nox storage in model pt/ba nsr catalysts: fabrication and reactivity of bao nanoparticles on...

Surface Science 537 (2003) 179–190

www.elsevier.com/locate/susc

NOx storage in model Pt/Ba NSR catalysts: fabricationand reactivity of BaO nanoparticles on Pt(1 1 1)

Peter Stone a,*, Masaru Ishii a,b, Michael Bowker a

a Department of Chemistry, Centre for Surface Science and Catalysis, University of Reading, Whiteknights,

Reading RG6 6AD, UKb Material Engineering Div. III, Component and System Development Centre, Toyota Motor Corporation,

Higashifuji Technical Centre, 1200 Mishuku, Susono, Shizuoka 410-1193, Japan

Received 5 February 2003; accepted for publication 23 April 2003

Abstract

We have employed high temperature scanning tunnelling microscopy to probe the adsorption of Ba on Pt(1 1 1) and

its reactivity towards oxygen and NO. The aim of this work is to study the NOx storage process. A novel approach to

prepare a model of the NOx exhaust catalyst has been used rather than the conventional deposition of a metal onto an

oxide substrate. Ba grows on the Pt(1 1 1) surface in a Stranski–Krastanov mode, at high doses Ba nanoparticles are

formed on a thin layer of Ba. The storage component of the catalyst has been prepared by reaction with oxygen at 573

K, which produces particles of BaO. The reactivity of this surface towards a mixture of O2 and NO results in a dramatic

increase in particle size which is attributed to the storage of NOx in the form of the nitrate, Ba(NO3)2.

� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Scanning tunneling microscopy; Alkaline earth metals; Platinum; Barium oxide; Low index single crystal surfaces; Catalysis;

Nitrogen oxides

1. Introduction

Due to environmental concerns, the permitted

level of exhaust emissions from motor vehicles is

continually reducing with the introduction ofsuccessively more stringent European and world

standards. 1 Of particular concern are the levels of

CO, NOx, hydrocarbons (HC) and particulate

* Corresponding author. Tel.: +44-1189-318289; fax: +44-

1189-316331.

E-mail address: [email protected] (P. Stone).1 For example under the 1997 Kyoto Agreement 38 nations

agreed to cut their emissions of greenhouse gases by 5.2%

relative to 1990 levels by 2008–2012.

0039-6028/03/$ - see front matter � 2003 Elsevier Science B.V. All rdoi:10.1016/S0039-6028(03)00614-9

matter (PM), and catalysts are used to reduce the

first three of these to acceptable levels. By the year

2006 the permitted levels of CO and NOx emis-

sions for newly built cars in Europe will be ap-

proximately half those currently allowed with afurther decrease due in 2009. Motor vehicle usage

is claimed to result in a net increase in the amount

of CO2 in the atmosphere and hence is a major

contributor to the greenhouse effect, therefore a

reduction of these emissions is essential. These can

be reduced by running the engine with an oxy-

gen excess, known as �lean-burn�, whereas con-ventional engines operate within a narrow air:fuel ratio close to the stoichiometric mix. Lean-

burn engines offer considerable energy efficiency

ights reserved.

mail to: [email protected]

180 P. Stone et al. / Surface Science 537 (2003) 179–190

(reducing the fuel consumption by 20–30% [1]),

however, the high concentration of oxygen in such

engine exhausts means that the three-way catalyst

(TWC) technology conventionally used to remove

NOx can no longer be used. Consequently the use

of lean-burn engines results in increased NOxemissions [2], requiring alternative methods of

NOx removal.

The direct reduction of NOx to N2 and O2, is a

thermodynamically favoured reaction, however

despite work in this area [3,4] no catalyst has been

found that gives sufficient conversion of NO in the

presence of high concentrations of oxygen. Alter-

natively selective catalyst reduction (SCR), inwhich the NOx is reduced with either ammonia or

hydrocarbons under oxygen excess can be used.

However, there are drawbacks associated with

each of these which mean that commercialisation

is problematic.

One way in which these problems have been

addressed is the introduction of a NOx storage

component, which is based on a mixed lean oper-ation in the engine. This was introduced by Toyota

[5,6] in the mid-1990s and is known as the NOxstorage reduction (NSR) catalyst. The catalyst is

based on three main components; a high surface

area material (Al2O3), a storage component (BaO)

and a noble metal (Pt). It works on the principle

that during the lean periods the NOx is stored and

released during the stoichiometric periods by re-duction on noble metal (Pt) sites in the catalyst.

