Catalytic oxidation of CO by platinum group metals: from ... files/406_Electrochimica Act… · Catalytic oxidation of CO by platinum group metals: from ultrahigh vacuum to elevated

Catalytic oxidation of CO by platinum group metals: from ultrahighvacuum to elevated pressures

A.K. Santra, D.W. Goodman *

Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842-3012, USA

Received 10 October 2001; received in revised form 15 February 2002

Abstract

CO oxidation over platinum group metals has been investigated for some eight decades by many researchers and is considered to

be the best understood catalytic reaction. Nevertheless, there has been a renewed interest in CO oxidation recently because of its

technological importance in pollution control and fuel cells. Removal of COx from automobile exhaust is accomplished by catalytic

converters using supported Pt, Pd and Rh catalysts. Catalysts are used in fuel cells to remove traces of COx from the H2 feed gas to

the few ppm level necessary for their efficient operation. Efforts have been made in our laboratory to understand the adsorption of

CO and the kinetics of CO-oxidation on both single crystals and supported metal catalysts over a wide temperature (100�/1000 K)

and pressure (1�/10�7�/10 Torr) range. By comparing the results of single crystals, model supported catalysts, and supported

technical catalysts the relationship between particle size and catalytic activity can be better understood. Also discussed is CO

oxidation on model supported Au catalysts, a promising new candidate for low temperature CO oxidation. # 2002 Published by

Elsevier Science Ltd.

Keywords: CO oxidation; Catalysts; Automobile exhaust

1. Introduction

Catalytic oxidation of CO over platinum group

metals (Pt, Ir, Rh and Pd) has been the subject of

many experimental and theoretical investigations [1�/46]

since the classic work of Langmuir [47] in 1922 and has

been extensively reviewed [10,48�/50]. Recently, CO

oxidation has attracted renewed attention due to its

technological importance in the area of pollution con-

trol [51] and fuel cells [52,53]. Currently the removal of

CO from automobile exhaust is accomplished by the

oxidation of CO in catalytic converters using supported

Pt, Pd and Rh catalysts. It is well established that for

optimum operation of low temperature fuel cells it is

essential to have a continuous supply of CO-free

hydrogen. Although, proton exchange membrane fuel

cells can tolerate a few ppm level of CO in the hydrogen

stream, alkaline fuel cells require CO-free hydrogen. The

conventional hydrogen production technologies such as

steam reforming, partial oxidation and autothermal

reforming of hydrocarbons produce large amounts of

CO as a by-product [52,53]. Therefore, it is extremely

important to have a CO oxidation catalyst with very

high efficiency and one that can preferentially oxidize

CO for the production of CO-free hydrogen stream.

Numerous adsorption and kinetic studies on single

crystals and supported metal catalysts have been re-

ported in the last two decades from our laboratory as a

function of O2 and CO partial pressure from ultrahigh

vacuum (UHV) to elevated pressures (10 Torr) over a

wide temperature range of 100�/1000 K [1�/6,10�/12,54�/

59]. Although the CO oxidation reaction is the best

understood among the industrially important catalytic

reactions, there are many complex aspects to be resolved

with respect to a unified reaction mechanism.

Historically, gold is chemically inert compared with

the other Pt group metals, however, recently it has been

shown that Au, deposited as finely dispersed, small

particles (B/5 nm diameter) on reducible metal oxides

like TiO2, is an excellent catalyst for CO oxidation at

relatively low temperatures [54,55,60�/63]. Furthermore,

* Corresponding author. Tel.: �/1-979-845-6822; fax: �/1-979-845-

0214

E-mail address: [email protected] (D.W. Goodman).

Electrochimica Acta 47 (2002) 3595�/3609

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this catalytic activity has been shown to be a critical

function of the Au cluster size.

In this article, we will compare CO adsorption and

oxidation on single crystals with model supportedcatalysts of Pt, Ir, Rh, Pd and Au. Primary emphasis

will be given to the results obtained in our laboratory

over wide temperature (100�/1000 K) and pressure

ranges (1�/10�7�/10 Torr). The goal of this work is

an understanding of the effects of temperature, pressure

and particle size on CO oxidation that spans the

material and pressure ‘gaps’ between ‘real world’

catalysis and ‘surface science’.

2. Experimental

The UHV systems used for this work were equipped

[64] with Auger electron spectroscopy (AES), a quadru-

pole mass spectrometer for temperature programmed

desorption (TPD), low energy electron diffraction

(LEED), scanning tunneling microscopy/spectroscopy(STM/STS) and an Ar ion sputter gun, contiguous to a

high-pressure reaction chamber. The single crystal

samples were mounted on a retractable manipulator,

allowing the sample to be moved between the two

chambers in situ. Pt(100), Ir(110) and Pd(110) crystals of

0.92 cm in diameter and 0.11 cm in thickness were used

for the experiments. The Ir(111) sample was elliptical in

shape and 0.75�/0.55�/0.03 cm3 in size. The sampleswere heated resistively by two high purity (0.051 cm)

tungsten leads spot-welded to the back of the crystal;

sample temperature was measured by a 0.08 cm

chromel-alumel thermocouple spot-welded to the sam-

ple edge.

AES was used to check the cleanliness of the samples.

