Chemical Physics Letters 469 (2009) 1–13

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Chemical Physics Letters

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FRONTIERS ARTICLE

New insights into catalytic CO oxidation on Pt-group metals at elevated pressures

Sean M. McClure, D. Wayne Goodman *

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

a r t i c l e i n f o

Article history:Received 2 December 2008In final form 12 December 2008Available online 25 December 2008

0009-2614/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.cplett.2008.12.066

* Corresponding author. Fax: +1 979 845 6822.E-mail address: goodman@mail.chem.tamu.edu (D

a b s t r a c t

Producing a definitive picture of the CO oxidation reaction (CO + O2 ? CO2) on Pt-group metals (Rh, Pd,Pt, and Ru) across the ‘pressure gap’ has proved to be a challenging task. Surface-sensitive techniquesamenable to high pressure environments (e.g. PM-IRAS) have sparked a renewed interest in this reactionunder realistic pressures. Here, we review recent work in our laboratory examining CO oxidation kineticson Pt-group single crystals using PM-IRAS, XPS, and mass spectrometry from low (10�8–10�3 Torr) tohigh (1–102 Torr) pressures. These studies have shown that at both low and high pressures (a) Lang-muir–Hinshelwood kinetics adequately describe CO oxidation kinetics on Pt-group metals (Pt, Pd, Rh)(i.e. there is no pressure gap) and (b) the most active surface is one with minimal CO coverage. Addition-ally, recent investigations of high pressure CO oxidation kinetics on SiO2 film supported Rh particles pre-pared in situ are discussed.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

CO oxidation (CO + O2 ? CO2) on Pt-group metal surfaces is of-ten viewed as an ideal reaction for fundamental investigations ofheterogeneous catalysis. This reaction features many of the funda-mental steps involved in a typical heterogeneous catalytic system:molecular adsorption/desorption of reactants (CO M COads), disso-ciative adsorption of a reactant (O2 ? 2Oads), surface reaction(COads + Oads ? CO2), and reaction inhibition/surface poisoning(oxide formation). As a result, researchers have found this seem-ingly ‘simple’ reaction to be quite complex, and bridging betweenthe low and high pressure kinetic regimes has been a challengingendeavor. Recent developments of surface analysis tools amenableto high pressure environments (sum frequency generation (SFG),polarization modulation infrared absorption spectroscopy (PM-IRAS), and high pressure scanning tunneling microscopy (STM))have enabled researchers to probe the surface state during reactionwithin the high pressure regime in ways not possible previously. Inparticular we wish to focus on two topics of current interest in thisLetter: (1) the applicability of the Langmuir–Hinshelwood mecha-nism at low and high pressures and (2) the nature of the active sur-face phase at high pressures and temperatures, which has been thesubject of recent research and debate. Thus, even though it hasbeen nearly 90 years since the classic investigations of CO oxida-tion on Pt by Langmuir [1], the CO oxidation reaction remainsone of the most investigated reactions in heterogeneous catalysis,with recent research producing insights and raising new questionsabout this well-studied prototype system.

ll rights reserved.

.W. Goodman).

In this Letter, we focus on recent work in our group [2–8] aimedat understanding details of CO oxidation reaction on Pt-group met-als under low (10�8–10�3 Torr) and high (1–102 Torr) reactantpressures. First, a short background of previous CO oxidation ki-netic investigations at low and high pressures will be presented.Second, recent data and current debate surrounding CO oxidationkinetics at high pressure will be discussed. Next, results of recentand ongoing work on CO oxidation on Pt-group single crystals(Rh, Pd, Pt and Ru) will be reviewed. Finally, we conclude with adiscussion of recent and ongoing work examining CO oxidationreaction kinetics on Rh/SiO2/Mo(112) model catalysts.

2. Background

2.1. Low pressure kinetics

Kinetics of CO oxidation on Pt-group metals (Rh, Pt, Pd and Ru)at low pressures (<10�3 Torr) has been well-studied using singlecrystal surfaces coupled with traditional ultrahigh vacuum (UHV)surface science probes [9,10]. Pt, Rh, and Pd surfaces, in general,behave similarly (with some minor subtleties), thus the followingbroad discussion can be regarded as applicable to Pt, Pd, and Rhsurfaces. It is generally accepted that at low pressure conditions,CO oxidation takes place via a Langmuir–Hinshelwood reactionmechanism, whereby co-adsorbed CO and O atoms react to formCO2. This reaction scheme is shown below in Eqs. (1)–(3) [9]

CO ¢K1

K2

COads ð1Þ

O2!K3 2Oads ð2Þ

COads þ Oads!K4 CO2 ð3Þ

Fig. 1. Measurement of CO oxidation rate by monitoring the total pressure change.(a) Total pressure decrease and change in the O2/CO ratio as a function of reactiontime. (b) CO2 formation rate and the change in the partial pressures of O2 and CO asfunction of reaction time. The reaction was carried out over a Pd(110) surface at525 K with an initial O2/CO ratio of 5 and a total pressure of 80 Torr. Data from Ref.[3].

2 S.M. McClure, D.W. Goodman / Chemical Physics Letters 469 (2009) 1–13

CO has been shown to inhibit O2 adsorption more effectivelythan surface oxygen inhibits the adsorption of CO [9–12]. Thus,at low temperatures, CO2 production is inhibited by adsorbed CO,with the CO2 production rate increasing concurrent with increasedCO desorption rate (i.e. higher temperatures). It thus follows thatthe maximum CO2 production rates occur at conditions that pro-duce minimal CO surface coverages [9–15]. The temperature atwhich maximum CO2 production occurs has generally been shownto increase with increasing CO partial pressures. This is directly re-lated to CO inhibition effects, which require increased tempera-tures to facilitate low steady-state CO coverages for optimalreactivity. At higher temperatures (high CO desorption rate), reac-tivity becomes inhibited due to oxygen inhibition of CO adsorptionand to decreased CO surface lifetimes [9,10,16–19]. Thus, the gen-eral findings of low pressure CO oxidation kinetics on Rh, Pd, and Ptmetals can be summarized as follows: (1) the reaction exhibits aLangmuir–Hinshelwood reaction mechanism, (2) the maximumCO2 production occurs on surfaces with minimal COads, (3) COinhibits reaction by inhibiting O2 adsorption and desorption at suf-ficiently low temperatures, and (4) high temperature reactivity de-creases due to CO adsorption inhibition by Oads and a decreasedsurface lifetime of CO.

Low pressure CO oxidation on Ru surfaces is characteristicallydifferent compared to Pd, Pt, and Rh surfaces. Ru at low pressuresexhibits the least activity of all the Pt-group metals [9,20,21]whereas at high pressures, Ru is the most active [22,23]. The higherreactivity of Ru at elevated reactant pressures is attributed to thetendency of the surface to be covered with oxygen atoms more fac-ilely than Rh, Pd, Pt.

2.2. High pressure CO oxidation kinetics

Due to its practical importance in industrial applications (e.g.catalytic converter technology), numerous kinetic studies of COoxidation reactions under high pressure conditions over Pt-groupsingle crystal surfaces [24–26] and supported technical catalystsamples [26–29] have been carried out. It has been shown thatCO + O2 ? CO2 reaction kinetics at high pressures exhibits first or-der dependence on O2 partial pressures and negative order depen-dencies on CO partial pressures at sufficiently low temperatures.Similar to the low pressure findings, this behavior is a direct resultof CO inhibition of O2 adsorption and dissociation. Hence, underCO-dominant conditions, the reaction is rate-limited by COdesorption, and the apparent activation energy (Ea) of the CO oxi-dation reaction is �110 kJ/mol on Pt, Pd and Rh under most con-ditions, a value very close to the corresponding CO desorptionenergies for these metals [9]. Additionally, under CO-dominantconditions, surfaces generally show structure-insensitivity (i.e. lit-tle dependence on the underlying crystal structure), shown by theexcellent agreement between results for supported and singlecrystal catalysts [24–28]. At higher oxygen partial pressures, CO2

formation becomes inhibited by surface oxygen, with Pd and Rhsurfaces deactivating due to oxidation [24,25,30]. Because of itsresistance to oxidation, Pt surfaces require extreme O2 pressuresand temperatures well above typical catalytic conditions [24].Low pressure titration measurements on Pd [19] and Rh [30] sup-ported and unsupported surfaces are also consistent with a re-duced reaction rate on oxidized surfaces. Overall, at relativelylow temperatures, CO inhibition dictates the oxidation kineticsat both low and high pressures. Under CO-dominant conditions,a Langmuir–Hinshelwood mechanism similar to that in Eqs. (1)–(3), has been shown to accurately predict the kinetics over singlecrystal and supported Rh samples within the higher pressure COinhibited reaction regime [26]. At high O2 pressures and tempera-tures, oxide deactivation can play an important role in the reac-tion kinetics.

