.

The transient methods can be classified into two main groups:

Advanced techniques for characterization of heterogeneous catalysts

the first one is when – due to the perturbation – the system has been transformed into another thermodynamic state. (For example the classical Temperature Programmed Desorption and concentration jump methods belong to this group. )

The second group is when the system remains in the same thermodynamic state during transient experiment. (Literally this means different kinds of labeling techniques.)



Kobayashi, H., Kobayashi, M.,Transient response method in heterogeneous catalysis, Catal. Rev. . Sci. Eng. 10 (1974) 139-176.

Furusawa,T., Suzuki, M., Smith, J. M., Rate parameters in heterogeneous catalysis by pulse techniques, Catal. Rev. . Sci. Eng. 13 (1976) 43-76.

Bennett, C. O., The transient method and elementary steps in heterogeneous catalysis, Catal. Rev. . Sci. Eng. 13 (1976) 121-147.

Bennett, C. O., Experiments and processes in the transient regime for heterogeneous catalysis, Adv. Catal. 44 (2002) 329-416.

Tamaru, K., Dynamic Heterogeneous Catalysis,Academic Press, New York, 1978.

Mirodatos, C., Use of isotopic transient kinetics in heterogeneous catalysis, Catal.Today 9 (1991) 83-95.

The transient response techniques applied to heterogeneous catalysis have been reviewed by:




The examples of transients combined with The examples of transients combined with IN SITU IN SITU or or operandooperando measurements measurements

Transient analysis of CO oxidation over AuTransient analysis of CO oxidation over Au--TiOTiO22 catalyst catalyst using using operandooperando DRIFTS and MS DRIFTS and MS





Transient analysis of CO oxidation over Au-TiO2 catalyst using operando DRIFTS and MS*

*[Catalysis Today 126 (2007) 135–142]

To probe aspects of the reaction pathways including desorption, oxygenstorage, reaction rates and the role of carbonates during CO oxidation reactions using supported Au nanoparticles (3.7 nm) as a catalyst.

Motivation



During the transients the FTIR spectrum of the pellet and the compositionof the gas flowing out of the reactor were continuously monitored.





A cylindrical, CaF2 rod (A) slides through internal O-rings located on both halves. A sample pellet is held between the CaF2 rods when the two halves are bolted together. Gas flows into and out of the cell through small holes (B) and is contained within the volume sealed by the O-ring (C). Chilling fluid is circulated through ports in the body of both halves (D).

Home made FTIR cell for fast transients

In order to observe fast surface transients, it is required that the reactor has a low dead volume, and a reactor was constructed with this as a major design criterion.

The reactor volume surrounding the wafer is theoretically <0.05 cm3 !!!




Analysis of cell transient parametersAnalysis of cell transient parameters

Ar → He The normalized Ar concentration profile, c(t), through the bypass can be fit well to an equation which contains an exponential decrease combined with a diffusive component in the source term:

where σ is a parameter describing the mixing at the interface following the gas switch, V – cell volume, F – gas flow.

σ = 2.1 s, V/F = 0.31 s

For the proposed reactor configuration most of a non-interacting gas can be eluted from the reaction cell in less than a few seconds (!!!), the time resolution limit of the mass spectrometer sampling system.




MS transient

FTIR transient

Switches He He →→ CO+OCO+O22+He+Ar +He+Ar →→ He He at T= -10oC

CO aCarbonates-bicarbonates

CO2 (a)




Spectral features of CO adsorptionSpectral features of CO adsorption--desorptiondesorption

Due to competing interactions

CO was adsorbed on metallic Au nanoparticles




Kinetics of CO desorptionKinetics of CO desorption

Heats of adsorption measured from CO adsorption isotherms on Au(110) surface is

7.8 7.8 ±± 1.3 kcal/mole1.3 kcal/mole

Because area under the peak Is promotional to the concentration it’s possible to calculate changes of surface coverage for different temperatures




• the close correspondence with Au single crystal data for the FTIR peak FTIR peak positions;positions;

• the coverage dependent shifts;shifts;• The desorption/adsorption energiesenergies together.

These features indicate that the CO adsorption is occurring on the Au particles and not the support.

So, there are the following features:

CO2 does not enhance CO desorption.




The different peak evolutions for adsorption compared to desorption indicates that these two processes are governed by different dynamics.