In addition, the Pt(100) sample was cleaned of carbon

by O2 adsorption/desorption and of Si and Ca impu-rities by high temperature oxidation (1123 K, 1�/10�7

Torr O2) and Ar sputtering. The Pd crystal was cleaned

in the reactor with 8 Torr of CO and 8 Torr of O2 and

heating to 600 K for 1�/2 min. One to three cycles of this

treatment produced a clean surface. Large carbon

impurities were cleaned from the Ir samples by oxida-

tion in 1�/10�5 Torr O2 at 1000 K for 5�/10 min,

followed by a 3 min anneal to 1600 K. Small traces ofcarbon were removed by reaction in 8 Torr O2 and 4

Torr CO for 2 min at 600�/625 K, followed by a brief

flash to 1600 K. The Pt sample was cleaned by

sputtering at 1100 K in 5�/10�5 Torr of Ar for 30

min (1 kV beam energy) mainly to remove Si and Ca

impurities. This treatment was followed by heating in

0.1 Torr of O2 at 1100 K for 30 min to remove traces of

carbon and then annealing at 1300 K. Repeated cyclesof this procedure produced a clean Pt surface, which

could not be oxidized under UHV conditions at high

temperature. Rh crystals were cleaned by oxidation (2�/

10�7 Torr O2 at 1300 K) followed by annealing at 1500

K. The Ru(0001) crystal was cleaned by oxidation (2�/

10�7 Torr O2 at 1450 K for 3 min) followed by

annealing at 1500 K.

The IR cell with CaF2 window is connected to the

UHV chamber through a double differentially pumped

sliding seal. The configuration of the elevated pressure

cell is similar to that described by Campbell et al. [65].

This arrangement allows IR experiments in the pressure

range of 10�8�/103 Torr and also provides convenient

access to the sample without opening the UHV cham-

ber. The pressure in the IR cell was determined using an

ionization gauge and a capacitance manometer at their

respective working pressure ranges.

TiO2(110) single crystals (Commercial Crystal La-

boratories) were used in these studies mainly due to their

suitability for atomically-resolved STM and STS experi-

ments. The n-type semiconductor form, sufficiently

conductive for STM and electron spectroscopic mea-

surements, was prepared by cycles of Ar� sputtering

and annealing to 700�/1000 K. Deposition of the metal

was typically accomplished by resistive evaporation of

high-purity metal wire wrapped around a W or Ta

filament in vacuum. Such dosers provide an excellent

means of obtaining a clean and stable metal flux after

thorough outgassing. By controlling the filament cur-

rent, doser to substrate distance and the substrate

temperature, fine control can be exercised over cluster-

size and density.

Gas chromatography with flame ionization detection

(FID) was used to analyze the reaction products in

which CO and CO2 were catalytically converted to

methane before analysis. Rates of reaction are expressed

as turnover frequencies (TOF), defined as the number of

CO2 molecules produced per active metal site per

second. For Pd, Rh or Ir, the entire crystal (front,

back and edge) were included in determining the total

number of sites; for Pt, only the front face was included,

as the back and edge of the crystal were not subjected to

the sputter cleaning procedure. Research grade CO

(99.99%) and O2 (99.995%) were supplied by Matheson.

The CO was further purified by slowly passing it

through a molecular sieve trap at 77 K. No metal

carbonyls (e.g. Ni(CO)4 were detected in any experiment

in post-reaction AES analysis.

The experimental procedure has been described in

detail elsewhere [64]. Briefly, after cleaning, the sample

was moved to the reactor and charged with reactants.

Most experiments were performed with 16 Torr of CO

and 8 Torr O2. The sample is heated to the desired

temperature for a specified time, the products are then

allowed to mix for 15 min and then an aliquot of the

product mixture was analyzed by GC. The reactor was

then evacuated, and the sample returned to the UHV

chamber for post-reaction analysis.

A.K. Santra, D.W. Goodman / Electrochimica Acta 47 (2002) 3595�/36093596

3. CO adsorption

Carbon monoxide adsorption under UHV conditions

especially on transition metal surfaces has been studied

extensively [10,48�/50,66�/68]. Three different types of

adsorption sites have been observed, in general, namely

a-top, bridge-bound and 3-fold hollow. The interaction

of a CO molecule with a transition metal can be

understood by a simple donor�/acceptor model first

described by Blyholder [69].

Both linear and bridge bonded CO has been observed

on Rh(100) for the entire coverage and temperature

range of 90�/300 K [70]. A transition from a well ordered

c (2�/2) structure at uCO�/0.5 ML to a p (4�2�/

4�2)R458 coincidence structure at uCO�/0.7 ML has

been observed by LEED and infrared reflection absorp-

tion spectroscopy (IRAS). On Pt single crystal surfaces,

however, all three types of CO have been observed

depending on the CO coverage (uCO), adsorption

temperature, pressure and crystal face [20,35,45,67,71�/

74]. Three different desorption features of CO have been

observed on the Ir(110) surface [75], whereas the

corresponding IR spectra showed a continuous increase

in the C�/O stretching frequency from 2001 cm�1 at the

lowest coverage to 2086 cm�1 at a coverage (uCO) near

0.86 ML. The species has been identified as the a-top

type and three different ordered structures at uCO�/

0.33, 0.5 and 0.80 ML have been identified by LEED.

