J. Chem. Phys. 114, 4669 (2001); https://doi.org/10.1063/1.1342240 114, 4669

© 2001 American Institute of Physics.

The CO oxidation kinetics on supported Pdmodel catalysts: A molecular beam/in situtime-resolved infrared reflection absorptionspectroscopy studyCite as: J. Chem. Phys. 114, 4669 (2001); https://doi.org/10.1063/1.1342240Submitted: 08 August 2000 . Accepted: 29 November 2000 . Published Online: 01 March 2001

J. Libuda, I. Meusel, J. Hoffmann, J. Hartmann, L. Piccolo, C. R. Henry, and H.-J. Freund

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The CO oxidation kinetics on supported Pd model catalysts: A molecularbeam Õin situ time-resolved infrared reflection absorptionspectroscopy study

J. Libuda,a) I. Meusel, J. Hoffmann, and J. HartmannFritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany

L. Piccolob) and C. R. HenryCRMC2-CNRS, Campus de Luminy, Case 913, F-13288 Marseille Cedex 9, France

H.-J. FreundFritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany and CRMC2-CNRS, Campus de Luminy, Case 913, F-13288 Marseille Cedex 9, France

~Received 8 August 2000; accepted 29 November 2000!

Combining molecular beam techniques and time-resolved infrared reflection absorptionspectroscopy~TR-IRAS! we have studied the kinetics of the CO oxidation reaction on analumina-supported Pd model catalyst. The Pd particles are deposited by metal evaporation underultrahigh vacuum~UHV! conditions onto a well-ordered alumina film, prepared on a NiAl~110!single crystal. Particle size, density and structure of the Pd deposits have been characterized inprevious studies. In the low temperature region, transient and steady-state experiments have beenperformed over a wide range of CO and oxygen fluxes by crossing two effusive molecular beamson the sample surface. We determine the steady-state CO2 production rate as a function of the COfraction in the impinging gas flux. Simultaneously, the occupation of CO adsorption sites understeady-state conditions is monitored byin situ IR spectroscopy. The origin of different types of CO2

transients is discussed. In particular we focus on the transient CO2 production after switching off theCO beam. For the model catalyst investigated, detailed reaction rate measurements in combinationwith time-resolved IRAS show that the origin of the particular transient behavior of the supportedmodel system is not due to the presence of specific adsorption sites on small particles, as has beenproposed previously. Instead, we show that the transient behavior can be semiquantitativelysimulated on the basis of a simple kinetic model considering a homogeneous surface, andaccounting for the inhibition of the dissociative adsorption of O2 at high CO coverage. Moreover,it is discussed how the inherent heterogeneity of the supported particle system can additionallyenhance the observed effect. ©2001 American Institute of Physics.@DOI: 10.1063/1.1342240#

I. INTRODUCTION

What is the molecular origin of the kinetics of heteroge-neously catalyzed reactions on supported metal systems be-ing in many cases drastically different from the correspond-ing single crystal surfaces? As possible answers to thisfundamental question in catalysis a number of concepts havebeen proposed. Among these are size effects, which mayalter the electronic properties of small metal particles1,2 andthus the adsorption properties, geometrical effects due to thepresence of specific adsorption sites or the combination ofadsorption sites~see, e.g., Refs. 3–7!, the influence ofboundary sites at the metal oxide interface,8 metal supportinteractions,9–11 or support effects involving spilloverfrom/to or reaction on the support~see, e.g., Refs. 4, 12, 13,and references therein!. Recently, Zhdanov and Kasemohave pointed out that there might be a substantial influence

on the kinetics due to the coupling between nonequivalentadsorption sites by diffusion.5–7 The presence of such non-equivalent sites such as different facets, edge, defect or par-ticle boundary sites is an inherent property of complex cata-lysts which cannot be modeled in studies on simple singlecrystal surfaces.

In spite of the numerous concepts, specific examples arescarce where there is clear experimental evidence concerningthe origin of the effects mentioned before. The reasons forthis lack of detailed knowledge is connected to what is usu-ally referred to as the ‘‘materials gap’’ between surface sci-ence and catalysis~see, e.g., Refs. 14, 15!. Real catalysts areusually hardly accessible by surface science methods. More-over, their vast complexity may preclude an assignment ofkinetic effects to specific molecular features of the system.On the other hand, the mentioned effects may not be presenton simple single crystals, but only on such complex surfaces.

As a possible solution to this materials problem, a vari-ety of supported model catalysts have been developed. Thesemodels allow us to introduce certain aspects of a real catalystin a well controlled fashion, while still maintaining the ex-perimental accessibility. Supported model catalysts have

a!Author to whom correspondence should be addressed. Electronic mail:libuda@fhi-berlin.mpg.de; Fax:149-30-8413-4309.

b!Present address: DSM/DRECAM/SPCSI, CEA Saclay, F-91191 Gif-sur-Yvette Cedex, France.

JOURNAL OF CHEMICAL PHYSICS VOLUME 114, NUMBER 10 8 MARCH 2001

46690021-9606/2001/114(10)/4669/16/$18.00 © 2001 American Institute of Physics

been prepared based on both, oxide single crystal surfaces~see Ref. 4 and references therein! and thin ordered oxidefilms.15–19 Here, we will employ a Pd/Al2O3 model systembased on a thin well-ordered alumina film prepared onNiAl ~110!. The structure and defect structure of the oxidefilm20,21as well as the growth, geometric structure, and elec-tronic structure and the adsorption of Pd particles and othermetals supported on this film have been extensively ad-dressed in a series of previous studies.15–17,19,22–24

Once the structural properties of the supported modelcatalyst are well characterized, the next step is to establish acorrelation between the structure and the microscopic kinet-ics. Here, molecular beam techniques represent the methodof choice,25–28 providing a maximum of control over the re-actant flux and its interaction with the sample. Ertl andEngel29,30 have early applied beam methods to study the ki-netics of the CO adsorption and oxidation and on Pd~111!. Inthis study, they were able to demonstrate that the reactionfollows a Langmuir–Hinshelwood~LH! mechanism and theysucceeded to determine the LH activation barriers in variouscoverage regions. A variety of similar studies on other singlecrystal surfaces have been published since~see, e.g., Refs.31–33!. The previous work on supported model systems hasbeen reviewed recently, and will not be discussed here indetail.4,22 It has to be pointed out, however, that, in spite ofthe successful experiments on single crystals, beam studieson the CO oxidation on supported catalysts are extremelyrare. This is mainly due to the experimental problems men-tioned before. As a consequence it remains unclear, to whatextent size effects might be present on the system.4 Amongthe few examples for beam studies on supported model cata-lysts, Stara´ et al. have studied the transient CO2 productionon Pd/Al2O3 as a function of particle size and substratestructure,34,35 and found indications for significant size ef-fects. No indications for a similar effect could be found in arecent study using the same model system as employed inthis work.36 Becker and Henry have investigated, by molecu-lar beam methods, the CO oxidation reaction on Pd/MgO~100! model catalysts.37,38 Variations in the steady state pro-

duction of CO2 with particle size were observed. However,these differences in activity were mainly due to the effect ofthe capture, by the Pd particles, of the CO physisorbed on thesupport~reverse-spillover! which is equivalent to an increaseof the effective pressure on the particles when their size de-creases~see Sec. III H!.38 In addition, the transient behaviorupon switching off the CO beam was considered in thiswork. A characteristic peak was found, the intensity of whichincreased with decreasing cluster size~for the same experi-mental conditions!. The origin of this peak was attributed tolow coordinated~edge and defect! sites, present on the sup-ported particles. As has been proposed previously,3 thosesites may bind CO more strongly and thus they may give riseto a pronounced transient CO2 production peak due to theincreased residence time for CO at these sites.