There is a large wealth of literature on NSR and

related issues [7–28]. Since 1996 the NSR catalyst

has been commercially available in the Japanese

market [15]. However a major problem with its

introduction into the European market is the

higher sulphur content in fuel, which results in

rapid deactivation as sulphate formation poisonsthe catalyst.

All of the work cited above involved investiga-

tion of the NOx storage/reduction on powdered

catalyst samples. In the work presented here we

have embarked on a fundamental approach, using

STM to study the NOx storage process at atomic

resolution. Early experiments involved the depo-

sition of Ba onto a TiO2(1 1 0) substrate but nowell-ordered structures were formed [29]. In this

work we have used inverse catalyst methods with a

single crystal of a noble metal (Pt(1 1 1)) as the

support onto which an alkaline earth metal (Ba) is

deposited. The oxidation of this to BaO produces

the storage component, thus allowing the NOxstorage reaction to be probed.

2. Experimental

STM and LEED experiments were performed

using a WA Technology variable temperature

STM contained within an ultra high-vacuum

(UHV) chamber equipped with facilities for Arþ

sputtering, low energy electron diffraction(LEED), Auger electron spectroscopy (AES), a

quadrupole mass spectrometer (QMS) and a Ba

evaporator. The chamber was ion pumped to

produce a typical base pressure of 1 · 10�10 mbarand has been described in detail elsewhere [30].

The Pt(1 1 1) crystal (Metal Crystals & Oxides

Ltd, UK) was cleaned by repeated cycles of Arþ

sputtering (up to 1.5 keV) at 1023 K with flashingto �1373 K during sputtering and flashing to�1373 K in UHV after sputtering. Initially anadditional cycle of annealing in 2 · 10�6 mbar ofoxygen at 723 K was used to remove residual

carbon and Ca. The surface cleanliness was

checked by LEED, AES and STM.

Ba was deposited on the Pt(1 1 1) surface at

room temperature by using a Ba evaporator(SAES Getters G.B. Ltd, UK). During evapora-

tion, the pressure in the chamber was �5 · 10�9mbar. NO (99.0%, ARGO International Ltd, UK)

and O2 (99.6%) were introduced via separate

variable leak valves at pressures between 1 · 10�9and 5 · 10�7 mbar.

3. Results and discussion

3.1. Ba deposition on Pt(1 1 1)

A clean Pt(1 1 1) surface was prepared that

showed no impurities detectable by AES, and

produced a sharp (1 · 1) LEED pattern (see insetin Fig. 1a). Upon inspection with STM, large ter-races were evident (up to 2000 �AA wide, Fig. 1a). Aninteresting feature of this image is the bright ap-

Fig. 1. STM images of the clean Pt(1 1 1) surface. The image in (a) shows the large terraces produced with the preparation procedure

outlined in Section 2, whereas the image in (b) shows atomic resolution of the surface. The inset in (a) shows the (1· 1) LEED patternof this surface recorded at 135 eV. Tunnelling parameters: (a) 5000 �AA, 2000 pA, 100 mV; (b) 30 �AA, 3500 pA, 74 mV.

P. Stone et al. / Surface Science 537 (2003) 179–190 181

pearance of the step edges. Double steps can be

ruled out by examination of the line profiles over

each of the steps. Alternatively these could be due

to electron standing waves on the clean Pt(1 1 1)

surface. Recent work by Leibsle [31] has shown

that it is possible to observe such structures on

metal surfaces at room temperature. In his workelectron standing waves were observed in clean

surface regions between islands of N on a Cu(1 1 1)

substrate. However, although this phenomenon

may explain the bright features close to the step

edges we are unable to unambiguously identify

them and we are unsure of their exact origin.

Despite the difficulties in obtaining atomic res-

olution on this surface at room temperature (dueto Pt atom mobility and the small lattice spacing)

we have achieved this. Fig. 1b shows an atomic

resolution image of the Pt(1 1 1) surface recorded

at room temperature, the bright protrusions evi-

dent in the image are Pt atoms and their separation

of 2.8 �AA compares favourably with the value of2.77 �AA expected [32].Ba was deposited onto this surface at room

temperature and the overlayer formed assessed

with STM. Only a small Ba signal was detected in

the AES, due to the low sensitivity to heavy atoms.