A regular increase in the C�/O frequency in the IRAS

data along with an increase in the CO desorption

temperature with respect to the increase in coverage

(uCO) have been observed and understood to be due to

lateral CO�/CO repulsion. On the reconstructed (5�/1)

as well as on the (1�/1) Ir(100) surface, formation of a

c(2�/2) overlayer structure has been reported [76]. In

this article, we would like to concentrate on the

adsorptive behavior of CO over a wide pressure range

from UHV to 10 Torr and a temperature range of 100�/

1000 K using Pd as an example. We will also discuss the

effect of particle size on the catalytic activity.

CO adsorption on Pd(100) and Pd(111), studied under

UHV conditions using an array of surface science

techniques, exhibits structure sensitivity depending on

the face of the Pd crystal. For example, on Pd(111), at a

CO coverage (uCO)B/1/3 ML, CO adsorbs at 3-fold

hollow sites, with a structure corresponding to (�3�/

�3)R308 as revealed by LEED and a carbon�/oxygen

stretching frequency of 1850 cm�1 [77,78]. However at a

coverage (uCO) of 0.5 ML, a c (2�/2) LEED pattern with

a C�/O stretching frequency of 1918 cm�1 has been

observed and is assigned to a CO adsorbed onto bridge-

bound sites. At very high coverages, a-top and bridge-

bound CO co-exist yielding a (2�/2) LEED pattern with

C�/O frequencies near 1951 and 2097 cm�1, respec-

tively. In contrast, only bridge-bonded CO has been

Fig. 1. IR spectra of CO on Pd(111) at Pco�/10.0 Torr as a function of

temperature. The spectra were collected from high temperature to low

temperature [85a,85b].

Fig. 2. IR spectra of CO on Pd(111) at Pco�/1.0�/10�6 Torr as a

function of temperature. The spectra were collected from high

temperature to low temperature [85a,85b].

A.K. Santra, D.W. Goodman / Electrochimica Acta 47 (2002) 3595�/3609 3597

observed on Pd(100) even up to uCO�/0.5 ML with a

c (2�2�/�2)R458 LEED pattern [79�/84]. In this study,

for uCO�/0.5 ML, a uniaxial compression takes place

resulting in the formation of incommensurate over-

layers. As a consequence of this compression of the

CO overlayer, a sharp decrease in the heat of adsorption

has been observed at uCO�/0.5 ML due mainly to lateral

repulsive interactions.

3.1. Effects of temperature and pressure

A series of IR spectra of adsorbed CO on Pd(111)

obtained at a CO pressure of 10 Torr is shown in Fig. 1

[85a,85b]. At high temperatures (i.e. low uCO) only 3-fold hollow sites are occupied. With decreasing tem-

perature (i.e. increasing uCO) the population at the

bridging sites increases. At temperatures near 200 K, a

sudden change in the spectrum is apparent, correspond-

ing to a transition from bridging sites to a mixture of a-

top and 3-fold hollow sites. The bridging0/a-top/3-fold

hollow transition takes place within a very narrow

temperature range while the transition from the 3-foldhollow to bridging sites is much less defined. The narrow

spectral line-width, due to the 3-fold hollow sites,

indicates the presence of a very well ordered CO

adsorbate layer, and has been confirmed by a very

sharp (2�/2) LEED pattern.

In addition to the 10.0 Torr data shown in Fig. 1,

isobaric adsorption data were collected at every decade

from 10�7 to 10 Torr. In each of these isobaric series,adsorption site progression was identical; the only

apparent difference was the temperature at which the

adsorption site transformation took place. As a repre-

sentative case, 1�/10�6 Torr data are shown in Fig. 2

[85a,85b]. Transition from 3-fold hollow to bridging

sites can clearly be seen as the C�/O stretching frequency

changes from 1855 to 1900 cm�1. As the population ofCO increases the frequency shifts toward higher values

and the peak becomes sharper. This blue-shift has been

attributed to repulsive lateral interactions between CO

molecules. The frequencies of the a-top and 3-fold

hollow sites are 2110 and 1895 cm�1. The highest

frequency observed for the bridge-bound CO was 1962

cm�1.

An equilibrium phase diagram for CO on Pd(111) isshown in Fig. 3, based on the adsorption data collected

at the temperature range 90�/1000 K and a CO pressure

range of 10�7�/10 Torr [85a,85b]. It is important to

mention that this phase diagram is valid only at

equilibrium conditions. This is the case for the

bridging0/a-top/3-fold hollow phase transition. Non-

equilibrium CO adsorption can also occur at low

adsorption temperatures (crosshatched region of Fig.3). In this region appropriate adsorption conditions

have to be used to ensure a fully equilibrated adsorbate

layer. This equilibrium phase diagram illustrates that

low pressure/low temperature adsorption information

can be extrapolated into the high pressure/high tem-

perature regime provided that certain initial adsorption

conditions are used.

In Fig. 4 two series of IR spectra of CO adsorption atPd(100) surface are shown at CO pressures of 1�/10�6

and 1 Torr [85a,85b], respectively. In agreement with

previous studies [83,84] only bridge-bound CO was

detected near the C�/O stretching frequency of 1895

cm�1 at the lowest coverage. The vibration frequency

that corresponds to a coverage near 0.5 ML was blue-

shifted to 1950 cm�1, which is further blue-shifted to

1995�/1998 cm�1 as the coverage is increased to 0.8 ML.