This result can be explained by a clustering model where increasing coverage of CO(a) first adsorbs on clean Au particles in widely spaced atop sites (2116 cm-1), followed by nucleation of denser CO(a) islands (2106 cm-1) which grow (with frequency shift) and dominate at higher coverage. Desorption (or reaction) occurs primarily from island edges causing a gradual frequency shift as the islands shrink.

Apparently, clustering ofCO and/or the distribution of the two different CO(a) species (island versus isolated) is not at equilibrium but depends upon whether the sites are populating or depopulating.

Difference in adsorption and desorption Difference in adsorption and desorption –– difference in peak evolutiondifference in peak evolution




Storage of CO and reactive oxygenStorage of CO and reactive oxygen

Now we’ll consider the relative rate of desorption of surface CO compared to the rate of its reaction on the surface.

Such information can be obtained by conducting a direct switch or a delayed switch from CO to O2.

In a direct switch, CO is flowed for a period of time and then the gas stream is switched abruptly to O2 and reaction of surface CO(a) begins.

CO CO →→ OO

In a delayed switch, the CO flow is switched to He, which flows for a timed interval before O2 is switched into the reactor. CO(a) desorbs during the He interval and then reaction commences when the oxygen reaches the reactor zone.

CO CO →→ He He →→ OO22




Transient data for CO desorptionTransient data for CO desorption

FTIR peak located near 2105 cm-1 is shown as a function of time after a direct or delayed switch to O2

•at 248 K the reaction rate of CO(a) is competitive with or faster than the desorption rate.

•CO may be stored briefly on the surface when oxygen is not present.

•Subtracting the observed desorption rate yields a reaction rate of about 0.043 s-1 which can be equated with a TOF. It is comparable with the TOF measured in a flow reactor being equal to 0.023 s-1 at 235 K.




Oxygen storageOxygen storage

Although adsorbed oxygen cannot be observed on the surface by FTIR, its presence may be detected indirectly by its effect on CO uptake.

O2 is flowed for 60 s prior to a direct switch to CO

The actual delay of less than 2 s implies that either less than 3.1 µmoles O/g-catal is stored anywhere on the catalyst surface or the oxygen desorption rate at 298 K is rapidcompared to the CO arrival rate.

Note, that there is 1.0 µmoles of surface Au




Surface carbonate speciesSurface carbonate species

Similar spectra both for Au-TiO2 catalyst and TiO2support !




Desorption of CODesorption of CO22

The position of this peak, assigned to adsorbed CO2(a) is independent of coverage, and occurs at the same frequency when the Au-free TiO2 is used in the reactor.




Desorption of CODesorption of CO22

Desorption of the CO2(a) is slower than for CO at comparable temperatures and appears to have both a fast and a slow component of desorption.

FAST - a very low activation energy of around 1–2 kcal/mole, which is characteristic for diffusion controlled removal process.

The similar peak positions, intensities and activation energies for TiO2 and Au-TiO2 suggest that the CO2(a) is primarily present on the support.




Desorption of CODesorption of CO22. Interconnection with surface carbonates. Interconnection with surface carbonates

Transients after switch CO/O2 to He at -20oC

• Fast response –desorption of adsorbed CO2.

• Slow response –decomposition of carbonates.




Pathways for CO oxidation on AuPathways for CO oxidation on Au--TiOTiO22

• Adsorption of O2 on WGC either occurs at a very low coverage, or its desorption is very fast.

• CO adsorbs on Au nanoparticles readily.

• CO(a) reacts rapidly in the presence of gas phase O2 to form CO2(a).

• The product CO2 can then desorb from the Au particle and interact with the support to form transient carbonate species which then slowly decompose to CO2.

• The desorption of CO2(a) is the rate limiting step in CO oxidation over Au-TiO2catalyst.

Study the storage and reduction of NOStudy the storage and reduction of NOXX over Pt(or Pd or Rh)/BaCOover Pt(or Pd or Rh)/BaCO33/Al/Al22OO33





Study the storage and reduction of NOX over Pt(or Pd or Rh)/BaCO3/Al2O3

NOX storage and reduction technology offers the possibility of reducing emissions of NOX from vehicles operating under lean-burn conditions.

Motivatiion

The concept is based on incorporating a storage material (commonly Ba) in the conventional three-way catalyst to store NOX (NO + NO2) under lean conditions until it is saturated with NOX.

Subsequently the stored NOX is released and reduced to N2 by turning the engine to rich operating conditions under a short period.