In this study, we apply an experimental approach com-bining molecular beam techniques and TR-IRAS. Wepresent a detailed study of the CO oxidation on well-shapedalumina supported Pd crystallites under steady-state andtransient conditions. It is shown that the transient CO2 peak,which is also found for the present model system, is notrelated to specific adsorption sites. Instead we discuss twopoints: First, we will show that the transient kinetics of theCO oxidation reaction—including the behavior of the CO2

peak under discussion—can be qualitatively simulated froma simple ‘‘mean-field’’ kinetic model based on the dataavailable on Pd~111!. Secondly, we will argue, how the in-herent heterogeneity of a supported catalyst may additionallyenhance the observed effect.

II. EXPERIMENT

All experiments were performed in a new UHV molecu-lar beam/surface spectroscopy apparatus at the Fritz-Haber-Institute~Berlin!, which has been described in the literature,recently.39 Schematically, the setup is displayed in Fig. 1.

The system has been specifically designed for kineticstudies on complex model systems. It offers the experimentalpossibility of up to three beams being crossed on the sample

FIG. 1. Schematic representation ofthe molecular beam/IR spectroscopyexperiment.

4670 J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 Libuda et al.

surface. The CO and O2 beams, which were mainly used inthe work presented here, are generated by two effusivesources based on multichannel arrays. This design allows aneasy variation of the beam intensities over several orders ofmagnitude, without any change in the beam properties. Beammodulation is provided by a computer-controlled shutter lo-cated inside the second pumping stage of the beam sources.To avoid artifacts due to only partial overlap, the beam di-ameter was chosen such that it exceeds the sample surface.The mean beam intensity variation over the sample surfacewas determined to be 6%. The average variation in theCO/O2 beam intensity ratio over the sample surface was es-timated to be less than 2%~from beam profile measure-ments!. Both sources were operated at room temperature.High purity oxygen ~AGA, .99.999%! and CO ~AGA,.99.996%! were used, the latter of which was further puri-fied using al 2N2 cold trap;. The beam intensities and pro-files were measured with a beam monitor as describedelsewhere.39 In order to avoid artifacts due to an effusive gasload upon closed beam at extreme CO/O2 flux ratio, the CO2

transients were recorded by opening/closing the inlet valvesof the beam sources~pressure rising/dropping time, 500/2000 ms!. The transient CO2 peaks observed were in allcases significantly slower than the CO pressure change. Atmoderate flux ratio transients were recorded by modulatingthe beam source via the internal shutter~opening/closingtime, 50 ms! to verify that there was no significant influenceof the modulation procedure on the transient behavior. Thethird source generates a beam from a supersonic expansionand has not been used in the present study.

Angular-integrated gas phase measurements were per-formed with a quadrupole mass spectrometer~ABB Extrel!which is not in line-of-sight of the sample. Additionally, thesystems allows time- and angular-resolved measurements viaa differentially pumped rotatable quadrupole mass spectrom-eter. The gas phase detection can be combined with time-resolved in situ FT-IR reflection absorption spectroscopy~TR-IRAS! employing a vacuum FT-IR spectrometer~Bruker IFS 66 v!. For the time resolved spectra with a tem-poral resolution of 1 s several transients were accumulated toimprove the signal to noise ratio. For this purpose the beamshutter was synchronized with the recording of the IR spec-tra. Additionally, a MIR polarizer was used to select only the

p-component of the IR light and thus further improve theS/N ration. For a detailed description of the experimentaldetails we again refer to the literature.39 All spectra wererecorded at a spectral resolution of 8 cm21. The total acqui-sition time for the in situ IR spectra was typically 50s/spectrum and for the TR-IR spectra 5 s/spectrum.

Briefly, the sample was prepared by sputtering and an-nealing of a NiAl~110! single crystal, followed by an oxida-tion and annealing procedure, the details of which are givenelsewhere.20,21 The cleanliness and quality of the oxide filmwas checked via LEED and AES. Before the actual experi-ment, the active metal component~Pd,.99.9%! was depos-ited under well controlled conditions by evaporation from arod ~1 mm diam! using a commercial evaporator~Focus,EFM 3! based on electron bombardment. During deposition,the crystal was biased with a retarding voltage in order toprevent ions from being accelerated towards the sample~point defect creation!. The evaporator flux was calibrated bya quartz microbalance prior to use. Details concerning typi-cal deposition rates and conditions are given in Table I. Afterpreparation the Pd particles were stabilized by oxygen andCO exposure as discussed previously.36

III. RESULTS AND DISCUSSION

A. Characterization of the structural and adsorptionproperties of the supported Pd model catalyst

The growth and structure of Pd deposits as well as othermetals on Al2O3 /NiAl ~110! has previously been studiedover a wide range of conditions and the results have beenextensively reviewed, recently.16,17,22–24In this work, we willfocus on one particular set of preparation conditions~substrate temperature, 300 K; Pd coverage, 2.731015atoms cm22!, for which detailed information on boththe particle size and density and on the particle structure isavailable. In the following, we will briefly review the mainfeatures of this system. In Figs. 2~a! and 2~b! STM images~CCT, Constant Current Topography, from Ref. 36! areshown. The Pd particles form three-dimensional islands;their density typically amounts to 131012 islands cm22.From this number it can be estimated that the average Pdparticle will contain about 2700 atoms, corresponding to anaverage particle size of 5.560.7 nm. Please note that in the

TABLE I. Preparation conditions and structural parameters for the Pd particles on Al2O3/NiAl ~110!.

Deposition parametersPd coverage~atoms•cm22! 2.731015

Deposition temperature~K! 300Deposition rate~atoms•cm22

•s21! 931012

Structural parametersIsland density~cm22! 1.0(60.2)31012 ~Refs. 22, 36!Number of Pd atoms/island ;2700 ~Refs. 22, 36!Fraction of support covered by Pd 0.20(60.02) ~Refs. 22, 36!Fraction of surface Pd atoms 0.20(60.03) ~Refs. 22, 36!Average island size 5.560.7 nm ~Refs. 22, 36!Epitaxial orientation ~111! ~Refs. 16, 22, 24!Island structure crystalline, predominantly~111! facets,

small fraction of~100! facets~Refs. 22, 24!