3.2. Particle size effects

As we have seen in the previous section CO adsorp-

tion behavior changes dramatically with respect to the

structure of the crystal surface indicating particle size

effects. In Fig. 5, IRAS data for CO adsorbed on Pd/

Al2O3/Ta(110) model catalysts for uPd�/5.0 and 1.0 ML

as a function of temperature is presented [86]. Theseresults were independent of the direction of temperature

variation provided the samples were annealed in CO to

600 K. On the uPd�/5.0 ML catalyst (Fig. 5a), a peak at

1894 cm�1 was observed at 500 K. With a decrease in

temperature, this peak shifted towards higher frequency,

became broader, and at 300 K split into two peaks at

1984 and 1942 cm�1. In addition, a peak at 2076 cm�1

appeared at 400 K. These three features continued togrow, gradually shifting to higher frequency until, at 150

K, a new peak appeared at 1890 cm�1. These peaks

have striking similarities to those observed on single

Fig. 3. CO�/Pd(111) equilibrium pressure�/temperature phase diagram

showing the different CO adsorption phases: a-top/3-fold hollow,

bridging, 3-fold hollow and non-adsorbed [85a,85b].


crystal Pd(100) and Pd(111) surfaces as discussed in the

previous section. The broad peak corresponding to 1894

cm�1 at 500 K can be attributed to a combination of 3-

fold hollow and bridge-bound CO species, shifting

Fig. 4. IR spectra of CO on Pd(100) as a function of sample temperature at CO pressures of (a) 1.0 Torr and (b) 1�/10�6 Torr [85a,85b].

Fig. 5. Temperature dependent IR spectra of CO adsorbed on uPd�/5.0 and 1.0 ML Pd/Al2O3/Ta(110) catalysts at 1�/10�5 Torr [86].


towards exclusively bridge-bound species with decreas-

ing temperature. At 300 K, the features at 1984 and 1942

cm�1 are assigned to contributions from bridging CO

from Pd(100) to Pd(111) facets, respectively. The feature

at 2076 cm�1 and 400 K is associated with an a-top

species on �111� facets. Near saturation coverage, the

peak at 1998 cm�1 corresponds to a bridging species on

�100� facets while the features at 2108, 1960 and 1890

cm�1 correspond to a-top, bridging and 3-fold hollow

sites, respectively, on �111� facets.

The results obtained for a uPd�/1.0 ML catalyst (Fig.

5b) differ significantly from the uPd�/5.0 ML sample.

The broad peak near the region of bridge-bound species

never splits cleanly into individual components as is the

case for larger particles. Moreover, no 3-fold hollow

feature is evident at saturation coverage and the ratio of

a-top/bridge intensity is higher compare to the larger

particles indicating that the smaller particles contain a

higher proportion of edge/defects sites. The broader

features observed at saturation coverage for the smaller

particles are due to higher surface curvature on the small

particles and a correspondingly less compressed CO

overlayer assuming roughly hemispherical shape of the

particles. That the types of transitions (bridging0/a-top/

3-fold hollow) occur on the Pd(111) single crystal and,

to a more limited extent, on the uPd�/5.0 ML catalyst

do not occur on the uPd�/1.0 ML particles is consistent

with the reduced CO density on the smaller particles.

The effect of particle size with respect to CO oxidation

will be discussed subsequently.

4. CO oxidation

4.1. Pt, Ir, Rh and Pd single crystals and model supported

catalysts

4.1.1. Steady-state reaction kinetics

The CO2 formation rate as a function of inverse

temperature (1/T ) for Pd, Pt and Ir single crystals is

shown [10] in Fig. 6a and compared with the data

obtained on several supported metal catalysts [38]. Data

obtained on Rh(100) and Rh(111) single crystals are

shown [11] in Fig. 6b. The Pd, Pt and Ir single crystaldata are for a (1:2) O2: CO mixture at a total pressure of

24 Torr, whereas, the Rh single crystal data are for a

(1:1) O2: CO mixture at a total pressure of 16 Torr.

Within this pressure range the reaction rate is zero-order

with respect to total pressure. The TOF for the single-

crystal catalysts traverse four orders of magnitude over

a temperature range of 450�/600 K. Kinetic measure-

ments over such a wide temperature range with sup-ported catalysts are not possible due to heat and mass

transfer limitations encountered at high temperatures.

Thus a direct comparison between the two types of

catalysts is limited to a relatively small temperature

range. Nevertheless, it is very clear from Fig. 6a and b

that there is excellent agreement between the single

crystal and model supported systems with respect to the

specific reaction rates and apparent activation energies[28,38].

Fig. 7a�/c [10,11] show reaction rate dependence on

CO partial pressure for Pd(110), Ir(111) and Rh(111)�/

Fig. 6. Arrhenius plot of the CO�/O2 specific rates of reaction (TOF) for (a) single crystal (Pd, Ir and Pt) and their supported catalysts and (b) for Rh

single crystals [10,11].