2.3. Recent findings and open questions: investigations with highpressure surface sensitive techniques

The recent development of high pressure analytical techniques(such as PM-IRAS, SFG spectroscopy, and high pressure STM) hasprovided new methods to investigate CO oxidation under elevatedpressure conditions. In particular, investigators now have a meth-od to probe surface adsorbates and phases during reactions athigh pressures. Recent investigations [2–6,31–38] into the CO oxi-dation reaction under high pressure and high temperature condi-tions have sparked renewed interest and debate over the behaviorand active phases in the reaction under conditions approachingthose of a working catalyst. In recent investigations, Frenkenand co-workers [31–33] have employed a coupled high pressureSTM-flow reactor system to investigate CO oxidation kinetics overPt(110) and Pd(100) surfaces under high pressure reaction condi-tions. Based on concurrent reaction rates and the change in sur-face morphology of the sample, these authors have proposedthat a surface oxide is the active phase for CO oxidation at ele-vated pressures and temperatures. Additional X-ray diffractionstudies by these investigators [34,35] have offered support foroxide phases being highly active for CO oxidation. Similarly, re-cent X-ray diffraction studies on Rh single crystal surfaces indicate

Fig. 2. (a) A correlation of reaction temperature and the critical (O2/CO) gas mixture ratio required to achieve the highly active ‘jump’ state over a Pd(110) single crystal. Datafrom Ref. [3]. (b) CO2 formation rates on bulk Al2O3 supported Pd nanoparticles of various sizes (mean diameter shown) and a Pd single crystal surface at 450 K as a function ofO2/CO ratio. Data from panel (a) were used to estimate the CO oxidation data for the Pd(110) single crystal at 450 K. Data from Ref. [3].

S.M. McClure, D.W. Goodman / Chemical Physics Letters 469 (2009) 1–13 3

that high CO2 formation rates appear concurrent with the forma-tion of oxide phases [36]. SFG experiments, conducted by Su et al.investigating high pressure CO oxidation kinetics over Pt, attrib-uted the high pressure reactivity to reaction at defect (non-regis-try) sites on the surface [37]; however, recent studies havedemonstrated that residual water can significantly alter reactivityat high reactant pressure [38].

Findings from recent studies of CO oxidation on Ru have alsogenerated interest and debate [2,6,20–23,39,40]. As discussed pre-viously, Ru has frequently been considered an example of a systemwith a ‘pressure gap’ between UHV and high pressures studies; Ruis the most least active Pt-group metal at low reactant pressures[20], whereas at high reactant pressures Ru is the most active[22,23]. Recent STM studies by Over and Muehler [39] togetherwith DFT calculations, have led these investigators to conclude thatthe active phase for CO oxidation at elevated pressures is a RuO2

oxide phase. These conclusions cannot be reconciled with previousresults on Ru single crystals, which show that the reactive surfaceat typical elevated reactant pressures is a 1 ML oxygen-covered Rusurface [23]. Furthermore, previous data acquired for technical cat-alysts demonstrate lower reactivity with high O2/CO ratios (>0.5stoichiometric) [22]. Recent low pressure studies on Ru(0001)have also demonstrated that formation of Ru oxide phases causesa decrease in CO2 formation during CO oxidation reaction [21].Additionally, recent STM studies suggest that carbonate formationcontributes to catalytic deactivation on RuO2 surfaces at moderatetemperatures [40]. The nature of the most active surface on Ru atelevated pressures remains a controversial topic. Later in thisLetter, we will discuss recent work in our laboratory addressingthese issues surrounding the kinetic behavior of CO oxidation onRu and RuO2 surfaces.

In a recent kinetic study [3] conducted in our laboratory, extre-mely active surfaces of Pt-group metals have been observed at highpressures (P = 1–300 Torr) and greater than stoichiometric gasmixture conditions (O2/CO > 0.5). Under these highly active reac-tion conditions, turnover frequency (TOF) values (CO2 moleculesformed per site per second) on the order of several thousand canoccur, 2–3 orders of magnitude greater than the rates observedat stoichiometric conditions on Pt-group metals. Shown in Fig. 1is an example of such an experiment, conducted on a Pd(110) sur-face at T = 525 K with a reactant gas mixture of (5/1) O2/CO at a to-tal pressure of P = 80 Torr. As the data illustrate, (red1 line panel

1 For interpretation of color in Figs. 1 and 10, the reader is referred to the webversion of this article.

(b)), a TOF approaching several thousand molecules of CO2 per siteper second is achieved when the O2/CO ratio reaches a criticalpoint (in the case of Fig. 1, O2/CO � 12 at T = 525 K). The O2/CO ra-tio required (Ru < Rh < Pd < Pt) for this ‘jump’ in the reactivity overeach metal corresponds inversely with the adsorption energies ofO2 on these surfaces (Ru (334 kJ/mol) > Rh (234 kJ/mol) > Pd(230 kJ/mol) > Pt (188 kJ/mol)) [3], suggesting that the oxygenchemical potential and generation of sufficient surface oxygen isa critical factor in forming this highly active state over a metal sur-face at a given temperature. Additionally, it was discovered thatthe temperatures required for the occurrence of the highly activestate were inversely related to the O2/CO ratio; i.e. higher O2/CO ra-tios required a lower reaction temperature to achieve the highlyactive state. This effect is observed in Fig. 2 panel (a), which showsdata for CO2 oxidation over Pd(110). Additionally, CO oxidationexperiments conducted on Al2O3-supported Pd nanoclusters dem-onstrate that O2/CO ratios required to achieve the highly activestate for small nanoclusters were lower than those required forthe Pd(110) single crystal, at a given temperature (Fig. 2b). TheO2/CO ratios required for the onset of the highly active state de-creased with decreasing particle size, consistent with the expectedability of small particles to oxidize more easily than larger particles[41,42]. Taken together, these results suggest that the highly activesurfaces consist of low CO coverages and are closely related to theability of the metal surface to stabilize surface oxygen. However,this does not necessarily mean that the surface oxygen phase un-der these conditions is an oxide. A better understanding of the nat-ure of the metal surface at this highly active ‘jump’ point will shedlight on (1) whether this highly active oxygen species is chemi-sorbed oxygen or an oxide and (2) the mechanism of the reactionunder these conditions.

Clearly, these recent studies have revealed interesting questionsrequiring additional research and investigation. First, does aLangmuir–Hinshelwood mechanism sufficiently capture thebehavior of CO oxidation kinetics at both low and high pressures?(i.e. is there a pressure gap?), and secondly, what is the nature ofthe highly active phase observed at elevated pressures (oxidephase or chemisorbed oxygen phase)? To directly address theseissues, analytical techniques are required that facilitate measure-ment of the surface adsorbates and reaction kinetics over a widepressure range (P = 10�8–102 Torr). To do so, we have recently[4,5,8] used PM-IRAS, XPS, and mass spectrometric techniques tostudy the CO oxidation reaction on Pd, Rh, Pt, and Ru surfaces overa wide temperature, pressure, and O2/CO reactant mixture range.PM-IRAS is a powerful tool which enables infrared spectroscopy

4 S.M. McClure, D.W. Goodman / Chemical Physics Letters 469 (2009) 1–13

measurements from UHV to atmospheric pressures. In the follow-ing sections we will review the recent progress in our group usingthese techniques in addressing questions (1) and (2) above. Addi-tionally, we will discuss current efforts aimed at investigating COoxidation kinetics on Rh/SiO2/Mo(112) model catalyst samples atelevated pressure conditions.

0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.010-5

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O2/CO (1/2) Mixtures over Pd(100)

P(CO) / Torr

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O2/CO (1/2) Mixtures over Pd(100)

P(CO) / Torr

108

O2/CO (1/1) Mixtures over Rh(111)

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1.0 x 10-8CO Flux @ 10-8 Torr

O2/CO (1/2) Mixtures over Pd(100)

P(CO) / Torr

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1.0 x 10-8CO Flux @ 10-8 Torr

O2/CO (1/2) Mixtures over Pd(100)

P(CO) / Torr

108

O2/CO (1/1) Mixtures over Rh(111)

a

b

3. Experimental approach

The experimental apparatus and measurement procedures havebeen described in-depth in previous publications [3–5]. Briefly, theapparatus consists of consists of a (1) UHV section equipped withAuger spectroscopy (AES), low energy electron diffraction (LEED),and a mass spectrometer for low pressure kinetic measurements;and (2) a contiguous high pressure cell equipped with PM-IRAS forhigh pressure kinetic measurements. Low pressure kinetic measure-ments (P < 10�3 Torr) are conducted under steady-state conditionsby backfilling of the UHV chamber with the desired gas mixtureand monitoring the CO and CO2 signals (correcting for fragmenta-tion). High pressure measurements are conducted using a Baratrongauge and a method previously described [3] (example in Fig. 1). Sin-gle crystal samples (Rh, Pd, Pt and Ru) were prepared using pub-lished procedures and the cleanliness verified by AES. In situ XPSmeasurements were conducted in a separate UHV apparatus.

Rh/SiO2/Mo(112) model catalyst samples were prepared as de-scribed previously [7]. Rh/SiO2/Mo(112) samples were synthesizedin UHV on a Mo(112) substrate by (1) preparation of �5 ML SiO2

thin film via vapor deposition of Si in an O2 ambient (P � 10�6 Torr)at T > 600 K, followed by anneal at T = 1000 K in O2 ambient(P � 10�6 Torr), then (2) vapor deposition of Rh metal. The cata-lysts were then transferred to a contiguous high pressure reactioncell for elevated pressure kinetic measurements using methodspreviously described [3]. CO temperature programmed desorption(TPD) and STM measurements (conducted in a separate STM/UHVsystem) were also conducted, as described in detail in a recentpublication [7].

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~110 kJ/mol

Fig. 3. (a) CO2 formation rate over Pd(100) from low (10�8 Torr) to high (8 Torr) COpressures as a function of reaction temperature. Reactant is an O2/CO (1/1) gasmixture. Dashed lines represent CO flux at given pressure calculated using theHertz–Knudsen equation. Data from Refs. [4,5]. (b) CO2 formation rate over Rh(111)from low (10�8 Torr) to high (1 Torr) CO pressures as a function of reactiontemperature. Reactant is an O2/CO (1/2) gas mixture. Dashed lines represent CO fluxat given pressure calculated using the Hertz-Knudsen equation. Data from Refs.[4,5].