4671J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 CO oxidation kinetics on supported Pd

STM image the islands appear larger than the above valuedue to an instrumental convolution with the tip shape. As aconsequence of preferential nucleation the island appear notto be completely homogeneously distributed over the supportbut will decorate oxide point and line defects~e.g., domainboundaries, see Ref. 21!. In the close-up shown in Fig. 2~b!,it becomes evident that most of the Pd particles exhibit thewell-defined morphology of a nanocrystallite. As indicatedin the schematical representation@Fig. 2~c!#, the particlesgrow in ~111!-orientation, preferentially exposing~111!-facets and only a small fraction of~100!-facets. The regularstructure of the Pd crystallites has been demonstrated in arecent STM study showing atomic resolution both on the topand side facets of the particles.24 This and other structuralinformation, which will be used in the following discussion,is summarized in Table I.

Before the kinetic experiments the particles were stabi-lized by extended oxygen exposure~typically ;100 L, 1 L5131026 Torr s!, followed by several CO oxidation cyclesof the type shown in this work. This stabilization procedurehas been shown to be necessary to obtain a stable oxidation

kinetics.36 As observed by STM,36 neither the particle den-sity nor the particle shape are significantly affected by theprocedure. The chemical processes related to the stabilizationprocedure will be discussed elsewhere.40

Two types of experiments were performed to ensure thatthe model surface principally resembles the behavior of atypical Pd surface:~1! We have probed the CO adsorption/desorption kinetics employing modulated molecular beamtechniques. In a temperature region from 430 to 530 K, wehave determined an activation energy for desorption of 13669 kJ mol21 and a pre-exponential factor of 1014.960.9s21

from the temperature dependence of the CO residence timein the low CO coverage region. Details of the experimentwill be given elsewhere.41 This value is in excellent agree-ment with experiments on a Pd~111! single crystal@desorp-tion energy, 13468 kJ mol21, pre-exponential factor,1014.460.8s21 ~Ref. 30!#. ~2! The activation barrier for theLangmuir–Hinshelwood reaction step has been determinedin a series of beam experiments at high oxygen~and CO!coverages.36 The apparent activation barriers of 6268 kJ mol21 ~and 5968 kJ mol21! are as well in goodagreement with Pd~111! single crystal data@5968 kJ mol21

~Ref. 29!# under similar conditions. Thus we conclude thatthe general adsorption and reaction behavior of the supportedmodel catalysts under consideration appears rather similar tothat of a Pd~111! single crystal surface In the following sec-tions we will investigate the steady state and in particular thetransient CO2 production in more detail. Thus we will at-tempt to elucidate, whether the reaction behavior of the sys-tem may still differ from the single crystal and how thesedifferences might be addressed experimentally.

B. General types of transient behavior

Before we come to a systematic discussion of the tran-sient and steady-state behavior of the CO oxidation, we willgive a short description on how most of the experiments inthis study were performed and how the different types oftransients can be qualitatively understood.

The general types of CO2 transients are shown in Fig. 3.The corresponding experiments were performed in the fol-lowing manner: After preparation and stabilization of themodel system, the surface is continuously exposed to an O2

beam with a beam fluxFO2, generated by one of the effusive

beam sources~see Fig. 1!. The integral CO2 production rateis recorded simultaneously. At a timetCO,on~see Fig. 3! a CObeam with a beam fluxFCO, generated by the second effu-sive source~see Fig. 1!, is switched on. As flux parameterswe define~1! xCO, the CO fraction with respect to the totalflux, as

xCO5FCO

FCO1FO2

~1!

and~2! ptotal, the total effective pressure of CO and O2 at thesample position,

ptotal5pCO1pO2with pi5Fi~2pmikT!0.5, ~2!

FIG. 2. ~a! STM image ~CCT, Constant Current Topography, 3000 Å33000 Å! of the Pd particles grown at 300 K on Al2O3 /NiAl ~110!, fromRef. 36; ~b! differentiated close-up STM image of the Pd particles~CCT,Constant Current Topography, 3000 Å33000 Å!, from Ref. 36;~c! sche-matic representation of a supported Pd particle.

4672 J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 Libuda et al.

where Fi is the partial flux of componenti at the sampleposition,mi represents its molecular mass, andTi is the tem-perature describing the velocity distribution~please note thatTi5300 K as there is no cooling of any degree of freedom inthe effusive sources!. Typically, in a series of experiments,the CO flux fraction was varied, whereas the other param-eters such as the total effective pressure, the sample tempera-ture etc. were kept constant.

In general two different types or transients can be ob-served as a function of the CO flux fraction, which, in thefollowing, we will denote as type 1 and type 2~see Fig. 3!,respectively. The initial transient behavior has recently beendiscussed by Piccoloet al.42,43

Type 1: If the experiment is performed at low CO andhigh O2 flux, a simple type of transient behavior is observedas displayed in the lower trace in Fig. 3 (xCO50.16). Westart from a situation, in which the system has reached itsoxygen saturation coverage under the given flux and tem-perature conditions. Upon switching on the CO beam(tCO,on), we find an instantaneous increase~on the time scaleof the experiment! in the CO2 production. As pointed out byPiccoloet al.43 this is due to the rapid formation of a steady-state precursor coverage. From this precursor, the CO mol-ecule may either desorb or chemisorb and subsequently reactto CO2. In the course of the reaction oxygen vacancies aregenerated, which lead to an increased adsorption probabilityfor the CO precursor and thus an increase in the reaction rate.After this transient period (t tr), the system finally reaches thesteady-state reaction conditions. Please note that the steady-state is characterized by a high oxygen coverage and low COcoverage. After switching off the CO beam (tCO,off), the ad-sorbed CO is consumed and the CO2 production rate de-creases continuously.

Type 2: If an equivalent experiment is performed at suf-ficiently high CO fraction~Fig. 3, upper trace,xCO50.42!, adifferent behavior is found. Again, starting from an oxygensaturated surface, we observe an instantaneous increase inthe CO2 production rate upon switching on the CO beam.Subsequently, the reaction rate increases slowly until it

reaches a first maximum~Fig. 3, tmax 1!. As for transient type1, the increasing reaction rate can be explained by an in-creasing chemisorption probability of the CO precursor, dueto the decreasing oxygen coverage.43 Once a certain CO cov-erage is reached, however, the CO oxidation rate decreasesagain. This is a consequence of the strongly inhibiting effectof CO on the O2 adsorption~for a general discussion see,e.g., Ref. 44, and references therein!. Finally, a steady-stateis reached, which is characterized by a high CO coverageand a low reaction rate~due to the inhibiting effect of CO!.Once the CO beam is terminated (tCO,off), the process isreversed: the adsorbed CO is consumed in the course of thereaction, leaving free adsorption sites for O2 adsorption.Thus the O2 adsorption is accelerated and so is the reactionrate, which reaches a maximum (tmax 2), before it finally de-creases due to depletion of the CO reservoir.