Rh(100), respectively. In these experiments the CO

pressure was allowed to vary keeping the oxygen

pressure fixed. In the case of Pd and Ir, up to a CO:

O2 ratio of approximately 1:12 the reaction was first-order with respect to CO partial pressure; whereas below

this ratio the reaction became negative-first-order with

respect to CO. For Rh(111) and Rh(100) such a role-

over with respect to the partial pressure of CO has been

observed with a maximum activity occurring approxi-

mately at a O2: CO ratio of 30:1. However, at a lower

partial pressure of O2 (8 Torr), for Pd(110), Rh(100) and

Rh(111), the reaction order has been observed to benegative-first-order with respect to CO partial pressure

(Fig. 3a and c, respectively). It is noteworthy (Fig. 7c)

that the rate of CO2 formation on Rh(100) is higher

compared with that on Rh(111) at any given CO partial

pressure indicating that the reaction may be structure

sensitive.

For Pd, Ir and Rh the reaction order with respect to

the partial pressure of O2 (Fig. 8a�/c) is generallypositive-first-order [10,11]. Above an O2:CO ratio of

12:1, the same ratio at which the CO order changes from

negative to positive, the reaction rate begins to decrease,

becoming negative-order in O2 partial pressures. For

example, at extremely high O2:CO ratios, the order of

reaction for O2 is �/0.79/0.2 on Ir(111). For Pd,

changing the CO partial pressure at a constant tem-

perature only shifts the curve, i.e. the maximum rate andthe ratio at which the rate turns over, remain un-

changed. Whereas, for Ir the O2:CO ratio at which the

rate varies from first-order oxygen dependency changes

somewhat (from 12:1 to 16:1) as does the order of the

reaction. The reaction rate on both Rh surfaces (Fig. 8c)

increases linearly at low oxygen partial pressures. This

first-order dependence is altered at high oxygen pres-

sures where the rates roll over and become negative-order with respect to the partial pressure of oxygen.

Note that this rollover occurs at different oxygen partial

pressures on the two single crystal surfaces indicating

the possibility of structure sensitivity.

On Pt(100), the order of the reaction with respect to

CO partial pressure changes with temperature (Fig. 9)

[10]. In the range where the activation energy is

changing (425�/490 K), the reaction order varies from0.0 to 0.6. Above 500 K the order becomes more

negative and rapidly approaches negative-first-order.

The reaction never becomes positive-first-order with

respect to CO partial pressure even at an O2: CO ratio of

200:1, as observed for Pd, Ir and Rh.

The Pt(100) surface shows (Fig. 10) [10] only positive-

first-order behavior with respect to the partial pressure

of oxygen. The range of O2:CO ratios studied at a giventemperature was limited to TOF’s where there was

differential conversion (B/5%) of CO. No decrease in

the order of reaction is observed for O2:CO ratios of 1:5

to 150:1, and temperatures from 475 to 650 K, indicat-Fig. 7. CO partial pressure dependence at constant oxygen pressure

and temperature: (a) on Pd(110), (b) on Ir(111) and (c) on Rh(111) and

Rh(100) [10,11].


ing that under these experimental conditions no strongly

bound, deactivating oxygen species is present. In order

to form a similar species on Pt, much higher O2 pressure

and/or temperatures would be necessary, conditions not

accessible in our experiments.

4.1.2. Structure sensitivity and particle size effects

Although it appears from the data in Fig. 1 that the

CO-oxidation reaction is structure insensitive, it should

be noted that the single crystal rates are compared withthe least dispersed supported catalyst. Very recently,

under UHV conditions structure sensitivity has also

been observed by Niemantsverdriet and co-workers [90]

on Rh single crystal surfaces wherein the CO2 deso-

rption temperature differs by �/50K on Rh(100) and

Rh(111) surfaces. The Rh(100) surface shows signifi-

cantly higher reaction rates compared with that on the

Rh(111) surface. The other difference between the two

systems is that on Rh(100) all CO is oxidized to CO2,

whereas on Rh(111) a fraction of the CO desorbs. The

reason for such high activity and selectivity on Rh(100)

towards CO2 formation is assumed to be due to the

surface reaction step, COads�/Oads0/CO2(g), being in-

trinsically faster on Rh(100) than on Rh(111). In

contrast to the high-pressure data (Fig. 7c and Fig.

8c), the reaction exhibits first order kinetics with respect

Fig. 8. O2 partial pressure dependence at constant CO pressure and temperature: (a) on Pd(110), (b) on Ir(111) and (c) on Rh(111) and Rh(100)

[10,11].


to both the partial pressure of CO and O2 under UHV

conditions, however, the structure sensitivity is consis-

tent with the high-pressure data (Fig. 7c and Fig. 8).

Dramatic structure sensitivity has been observed on

Ir/SiO2 catalyst [38] (Fig. 11). It is clear from Fig. 11

that larger particles are more active for CO oxidation

and the data more comparable to the single crystal data.

The data for the larger particles were calculated from

the dispersion values obtained by Cant and co-workers

[38]. It is noteworthy that the single crystal data in Fig.

11 corresponds to the supported data extrapolated to a

particle size of 40 nm. This is close to the ‘effective’

particle size of the single crystals used in this study,

Fig. 9. CO partial pressure dependence as a function of temperature at constant oxygen pressure on Pt(100). (a) The Arrhenius plot for CO oxidation

on Pt(100) is shown to illustrate the temperature regimes in which (b) the CO pressure dependence data were obtained [10].

Fig. 10. O2 partial pressure dependence at constant CO pressure and

temperature on Pt(100). In all cases the reaction order is 1.09/0.1 [10].