4. Discussion

4.1. CO Oxidation kinetics at low (10�8–10�3 Torr) and nearatmospheric (1–102 Torr) Pressure Ranges

Shown in Fig. 3a and b are reaction rates (TOF) vs. temperatureof O2/CO (1/2) mixtures on Pd(100) and O2/CO (1/1) mixtures onRh(111) [4,5], plotted in Arrhenius form, over a wide pressurerange (PCO = 10�8–8 Torr). The CO fluxes (CO molecules strikingeach surface site per sec), shown by the horizontal dashed linesin the figures at a given pressure conditions, were calculated usingthe Hertz–Knudsen equation: Flux ¼ P

ffiffiffiffiffiffiffiffiffiffiffi

2pmkTp [43]. One can view

these flux lines as the maximum CO2 rate achievable at the CO col-lision limit, and the ratio of the actual reaction rate to this flux va-lue at a given pressure and temperature can be regarded as thereaction probability under the selected (P,T) conditions.

Immediately apparent from the Arrhenius plots of Fig. 3a and bis the similarity between the low and high pressure reactivity re-gimes at low temperatures (region following the linear dashedline). At sufficiently low temperatures, both pressure regimes con-tain a CO inhibited regime, wherein the reaction is inhibited by ad-sorbed CO preventing the adsorption and dissociation of incomingO2. In the high pressure regime, (P > 1 Torr), the reactivity withinthis CO inhibited regime exhibits an activation energy of�110 kJ/mol (dashed line), very close to the desorption energy ofCO from these Pt-group metal surfaces. At lower pressures,although the reaction is clearly inhibited by adsorbed CO, the acti-vation energy varies slightly from the dashed line. This is a known

phenomenon which occurs at lower pressure due to subtle CO andO co-adsorbed phases and compressed structures which can formon the surface [9,10]. From these data, it is apparent that, underthe CO inhibited regime, CO reaction kinetics behave similarly athigh and low pressures.

As the reaction temperature is increased under low pressureconditions (P < 10�3 Torr), the reaction rate increases, reaches amaximum value, then decreases as the temperature is further in-creased. The temperature at which this ‘rollover’ occurs increases

S.M. McClure, D.W. Goodman / Chemical Physics Letters 469 (2009) 1–13 5

with increasing reactant pressures, PCO. Investigations using PM-IRAS can shed further light on this behavior. Shown in Fig. 4 arelow pressure PM-IRAS spectra obtained at low pressures(P = 10�8, 10�5, 10�3 Torr) during CO oxidation over Pd(100) witha O2/CO (1/2) mixture. Similar trends are observed on Pt and Rhsurfaces, such that this discussion will serve as a broad example.

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Temp. / K

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Temp. / K

P(CO) = 1 x 10-3 Torr

a

b

c

Fig. 4. CO PM-IRAS as a function of reaction temperature for O2/CO (1/2) mixturesat various low CO pressures (a) 10�8 Torr, (b) 10�5 Torr, and (c) 10�3 Torr, overPd(100). Data from Refs. [5,4].

The PM-IRAS data illustrate that, with an increase in the reactiontemperature, the CO PM-IRAS signal decreases due to increasedCO desorption. As the CO pressure is increased, additional COdesorption (higher temperature) is required to reduce the surfaceCO coverage. Fig. 5 combines low pressure PM-IRAS data and ki-netic data obtained over Pd(100) for a O2/CO (1/1) mixture. In pa-nel (a), the CO PM-IRAS integrated spectral area during reaction areshown for P = 10�8–10�3 Torr; in panel (b) the CO reaction proba-bilities for each reaction temperature are shown. Recall that theCO reaction probability can be obtained simply by dividing the ob-served reaction rate by the CO flux value, at a given temperatureand pressure. As these results indicate, the CO reaction probabilityincreases to its maximum value as the CO PM-IRAS signal ap-proaches zero for each of the pressures shown. Furthermore, asthe CO integrated area approaches zero (and hence approachesthe most reactive surface), no vibrational features associated withCO near O adatoms are observed (for Pd, vCO > 2087 cm�1), thepresence of which indicate a large surface concentration of Oatoms. These observations are consistent with the inhibiting effectof surface CO on the reaction rate, and clarifies a point which willbe revisited during the discussion of the high pressure/high tem-perature reaction regime; the highest reactivity is achieved undersurface conditions which exhibit low to negligible surface CO cov-erages. This finding is consistent with previous low pressure re-sults on Pt-group metals [11–15].

As the temperature is increased beyond the rollover point, thelow pressure reactivity begins to decrease. This reactivity decreasecan be attributed to the inhibition of reaction by surface oxygen athigher coverages (which inhibits CO adsorption) and the shortersurface lifetime of CO at higher temperatures [9,10,16–19]. Indeed,when comparing reactivity data at low pressures across the Pt-group metals, the onset of oxygen inhibition occurs in the orderRh > Pd > Pt, consistent with the oxygen adsorption energies on

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0.4

0.5

1×10-8 Torr

1×10-7 Torr

1×10-6 Torr

1×10-5 Torr

1×10-4 Torr

1×10-3 TorrCO

Sig

nal A

rea

/ a.u

.

CO Pressure

CO

Rea

ctio

n P

roba

blit

y

Temperature / K

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.3

1×10-8 Torr

1×10-7 Torr

1×10-6 Torr

1×10-5 Torr

1×10-4 Torr

1×10-3 Torr

CO Pressure

Temperature / K

1×10-8 Torr

1×10-7 Torr

1×10-6 Torr

1×10-5 Torr

1×10-4 Torr

1×10-3 Torr

CO Pressure

Temperature / K

a

b

Fig. 5. CO PM-IRAS signal area (upper panel (a)) and reaction probability (bottompanel (b)) as a function of reaction temperature for O2/CO (1/1) mixtures at variousCO pressures over Pd(100). Data obtained at different CO pressures are presentedwith different symbols. Data from Refs. [5,4].

6 S.M. McClure, D.W. Goodman / Chemical Physics Letters 469 (2009) 1–13

these surfaces (Rh > Pd > Pt) [3–5]. Thus, in the low pressure re-gime, there exists a CO inhibited regime at low temperatures andan oxygen inhibited regime at high temperatures. The highest reac-tivity (CO reaction probability) occurs on samples with low to neg-ligible CO coverages.

4.2. High pressure regime and mass transfer limitations

As discussed in the background section, recent studies in thehigh pressure CO-uninhibited regime have opened new questionsregarding the active surface and reaction mechanisms on Pt-groupmetals under these reaction conditions. Upon examination ofFig. 3a–c, the high pressure–high temperature reactivity data(P > 1 Torr) become limiting and temperature-invariant at ratesapproaching a TOF of 1000. Additionally, the CO reaction probabil-ity begins to rapidly decrease within the 10�4 Torr to 102 Torr re-gime, as illustrated in Fig. 6. Data in Fig. 6 show the CO reactionprobability at various temperatures as a function of pressure forO2/CO equal to 0.5 over Pd(1 0 0). As the pressure is increasedabove P � 10�4 Torr at higher temperatures (T > 550 K), the COreaction probability begins to decrease as the reactant pressure isincreased. Whereas CO reaction probabilities within the 10�8–10�4 Torr range are generally around 40%, the reaction probabilitydrops to less than 1% at pressures greater than 1 Torr. This effect isdue primarily to mass transfer limitations which become increas-

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

1E-4

1E-3

0.01

0.1

1

10

100

CO

Rea

ctio

n P

roba

bilit

y (%

)

Pressure (Torr)

Max at Pressure T=500 K T=550 K T=600 K T=700 K T=800 K

-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1

Max at Pressure T=500 K T=550 K T=600 K T=700 K T=800 K

Fig. 6. CO reaction probabilities for Pd(100) O2/CO (1/2) calculated from the data ofFig. 5a, as a function of pressure for several temperatures: 500 K (j), 550 K (d),600 K (N), 700 K (.) and 800 K (r). The max CO reaction probability at each of thepressures (corresponding to rollover) is shown by open circles (s).

ingly important at higher pressures. As the mean free path (k / 1P)

of an ideal gas becomes shorter with an increase in the gas pres-sure, the gas collisions and gas-phase transport resistance in-creases. From kinetic theory, as the pressure of an ideal gasincreases, the gas phase diffusivity also decreases (D / 1

P), propor-tional to 1

P [44]. A general analysis of mass transfer coefficients overmetal wire samples by Schmidt and coworkers indicates that masstransfer effects can become important at pressures greater than 1Torr and reaction rates greater than 1019 molecules/cm2 s (�103–104 TOF, for a typical metal surface with �1015 sites/cm2) [45].Thus, mass transfer effects can clearly become an important factorwhen studying reaction kinetics at elevated pressures and highTOF values, and care must be taken to consider this factor whenconducting experiments within the high reaction rate regime.