NO + NO2

NOX

N2

R

R = CO, H2, C3H6, or C3H8

The concept was introduced by Toyota in the beginning of the 1990s.



NONOXX storage and reduction cycles for Pt/BaCOstorage and reduction cycles for Pt/BaCO33/Al/Al22OO33

Gas phase analysisCO

H2lean

richrich rich

lean lean

• The efficiency for NOx reduction changes with T and reducing agent.• CO can block some Pt sites and decrease efficiency of reduction.



The in situ DRIFTS experiments were performed using a BioRad FTS6000 FTIR spectrometer equipped with DRIFTS optics and a heated reaction chamber(Harrick Scientific Praying Mantis with a DRIFTS cell).

• Temperatures: 350, 250,and 150 ◦C• Total flow rate: 200 ml/min, • Space velocity: 106,000 h−1.

• The pretreated catalyst was first saturated with NOX by exposing it to 500 ppm NO2 in Ar for 19 min. The stored NOX was subsequently reduced by introducing a reducing agent to the NO2/Ar flow.

• The regeneration gas mixture consisted of 500 ppm NO2 and 4000 ppm CO, 4000 ppm H2.

• Two consecutive storage–reduction (lean–rich) cycles were conducted to follow the evolution of the surface species under the second cycle.

• The spectra were collected during the entire NOX storage–reduction cycles with a time resolution of 0.5 s.

Experimental steps

Experimental conditions



In Situ DRIFTS spectraIn Situ DRIFTS spectra

Regeneration by CO

bidentate nitrate over Al2O3

monodentate nitrate over BaO

bridge-bonded bidentate nitrite over BaO

monodentate nitrite over Al2O3

monodentate nitriteover BaO

• NOX storage occurs via the formation of nitrites and nitrates of both barium and alumina.

• With increasing temperature, NOXstorage on barium becomes more significant.

An operando UVAn operando UV––vis spectroscopic study of the vis spectroscopic study of the catalytic decomposition of NO and Ncatalytic decomposition of NO and N22O over CuO over Cu--ZSMZSM--5*5*





*[Journal of Catalysis 220 (2003) 500–512]

An operando UV–vis spectroscopic study of the catalytic decomposition of NO and N2O over Cu-ZSM-5*

MotivationMotivationoperando UV–vis spectroscopy in combination with on-line GC analysis allows a direct study of the role of the bis(μ- xo)dicopper core in the catalytic NO and N2O decomposition cycles

N2O → N2 + ½ O2

2 NO → N2 + O2

The decomposition of NO is thermodynamically favored at temperatures below 1000 K. However, the reaction is kinetically retarded due to the very high activation energy of about 300 kJ mol−1.




A 100% selective conversion of NO into N2 and O2 can be achieved over Cu-ZSM-5 above 350oC with maximum activity in the temperature range 450–500oC.

Several explanations for this profile have been proposed:

Much research has been dedicated to the identification of the active sites in Cu-ZSM-5 and the NO decomposition reaction mechanism.

WHY ?

• the desorption of oxygen, • the instability or surface nitrates,• the adsorption equilibrium of NO.

Different active species were proposed:• Cu(I) and Cu(II)–ELO species (ELO, extralattice oxygen);• Mononuclear species Cu2+–O− or Cu2+–O2−; • [CuOCu]2+;• According to XAFS



An operando UV–vis spectroscopic study of the catalytic decomposition of NO and N2O over Cu-ZSM-5Experimental setExperimental set--upup



An operando UV–vis spectroscopic study of the catalytic decomposition of NO and N2O over Cu-ZSM-5

The evolution of the in situ UV–vis spectraof Cu-ZSM-5 collected during calcination in oxygen flow.

Ozeolite →Cu(II) CT transitions

d–d transitions of (partially hydrated) Cu(II) ions

Cu CuO

O




Transient response after switch OTransient response after switch O22 →→ NONO

They can be assigned to a single Cu-ELO species




Time dependence of the NO decompositionTime dependence of the NO decomposition




Proposed reaction mechanism for the decomposition of NO and NProposed reaction mechanism for the decomposition of NO and N22OO



Transient Technique for Identification of True Reaction IntermedTransient Technique for Identification of True Reaction Intermediates: iates: Hydroperoxide Species in Propylene Epoxidation on Gold/TitanosilHydroperoxide Species in Propylene Epoxidation on Gold/Titanosilicate icate

Catalysts by XCatalysts by X--ray Absorption Fine Structure Spectroscopyray Absorption Fine Structure Spectroscopy



• Propylene oxide (PO) is a well-known chemical intermediate used in the synthesis of polyether polyols, propylene glycols, and propylene glycol ethers, which are subsequently employed in the preparation of many end products such as polyurethanes, cosmetics, hydraulic, antifreeze and brake fluids, coatings, inks, textile dyes, and solvents.