It is clear from the STM images that Ba has been

deposited and a thin film formed. A large-scale

image (1000 �AA), shown in Fig. 2a demonstrates

this and upon closer inspection (Fig. 2b) a number

of features are observed that may be attributed to

individual Ba atoms. One such area is marked by

the white circle in the figure. Consideration of the

atomic radii of Ba dictates that the closest packing

allowed is a (2 · 2) structure. Therefore based onthis assumption and that any dark areas are holesin the Ba overlayer a coverage of 0.15–0.20 ML

(where 1 ML is the number of surface Pt atoms)

for Fig. 2a is estimated.

In summary the Ba grows on the Pt epitaxially

as the underlying fcc Pt dictates the growth of the

Ba. The domain sizes of these regions are ex-

tremely small (typically �10 atoms) due to strainbetween the Pt and Ba, therefore no LEED patternwas observed. The only change in the LEED on

deposition of Ba was an increase in the back-

ground intensity.

3.1.1. Annealing of the Ba overlayer

To produce more order in the Ba overlayer

structure, the surface was annealed in order to

promote adatom mobility. At 500 K atom andnanoparticle mobility is expected [33]. STM im-

ages of the surface recorded at room temperature

after annealing to 500 K (a,b), 573 K (c) and 673 K

(d) for 15 min each are shown in Fig. 3. In contrast

to the disordered film observed for the as-depos-

ited surface, the Ba has now formed a number of

Fig. 2. STM images of the Pt(1 1 1) surface after deposition of Ba at room temperature. The overlayer film formed by Ba is fairly

disordered although evidence for some order is observed. Tunnelling parameters: (a) 1000 �AA, 1000 pA, 1000 mV; (b) 250 �AA, 1000 pA,

500 mV. Images recorded at room temperature.


small islands on which atomic resolution is ob-

served (Fig. 3a). A smaller scale image, Fig. 3b

allows us to determine the separation of the fea-

tures within these islands. The bright protrusions

in the central island in Fig 3b have a separation of9.6 �AA, which is considerably greater than the 7.10�AA expected for the (1 1 1) plane of Ba. The differ-ence in spacing from that observed and that ex-

pected for the (1 1 1) plane of Ba can be tentatively

explained as follows. The first layer of Ba on the

Pt(1 1 1) attempts to arrange itself in the fcc (1 1 1)

plane but due to the large lattice mismatch (and

difference in crystal packing) between the Pt andBa this is unfavourable. Therefore a structure is

formed upon annealing, which manifests itself on

the surface as islands with a 9.6 �AA atomic spacing.Further annealing (Fig. 3c/d) results in the island

number density decreasing and their average size

increasing, which indicates that a kind of sintering

has occurred.

3.2. Reactivity of the Ba/Pt(1 1 1) surface towards

oxygen

The reactivity of the Ba surface prepared in

Section 3.1 towards oxygen has been investigated

at 573 K under two pressure regimes. The se-

quence recorded in an oxygen pressure of 1 · 10�9mbar is shown here as it displays all relevant fea-

tures. Prior to imaging at 573 K the surface had

undergone an annealing cycle at a higher temper-

ature (673 K). This is to avoid any further sinter-

ing of the Ba before and during the reaction with

oxygen, which could complicate image interpreta-tion. Based on the STM images the coverage of Ba

here is 0.25–0.30 ML. An interesting feature (more

evident in Fig. 4a due to a higher defect density) is

the periphery of these particles where the defects

appear to have aggregated. This is visible as a ring

of dark features surrounding the particles. The

inset in Fig. 4a shows this at greater magnifica-

tion. A possible explanation for this is that as theparticles form, the defects are pushed out of the

particle area as the strain is being relieved by

the formation of the particle, which is probably

of a different structure. Alternatively they could

form as a result of Ba from the surrounding area

being effectively consumed by the formation of the

particle.