4.3. The nature of the highly active surface at high pressures

PM-IRAS is a powerful tool that can be used to investigate sur-face species under elevated pressure conditions. Coupled with ki-netic measurements, it is an ideal analytical tool to provideinsight into the nature of the active surface phases during thehighly active regime observed at high pressures (Fig. 1). Fig. 7a–cshow kinetic data obtained from Pt(110), Rh(111), and Pd(100)surfaces at various O2/CO ratios ((10/1) through (1/2)), at highpressure (PCO = 8 Torr). As the data illustrate, there exists three pri-mary regimes for reaction at higher pressures for the three Pt-group metals shown. First, at lower temperatures, there exists aCO inhibited regime in which TOF increases with increasing tem-perature. This regime, exhibiting an activation energy (Ea) of�110 kJ/mol (approximately equal to the CO binding energy onthe Pt, Pd, Rh surfaces) is due to the rate-limiting step being COdesorption, as discussed in the previous section. As the tempera-ture is increased in this regime, the TOF increases until theshort-lived, highly active ‘jump’ region is observed (denoted bythe arrows in the reactivity data of Fig. 7). This highly active ‘jump’is the same phenomena displayed in Fig. 1, where the TOF canreach values of several thousand CO2 molecules formed per siteper second. Note that the temperature dependence of the jumppoints (arrows) correspond inversely with the O2/CO ratios, as dis-cussed in Fig. 2. Higher temperatures are required for lower O2/COratio reactant mixtures to reach the highly active ‘jump’ state. Aswe will show, CO PM-IRAS spectra show a rapid decrease in theCO IRAS stretching feature when passing through the jump point.This highly reactive phase is transient in nature and quickly be-comes mass transfer limited in the reactants within the boundarylayer near the sample surface. If the mass transfer limit can be cir-cumvented, this transient phase should last longer, as has been ob-served in recent experiments [4,5]. In these experiments, highpressure experiments conducted over Pd(100) in the entire cham-ber (UHV volume of 61.6 L + high pressure cell volume of �0.6 L)show identical kinetics below the jump point to the kinetics ob-served in experiments in the high pressure cell volume only. How-ever, at the jump point, the transient regime in the full volume(UHV section + high pressure cell) experiment lasts much longerdue to the increased volume of reactants and to stirring from ther-mal convection. The latter results in a partial breakdown of themass transfer limitation. Similar results have been observed bySu et al. over Pt(111) [37]. Based on these findings, we concludethat the ‘jump’ point is associated with a highly reactive surfacewhich rapidly consumes reactants in the near surface region ofthe catalyst. That is, the CO2 formation rate after the ‘jump’ pointbecomes rate limited by the mass transport of reactants from thebulk gas phase to the near surface boundary layer. Indeed, this ismanifested in the data of Fig. 7 by the appearance of the third

1.2 1.4 1.6 1.8 2.0 2.210

1

102

103

104

P(CO) = 8 Torr

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K

O2/COMixtures over Pd(100)

500600700800 550650Temperature / K

P(O2)/P(CO)

1.2 1.4 1.6 1.8 2.0 2.210

1

102

103

104

P(CO) = 8 Torr

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K

O2/COMixtures over Pd(100)

500600700800 550650Temperature / K

1.2 1.4 1.6 1.8 2.0 2.210

1

102

103

104

P(CO) = 8 Torr

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K

O2/COMixtures over Pd(100)

500

1.2 1.4 1.6 1.8 2.0 2.210

1

102

103

104

P(CO) = 8 Torr

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K

O2/COMixtures over Pd(100)

500600700800 550650Temperature / K

P(O2)/P(CO)

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4100

101

102

103

104

P(CO) = 8 Torr

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K

O2/CO Mixtures over Pt(110)

500600700800 550650Temperature / K

450900

1.2 1.4 1.6 1.8 2.0 2.2101

102

103

104

∗

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K

O2/CO Mixtures over Rh(111)P(CO) = 8 Torr P(O

2)/P(CO)

∗

500550600700800 650

Temperature / K

P(O2)/P(CO)

1.2 1.4 1.6 1.8 2.0 2.210

1

102

103

104

P(CO) = 8 Torr

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K

O2/COMixtures over Pd(100)

500600700800 550650Temperature / K

P(O2)/P(CO)

1.2 1.4 1.6 1.8 2.0 2.210

1

102

103

104

P(CO) = 8 Torr

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K

O2/COMixtures over Pd(100)

500600700800 550650Temperature / K

1.2 1.4 1.6 1.8 2.0 2.210

1

102

103

104

P(CO) = 8 Torr

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K

O2/COMixtures over Pd(100)

500

1.2 1.4 1.6 1.8 2.0 2.210

1

102

103

104

P(CO) = 8 Torr

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K-1)

O2/COMixtures over Pd(100)

500600700800 550650Temperature / K

P(O2)/P(CO)

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4100

101

102

103

104

P(CO) = 8 Torr

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K

O2/CO Mixtures over Pt(110)

500600700800 550650Temperature / K

450900

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4100

101

102

103

104

P(CO) = 8 Torr

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

100

101

102

103

104

P(CO) = 8 Torr

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K-1)

O2/CO Mixtures over Pt(110)

500600700800 550650Temperature / K

450900

1.2 1.4 1.6 1.8 2.0 2.2101

102

103

104

∗

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K

O2/CO Mixtures over Rh(111)P(CO) = 8 Torr P(O

2)/P(CO)

∗

500550600700800 650

Temperature / K

1.2 1.4 1.6 1.8 2.0 2.2101

102

103

104

∗

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K

O2/CO Mixtures over Rh(111)P(CO) = 8 Torr P(O

2)/P(CO)

∗

500550600700800

1.2 1.4 1.6 1.8 2.0 2.2101

102

103

104

∗

1/21/12/15/110/1

TO

F (

CO 2

mol

ecul

e si

te-1s-1

)

1000/T (K-1)

O2/CO Mixtures over Rh(111)P(CO) = 8 Torr P(O

2)/P(CO)

∗

500550600700800 650

Temperature / K

P(O2)/P(CO)

a

b

c

Fig. 7. Arrhenius plots of CO2 formation of 1/2 (j), 1/1 (d), 2/1 (N), 5/1 (.) and 10/1 (�) O2/CO mixtures at initial CO pressure of 8 Torr over (a) Rh(111), (b) Pt(110)and (c) Pd(100). Data from Refs. [4,5].

S.M. McClure, D.W. Goodman / Chemical Physics Letters 469 (2009) 1–13 7

regime, which occurs after the jump point at high temperatures, aregime largely invariant with temperature. In this mass transfer re-gime, the reactivity becomes largely independent of sample tem-perature under mildly oxidizing conditions (O2/CO < 2), as it isprimarily dictated by transport of reactant to the surface fromthe gas phase. Under highly oxidizing conditions (O2/CO > 5), Rhand Pd surfaces begin to show a decrease in reactivity at highertemperatures. These decreases are attributed to reaction inhibitiondue to oxide formation. At these points, reactivity over the oxidesurface is decreased below the mass transfer limit due to sampleoxidation.

Thus, to summarize, these data demonstrate that at higher pres-sures, there exist three distinct phases for reaction: (1) a CO inhib-ited phase, where reactivity is limited by CO desorption, (2) ahighly reactive transient phase, which consists of a CO un-inhib-ited surface and is terminated by mass transfer limitations (dueto depletion of boundary layer material), and (3) a high tempera-ture, mass transfer limited phase wherein reaction is limited bytransport of reactants from the bulk gas phase to the near surfaceregion. This results in a temperature-invariant reaction rate formildly oxidizing conditions; however, for Rh and Pd surfaces, de-creases in reactivity for higher oxidizing conditions can result athigh temperatures, likely due to oxidation induced deactivationof the sample.

Complementary PM-IRAS spectra have been collected for the ki-netic data of Fig. 7 and have produced insights into the nature ofthe surface near the jump point. Shown in Fig. 8 are a series ofPM-IRAS spectra acquired for Pd(100) at the indicated tempera-tures and gas mixtures of Fig. 7. Spectra for Rh and Pt are similar,with minor differences to be discussed below. Thus, the followingdiscussion can be regarded as a general one for Pd, Pt, Rh. Shaded inthe figures are the corresponding PM-IRAS spectra acquired beforeand after the jump point. As the data illustrate for Pd(100), CO PM-IRAS signal is rapidly attenuated in passing through the jumppoint, with no evidence for any oxygen-shifted CO PM-IRASstretching features for mildly oxidizing reaction mixtures (O2/CO > 2). For more strongly oxidizing conditions, (O2/CO > 5), theCO stretching frequencies (vCO > �2087 cm�1) associated withneighboring O species are observed after the jump on both Pdand Rh surfaces, although normal CO stretching features are ob-served before the jump. Complementary XPS studies conductedin our laboratory for Rh(111) [4] demonstrate that the normalCO stretching features near the jump point are associated withoxygen coverages no higher than �0.5 ML, whereas higher vCO

stretching frequencies are associated with oxygen coveragesapproaching �1 ML. A similar behavior is observed for PM-IRASspectra obtained for Pt(110), i.e., before and after the jump point,where the CO PM-IRAS signal falls rapidly to zero. However, for allthe (O2/CO) jump points in the Pt(110) data, no evidence for IRstretching frequencies associated with CO near O atoms isapparent.

Taken together, these data strongly suggest that, similar to lowpressures, the most reactive surface (surface approaching the jumppoint) at high pressures is a surface with a low to undetectable COcoverage and O coverages below that characteristic of an oxidephase. Furthermore, Langmuir–Hinshelwood kinetics appear toadequately describe reaction kinetics from low to high pressures,when taking into account oxidation and mass transfer limits. Inother words, there does not appear to be a pressure gap with re-gards to the CO oxidation reaction mechanism on Pt, Rh, and Pdacross the pressure continuum (10�8–102 Torr).