C. Catalyst stability under reaction conditions

In a second step, we will briefly consider the catalyststability under the reaction conditions applied in this study.There are two main processes which may have an influenceon the catalytic activity:~1! Prolonged exposure to the reac-tant gases in combination with elevated reaction tempera-tures may lead to structural rearrangements or changes in theparticle dispersion. For example, in a recent study it has beenshown that Pd particles may undergo drastic shape variationsunder the influence of oxygen.45 ~2! CO dissociation maylead to a build-up of surface carbon, which may result in adeactivation of the catalyst. The formation of surface carbonhas previously been observed for the CO oxidation reactionon small Pd particles46 and there have been indications forcarbon formation on Pd single crystal surfaces at higher COpressure.47

Concerning a possible restructuring of the particles werefer to a previous study focusing on the interaction of oxy-gen with the same Pd model catalyst and its influence on thekinetics of the CO oxidation.36 It was shown that under thereaction conditions applied here, neither the Pd particleshape nor the particle dispersion are significantly affected bythe oxygen pretreatment or by subsequent oxidation cycles.With respect to the possibility of surface carbon formation,we consider the experiment displayed in Fig. 4. Four differ-ent transients at high CO flux fraction~transient type II! areshown for comparison. The transients were recorded underidentical conditions directly after an initial extended oxygenexposure~oxidation cycle 1!, and after 1 to 22 subsequenttransient experiments~see Fig. 4!. It is found that neither thesteady-state CO2 production rate nor the shape or the inten-sity of the second CO2 peak ~after termination of the CObeam! changes as a function of reaction time. The only ob-servable change is related to the initial CO2 production peak,which is slightly smaller during the first oxidation cycles.

As mentioned before, in a recent work employingSTM,36 we could exclude major structural rearrangementssuch as a spreading of the Pd under the influence of theoxygen as an origin of the increased adsorption capacity afterseveral adsorption/reaction cycles. In the same study, how-ever, a strongly reduced CO2 peak was found before ex-

FIG. 3. Transient behavior of the CO2 production rate~continuous O2 beamand a modulated CO beam! under CO-rich conditions~upper trace, transienttype II! and O2-rich conditions ~lower trace, transient type I! ~TSample

5415 K, ptotal51.031026 mbar!.

4673J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 CO oxidation kinetics on supported Pd

tended oxygen exposure. This observation suggests that alsoin the initial cycles shown in Fig. 4 the stabilization process~see Sec. III A! is not fully completed, leading to a loss ofoxygen on the Pd particles during the experiment. After theinitial cycles a stable oxidation kinetics evolves, showingglobal desorption and reaction rates consistent with previousexperiments on Pd~111! surfaces~see Sec. III A!.

Another indication that there is no significant carbon for-mation, is derived from thein situ IR spectra, which will bediscussed in Sec. III E. For CO dissociation on Pd~111!Hammoudeh47 has shown that surface carbon formation re-sults in a strongly enhanced occupation of on-top sites. Noindications for such changes have been found in the presentexperiments.

Thus we may conclude that under the reaction conditionsand time-scales applied here, there is no substantial deacti-vation mechanism present, which may significantly affect theresults presented in the following sections.

D. Systematic studies of the transient behavior andsteady-state CO 2 production

In order to obtain a general overview over the reactionbehavior we have performed a series of transient beam ex-periments at different reaction temperatures. A typical seriesof experiments at a sample temperature of 440 K is displayed

in Fig. 5. The sample was exposed to a continuous oxygenbeam. Its intensity was chosen such that the total effectivepressure after switching on the CO beam wasptotal51.031026 mbar. The CO flux fractionxCO was varied between0.05 and 0.95 and for eachxCO a transient was recorded.

In general, we find the two types of transient behavior,discussed in Sec. III B. ForxCO<0.37 the transients are oftype 1 ~oxygen-rich steady state, see Fig. 3!, whereas forxCO>0.47 we find a CO rich steady-state, connected with thecharacteristic CO2 production rate maxima~transient type 2!.The transition region between the two regimes will be dis-cussed separately in Sec. III F.

We start considering some basic features of the experi-ment. With respect to the characteristics of the first transientCO2 peak, it is found that the time constant at which thesteady-state is reached after switching on the CO beam di-verges close to the transition region. This is easy to under-stand as the CO coverage is build up by the excess of ad-sorbed CO ~not consumed during the reaction!, whichvanishes at the transition between the two regimes. The in-tegral below the transient peak, however, remains nearlyconstant forxCO>0.52. This is explained by the fact that thepeak integral roughly corresponds to the oxygen precover-age, which is constant and will be fully consumed during thereaction. Contrary to the first transient peak, the width of thesecond transient peak~upon switching off the CO beam! in-creases with increasingxCO since the O2 flux and thereforethe reaction rate decreases. The integral over the second tran-sient peak as a function ofxCO will be considered in detail inSec. III F. It corresponds to the fraction of CO adsorbed inthe steady-state that upon termination of the CO beam willnot desorb but react to CO2. In general, it will show a step-like behavior similar to the first transient peak. In contrast tothe adsorbed oxygen, the CO, however, may desorb on thetime-scale of the reaction at high reaction temperatures andlow O2 flux, which may lead to a decreasing peak integral athigh xCO.

From the transients in Fig. 5 we can derive a plot of thesteady state reaction rate as a function of the CO flux fractionxCO as displayed in Fig. 6. Qualitatively, it can be understoodas follows: At low xCO ~on the oxygen-rich side!, the CO2

production is proportional to the CO partial pressure. It re-flects the sticking coefficient of CO under the particularsteady-state conditions, i.e., under high oxygen and low COcoverage. At a certain criticalxCO, the system switches from

FIG. 4. Comparison of the transient behavior of the CO2 production rateunder identical conditions as a function of reaction time~TSample5415 K,ptotal51.031026 mbar!.

FIG. 5. Transient behavior of the CO2

production rate as a function of theCO fraction in the impinging gas fluxxCO ~TSample5440 K, ptotal51.031026 mbar!.

4674 J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 Libuda et al.

an oxygen-rich to a CO-rich steady-state. This transition ap-pears to occur continuously over a significant range of fluxratios, which will be discussed in detail in Secs. III F–III H.The transition is connected to a sudden decrease in the reac-tion rate as a consequence of the inhibiting effect of CO onthe dissociative adsorption of O2 ~see, e.g., Ref. 44!.