Fig. 11. Effect of particle size on the CO2 formation rate on Ir/SiO2

and Ir single crystal catalysts [38].


taking into account step densities and edge effects. The

decreasing activity of the reaction with Ir particle size

can be understood as due to a preferential poisoning of

the active sites by carbon formed by CO dissociation, a

competing process to CO oxidation under such high-

pressure conditions. This reaction would be expected to

occur more rapidly on defect sites and step edges,

present at much higher concentration on the smaller

particles.

CO oxidation has also been investigated over Pd/

SiO2/Mo(100) model catalyst [56,86]. The reaction

conditions for the catalysts were 10 Torr CO, 5.0 Torr

O2, and reaction temperatures in the range 540�/625 K.

The conversions were maintained at less than 50% and

were measured by monitoring the pressure decrease in a

static reactor of known volume (750 cm3). The average

cluster sizes shown in Fig. 12 were determined by CO

TPD, O2 TPD and ex situ STM/AFM and all were in

good agreement. The specific reaction rates were some-

what higher for the model catalysts than the high-

surface-area catalysts, but the activation energies are

remarkably similar. There was no noticeable depen-

dence of the CO2 formation rate on the Pd cluster size,

indicating that CO oxidation over Pd/SiO2 is structure

insensitive.

Structure sensitivity has also been observed for Pt.

Yates and co-worker [91�/93] using IRAS and TPD have

shown that on Pt(335) surface dissociation of CO and

O2 occurs preferentially at step sites. It also has been

observed that CO adsorbed at �111� terraces is more

active compared with CO at �100� step sites whereaschemisorbed oxygen atoms at step sites is more active

compared with CO adsorbed at terraces. Using isotopi-

cally labeled 13C18O molecules, it has been shown that

the oxidation rate at step sites is twice the oxidation rate

at terrace sites. However, recent STM studies of CO

oxidation on Pt(111) have shown that the reaction

occurs exclusively at the boundaries between (2�/

2)Oads and c(4�/2)COads domains when co-adsorbedunder UHV condition [17,87�/89]. Therefore, to increase

the reactivity of the surface it is necessary to increase the

interface between the Oads and COads domains, i.e. to

increase the coverage of Oads.

4.1.3. CO oxidation versus metal�/oxygen bond energy

The oxidation rate of CO under steady-state condi-

tions on various Pt group metals are compared in Fig.

13 [94]. The temperature of the reaction was chosenspecifically as 793 K in order to avoid surface site

blocking due to CO adsorption; the pressure of CO and

O2 was 1�/10�7 Torr. From the data of Fig. 13 it is

clear that the most active metals (Pd, Pt, Ir and Rh) for

CO oxidation have M�/O bond energies within the range

320�/390 kJ mol�1. In the case of metals having M�/O

bond energies less than 320 kJ mol�1, e.g. Ag or Au, the

rate-determining step for CO oxidation is the adsorptionand dissociation of oxygen. On the other hand, for

metals having M�/O bond energies larger than 390 kJ

mol�1, the rate-determining step is the reaction between

COads and Oads. In other words those metals on the right

hand side of Fig. 13 have a higher tendency to form

stable oxides. In fact, on Rh(111) and (100) single

crystals, high O2:CO ratios result in a decrease in the

overall rate and a change from positive-order in oxygento negative-order. This change was directly correlated

with the formation of an oxide-like species, as deter-

mined by post-reaction AES and TPD [10,11]. Pd(110),

Ir(111) and Ir(110) exhibit partial pressure dependence

and high oxygen pressure behavior similar to Rh. For

Rh, the formation of a near surface oxide (probably

Rh2O3 [10,11] which is much less active) is responsible

for the deactivation. The similar behavior of Pd and Irsuggests a similar deactivation mechanism. In contrast,

on Ru the oxide was found to be substantially more

active than the clean surface, and the reaction order in

oxygen pressure increases by approximately 3-fold

[9,13,80,95].

4.1.4. Reaction mechanism

The Langmuir�/Hinshelwood reaction between COads

and Oads is well established as the dominant reaction

mechanism for conditions where CO is the primary

surface species [48,49]. This mechanism has been con-

Fig. 12. CO oxidation with O2 over model Pd/SiO2/Mo(100) and a

conventional 5% Pd/SiO2 catalyst. Reaction conditions were PTorr�/

0.5 Torr and CO�/O2�/0.2 [56,86].

Fig. 13. Dependence of the rate of CO oxidation on metals on the

oxygen bond energy EM�O (793 K, PO2�/PCO�/10�7 Torr) [94].


firmed by numerous UHV studies of the co-adsorptionof reactants, transient kinetic studies, and steady-state

kinetics [1�/46]. The reaction steps can be written as

CO(g) 0 COads (1)

O2(g) 0 2Oads (2)

COads�Oads 0 CO2(g) (3)

where the recombinative desorption of adsorbed Oads

atoms (reverse of reaction 2) and the dissociative

chemisorption of CO2 (reverse of reaction 3) are

neglected. It is assumed that CO is the dominant surfacespecies [96]. Considering the reaction steps (1�/3) and

using the above assumptions, an approximate rate

expression, originally proposed by Langmuir [47] for

Pt can be derived as:

d[CO2]=dt�k exp (�Edes;CO=F )PO2=PCO (4)

where the reaction rate is independent of total pressure,

first-order in O2 pressure and negative-first-order with

respect to the CO pressure. The rate of the reaction is

then governed by the desorption of CO or the lifetime of

CO (tCO) on the surface [48], depending on the reactiontemperature, whereas the pressure dependence simply

reflects the competition for adsorption sites between

oxygen and CO. It has been found [10] that the kinetics

on Pd, Ir and on Pt at high temperatures, are consistent

with this model in that the pressure dependencies

predicted by equation 4 are observed. In addition, a

correlation of the activation energies between supported

and single crystal data, and among different single-crystal planes [28,38] reflect the fact that the binding

energy of CO does not vary greatly among these metal

catalyst surfaces.