Very recent work [8] in our laboratory has focused on under-standing CO oxidation kinetics on Ru and RuO2 surfaces at elevatedpressures (P = 10–100 Torr). As discussed earlier, many open

2200 2100 2000 1900 1800

2147

2145

2142 2087

1980

1984

1989

1991

1994

1996

O2/CO = 10/1, P(CO) = 8 Torr

Frequency / cm-1

600

575

550

525

500

475

450

425

400

Temp. / K

350

2200 2100 2000 1900 1800

O2/CO = 1/1, P(CO) = 8 Torr

1968

1971

1976

1980

1984

1990

1996

1999

Frequency / cm-1

600

575

550

525

500

450

400

350

Temp. / K

310

2200 2100 2000 1900 1800

O2/CO = 2/1, P(CO) = 8 Torr

Frequency / cm-1

575

550

525

500

450

400

350

Temp. / K

310

1969

1975

1979

1987

1993

1998

1999

2200 2100 2000 1900 1800

650

625

600

575

550

500

450

400

3501995

1987

1978

1982

1980

1977

1972

1960

PM

-IR

AS

Sign

al I

nten

sity

/ a.

u.

Frequency / cm-1

1999 Temp. / K

310

O2/CO = 1/2, P(CO) = 8 Torr

2200 2100 2000 1900 1800

2142

O2/CO = 5/1, P(CO) = 8 Torr

PM

-IR

AS

Sign

al I

nten

sity

/ a.

u.

Frequency / cm-1

600

575

550

525

500

450

400

350

Temp. / K

310

2087

1975

1978

1988

1994

1997

1999

2200 2100 2000 1900 1800

2147

2145

2142 2087

1980

1984

1989

1991

1994

1996

O2/CO = 10/1, P(CO) = 8 Torr

Frequency / cm-1

600

575

550

525

500

475

450

425

400

Temp. / K

350

2200 2100 2000 1900 1800

O2/CO = 1/1, P(CO) = 8 Torr

1968

1971

1976

1980

1984

1990

1996

1999

Frequency / cm-1

600

575

550

525

500

450

400

350

Temp. / K

310

2200 2100 2000 1900 1800

O2/CO = 2/1, P(CO) = 8 Torr

Frequency / cm-1

575

550

525

500

450

400

350

Temp. / K

310

1969

1975

1979

1987

1993

1998

1999

2200 2100 2000 1900 1800

650

625

600

575

550

500

450

400

3501995

1987

1978

1982

1980

1977

1972

1960

PM

-IR

AS

Sign

al I

nten

sity

/ a.

u.

Frequency / cm-1

1999 Temp. / K

310

O2/CO = 1/2, P(CO) = 8 Torr

2200 2100 2000 1900 1800

2142

O2/CO = 5/1, P(CO) = 8 Torr

PM

-IR

AS

Sign

al I

nten

sity

/ a.

u.

Frequency / cm-1

600

575

550

525

500

450

400

350

Temp. / K

310

2087

1975

1978

1988

1994

1997

1999

Fig. 8. PM-IRAS spectra as a function of reaction temperature over the various O2/CO mixtures of Fig. 7c at initial P(CO) = 8 Torr over Pd(100). O2/CO ratios are displayedwithin the top part of each panel and sample temperatures are marked adjacent of each spectrum. Spectra for the temperatures occurring directly before and after the jumppoint are shaded. Data from Ref. [4].

8 S.M. McClure, D.W. Goodman / Chemical Physics Letters 469 (2009) 1–13

questions remain regarding the reactivity of Ru surfaces under ele-vated pressures. While Ru has been shown to be the least reactivePt-group metal under UHV conditions [20], it exhibits the highestreactivity at elevated pressures [22,23]. Questions surround thenature of the most active phase (chemisorbed oxygen vs. oxidesurface) on Ru surfaces [21–23,39,40]. In our recent studies,CO reactivity (coupled with PM-IRAS measurements) has beeninvestigated at elevated pressures on Ru(0001) single crystal,(1 � 1)-O/Ru(0001) and RuO2(110) surfaces formed on theRu(0001) surface. Reactions were investigated under net reducing(O2/CO < 2), stoichiometric (O2/CO = 2), and net oxidizing(O2/CO > 2) reaction conditions. These studies have demonstratedthat CO oxidation on (1 � 1)-O/Ru(0001) surfaces occurs onsurface defects (concentration: 0.01–10�5 ML active sites); thisCO species is weakly bound (Ea � 68 kJ/mol). When a (1 � 1)-O/Ru(0001) surface is reacted under oxidizing conditions, the sur-face eventually oxidizes to form RuO2(110). Although the totalreactivity (CO2 molecules formed per sec) of the RuO2(110) surface(concentration: �0.33 ML active sites) is higher (�4�) than reac-tivity on the (1 � 1)-O/Ru(0001) surface, if reactivity is assessedon a per site basis (TOF), the reactivity on the RuO2 surface cannotbe concluded to be higher than the (1 � 1)-O/Ru(0001) surface.

When a RuO2(110) surface is prepared and reacted underreducing and stoichiometric reaction conditions (T = 400–600 K),

the surface reduces (under reaction conditions) gradually to a sur-face oxide, then to a chemisorbed oxygen covered surface. Thisindicates that the chemisorbed oxygen surface is the more thermo-dynamically stable phase under most relevant reaction conditions.The CO oxidation rate increases as the surface is reduced, indicat-ing that chemisorbed oxygen is more reactive than the oxide sur-face, due to the lower CO binding energy on (1 � 1)-O/Ru(0001)(Ea � 68 kJ/mol) than on the RuO2(110) surface (Ea � 130 kJ/mol).Under highly oxidizing (O2/CO > 2) and low temperature condi-tions, RuO2 surfaces can remain stable under reaction conditionsand can exhibit high reactivity for an initial period of time. Thisis presumably due to a reactive, weakly bound oxygen speciespresent on the RuO2 surface; deactivation eventually occurs grad-ually due to the buildup of carbon containing (likely surface car-bonate) species.

These experiments have provided insight into some of the morepuzzling issues surrounding Ru reactivity. First, these findings indi-cate that the chemisorbed oxygen-covered surface is the mostreactive surface (compared to oxide surface) at elevated pressuresunder most reaction conditions. Additionally, a chemisorbed oxy-gen-covered surface is the most thermodynamically stable surfaceunder typical stoichiometric and reducing conditions. Under highlyoxidizing and low temperature conditions, RuO2 exhibits a veryhigh activity (initially), likely due to a weakly bound oxygen

1.6 1.7 1.8 1.9 2.0 2.1 2.21

10

100

Calculated Rh activesites from normalization

0.25 ML; 8.5 x 1013

0.5 ML; 1.8 x 1014

1.0 ML; 3.8 x 1014

2.0 ML; 6.3 x 1014

4.0 ML; 9.3 x 1014

10 ML; 9.6 x 1014

Rh/SiO2/Mo(112)

θRh

0.25 ML 0.5 ML 1.0 ML 2.0 ML 4.0 ML 10 ML Rh(111)

TOF(

CO

2 Mol

ecu

les/

site

/sec

)

1000/T(K)

1.6 1.7 1.8 1.9 2.0 2.1 2.2

1015

1016

475K

500K

525K

550K

575K

600K

P=8 Torr(1/10) O2/CO

Rh/SiO2/Mo(112)

θRh

0.25 ML 0.5 ML 1.0 ML 2.0 ML 4.0 ML 10 ML Rh(111)

Rat

e (C

O2 m

olec

ule

s fo

rmed

/sec

)

1000/T(K)

a

b

Fig. 9. (a) CO2 reaction rate data, (CO2 molecules formed/s) vs. 1000/T(K), forvarious Rh coverages (hRh) on Rh/SiO2(�5 ML)/Mo(112) model catalyst samples:(j) 0.25 ML; (d) 0.5 ML; (N) 1.0 ML; (.) 2.0 ML; (r) 4.0 ML; (+) 10 ML. CO2

reaction rate data obtained over Rh(111) (s), conducted under the same reactionconditions. (b) CO2 reaction rate data, from (a), normalized to Rh(111) data toobtain reaction rate in terms of TOF [CO2 molecules formed/(site s)]: (j) 0.25 ML;(d) 0.5 ML; (N) 1.0 ML; (.) 2.0 ML; (r) 4.0 ML; (+) 10 ML; (s) Rh(111). Each set ofRh coverage reaction data was linear best fit to the Rh(111) reaction data. Datafrom Ref. [7].

S.M. McClure, D.W. Goodman / Chemical Physics Letters 469 (2009) 1–13 9

species present on this surface; deactivation occurs due to carbo-naceous buildup. Second, it is apparent that the number of activesites, oxygen binding energy, and CO binding energy play a criticalrole in understanding the reactivity of chemisorbed oxygen andoxide phases on Ru surfaces. The apparent ‘pressure gap’ differ-ences between Ru reactivity at UHV and high pressure need notbe explained by invoking a new active surface phase at high pres-sures (i.e., RuO2). Under UHV conditions, CO oxidation rates on Rusurfaces are lower (than for other Pt-group metals) due to the highbinding energy of oxygen atoms on low oxygen coverage Ru sur-faces. Under elevated pressures, the high reactivity of Ru surfaces(compared to Pt, Pd, and Rh) occurs because Ru surfaces (whichare more facilely covered with O atoms) do not become CO inhib-ited to the extent that Pt, Pd, and Rh surfaces do.

4.4. CO oxidation kinetics at elevated pressures on Rh/SiO2 modelcatalysts

Recent progress and research in surface science techniques haveallowed for investigations of more realistic and complex modelcatalyst samples prepared and characterized in a UHV environ-ment – oxide supported metal particles designed to better mimicthe active surfaces of actual working catalysts [46]. Recipes devel-oped by several groups have demonstrated the ability to preparean assortment of thin metal oxide films (such as SiO2 [47–50])on single crystal substrates under UHV conditions. These films,coupled with vapor deposition of metal particles, allow for the cre-ation of model surfaces for use in investigations of supported metalparticles – surfaces which are amenable to traditional surface sci-ence spectroscopies. These surfaces enable fundamental investiga-tions of variables such as support effects, particle size/morphologyeffects, and particle sintering behavior, which can exert influenceon the kinetics of catalytic reactions on supported catalysts.