Transient Technique for Identification of True Reaction Intermediates: Hydroperoxide Species in Propylene Epoxidation on Gold/Titanosilicate

Catalysts by X-ray Absorption Fine Structure Spectroscopy

MotivationMotivation

• Because of the high selectivity to PO (>90%) and the lower cost of the feedstocks, the hydrogen-oxygen route over Au/Ti-SiO2 catalysts has attracted great attention.



Possible sequence of steps for propylene epoxidation with H2 and O2on Au-supported titanosilicates

• synthesis of hydrogen peroxide from hydrogen and oxygen on gold nanoparticles;

• formation of Ti-hydroperoxo or peroxo species from hydrogen peroxide on tetrahedral Ti centers;

• reaction of propylene with the Ti-hydroperoxide species to form PO;

• decomposition of hydrogen peroxide to water.

The important steps during PO synthesis consist of:



In situ In situ transient UVtransient UV--vis data vis data are used to identify the formation of Ti-hydroperoxide species and to verify its characteristic behavior as a reaction intermediate.

In situ In situ XAFS spectroscopy XAFS spectroscopy is another powerful technique that can be used to obtain atom-specific structural and electronic information from solid inorganic catalysts under operating conditions.

Techniques to be used: Techniques to be used:

Transient Technique for Identification of True Reaction Intermediates: Hydroperoxide Species in Propylene Epoxidation on Gold/Titanosilicate

Catalysts by X-ray Absorption Fine Structure Spectroscopy

Short presentation of XAFS technique



In situ UVIn situ UV--vis spectra for Auvis spectra for Au--Ba/TiBa/Ti--OO under under HH22/O/O22/Ar (1/1/8) /Ar (1/1/8) gas mixture at 150gas mixture at 150ooCC

3.82 eV -Ti-hydroperoxospecies,

3.96 eV (313 nm) and 4.18 eV (297 nm) -water coordinated to Ti sites.



In situ UVIn situ UV--vis spectroscopy results for Auvis spectroscopy results for Au--Ba/TiBa/Ti--O O under propylene epoxidation conditions under propylene epoxidation conditions (C(C33HH66/H/H22/O/O22/Ar ) 1/1/1//Ar ) 1/1/1/7) 7) at 150 at 150 ooCC

3.82 eV -Ti-hydroperoxospecies,

3.96 eV (313 nm) and 4.18 eV (297 nm) -water coordinated to Ti sites.



Intensity of band corresponding to titanium peroxide vs time inIntensity of band corresponding to titanium peroxide vs time in flowflow

C3H6/H2/O2/Ar = 1/1/1/7

H2/O2/Ar = 1/1/8



Intensity of band corresponding to titanium peroxide vs time Intensity of band corresponding to titanium peroxide vs time after different switchesafter different switches



In situ Ti KIn situ Ti K--edge XANES spectroscopy data as a function of time edge XANES spectroscopy data as a function of time in flow of reaction mixturein flow of reaction mixture

(C3H6/H2/O2/Ar = 1/1/1/7)

4-fold coordinated Ti



Rates of formation of 4Rates of formation of 4--fold coordinated Ti under the treatmentfold coordinated Ti under the treatmentin the different gasesin the different gases

H2/O2/Ar = 1/1/8

C3H6/H2/O2/Ar = 1/1/1/7



This reaction rate closely matched the TOF (2.5 10-4 s-1) obtained for the Au-Ba/Ti-O catalyst at steady-state conditions.

So, measurement of the changes in Ti-hydroperoxo coverage under transient experiments at reaction conditions with H2/O2/Ar and C3H6/H2/O2/Ar gas mixtures, allowed the estimation of the initial net rateof propylene epoxidation (3.4 10-4 s-1).

Ti-hydroperoxo species formed during propylene epoxidation conditions is a true reaction intermediate.

advanced techniques for characterization of heterogeneous...

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