Fig. 4 starts part way through the reaction andFig. 4a shows that the step edges have become

pinned, which has occurred after an oxygen ex-

posure of only �3 L (where 1 L¼ 1.3 · 10�6mbar s). This is probably due to the fast dissocia-

tion of O2 at the step sites. Images from Fig. 4a to

4k were recorded successively during the reaction

at an interval of 264 s, approximately 0.4 L O2 per

image. Individual particles suddenly, and inde-

Fig. 3. STM images after annealing the as deposited surface to (a,b) 500 K, (c) 573 K and (d) 673 K for 15 min. The white box marked

in (a) signifies the area shown in (b). On this scale atomic resolution on the Ba islands is observed and displays a separation of 9.6 �AA.

Tunnelling parameters: (a) 1000 �AA, 100 pA, 1000 mV; (b) 200 �AA, 100 pA, 1000 mV; (c) 1000 �AA, 100 pA, 1000 mV; (d) 1000 �AA, 1000 pA,

1000 mV.


pendently of each other, increase in brightness,

(apparent height) throughout the sequence. How-

ever, their size (diameter) displays little change.

Their average height increases from 2.35 �AA inFig. 4a, to 6.96 �AA in Fig. 4l, with height ranges of1.8–2.8 and 4.8–10.4 �AA respectively. The white cir-cles in the figure signify particles that have un-

dergone this transformation since the previous

image and, once changed, the particle remains in

its new state. Tip-induced effects or tip artefacts

can be ruled out for the following reasons. Firstly,

if a particle change was induced by the STM tip wewould expect this to coincide with a tip change in

the image, which would appear as a horizontal

streak (the fast scan direction). Second, if the

change in appearance was due to a tip artefact we

might expect all the particles to change their ap-

pearance at the same time and not in sequence.These features were not observed in conjunction

with particle transformation. As further confir-

mation that the effect is not tip induced, �panningout� to a different area on the surface shows that allparticles have changed, not just those within the

scan area. Another aspect of the reaction occurs

after the pressure is increased to 5 · 10�9 mbar(�half way through the image in Fig. 4i). Towardsthe top left of Fig. 4i an island begins to grow

within the first Ba layer. It is postulated that this is

Fig. 4. A sequence of STM images recorded during the exposure of the Ba covered Pt(1 1 1) surface to �1· 10�9 mbar O2 at 573 K.This sequence starts mid-way through the reaction and by (a) the surface has been exposed to approximately 3 L of O2. There is also

one pressure change, mid-way through the image in (i), where the O2 pressure is increased to �5· 10�9 mbar. All images are 1000 �AA,tunnelling parameters: (a,b) 100 pA, 1000 mV; (c) 80 pA, 1300 mV; (d,e) 80 pA, 1500 mV; (f–j) 70 pA, 1500 mV; (k) 100 pA, 1500 mV;

(l) 70 pA, 1500 mV. Inset size in (a) 300 �AA.


due to the formation of barium peroxide, BaO2.

The decomposition temperature of the bulk per-

oxide is cited as 723 K [32] and therefore it is likely

that such a species will be metastable at the tem-


perature of 573 K used here. Due to this low de-

composition temperature the peroxide islands are

only visible in the presence of gas phase oxygen,

therefore once the oxygen over-pressure is re-

moved the islands shrink and ultimately disappear.

In addition, atomic resolution at 573 K is notpossible due to a high rate of diffusion and thermal

drift. Under certain conditions (higher exposure to

oxygen) we have been able to produce a surface

with peroxide islands that are stable after the re-

moval of oxygen and cooling. Atomically resolved

images of the BaO2 structure have been obtained

at room temperature and are shown in Fig. 5. The

most striking feature of this structure is the ap-pearance of a bright atom in a regular pattern

Fig. 5. Atomically resolved STM images of the BaO2 surface. (a) sh

marked by the black box in (b). A schematic diagram of the [1 0 0] plan

in (d) (see text for greater detail). Tunnelling parameters: (a) 150 �AA,

(Fig. 5a). A positive sample bias has been used for

all images and therefore the assumption is made

that Ba is imaged as the bright points as tunnelling

will be from tip to sample. A unit cell is marked by

the black box in Fig. 5b and has dimensions (av-

eraged over many unit cells) of 13.4 �AA · 12.7 �AA. Inaddition this structure has a square arrangement,

in contrast to the hexagonal arrangement of the

underlying substrate. This can be explained by

considering the crystal structures of BaO and

BaO2. The main difference between the two is the

elongation of the unit cell by the peroxide unit in

BaO2, this has the effect that a hexagonal (1 1 1)

plane of Ba atoms is not possible in BaO2 whereasit is in BaO. A re-scaled STM image is shown in

ows the longer range order of the structure and a unit cell is

e of BaO2 is shown in (c) and is compared with a re-scaled STM

2499 pA, 198 mV; (b) 50 �AA, 2499 pA, 198 mV.