Due to their fundamental and practical importance, metal oxidesupported Rh model catalysts have been prepared and character-ized by a number of groups. Insights into particle morphology, par-ticle sintering, interaction with CO and O2 ambients, and lowpressure CO oxidation kinetics have been acquired for a varietyof oxide supported Rh systems (alumina [51–57], silica [58–60],titania [61–64], and magnesium oxide [53,57,65] single crystaland thin film substrates). Additionally, recent solution-basedmethods for generating mono-dispersed Rh nanocubes supportedon Si have allowed for ex situ CO oxidation kinetic measurementson well-defined Rh particles [66]. Combined, these previous stud-ies have demonstrated that Rh particles on model catalyst samplescan undergo morphological, structural and chemical changes un-der even low pressure CO and O2 ambients and annealing condi-tions, and can exhibit chemical properties not observed on lowindex Rh single crystals (i.e. CO dissociation). However, extensivekinetic studies of CO oxidation under elevated pressures on modelcatalyst samples prepared in situ have to this point been limited.Understanding the quantitative and qualitative agreement of COoxidation reactivity studies conducted on model catalyst samples,with respect to single crystal and technical catalyst data undersimilar conditions, requires investigation.

Clearly, additional studies are necessary to better understandelevated pressure CO oxidation kinetics on Rh supported modelcatalyst samples and how these systems behave with respect totraditional single crystal and bulk catalyst samples. Furthermore,kinetic studies of supported particles can potentially produce fur-ther insights into mass transfer limitations and highly active statesfound at elevated pressures. Thus, current [7] and ongoing work inour laboratory are aimed at investigating the reactivity of Pt-groupmetal particles (in particular Rh) supported on thin (�15 Å) SiO2

films grown on single crystal substrates (e.g. Mo(112)) under ele-vated pressure conditions (P = 8 Torr). Silica thin films have been

well characterized in recent studies and have been shown in previ-ous work by Knözinger and coworkers [59] to provide an inert sur-face for Rh at high temperatures (no Rh silicide formation belowT < 873 K, well below the highest reaction temperature exploredin this study). Discussed below are results from a recent study[7] aimed at developing a fundamental understanding of CO oxida-tion kinetics on supported particle surfaces at elevated pressures.Our initial objectives have been to address the following issues:First, what is the qualitative behavior of CO oxidation reactivity(with respect to PCO, PO2, structure insensitivity) within the COinhibited regime? Secondly, what is the extent of quantitativeagreement between TOF values obtained on Rh particles and how

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Fig. 10. (a) Number of active Rh sites per cm2 vs. Rh coverage (ML equivalent), asestimated from TPD spectra (j), (1/10) O2/CO reactivity data (d), and STM data (N),as described in the text. (b) Average particle diameter (in Angstroms, Å) (j), asdetermined from histogram data obtained from STM images. Shown by dotted lines(– – –) are the minimum and maximum Rh particle diameter observed for a givenRh coverage. (c) STM image for a 1 ML Rh/(�1 ML SiO2)/Mo(112) film. Panel is a 50nm � 50 nm image. Data from Ref. [7].

10 S.M. McClure, D.W. Goodman / Chemical Physics Letters 469 (2009) 1–13

do they relate to quantitative data obtained on single crystalsurfaces.

As discussed briefly in the experimental section, a Mo(112) sin-gle crystal is used as a substrate for SiO2 thin film growth (�15 Å),and was grown based on vapor deposition recipes described previ-ously [47,50]. Rh particles are generated on the SiO2 film by Rh va-por deposition (at a sample temperature of T = 300 K). The Rhdosing rate on the SiO2 was calibrated with a breakpoint analysisof the primary Rh AES feature (302 eV) on the Mo(112) surface.The Rh growth on Mo(112) at T = 300 K exhibited a breakpointat a Rh(302 eV)/Mo(187 eV) AES ratio of �0.5. For the purpose ofdose calibration, this ratio was assumed to correspond to a mono-layer equivalent coverage of Rh; reported coverages on SiO2 filmswill be referenced to this amount. Typical Rh dose rates were�0.25 ML/min and based on Auger electron spectroscopy (AES)measurements across the sample, we estimate that the Rh cover-age varies no more that �15% from the stated coverage acrossthe sample face.

Shown in Fig. 9a are reactivity measurements [molecules CO2

formed/s vs. 1000/T(K)] obtained from a series of Rh/SiO2 model cat-alyst samples with varying Rh coverages (hRh = 0.25–10) using a (1/10) O2/CO gas mixture, along with comparable data under identicalconditions obtained on a Rh(111) single crystal. As the data illus-trate, increasing Rh coverages result in an increased rate of CO2 for-mation, as expected. Temperature dependence shows Arrheniusbehavior for the temperatures and coverages studied, exhibitingan activation energy of 100–110 kJ/mol for all coverages studied.This value is in good agreement with the consensus of literature val-ues (�110 kJ/mol) obtained previously for a Rh single crystal and atechnical catalysts, and indicates that under the CO-dominant con-ditions used in these measurements, Rh particles exhibit CO inhib-ited reaction kinetics similar to Rh single crystal samples. Additionalexperiments, conducted with a (1/10) O2/CO mixture under the con-ditions shown in the figure, demonstrated repeatable behavior (i.e.samples could be run repeatably throughout the temperaturesranges without irreversible changes to the reaction rate). Thisobservation suggests that for the CO rich conditions (1/10) O2/COof Fig. 9a, changes in particle reactivity due to reaction are minimal.As will be discussed shortly, irreversible changes occurred to thesample reactivity as temperature and/or the O2 partial pressure ofthe gas mixture are increased to a critical point.

Catalytic activity is typically defined in terms of turnover fre-quency, TOF, (molecules CO2 produced per site per second). As dis-cussed previously, CO oxidation on Rh single crystal and technicalcatalysts under similar CO dominant conditions has been shown toexhibit structure insensitivity. A plot of the Rh/SiO2/Mo(112) andthe Rh(111) data of Fig. 9a in terms of TOF should be coincidentfor a structure insensitive reaction. Thus, the Rh/SiO2/Mo(112)and Rh(111) reactivity data of Fig. 9a can be utilized to obtainan estimate of the number of active of sites present on the Rh/SiO2 as a function of Rh coverage, by simply normalizing the parti-cle data to the Rh(111) single crystal data.[7] (The Rh(111) surfacedensity is 1.6 � 1015 atoms/cm2 and the single crystal area is�1.5 cm2 front and back.) This exercise is visualized in Fig. 9b,which is an Arrhenius plot of the data of Fig. 9a in terms of TOF,where the Rh/SiO2/Mo(112) reactivity data for the various cover-ages has been normalized by an appropriate number of sites suchthat the plots are normalized to the Rh(111) single crystal data.The number of active Rh sites (per cm2) obtained from this normal-ization is shown in Fig. 10a (d, red circle symbol) for hRh = 0.25–10.As anticipated, the number of active Rh sites per cm2 increaseswith increasing Rh coverage, and approaches the value expectedfor the Rh(111) sample using a Rh(111) surface atom densityequal to 1.6 � 1015 atoms/cm2.

The number of Rh active sites on the Rh/SiO2 samples was alsoestimated with two additional methods: (1) CO TPD measurements

and (2) STM images from Rh particles deposited on ultra-thin SiO2

films grown on Mo(112) (conducted in a separate STM chamber)[7]. CO TPD measurements were conducted for a variety of Rh/SiO2 samples and Rh coverages (hRh = 1–10), with the Rh(111) sin-gle crystal sample providing an additional measure of the numberof active sites present on the Rh/SiO2/Mo(112) samples [7]. Bycomparing the integrated CO TPD coverages from the Rh particlesand the Rh(111) single crystal samples, estimates of CO adsorp-tion/reactive sites were obtained and are shown in Fig. 10a in

S.M. McClure, D.W. Goodman / Chemical Physics Letters 469 (2009) 1–13 11

terms of Rh sites per cm2 (j, black square symbol). STM imageswere obtained (in a separate system) for various Rh coverages(hRh = 0.25–4) on ultrathin SiO2 films grown on Mo(112).Fig. 10b shows the mean particle diameter (in Å) and min andmax particle diameters (red dotted lines) observed in the imagesfor each of the coverages studied (hRh = 0.25–4). Shown inFig. 10c is a representative image for these studies for a 1 ML Rh/SiO2/Mo(112) surface. Using particle diameter, z-height measure-ments and assuming hemispherical shaped particles, STM imagescan be utilized to obtain an additional estimate of the number ofRh active sites per cm2. Estimates based on hRh = 1–4 images areshown in Fig. 10a (N, blue triangle symbol). As the data demon-strate, reasonable agreement is achieved between the reactivity,TPD, and STM estimations of Rh active sites per cm2. These obser-vations provide evidence supporting the surface insensitivity of theCO oxidation reaction on Rh/SiO2 model catalyst samples under COdominant conditions.