Comparing the three different reaction temperatures dis-played, we find a behavior which is easily explained quali-tatively: In the low CO coverage region, the reaction rate isalmost independent of reaction temperature. This indicatessimilar steady-states under these conditions which are closeto oxygen saturation. Generally, the CO sticking coefficient,which governs the reaction rate, is expected to decreaseslightly with increasing temperature due to the decreasinglifetime of the CO precursor state both on the alumina sub-strate and on the Pd particles. This effect, however, is tooweak to be identified clearly from the present data. Note thatalthough the CO steady-state coverage is small, it will de-pend on the reaction temperature, i.e., the steady-state COcoverage will increase with decreasing surface temperature.Therefore, at low temperature the transition point, at whichdue to inhibition of oxygen adsorption the system switches tothe CO-rich state, is reached at lowerxCO. In the high COcoverage region the rate is limited by the oxygen adsorptionstep. The reaction rates do not coincide due to the increasedCO steady-state coverage at lower temperatures~slower de-sorption and reaction rates! which give rise to an increasedinhibition of the O2 adsorption.

Similar data have been derived for the CO oxidation ona number of catalytically active transition metal surfaces. Asthe most relevant reference case we briefly consider the workof Hammoudeh,47 who has studied the CO and O2 adsorptionand CO oxidation on Pd~111! applying in situ IR and work-function measurements. It was shown that the reaction rateon Pd~111! exhibits a rather complex behavior, showing twomaxima/hysteresis regions as a function of the reactantfluxes. Both strongly depend on certain internal parameterssuch as the pretreatment of the sample. Hammoudeh con-nected the first maximum to the inhibition effect explained

above and tentatively assigned the second maximum to reac-tivity variations related to changes in the dispersion of ad-sorbed CO islands. In the case of small metal particles, theformation of large adsorbate islands may be prevented by thelimited size of the crystallite facets. This, as a general differ-ence between single crystal and supported particle systems,may—among other effects—give rise to a suppression of asecond reactivity maximum and thus to a simpler overallbehavior of the reaction system.

Moreover, it was observed in the same study47 that thekinetics is not stable under certain reaction conditions, butgradually changes during the reaction itself. This behaviorwas related to the slow formation of subsurface and bulkoxygen and the situation was found to be stabilized afterprolonged oxygen exposure. In accordance with this inter-pretation, we find that the oxygen-pretreated particles used inthis study, show a stable oxidation kinetics, even after re-peated oxidation-reduction cycles.

In future work, the data of the type presented here, mayprovide a basis for more extensive microkinetic simulationsof the steady state and transient reaction behavior. It has tobe pointed out that such simulations—in connection withsteady-state reaction rate measurements for different types ofstructurally well-characterized supported model catalysts—may be an excellent tool to identify kinetic effects character-istic for supported nanoparticles.5 In a recent series of papersZhdanov and Kasemo have discussed this issue5–7 in connec-tion with two types of effects, namely, the influence of com-munication between different facets on the nanocrystallitesand the role of support mediated adsorption.

E. In situ IR absorption spectroscopy under steady-state conditions

In order to obtain further insight into the occupation ofCO adsorption sites under steady-state conditions, we haveacquired IR reflection absorption spectra under reaction con-ditions. The spectra were taken after the steady-state wasreached in transient experiments of the type shown in Fig. 5.For the example displayed in Fig. 7 the surface temperaturewas 415 K.

Based on a comparison with IR spectra of CO on differ-ent Pd single crystal surfaces,48–55 we may assign the ob-served features. Tu¨shaus,52 for example, has studied CO onclean and roughened Pd~111! surfaces. Hammoudeh47 hasinvestigated the coadsorption of oxygen and CO using IRspectroscopy. The spectral features observed for CO ad-sorbed on Pd particles supported on Al2O3 /NiAl ~110! werepreviously investigated by Wolteret al.23 over a large rangeof surface coverages and at different sample temperatures. Atroom temperature~and above! three types of adsorbed COcan be identified. At low coverage CO adsorbed in hollowsites is the dominating species~;1830 cm21!, mainly fromthreefold hollow sites on the~111!-facets, since there will beonly a small fraction of CO adsorbed on the minority of~100! sites and the signal will be additionally damped due tothe inclination angle with respect to the sample. With in-creasing coverage an increasing occupation of bridge sites~;1900–1950 cm21! is observed. In contrast to low sampletemperatures, however, thermal line-broadening at room

FIG. 6. Steady-state CO2 production rate as a function of the CO fraction inthe impinging gas fluxxCO (ptotal51.03106 mbar).

4675J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 CO oxidation kinetics on supported Pd

temperature and higher prevents a differentiation betweenCO bonded to regular and defect sites.23 With the surfacecoverage increasing further, a weak absorption signal in theenergy region typical for terminally bonded CO appears~;2060 cm21!.

Qualitatively, the CO spectra under steady state reactionconditions~Fig. 7! appear very similar to the observationsfor adsorption of pure CO described above. CO bonded inhollow sites~Fig. 7, No. 3!, in bridge sites~Fig. 7, No. 2! andon-top CO~Fig. 7, No. 1! can be distinguished. The quanti-tative development of the IR spectra as a function ofxCO isdisplayed in Fig. 8. The integral CO absorption signal@Fig.8~a!, regime I# initially ~below xCO50.2! increases almostlinearly with increasing CO flux fraction. This observation isin line with a LH rate constant being largely independent ofCO coverage in the low CO flux limit~note that the IRabsorption will be almost linear with the coverage in the lowcoverage limit, whereas at high coverages strong deviationsmay occur, see, e.g., Refs. 48, 56–58!. In the transition re-gion ~Fig. 8, regime II!, where the systems switches to theCO-rich reaction regime, the CO absorption signal increasesrapidly, indicating also a sudden increase in CO coverage.From this point on~Fig. 8, regime III,xCO.0.4!, hardly anyfurther changes can be detected in the CO absorptionspectrum.

Figure 8~b! shows the relative fraction of the three prin-cipal CO species as derived from a fit of the spectra dis-played in Fig. 7. At lowxCO, CO in hollow sites representsthe majority species. The fraction of bridge CO increasesslowly with increasing CO fraction, before in the transitionregime a sudden increase in the population of bridge sites isobserved, whereas the signal from the CO in hollow sitesremains almost constant. Simultaneously, a small fraction ofCO populates on-top sites. In comparison to single crystals,this on top signal under steady-state conditions appears atlower energies~2062 cm21! and is present at lower sampletemperatures~Ref. 47, see also Refs. 51 and 53!. This effectmay be related to adsorption at typical defect sites on thesupported particles such as steps or edges.

FIG. 7. In situ IR reflection absorption spectra under steady-state reactionconditions as a function of the CO fraction in the impinging gas fluxxCO

~TSample5415 K, ptotal51.031026 mbar!.

FIG. 8. ~a! Integral absorption for the IRA spectra presented in Fig. 7;~b!relative absorption in the regions characteristic for CO adsorbed in~1! on-top, ~2! bridge-bonded, and~3! hollow sites;~c! vibrational frequencies ofthe CO species marked in~b!. Additionally, the oxygen-rich region~I!, theCO-rich region~III !, and transition regime~II ! are indicated.