4.1.5. Angle-resolved temperature programmed

desorption

In search of the possible reaction mechanism of CO

oxidation on both polycrystalline and Pt(111) surfaces,angle resolved reactive temperature programmed deso-

rption (RTPD) has been performed using time of flight

(TOF) mass spectrometry [32,97�/99]. The results show

dramatic angular and velocity dependence of CO2

desorption from the reaction of CO and oxygen co-

adsorbed on a Pt(111) surface at 100 K. The velocity

integrated desorption spectra [99] (Fig. 14) show four

different peaks (a, b3, b2 and b1) for CO2 formation at145, 210, 250 and 330 K, respectively. These feature

have been attributed to four different reaction mechan-

isms operating at various temperatures depending upon

the relative binding of oxygen and the geometric

arrangement and coverage of the adsorbed species.

Although the precise origin of b3 and b2 processes are

not clear at present, the a-CO2 formation temperature

coincides with that of the molecular O2 desorption. Theb1-CO2 formation is most likely the mechanism pro-

posed in the previous section due to the reaction at the

overlapping regions of COads and Oads island bound-

aries. Very interesting oscillatory behavior of the CO

oxidation reaction has been observed on Pt single crystal

under UHV conditions [15,16,100�/102], however, this

behavior is beyond the scope of the present article.

4.2. CO oxidation on Au

In the bulk form, Au is known to be chemically inert

compared with the other Pt group metals. Howeverrecently it has been shown that Au clusters, deposited as

finely dispersed, small particles (B/5 nm diameter) on

reducible metal oxides like TiO2, Fe2O3 and Co3O4,

dramatically enhance the rate of a number of indust-

rially important reactions. Reactions catalyzed by Au

particles on TiO2 supports are CO oxidation, hydro-

genation and partial oxidation of hydrocarbons and the

selective oxidation of higher alkenes [54,55,60�/63]. Ithas also been observed that catalytic activity of these

catalysts is a function of cluster size.

4.2.1. Characterization of the Au clusters by STM and

STS

The constant current STM micrographs in Fig. 15

show [103] changes in the clusters with respect to the

amount of Au deposited. At a relatively low coverage of

Au (uAu�/0.1 ML) hemispherical 3D clusters are

observed with diameters of 2�/3 nm and heights of 1�/

1.5 nm. Interestingly the clusters mainly grow along the

step edges. Well-dispersed quasi-2D clusters, having aheight of 0.3�/0.6 nm and a diameter of 0.5�/1.5 nm, can

be seen on the terraces. With increasing Au coverage

(uAu), the clusters steadily grow larger while the increase

Fig. 14. Reactive temperature programmed desorption (RTPD) of

CO2 as a function of emission angle after predosing 1.5�/1015 cm�2

molecules of O2 and 2.7�/1015 cm�2 molecules of CO successively at

Ts�/100 K [99].


in cluster density is minimal. However, even at uAu�/4.0

ML, some portions of the TiO2 substrate are still visible.

In Fig. 16A, the constant current STM micrograph of

Au (uAu�/0.25 ML) deposited onto single crystal

TiO2(110)�/(1�/1) [55,103] is shown. The metal deposi-

tion was performed at 300 K, followed by annealing to

850 K for 2 min to stabilize the clusters. In Fig. 16A only

the Ti cations are visible; whereas the O2� anion are not

seen. The inter-atomic distance between the �001� rows

is �/0.65 nm, which can be observed along the terraces

corresponding to the length of the unit cell along the

[110] direction of the unreconstructed TiO2(110)�/(1�/

1). Three-dimensional Au clusters, imaged as bright

protrusions, have average diameters of �/2.6 nm and

heights of �/0.7 nm (corresponding to 2�/3 atoms thick)

and are preferentially nucleated at the step edges. Quasi-

two-dimensional clusters are characterized by heights of

1�/2 atomic layers. Previous studies have shown that the

Au clusters upon annealing form large microcrystals

with well-defined hexagonal shapes.Fig. 16B shows STS taken over various clusters on the

surface, where the tunneling current (I) as a function of

bias voltage (V) across the STM tip is measured. The I�/

V curves correlate with the Au cluster size on the TiO2

Fig. 15. A set of 50�/50 nm2 STM images (2.0 V, 1.0 nA) of TiO2(110)�/(1�/1) with different Au coverage (uAu): (A) 0.10 ML, (B) 0.25 ML, (C) 0.50

ML, (D) 1.0 ML, (E) 2.0 ML and (F) 4.0 MLE. With increasing coverage, Au clusters grow and gradually cover the surface [103].


surface. The length of the observed plateau at the zero

tunneling current is a measure of band-gap (along the

bias voltage axis) of electrons tunneling between the

valence and conduction band of the cluster and tip. The

electronic character of these clusters vary between that

of a metal and a non-metal depending on their size.