Reactivity measurements were conducted using various gasmixture compositions (O2/CO) to investigate the PCO and PO2 partialpressure dependencies of the CO2 oxidation reaction on the Rh/SiO2/Mo(112) catalyst samples. Shown in Fig. 11 are reactivitymeasurements (TOF vs. 1000/T(K)) obtained on Rh/SiO2/Mo(112)samples (with a constant Rh coverage of hRh = 0.25) and compara-

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Fig. 11. CO2 reaction rate data (TOF (CO2 molecules formed/site sec) vs. 1000/T(K))performed for various O2/CO gas mixture ratios over a Rh/SiO2(�5 ML)/Mo(112)surface with hRh � 0.25 ML Rh coverage: (j) (1/10) O2/CO; (d) (1/5) O2/CO; (N)(1/2) O2/CO (.) (1/1) O2/CO. Shown as open symbols are corresponding reactiondata taken on Rh(111) single crystal sample: (h) (1/10) O2/CO; (s) (1/5) O2/CO; (D)(1/2) O2/CO (r) (1/1) O2/CO. Particle data are normalized using the number of siteestimate obtained for hRh � 0.25 ML coverage in Fig. 9. Data from Ref. [7].

ble data for a Rh(111) single crystal for various gas mixtures (1/1,1/2, 1/5, 1/10) O2/CO mixtures. Particle data are normalized to thenumber of sites calculated from the data of Fig. 9b for hRh = 0.25equivalent sample (8.5 � 1013 Rh sites). As Fig. 11 illustrates, thedata exhibit similar partial pressure dependencies (positive orderin O2, negative order in CO) as seen on the Rh single crystal dataand technical catalyst data, as evidenced by an increase in the reac-tion rate as a function of increased O2 content. Additionally, nor-malization of the particle data, using site estimates obtainedwith (1/10) O2/CO measurements (Fig. 9), produce reasonableagreement with the Rh single crystal data. Rh(111) single crystalTOF values obtained at (1/1) O2/CO exhibit TOF values consistentwith previous studies [25]. Thus, Rh/SiO2/Mo(112) samples exhibitsimilar behavior within the CO inhibited regime as does Rh(111)[7].

As the O2/CO ratio is increased and reaction temperatures areincreased, Rh particles begin to exhibit deactivation, with repeatedreactions showing decreasing TOF values. Exhibited in Fig. 12a andb is a series of measurements conducted on hRh = 0.25 Rh/SiO2/Mo(112) samples. In panel (a), Rh reactivity measurements showrepeatable behavior over the linear, CO inhibited range. Reactivitymeasurements as shown can be taken repeatably over this rangewith TOF values within measurement error. As the reaction tem-perature is increased to T � 600 K, the reaction rate begins to roll-over; subsequent repeated reactions (denoted by numeric order) athigh temperatures begin to decrease. To further investigate this ef-fect, a new sample hRh = 0.25 Rh/SiO2/Mo(112) was prepared andreactions run in a similar fashion, as shown in panel (b). Reactionswere repeatable over the linear CO inhibited range as shown in pa-nel (a). A reaction was then immediately run at high temperature(T = 650 K), followed by a subsequent reaction at T = 525 K. Asthe data illustrate, the reactivity of the sample has decreased.The sample is then reduced in pure CO (red arrow; 20 min inP = 8 Torr CO at T = 525 K) and a reaction is the run again on thesample at T = 525 K; as the results show nearly �80–90% of thereactivity was reclaimed due to the CO reduction treatment. Theseresults indicate that the Rh particles undergo deactivation at suffi-ciently high temperatures and oxidizing conditions, consistentwith observations on Rh single crystal surfaces (albeit at lowerO2/CO gas mixture ratios). The reclamation of catalytic activity,upon reduction in pure CO, suggests that a substantial portion ofthis deactivation is due in part to particle oxidation. For the caseof hRh = 0.25 Rh particles, this point appears to occur at T > 600 Kfor (1/1) O2/CO mixtures; deactivation conditions could be ex-pected to differ for different Rh coverages.

Oxidation and reduction cycling of Rh and Pd particles has beenobserved and studied in previous investigations. Electron micros-copy and diffraction studies by Rupprechter et al. [51] of aluminasupported Rh particles and investigations by Dudin et al. [65] ofMgO-supported Rh particles have demonstrated oxidation/reduc-tion of Rh particles by treatments in O2/O (oxidative) and H2

(reductive) environments. Investigations by Penner et al. [41] havedemonstrated oxidation of Pd particles on Al2O3(0001) to PdO(small particles) and the existence of a thin PdO film (on large par-ticles) under high (25 Torr) O2 pressures and moderate tempera-tures (T � 400 K). Heating of the sample (under ambient10�7 Torr O2) indicates that this oxide decomposes nearT � 750 K, roughly �200 K higher than expected for bulk PdO.Experiments conducted by Hayek and coworkers [19,67] demon-strate using selected-area electron diffraction (SAED) and trans-mission electron microscopy (HR-TEM) that under P = 1 bar O2,T = 673 K conditions, Pd/SiO2 particles disperse and form a PdOphase, with PdO growth occurring on top of Pd particles [67].Reduction in pure CO (P � 10 mbar at T = 525 K) was required toachieve reduced particles. Consistent with lowered reactivity ofoxidized particles, recent molecular beam experiments probing

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Fig. 12. (a) High temperature reactions run over hRh = 0.25 ML Rh/SiO2(�5 ML)/Mo(112) sample with (1/1) O2/CO. Deactivation at higher temperatures run insequential order (1–5). TOF normalized to number of Rh sites obtained from (1/10)O2/CO experiments over hRh = 0.25 ML. (b) hRh = 0.25 ML Rh/SiO2(�5 ML)/Mo(112)sample run with (1/1) O2/CO mixtures. Deactivation at higher temperatures run insequential order (1)–(3). Between (2) and (3) a 20min reduction in P = 8 Torr pureCO at T = 525 K is conducted. TOF normalized to number of Rh sites obtained from(1/10) O2/CO experiments over hRh = 0.25 ML. Data from Ref. [7].

12 S.M. McClure, D.W. Goodman / Chemical Physics Letters 469 (2009) 1–13

the reactivity of Pd surfaces (single crystal and particles) byGabasch et al. [19] have demonstrated that CO oxidation reactivitydecreases with increased oxidation state (chemisorbed oxy-gen > surface oxide > bulk oxide), under low pressure conditions.Current kinetic measurements are being conducted on Pt/SiO2

model catalyst samples to investigate this oxygen deactivationbehavior; if it is indeed related to the oxygen adsorption energy,one would expect this deactivation to be less pronounced for Pt(Rh < Pd < Pt). This may enable reactions to be run on Pt particlesat higher temperatures and TOF, without substantial particle deac-tivation due to oxidation.

As the results indicate, CO oxidation kinetics under elevatedpressures on Rh/SiO2/Mo(112) appear to exhibit similar activationenergies, and PO2, PCO partial pressure dependence as observed inprevious studies on Rh single crystals and technical catalysts, un-der CO dominant reaction conditions. Similarly, reasonable agree-ment between measurements of reactive Rh sites per cm2 viaTPD, STM, and O2/CO (1/10) reactivity data (Fig. 10a) support thestructure insensitivity of the CO oxidation reaction under CO dom-inant conditions. Substantial particle deactivation, consistent withoxidation, is observed on Rh particles under higher temperatures(T > 600 K) for O2/CO ratios > (1/1). Care must be taken to under-stand the conditions and temperatures at which particles oxidizeand exhibit irreproducibility, as these can directly affect the TOFvalues at high temperatures and high O2/CO reaction conditions.These findings indicate that model catalyst samples can providea useful means for studying reaction kinetics at elevated pressures.In this sense, CO oxidation reactions, conducted under CO domi-nant conditions, can provide a useful analytical tool to characterizeactive sites on model catalyst samples for investigations of surfacesensitive reactions (e.g., NO + CO) [25] on Rh particles.

While these results have provided insights into the reactionkinetics on model catalyst samples under elevated pressures, morequestions remain. Ongoing work is aimed at the characterization ofother Pt-group metal particles (Pt and Pd), understanding the hightemperature deactivation, and understanding the relation betweenparticle deactivation and oxygen adsorption energies of these me-tal particles.

5. Concluding remarks

Recent and ongoing investigations of CO oxidation at elevatedpressures using high pressure surface-sensitive techniques (PM-IRAS) have produced new insights into this well-studied reaction.Recent results in our laboratory, conducted on Pt-group metals(Pd, Pt, Rh) from UHV to high pressures, suggest that Langmuir–Hinshelwood kinetics can adequately describe reaction kineticsacross the pressure range, when mass transfer and oxidation ef-fects are appropriately considered. The most highly active surfaceat low and high pressures appears to be a CO uninhibited surfacecontaining chemisorbed oxygen. In other words, there appears tobe no ‘pressure gap’ with respect to the active mechanism for COoxidation on Pt-group metals. CO oxidation on Ru surfaces (Ru,(1 � 1)-O/Ru(0001), RuO2(110)) exhibits a more subtle kineticpicture. Under most relevant reaction conditions, a chemisorbedoxygen surface is the most thermodynamically stable phase andthe most active phase for CO oxidation; under highly oxidizingand low temperature conditions, RuO2 can exhibit high activity fora brief period of time. This activity is likely due to a weakly boundoxygen species and is deactivated due to carbonate formation.These results indicate that the differences in Ru reactivity at highand low pressure can be understood in terms of CO and oxygenbinding; thus, like Pd, Pt, and Rh, there exists no ‘pressure gap’for Ru surfaces. Recent investigations on Rh/SiO2/Mo(112) modelcatalysts prepared in situ under UHV conditions exhibit similar ki-netic behavior and structure insensitivity within the CO inhibitedregime compared to Rh single crystals. These findings demonstratethat Rh/SiO2/Mo(112) model catalysts are ideal for investigationsof CO oxidation kinetics over a wide dynamic range of reactionconditions.