4676 J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 Libuda et al.

In Fig. 8~c! we investigate the influence of the CO fluxfraction xCO on the CO stretching frequency. The data wasalso derived from fits of the spectra displayed in Fig. 7. Ashas been discussed early~see, e.g., Ref. 59, and referencestherein!, the CO stretching frequency may serve as a measureof the local CO density. The shifts are mainly due to twoeffects, namely changes in the CO–Pd bonding induced byneighboring CO molecules~chemical effects! or by couplingof the dynamic dipoles.59 Quantitatively, the contributionsfor Pd~111! have been estimated by Tu¨shaus.52 It has to bepointed out that a quantitative relation of the CO stretchingfrequency to the local CO coverage is complicated by theinfluence of coadsorbed oxygen, which also perturbs theelectronic structure of Pd and thus the Pd–CO bond. Forexample, Hammoudeh47 has shown that preadsorbed oxygenon Pd~111! may result in a shift of the CO singleton fre-quency from 1803 cm21 to 1822 cm21.

Although these effects preclude a straightforward quan-titative analysis, we may, however, derive some indicationson the CO distribution from the vibrational frequencies dis-played in Fig. 8~c!. Up to a xCO of 0.2, the shift in thestretching frequency of CO bound to hollow sites is small~;1 cm21!, indicating that there are only minor changes inthe local environment of adsorbed CO molecules. Similarly,this observation holds for CO bound to bridge sites up toxCO<0.2. Thus, we may conclude that the formation of in-creasingly large CO islands under these conditions is un-likely and the CO can be anticipated to exist in a largelydiluted form in an oxygen-rich surrounding. The local COdensity changes suddenly and almost exclusively in the tran-sition range between the oxygen-rich and the CO rich re-gime. This is indicated by a shift of approximately 28 cm21

to higher frequency, both for the CO in bridge and hollowsites.

The transition between the two regimes, however, doesnot show a step-like behavior, but continuously extends overa finite range ofxCO values. This effect will be further dis-cussed in the following sections.

F. Transient CO switching-off behavior

Among the different kinetic effects on supported cata-lysts, surface diffusion mediated coupling between differenttypes of adsorption sites has recently attracted considerableattention.5–7 The presence of different types of adsorptionsites in close proximity is an intrinsic property of complexcatalytic systems. Such types of adsorption sites may, e.g.,be different facets on nanocrystallites, step, edge or otherdefect sites, interface sites located at the particle boundary oradsorption sites on the support material itself. Zhdanov andKasemo5 have extensively reviewed simulations of such cou-pling processes. They have shown that relatively moderatechanges in the sticking coefficients or reaction/adsorption en-ergetics may drastically change the reaction rate under cer-tain conditions.

Still, the experimental verification of these effects is notstraightforward and very few examples exist in the literature.Recently, Becker and Henry have observed a transient be-havior in an experiment, similar to the transient experimentsshown in this work, which was related to such an heteroge-

neity effect.4,37,38Upon switching-off the CO beam, they ob-served a sudden decrease of the CO2 production rate, fol-lowed by a smaller CO2 production peak.

Becker and Henry originally suggested anexplanation,4,37,38followed by simulations,60 which involvesrapid desorption of CO from regular sites~resulting in therapid decrease in CO2 production!. Based on previousstudies,3,61 CO adsorbed at low coordinated edge sites~theproportion of which is increasing with decreasing clustersize!, is assumed to be bound more strongly. Thus these siteswill remain fully saturated until the CO reservoir on the fac-ets is largely depleted. Only after vacancies among thesedefect are generated, the reaction rate increases again. Thiseffect is assumed to result in the smaller maximum in theCO2 production rate.

Since the Pd crystallites in the present study are com-posed of similar facets~111! and ~100!, a similar effect isexpected for the present model system, but with a weak sec-ond peak due to the rather large particle size. Therefore, wehave investigated the transient behavior under a variety offlux and temperature conditions~Fig. 9!. Indeed, we find thatunder certain flux conditions we observe a transient CO2

peak, which appears very similar to the effect originally re-ported by Becker and Henry. After an initial sudden decreasein the reaction rate, the CO2 production shows a peak, whichstrongly depends on the experimental conditions. The effectonly starts to appear close to the transition between theoxygen-rich and the CO-rich reaction regime.

To further investigate the origin of the peak, we havesystematically varied the CO flux fractionxCO between bothreaction regimes. The result for two reaction temperatures isdisplayed in Fig. 9. The inset shows the integral CO2 pro-duction in the switching-off peak. As a function ofxCO theintensity varies continuously. At high CO ratio it developsinto the typical transient peak observed at high CO coverage,which is a simple consequence of the inhibiting effect of COon the O2 adsorption. At low CO fraction it first decreasesand finally vanishes completely.

In contrast to the previously suggested explanation, thiscontinuous intensity variation indicates that the effect, atleast for the present supported model catalyst is not related toa characteristic type of defect sites connected to the Pd par-ticle size. Instead, the effect appears to involve all CO ad-sorption sites available on the particle.

The crucial point with respect to the previous explana-tion is related to the assumption that the sudden decrease inthe reaction rate is related to the spontaneous desorption ofCO upon termination of the CO beam. To verify, whethersuch an effect exists, we have performed a time-resolvedinsitu IRAS experiment@see Fig. 10~b!# simultaneously withthe reactivity measurements@Fig. 10~a!#. Under conditions,where the sudden decrease and transient peak are observedwe have recorded IR absorption spectra with a temporalresolution of 1 s. In order to improve the signal/noise ratiothe full reaction sequence was automatically repeated. In Fig.10, spectra which were obtained by accumulation of 5 tran-sients are displayed. The IRA spectra~bridge and hollowabsorption peaks! were integrated and the results are shownin the inset@Fig. 10~c!#. No indication for a sudden decrease

4677J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 CO oxidation kinetics on supported Pd

in the CO absorption signal is found. Instead the absorptionpeaks decreases continuously with a slope roughly resem-bling the reaction rate~note that due to the nonlinearcoverage-absorption dependency there is no simple corre-spondence between the two quantities, see Sec. III E!

This result, in line with the previous intensity argument~Fig. 9!, clearly demonstrates that the observed behavior can-not be a result of spontaneous CO desorption from regularsites and strong binding to defect sites. Instead, the effectappears to be related to the simple CO2 production peakunder conditions of high CO flux. This suggestion is consis-tent with an estimate of the CO surface residence times basedon the desorption data given in Sec. III A~Ea,des513669 kJ mol21, Ades51014.960.9s21!. Using these values, wecalculate a residence times of 200 s~20 s! for a surface

temperature of 415 K~440 K! at low CO coverage. Indeed,this estimate indicates that no sudden desorption is expectedon the time scale under discussion. In the following we willdiscuss, which alternative effects may have to be taken intoconsideration in order to account for the observed transientbehavior.