Fig. 5d with a model of the [1 0 0] plane of BaO2 in

Fig. 5c. The dimensions marked A and B in Fig. 5care A ¼ 15:24 �AA, B ¼ 21:52 �AA (for the [1 0 0] planeof BaO2 and A ¼ 18:63 �AA, B ¼ 25:28 �AA for thestructure observed in STM. The structure ob-

served in STM is therefore 20% larger than thatexpected for the [1 0 0] plane of BaO2. The most

likely explanation for this discrepancy is the in-

fluence the underlying [1 1 1] substrate has on the

overlying [1 0 0] structure. Evidence for this is

further supported by the alternately rotated nature

of the structure in the STM, which indicates a

considerable amount of strain within the structure.

Returning to the reaction sequence in Fig. 4, theparticles cover 12.4% (±0.5%) of the total image

area in Fig. 4a, compared with 14.8% (±0.5%) in

Fig. 4l, although their average apparent height

increases by 4.61 �AA (a factor of three increase).What happens to the particles during their expo-

sure to oxygen? The simplest explanation is the

conversion of Ba to BaO. However, from the re-

action sequence a number of further questionsarise:

1. Why does a whole particle change spontane-

ously (or on the time-scale of imaging)?

2. Is there any pattern to the order of the particle

transformation?

3. Is the same structure formed at the step edges

and particles?4. Why is the height of the particle increased far

more than the lateral size (diameter)?

Considering the first point, it is unlikely that the

particles do in fact change spontaneously, the scan

time of each image in the sequence is 264 s, which

must therefore be greater than the time-scale for

particle transformation. However a clue to whyand when the particles transform may come from

the density of defect regions surrounding them.

Assuming that these defect regions are clean areas

of Pt, it is likely that oxygen can dissociatively

adsorb and thus create a high local coverage of

oxygen surrounding the particles. Therefore once

enough oxygen is present, the particle can convert

to the oxide.On the second point, a closer look at the se-

quence in Fig. 4 reveals that the particles adjacent

to the step edges are the slowest to react. This can

be explained by considering that the oxygen reacts

with the Ba at the step edges first due to the lower

co-ordination of Ba atoms. Therefore only when

the step sites have converted to BaO is there a high

enough concentration of oxygen around the par-ticles close to the step edge for them to react.

On the third point, line profiles recorded over

step edges and particles display the same increase

in �apparent� height and we therefore tentativelyattribute this as formation of BaO.

Regarding point 4 above which concerns the

large increase in apparent height of the particles

during oxygen treatment. Due to the basic princi-ples underlying the operation of an STM, it is

likely that the change observed here is not entirely

due to an increase in topographical height. A

dominant factor in STM images is the electronic

state of the surface, consequently it is possible to

observe bright features in an STM image, which

are in fact topographically lower than adjacent

dark features. For example on the TiO2(1 1 0)surface the Ti atoms appear bright in the STM

even though they are 1 �AA lower than the O atoms[34,35]. A change in electronic state can arise from

a change in chemical species and subsequent

change in work-function. Therefore it is possible

that the sudden bright appearance of these parti-

cles is due to their transformation from metal to

oxide. In support of this, Pt(1 1 1) has a work-

function of 5.7 eV [36], Ba(1 0 0) is 5.67 eV [36],

whereas polycrystalline Ba is cited to be 2.7 eV[36]. BaO has a work-function of 1.5 eV [37],

considerably lower than Pt or Ba surfaces. In the

STM a region on the surface with a very low work-

function (BaO) will appear bright as the tip has to

withdraw away from the surface to maintain

constant current. In addition a real increase in

height from BaO formation may also contribute

to the apparent height increase observed in theSTM.

Based on the reaction sequence shown in Fig.