S.M. McClure, D.W. Goodman / Chemical Physics Letters 469 (2009) 1–13 13

Acknowledgements

We gratefully acknowledge the support for this work by theDepartment of Energy (DOE), Office of Basic Energy Sciences, Divi-sion of Chemical Sciences under Grant Number DE-FG02-95ER14511, and the Robert A. Welch Foundation.

References

[1] I. Langmuir, Trans. Far. Soc. 17 (1921) 621.[2] D.W. Goodman, C.H.F. Peden, M.S. Chen, Surf. Sci. 601 (2007) L124.[3] M.S. Chen, Y. Cai, Z. Yan, K.K. Gath, S. Axnanda, D.W. Goodman, Surf. Sci. 601

(2007) 5326.[4] F. Gao, Y. Cai, K.K. Gath, Y. Wang, M.S. Chen, Q.L. Guo, D.W. Goodman, J. Phys.

Chem. C, in press.[5] F. Gao, S.M. McClure, Y. Cai, K.K. Gath, Y. Wang, M.S. Chen, Q.L. Guo, D.W.

Goodman, Surf. Sci. 603 (2009) 65.[6] D.W. Goodman, C.H.F. Peden, M.S. Chen, Surf. Sci. 601 (2007) 5663.[7] S.M. McClure, M. Lundwall, F. Yang, Z. Zhou, D.W. Goodman, J. Phys. Chem. C,

submitted for publication.[8] F. Gao, Y. Wang, Y. Cai, D.W. Goodman, Surf. Sci., submitted for publication.[9] T. Engel, G. Ertl, Adv. Catal. 28 (1979) 1.

[10] T. Engel, G. Ertl, J. Chem. Phys. 69 (1978) 1267.[11] I.Z. Jones, R.A. Bennett, M. Bowker, Surf. Sci. 439 (1999) 235.[12] S.B. Schwartz, L.D. Schmidt, G.B. Fisher, J. Phys. Chem. 90 (1986) 6194.[13] J.H. Miners, S. Cerasari, V. Efstathiou, M. Kim, D.P. Woodruff, J. Chem. Phys. 117

(2002) 885.[14] X. Xu, D.W. Goodman, J. Phys. Chem. 97 (1993) 7711.[15] L.W.H. Leung, D.W. Goodman, Catal. Lett. 5 (1990) 353.[16] Y. Kim, S.K. Shi, J.M. White, J. Catal. 61 (1980) 374.[17] E. Lundgren et al., J. Elec. Spec. Rel. Phenom. 144 (2005) 367.[18] B. Brandt, T. Schalow, M. Laurin, S. Schauermann, J. Libuda, H.-J. Freund, J. Phys.

Chem. C 111 (2007) 938.[19] H. Gabasch, A. Knop-Gericke, R. Schlögl, M. Borasio, C. Weilach, G. Rupprechter,

S. Penner, B. Jenewein, K. Hayek, B. Klötzer, Phys. Chem. Chem. Phys. 9 (2007)533.

[20] H.-I. Lee, J.M. White, J. Catal. 63 (1980) 261.[21] A. Böttcher, H. Conrad, H. Niehus, J. Chem. Phys. 112 (2000) 4779.[22] J.T. Kiss, R.D. Gonzalez, J. Phys. Chem. 88 (1984) 892.[23] C.H.F. Peden, D.W. Goodman, J. Phys. Chem. 90 (1986) 1360.[24] P.L. Berlowitz, C.H.F. Peden, D.W. Goodman, J. Phys. Chem. 92 (1988) 5213.[25] C.H.F. Peden, D.W. Goodman, D.S. Blair, P.J. Berlowitz, G.B. Fisher, S.H. Oh, J.

Phys. Chem. 92 (1988) 1563.[26] S.H. Oh, G.B. Fisher, J.E. Carpenter, D.W. Goodman, J. Catal. 100 (1986) 360.[27] N.W. Cant, P.C. Hicks, B.S. Lennon, J. Catal. 54 (1978) 372.[28] J.T. Kiss, R.D. Gonzalez, J. Phys. Chem. 88 (1984) 898.[29] S.H. Oh, C.C. Eickel, J. Catal. 128 (1991) 526.[30] C.H.F. Peden, P.L. Berlowitz, D.W. Goodman, in: Proceedings of the Ninth Int.

Conf. on Catal., 1988, p. 1214.[31] B.L.M. Hendriksen, S.C. Bobaru, S.C.J.W.M. Frenken, Catal. Today 105 (2) (2005)

234.[32] B.L.M. Hendriksen, J.W.M. Frenken, Phys. Rev. Lett. 89 (2002) 046101.[33] B.L.M. Hendriksen, S.C. Bobaru, J.W.M. Frenken, Surf. Sci. 552 (2004) 229.[34] M.D. Ackermann et al., Phys. Rev. Lett. 95 (2005) 255505.[35] M.D. Ackermann, Ph.D. thesis, Leiden University, 2007. A digital version of this

thesis is available at <http://www.physics.leidenuniv.nl/sections/cm/ip>.[36] J. Gustafson et al., Phys. Rev. B 78 (2008) 045423.[37] X. Su, P.S. Cremer, Y.R. Shen, G.A. Somorjai, J. Am. Chem. Soc. 119 (1997) 3994.[38] G. Rupprechter, T. Dellwig, H. Unterhalt, H.-J. Freund, J. Phys. Chem. B 105

(2001) 3797.[39] H. Over, M. Muehler, Prog. Surf. Sci. 72 (2003).[40] R. Rössler, S. Günther, J. Wintterlin, J. Phys. Chem. C 111 (2007) 2242.[41] S. Penner, P. Bera, S. Pedersen, L.T. Ngo, J.T.W. Harris, C.T. Campbell, J. Phys.

Chem. 110 (2006) 24577.

[42] V.P. Zhadnov, B. Kasemo, J. Catal. 170 (1997) 377.[43] H.K. Cammenga, Evaporation mechanisms of liquids, in: E. Kaldis (Ed.), Current

Topics in Materials Science, North-Holland, Amsterdam, vol. 5, 1980, p. 335.[44] P. Atkins, Physical Chemistry, second ed., W.H. Freeman and Co, New York,

2000.[45] D.G. Lofler, L.D. Schmidt, Ind. Eng. Chem. Fund. 3 (1977) 362.[46] C.R. Henry, Surf. Sci. Rep. 31 (1998) 231.[47] M.S. Chen, A.K. Santra, D.W. Goodman, Phys. Rev. B 69 (2004) 155401.[48] T. Schroeder, J. Giorgi, M. Bäumer, H.-J. Freund, Phys. Rev. B 66 (2002) 165421.[49] T. Schroeder, M. Adelt, B. Richter, M. Naschitzki, M. Bäumer, H.-J. Freund, Surf.

Rev. Lett. 7 (2000) 7.[50] X. Xu, D.W. Goodman, Appl. Phys. Lett. 61 (7) (1992) 774.[51] G. Rupprechter, K. Hayek, H. Hofmeister, J. Catal. 173 (1998) 409.[52] D.N. Belton, S.J. Schmeig, Surf. Sci. 202 (1988) 238.[53] V. Matolín, K. Mašek, M.H. Elyakhloufi, E. Gillet., J. Catal. 143 (1993) 492.[54] S. Andersson et al., J. Chem. Phys. 108 (1998) 2967.[55] M. Frank et al., Chem. Phys. Lett. 279 (1997) 92.[56] V. Nehasil, I. Stará, V. Matolín, Surf. Sci. 352 (1996) 305.[57] V. Matolín, M.H. Elyakhloufi, K. Mašek, E. Gillet, Catal. Lett. 21 (1993) 175.[58] Y. Zhu, L.D. Schmidt, Surf. Sci. 129 (1983) 107.[59] S. Labich, A. Kohl, E. Taglauer, H. Knözinger, J. Chem. Phys. 109 (1998) 2052.[60] A. Kohl, S. Labich, E. Taglauer, H. Knözinger, Surf. Sci. 454 (2000) 974.[61] S. Labich, E. Taglauer, H. Knözinger, Top. Catal. 14 (1–4) (2001) 153.[62] A. Berkó, F. Solymosi, J. Catal. 183 (1999) 91.[63] L. Óvári, J. Kiss, Appl. Surf. Sci. 252 (2006) 8624.[64] J.B. Park, J.S. Ratliff, S. Ma, D.A. Chen, J. Phys. Chem. C 111 (2007) 2165.[65] P. Dudin, A. Barinov, L. Gregoratti, D. Scaini, Y.B. He, H. Over, M. Kiskinova, J.

Phys. Chem. C 112 (2008) 9040.[66] Y. Zhang et al., J. Am. Chem. Soc. 130 (2008) 5868.[67] S. Penner et al., J. Chem. Phys. 125 (2006) 094703.

Sean M. McClure received his BS in Chemical Engi-neering from the University of Iowa in 2001. In 2006, heobtained his PhD in Chemical Engineering from theUniversity of Texas at Austin under the supervision ofProf. C. Buddie Mullins. In 2008 he joined the researchgroup of Prof. D.W. Goodman in the Department ofChemistry at Texas A&M University as a post-doctoralresearch associate. His primary research interests arekinetic investigations of heterogeneous catalysis onmodel catalyst surfaces at elevated pressures.

D. Wayne Goodman received his BS in Chemistry fromMississippi College and his PhD from the University ofTexas studying with M.J.S. Dewar. After NATO post-doctoral studies in Germany and at NBS (now NIST) hejoined the research staff at NBS in 1978. In 1980 hebecame a research scientist at Sandia National Labora-tories where he served as Head of the Surface ScienceDivision. In 1988 he joined the faculty of the ChemistryDepartment at Texas A&M where he is currently Dis-tinguished Professor and Robert A. Welch Chair.