The experiments discussed before, have shown that theCO2 peak~1! starts to appear only in the transition betweenthe two reaction regimes and~2! is not related to specificdefect sites~at least for the model system studied here!. Tak-ing this into account we may consider the following alterna-tive explanations, which may give rise or additionally con-tribute to the observed effect:

~1! Local beam intensity variations:Due to beam intensityvariations in the CO and O2 beam, the sample surfacewill locally be exposed to different effective pressuresand flux ratios. From beam profile measurements, a stan-dard deviation of the local pressurepeff of typically66% can be estimated. The standard deviation of theflux ratio xCO is typically 62%. The width of the transi-tion region~see Fig. 9!, however, is significantly broader~typically in the order of610% of the xCO value atwhich the transition occurs!. Thus, in a first approxima-tion this contribution may be neglected.

~2! CO poisoning induced peaks:As discussed in Sec. III G,the transient behavior can be semiquantitatively simu-lated using a simple homogeneous ‘‘mean-field’’ modelbased on kinetic parameters from separate adsorptionand reactivity studies on Pd~111! and the supported Pdparticles. It is shown that the transient CO2 productionrate in the transition region may show a behavior similarto the one observed experimentally, as a consequence ofthe competition between the decreasing CO coverageand increasing O2 sticking probability.

~3! Effects induced by the heterogeneity of the system:Theeffect, described in~2! may be additionally enhanced bythe inherent heterogeneity of the supported model cata-lyst. On the model surface, every Pd particle represents alargely isolated reaction system. Yet, the individual ad-sorption and reaction rates may differ considerably.Thus, different steady states may coexist under givenconditions, with the total reaction rate being a superpo-sition of the individual states. The influence of this effecton the transients will be discussed in Sec. III H.

G. Simulations

It has been shown in the previous section that the pecu-liar transient behavior of the reaction rate was not related tothe presence of specific highly-binding sites on the clusters.At this point it is tempting to simulate the steady state andtransient experiments on the basis of a ‘‘mean-field’’ kineticmodel of a homogeneous surface. Such modeling may pro-vide some indications, which features may at least qualita-tively be described by the simple kinetic model and wherethe complexity of the system will have to be explicitly con-sidered in the model.

Let us consider a Pd~111! single-crystal surface. Adsor-bates are supposed to be randomly distributed on this sur-

FIG. 9. Transient behavior of the CO2 production rate as a function of theCO fraction in the impinging gas fluxxCO close to the transition between thetwo reaction regimes (ptotal53.431027 mbar). The inset shows the integralover the peak which appears upon termination of the CO beam.

4678 J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 Libuda et al.

face. The overall CO oxidation reaction can be expanded inits elementary steps:

CO1sad2k1→COad, ~3!

COad2k2→CO1sad, ~4!

O212sad2k3→2Oad, ~5!

COad1Oad2k4→CO212sad. ~6!

sad refers to a free adsorption site.ki is the rate constant ofstepi.

The CO2 desorption step (CO2ad→CO2) being muchfaster than the other steps,29 it is not necessary to expand step~4!. Equations~3!–~6! can be converted to the following sys-tem of coupled differential equations, which describes thetime evolution of the CO coverageuCO and of the oxygencoverageuO ~see, e.g., Ref. 42!,

duCO

dt5k1SCO2k2uCO2k4uCOuO

with

SCO5SCO0 S 12

uCO

uCOmax2CTs

uO

uOmaxD , ~7!

duO

dt52k3SO2

2k4uCOuO

with

SO255 SO2

0 S 12uCO

uCOmax2

uO

uOmaxD 2

; 12uCO

uCOmax2

uO

uOmax>0

0; 12uCO

uCOmax2

uO

uOmax,0

.

~8!

SCO0 and SO2

0 are the initial~i.e., at zero coverage! sticking

coefficients of CO and O2, respectively. We will useSCO0

50.96(;Ts) andSO2

0 50.78– 7.431024 Ts , whereTs is the

sample temperature.30 For the maximum coverages, we as-sumeuCO

max50.5 anduOmax50.25. Those values correspond to

the saturation coverages on Pd~111! at Ts>300 K.62

It should be noted that we have chosen simple~not al-ways realistic! Langmuir’s Laws for the sticking coefficients.The only correction concerns the influence of preadsorbedoxygen on sticking of CO@CTs

(uO/uOmax), Eq. ~7!#. It has

early been recognized that there is no strong inhibiting effectof preadsorbed oxygen on the sticking coefficient of CO onPd~111!.29 However, to account for the reaction rate at highoxygen coverage and, therefore, to produce a qualitativelycorrect transient behavior upon impingement of the CObeam on the oxygen saturated sample, this minor effect hasto be taken into account. Erikson and Ekedahl63 and Piccoloet al.43 have described the inhibiting effect of oxygen intro-ducing a temperature dependent coefficientCTs

@see Eq.~7!#,

which is physically related to the presence of a precursor toCO chemisorption. Over the limited temperature range inthis study, we have neglected its moderate temperature de-pendence and have chosenCTs

50.3 to fit the experimental

transient behavior in Fig. 5. Note that the exact choice of thisparameter is not of relevance with respect to the followingsemiquantitative discussion.

In the case of the present experiments, the initial condi-tions ~i.e., just before switching on the CO beam! areuCO(t5tCO,on)50 and uO(t5tCO,on)5uO

max50.25. Besides,step 1@Eq. ~3!# no longer exists att.tCO,off . Thus, the ad-sorption term in Eq.~7! disappears during this period. Therate constants of the four steps are

FIG. 10. Combined reactivity~a! andtime-resolved IR reflection absorptionexperiment ~b! during the transientregion upon termination of the CObeam ~TSample5415 K, ptotal53.431027 mbar!. ~c! Integral absorptionfor the IRA spectra presented in~b!and partial absorption in the bridge~2!and hollow~3! regions.

4679J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 CO oxidation kinetics on supported Pd

k15FCO

nPd, ~9!

k25v2 expS 2E2

kBTsD , ~10!

k35FO2

nPd, ~11!

k45v4 expS 2E4

kBTsD . ~12!

nPd is the surface density of Pd atoms on a~111! face: nPd

51.5331015cm22 s21. v i and Ei are the frequency factorand the activation energy related to stepi, respectively. Onthe basis of the available literature, these values have beenchosen asv251015.0s21, E25136 kJ/mol, v45107.9s21,and E4560 kJ/mol. Engel and Ertl have determinedE4

559 kJ/mol~Ref. 29! for the oxidation step at high coverageandv251014.460.8s21 andE2513468 kJ mol21 for the COdesorption on Pd~111!.30 For the model system used here, wehave recently measuredv251014.960.9s21 and E2513669 kJ mol21 for the CO desorption~see Sec. III A! and v4

5107.061.0s21 and E456268 kJ/mol at low reaction tem-peratures and high oxygen and moderate CO coverages.36,40

Numerical integration of Eqs.~7! and ~8! allows us toderive the temporal evolution of CO and O coverages. TheCO2 production rate is then given by

r CO25k4uCOuO. ~13!