With an increase in size the clusters gradually exhibit

metallic character with an enhanced density of states at

the Fermi level. Note that clusters of 2.5�/0.7 nm2 size

have a larger band gap than that for a cluster 5.0�/2.5

nm2 in size. Smaller clusters have a non-metallic

character resulting in significant band-gap and a re-

duced density of states near the Fermi level. A similar

metal to non-metal transition with respect to cluster size

has also been observed for Fe clusters deposited on

GaAs(110) [104]. Very small clusters then are non-metallic and exhibit electronic and chemical properties

unlike those of the corresponding bulk metal.

4.2.2. Particle size effects

Interestingly, a marked correlation between the clus-

ter size and catalytic activity has been observed for CO

oxidation over Au/TiO2 system [54,55,60�/63]. Studies

have been carried out on Au/TiO2/Mo(100) as well as on

Au/TiO2(110)�/(1�/1) for comparison. Fig. 17a and bshow plots of CO oxidation activity (TOF) at 350 K as a

function of Au cluster size supported on TiO2(110)�/

(1�/1) and TiO2�/Mo(100) substrates, respectively.

Fig. 16. (a) A CCT�/STM image of a Au (uAu�/0.25 ML) deposited

onto TiO2(110)�/(1�/1) prepared just prior to a CO:O2 reaction. The

sample had been annealed to 850 K for 2 min; (b) STS data acquired

for Au clusters of varying sizes on the TiO2(110)�/(1�/1). An STS of

TiO2 substrate, having a wider band-gap than the Au cluster, is also

shown as a point of reference [55].

Fig. 17. CO oxidation TOFs as a function of the Au cluster size

supported on TiO2. (A) The Au/TiO2 catalysts were prepared by a

precipitation method, and the average cluster size was measured by

TEM, 300 K. (B) The Au/TiO2 catalysts were prepared by vapor-

deposition of Au on planner TiO2 films on Mo(100). The CO�/O2

mixture was 1:5 at a total pressure of 40 Torr, 350 K [55].

Fig. 18. The specific activity for CO conversion as a function of

reaction time at 300 K on a model Au/TiO2/Mo(100) catalyst. The Au

coverage (uAu) was 0.25 ML, corresponding to an average cluster size

of �/2.4 nm [103].


These results show similarities in the structure sensitivity

of CO oxidation with a maximum activity evident at �/3

nm Au cluster size on both TiO2 supports. For each

catalyst, the activity and the selectivity of the supportedAu clusters are markedly size-dependent. Although the

TiO2 supported Au catalysts exhibit a high activity for

the low-temperature CO oxidation, rapid deactivation

was observed as a function of reaction time. Fig. 18

shows a plot of TOF versus time for CO oxidation at

300 K on a Au(uAu�/0.25 ML)/TiO2/Mo(100) model

catalyst. The model catalyst, which exhibited a high

initial activity, deactivated after a CO�/O2 (1:5) reactionof �/120 min at 40 Torr. This deactivation is due to

agglomeration of the Au clusters with reaction time, as

detailed by post-reaction STM measurements. The STM

data clearly demonstrate [54,55,60�/63] that under reac-

tion conditions the Au clusters ripen via an Ostwald

mechanism; i.e. large clusters grow at the expanse of

small ones leading to a bimodal size distribution. This

ripening mechanism depends upon the strength of thecluster�/support interaction as well as gas pressure. The

TOF of CO oxidation, which maximizes with respect to

cluster size, correlates with a metal to non-metal

transition at a particle size of �/3 nm, as revealed by

a detailed STM�/STS investigation. This behavior has

been discussed in the previous section and there is no

unified theory to understand the catalytic activity of the

small gold particles at the moment. However, the factthat CO oxidation activity increases with decreasing

cluster size as long as the particles are metallic suggests

that an overall catalytic activity that depends on both

electronic as well as geometric factors. Electronic factor

means the size-induced changes in the electronic levels of

the small clusters namely electronegativity, charge state

and metallicity leading to changes in the cluster �/CO

and �/O2 interaction strength thereby the overall reac-tion itself. On the other hand, geometric factor leads to

changes in the shape with respect to the changes in the

particles size leading to change in the number of steps,

size of the terraces and facetes.

5. Conclusions

An approach that combines both surface science andtraditional catalytic methodologies has been shown to

be extremely advantageous in bridging the material and

pressure ‘gaps’ between ‘real world catalysis’ and ‘sur-

face science’. Although, it appears that the CO oxida-

tion reaction follows a simple Langmuir-Hinshelwood

mechanism, many complicating factors influence the

overall activity. The example of Pd shows that the

knowledge acquired from single crystal data can be usedto understand the results for model supported catalysts,

including structure sensitivity, reaction order determina-

tion, nature of the bonding of adsorbed molecules, the

optimum reaction temperature, deactivation, etc. Fi-

nally it has been shown that ultra-small gold particles,

unlike bulk gold metal, are promising catalysts for low

temperature CO oxidation.

Acknowledgements

The support of this work by the Department of

Energy, Office of Basic Energy Sciences, Division of

Chemical Sciences, and the Robert A. Welch Founda-

tion is gratefully acknowledged.

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