4a reaction mechanism is shown in Eqs. (3.1)–(3.4)

below:

BaðseÞ þ 12O2 ! BaOðseÞ ð3:1Þ

BaðslÞ þ 12O2 ! BaOðslÞ ð3:2Þ


BaðtÞ þ 12O2 ! BaOðtÞ ð3:3Þ

BaOðtÞ þ 12O2 ! BaO2ðtÞ ð3:4Þ

where, se refers to a step edge, sl to the second

layer of Ba and t Ba on the terrace.

3.3. Reactivity of BaO/Ba/Pt(1 1 1) towards O2 and

NO

The reactivity of the surface prepared in Section

3.2 towards oxygen and NO has been investigated

with the STM at 573 K. Fridell et al. reported

maximum NOx storage at about 650 K [9]. We

studied this reaction at 573 K as it is close to the

temperature at which maximum NOx storage isobserved and the Ba covered surface was oxidised

at 573 K. The tip was withdrawn overnight after

the sequence in Fig. 4 and the sample temperature

kept at 573 K; upon re-approaching, an area with

a similar particle density was found before com-

mencing the experiment. Taking into account

simple stoichiometric arguments and assuming

that each gas has an equal sticking probability itcan be seen from Eq. (3.5) that it is appropriate to

dose NO and O2 in a 4:3 ratio to allow the nitrate

to form.

BaOþ 2NOþ 32O2 ! BaðNO3Þ2 ð3:5Þ

The reactivity of this surface to O2 and NO isshown in Fig. 6. NO and O2 were admitted to the

chamber via separate leak valves and therefore in

the first part of this sequence only NO was ad-

mitted (to determine its pressure) before O2 was

admitted using the second leak valve. Throughout

the course of this reaction there were a number of

pressure changes of both reactant gases. Between a

and b in Fig. 6 only NO was present. Then O2 wasadmitted to the chamber and the NO/O2 ratio

slowly increased (with NO in excess) until Fig. 6d

at which point the oxygen pressure was increased

further and was in excess of the NO.

After the admission of NO a small amount of

additional growth was observed at the step edges.

This can perhaps be attributed to additional for-

mation of BaO, which may have occurred by NOdissociation. From Fig. 6a to b the area covered by

particles and step edge growth increases from

13.8% to 16.0%, which can be largely attributed to

the oxide formation at the step edges. In powdered

catalyst studies it is observed that no NOx storage

occurs with just NO; O2 is also required [38]. Once

both were in the chamber the reaction proceeded

and an increase in particle size (diameter) wasobserved. The particles slowly increased in size

until they occupied 27.7% of the surface area,

approximately twice that before reaction. We at-

tribute this to the reaction of the BaO particles

with the NO and O2 to form the nitrate, Ba(NO3)2.

It is also possible for the nitrite to form, Ba(NO2)2,

Eq. (3.6) although we are unable to determine this

from the STM images alone.

BaOþ 2NOþ 12O2 ! BaðNO2Þ2 ð3:6Þ

To determine this conclusively it would be neces-

sary to use XPS to identify the chemical species

present on the surface. In XPS we would expect to

detect a change in the Ba 3d5=2 binding energy of

about 1.3 eV in going from the oxide (779.5 eV) to

the nitrate (780.8 eV) [39]. However, we need to

determine whether these particles are the nitrate byusing indirect methods. This can be achieved by

looking at the crystal structures of the oxide and

nitrate and determining whether the increase in

particle size is consistent with the conversion of

oxide to nitrate. BaO has a cubic crystal structure

with a lattice constant of 5.52 �AA [36], therefore theunit cell volume would be (5.52 �AA3), or 168 �AA3.Ba(NO3)2 also has a cubic crystal structure butwith a lattice constant of 8.11 �AA. Consequently thevolume occupied by a unit cell of the nitrate would

be 533 �AA3, which is approximately three times thatof the oxide. During the reaction sequence in Fig.

6 the particle height remains fairly constant but the

area increases by a factor of two, which would be

expected for the conversion of oxide to nitrate. An

area increase will give the best indication of anychange that is occurring as there may be a work-

function change associated with the conversion of

oxide to nitrate, which will affect the apparent

height of the particles. Therefore, if we assume

that the increase in area is due to the formation of

nitrate then the changes observed are consistent

with most of the oxide being converted to nitrate.

Exact conversions of oxide to nitrate are difficult

Fig. 6. A sequence of STM images recorded during co-dosing NO and O2 at 573 K onto the surface prepared in Fig. 4. The total

pressure gradually rises throughout the sequence up to a maximum value of approximately 3· 10�7 mbar. All images are 1000 �AA,

tunnelling parameters: (a,b) 100 pA, 1500 mV; (c) 80 pA, 1600 mV; (d) 80 pA, 1500 mV; (e,f) 80 pA, 1700 mV.


as particle area measurements in the STM have

some error, we can however say that the particles

increase in area by at least a factor of two. How-

ever, to obtain a quantitative value on the amount

of oxide and nitrate present the use of XPS in situ

would be advantageous.

Fig. 7. Schematic diagram of the preparation of the model

catalyst used in this work and the storage of NOx observed. See

text for greater detail.


A schematic diagram depicting the reactivity of

the Ba covered Pt(1 1 1) surface is shown in Fig. 7.

A Stranski–Krastanov growth mode is observed

for Ba on Pt(1 1 1) and this produces a surface withBa in two different environments; as a thin layer

and as particles. These display differing reactivity

towards oxygen at 573 K. The particles irreversibly

transform into BaO, whereas the thin film partially

converts into metastable BaO2 which is lost when

the oxygen is removed. The BaO particles remain

and if treated with NO at 573 K there is no evi-

dence for a reaction occurring. However, using aco-dosing mixture of NO and O2 allows the NOxto be stored in the form of the nitrate.

4. Conclusions

In this work we have succeeded in preparing an

ordered overlayer of Ba on a Pt(1 1 1) substrate.Room temperature deposition results in a largely

disordered layer on the surface, although local

order is observed in places. Annealing increases

the order and produces an array of Ba islands

across the surface. Exposure to oxygen at elevated

temperatures (573 K) allows the formation of

BaO, which is our storage component. This

transformation is imaged in situ and is seen as asudden change in apparent height of the Ba par-

ticles on the surface, which is attributed to the

lower work-function of the oxide compared to the

metal. A metastable form of oxygen on the surface

is also observed and is attributed to BaO2 forma-

tion. It is found that NOx is only stored on this

surface when both NO and O2 are co-dosed. At

573 K, and with the appropriate NO/O2 ratio, theincrease in particle area of approximately 100% is

attributed to the storage of NOx in the form of the

nitrate.

Acknowledgements

The authors would like to thank the EPSRCand the Toyota Motor Corporation for funding

this work.

References

[1] R.M. Heck, R.J. Ferrauto, Catalytic Air Pollution Con-

trol, Van Nostrand Reinhold, New York, 1995.

[2] G. Ertl, H. Kn€oozinger, J. Weitkamp (Eds.), Handbook of

Heterogeneous Catalysis, vol. 4, VCH Verlagsgesellschaft

mbH, 1997.

[3] A. Fritz, V. Pitchon, Appl. Catal. B 13 (1997) 1.

[4] M. Shelaf, Chem. Rev. 95 (1995) 209.

[5] N. Miyoshi, S. Matsumoto, K. Katoh, T. Tanaka, J.

Harada, N. Takahashi, K. Yokota, M. Sugiura, K.

Kasahara, SAE technical paper series, No. 950809, 1995.

[6] N. Miyoshi, S. Matsumoto, Sci. Technol. Catal. (1998)

245.

[7] W. B€oogner, M. Kr€aamer, B. Krutzsch, S. Pischinger, D.

Voigtl€aander, G. Wenninger, F. Wirbeleit, M.S. Brogan,

R.J. Brisely, D.E. Webster, Appl. Catal. B 7 (1995) 153.

[8] N. Takahashi, H. Shinjoh, T. Iijima, T. Suzuki, K.

Yamazaki, K. Yokota, H. Suzuki, N. Miyoshi, S. Mat-

sumoto, T. Tanizawa, T. Tanaka, S. Tateishi, K. Kasa-

hara, Catal. Today 27 (1996) 63.

[9] E. Fridell, M. Skoglundh, B. Westerberg, S. Johansson,

G. Smedler, J. Catal. 183 (1999) 196.

[10] E. Fridell, H. Persson, B. Westerberg, L. Olsson, M.

Skoglundh, Catal. Lett. 66 (2000) 71.

[11] P. Engstr€oom, A. Amberntsson, M. Skoglundh, E. Fridell,

G. Smedler, Appl. Catal. B 22 (1999) L241.

[12] A. Amberntsson, B. Westerberg, P. Engstr€oom, E. Fridell,

M. Skoglundh, Catal. Deactivation (1999) 317.

[13] S. Erkfeldt, M. Skoglundh, M. Larsson, Stud. Surf. Sci.

126 (1999) 211.

[14] S. Matsumoto, Y. Ikeda, H. Suzuki, M. Ogai, N. Miyoshi,

Appl. Catal. B 25 (2000) 115.

[15] Y. Ikeda, K. Sobue, S. Tsuji, S. Matsumoto, Technical

paper series, No. 1999-01-1279, 1999.


[16] Y. Li, S. Roth, J. Dettling, T. Beutel, in: Proceedings from

the 5th International Congress on Catalysis and Automo-

tive Pollution Control (CAPOC 5), vol. 1, Brussels,

Belgium, 12–14 April 2000, p. 167.

[17] H. Mahzoul, J.H. Brilhac, P. Gilot, Appl. Catal. B 20

(1999) 47.

[18] S. Balcon, C. Potvin, L. Salin, J.F. Temp�eere, G. Dj�eega-Mariadassou, Catal. Lett. 60 (1999) 39.

[19] T. Kobayashi, T. Yamada, K. Kayano, SAE technical

paper series, No. 970745, 1997.

[20] S. Erkfeldt, E. Jobson, M. Larsson, in: Proceedings from

the 5th International Congress on Catalysis and Automo-

tive Pollution Control (CAPOC 5), vol. 1, Brussels,

Belgium, 12–14 April 2000, p. 143.

[21] S. Matsumoto, Catal. Today 29 (1996) 43.

[22] F. Prinetto, G. Ghiotti, I. Nova, L. Lietti, E. Tronconi, P.

Forzatti, J. Phys. Chem. B 105 (2001) 12732.

[23] L. Olsson, H. Persson, E. Fridell, M. Skoglundh, B.

Andersson, J. Phys. Chem. B 105 (2001) 6895.

[24] D. Uy, K.A. Wiegand, A.E. O�Neill, M.A. Dearth, W.H.Weber, J. Phys. Chem. B 106 (2002) 387.

[25] P. Broqvist, I. Panas, E. Fridell, H. Persson, J. Phys.

Chem. B 106 (2002) 137.

[26] P.J. Schmitz, R.J. Baird, J. Phys. Chem. B 106 (2002) 4172.

[27] J.P. Breen, M. Marella, C. Pistarino, J.R.H. Ross, Catal.

Lett. 3–4 (2002) 123.

[28] D. James, E. Fourr�ee, M. Ishii, M. Bowker, Appl. Catal. B:Environ., in press.

[29] R.A. Bennett, M. Ishii, C.L. Pang, M. Bowker, unpub-

lished results.

[30] M. Bowker, S. Poulston, R.A. Bennett, P. Stone, A.H.

Jones, S. Haq, P. Hollins, J. Mol. Catal. A 131 (1998)

185.

[31] F.M. Leibsle, Surf. Sci. 514 (2002) 33.

[32] David R. Lide (Ed.), Handbook of Chemistry and Physics,

73rd ed., CRC Press, pp. 12–29.

[33] G. Ertl, H. Kn€oozinger, J. Weitkamp (Eds.), Handbook of

Heterogeneous Catalysis, Wiley-VCH, New York, 1997.

[34] U. Diebold, J.F. Anderson, K-On Ng, D. Vanderbilt, Phys.

Rev. Lett. 77 (1996) 1322.

[35] H. Onishi, K. Fukui, Y. Iwasawa, Bull. Chem. Soc. Jpn. 68

(1995) 2447.

[36] David R. Lide (Ed.), Handbook of Chemistry and Physics,

73rd ed., CRC Press, pp. 12–109.

[37] G.A. Haas, A. Shih, Appl. Surf. Sci. 8 (1981) 145.

[38] S. Matsumoto, Cattech 4 (2000) 102.

[39] Handbook of X-ray Photoelectron Spectroscopy, Perkin-

Elmer Corporation, 1992.

nox storage in model pt/ba nsr catalysts: fabrication and reactivity of bao nanoparticles on...

Documents