Results of such calculations for typical reaction conditions~Tsample5440 K, ptotal51.031026 mbar! are plotted in Figs.11 and 12. As an example, we display the two principle typesof transients, i.e., under oxygen-rich conditions~Fig. 11,lower trace,xCO50.2, transient type 1! and under CO-richconditions~Fig. 11, upper trace,xCO50.5, transient type 2!.The additional traces in Fig. 11 represent the CO and Ocoverages. A comparison with the experimental data in Figs.3, 5, and 13 shows that the main features of the shape of theexperimental reaction rate curve are well reproduced. In par-ticular, the initial CO2 peak~after CO beam switch-on! andthe second CO2 peak~after CO beam switch-off! under COrich conditions are well described in a semiquantitative man-ner. A close comparison with the transient in Fig. 13 re-corded under roughly the same conditions~second curve

FIG. 11. Simulated transient behavior of the CO2 production rate, the COcoverage and the oxygen coverage for oxygen rich~bottom tracexCO50.2!and CO rich ~top trace, xCO50.5! reaction conditions~TSample5440 K,ptotal51.031026 mbar!.

FIG. 12. Simulated transient behaviorof the CO2 production rate as a func-tion of the CO fraction in the imping-ing gas flux xCO ~TSample5440 K,ptotal51.031026 mbar!. The insetshows the transient behavior in thetransition region between the oxygenrich and the CO rich reaction regime.

4680 J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 Libuda et al.

from top, TSample5440 K, ptotal51.0331026 mbar, xCO

50.52! reveals slight deviations with respect to the shapeand the width of both peaks.

In general, these deviations can be considered to be aconsequence of the simplistic kinetic model:~a! The exactcoverage dependence of both the CO and O2 sticking coef-ficients is not taken into account, e.g., the complex influenceof precursor states of Ref. 43,~b! the coverage-dependencyof energy barriers is not considered, and~c! any influence ofthe microscopic arrangement of the adsorbates~islanding,etc.! is completely neglected~see, e.g., Ref. 64!. Addition-ally, we disregard all heterogeneity effects, which may existon the supported model catalyst. Again, it must be under-lined that our purpose is not to fit the experimental data, butto explore to what extend the kinetics can be qualitativelyexplained by a simple homogeneous surface model. It has tobe pointed out that considering the present lack of detailedand quantitative information on the reaction system, any fur-ther fit of the many parameter system, in particular introduc-ing more complex adsorption and reaction kinetics, does notappear reasonable.

From the transient behavior of the calculated coveragesof CO and oxygen~Fig. 11!, the first CO2 peak is easilyexplained by the fact that, in the considered excess-CO re-gime ~at steady state!, the initially higher oxygen coverageand lower CO coverage~as compared to the steady state! are

favorable for the reaction. The same phenomenon can ex-plain the second CO2 peak: when the CO beam is closed, COdepletion occurs due to reaction or desorption. Subsequently,the adsorption rate of O2 molecules increases, which resultsin a transient maximum of the CO2 production. When nomore CO is adsorbed, the CO2 peak vanishes. This peak canonly be observed in the CO-rich regime, where the dissocia-tive chemisorption of O2 is inhibited by chemisorbed CO.

In Fig. 12, a simulated series of CO2 transients is shown,for the same reaction conditions as the ones used in the ex-periment of Fig. 5~TSample5440 K, ptotal51.031026 mbar!.Considering the simplicity of the model, the agreement be-tween the experiment and the simulation is surprisinglygood. The oxygen-rich regime~corresponding to the absenceof any CO2 peak during the experiment, i.e., to transients oftype 1, see Fig. 3! is present untilxCO'0.35. The CO-richregime~corresponding to the presence of a CO2 peak duringthe closed-beam period, i.e., to transients of type 2! isreached fromxCO>0.45.

In the inset in Fig. 12 we consider the transient behaviorin the transition region between the oxygen-rich and the CO-rich regime in detail. It is noteworthy that upon terminationof the CO beam, a behavior is found which is qualitativelysimilar to the effect discussed in the previous section. Over asmall regime ofxCO values between 0.39 and 0.44 we find asudden decrease of the CO2 production followed by a peak inthe CO2 production. This subtle effect is a consequence ofthe competition between two contributions: On the one hand,the reaction rate immediately decreases due to the decreasingCO coverage upon termination of the beam. On the otherhand, the oxygen adsorption rate increases due to the de-creasing CO coverage. Due to the different kinetics of bothprocesses, the product of the coverages might reach a maxi-mum, giving rise to the CO2 peak at later time. Although thetransient behavior is qualitatively reproduced, some discrep-ancies remain, comparing the simulation in Fig. 12 with theexperimental results shown in Fig. 9. In general, the CO2

production dip in the simulations appears less pronouncedthan in the experiment. Similar to the discussion above, thediscrepancy may be partially related to the simplifications ofthe kinetic model. Additionally, the influence of the systemheterogeneity may result in an enhancement of this kineticeffect. This will be discussed in detail in Sec. III H.

Finally, we have simulated the experiments of Fig. 6,concerning the steady-state reaction rate at various flux ratiosand sample temperatures~for ptotal51.031026 mbar!. Asseen in Fig. 14, the result of such a simulation is again quali-tatively satisfying. The general shapes of the curves, the al-most linear increase in the high oxygen coverage region, andthe position of the maximum as a function of the surfacetemperature are rather well reproduced. The main differenceis related to the high CO-flux side. Whereas the inhibitingeffect of CO is reasonably well described for a surface tem-perature of 440 K, the inhibition is overestimated for lowersurface temperatures and underestimated for higher tempera-tures. To a great extent, this is a consequence of the fact thatthe current model disregards coverage dependencies of theCO adsorption energy and thus overestimates the CO desorp-tion rate at high temperature and underestimates desorption

FIG. 13. Transient behavior of the CO2 production rate as a function of theeffective total pressureptotal , close to the transition between the two reac-tion regimes (TSample5415 K).

4681J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 CO oxidation kinetics on supported Pd

at low temperature. On a supported model catalyst, this ef-fect may be particularly relevant due to the variety of avail-able adsorption sites and thus the wider range of adsorptionenergies.

In conclusion of this section, simple kinetic calculations,assuming a homogeneous surface, and using only a limitedset of parameters issuing from literature data, allow to simu-late the experiments with a good semiquantitative agreement.The behavior of the transient and steady-state rates as a func-tion of the gas partial pressures and of the surface tempera-ture is well described. In particular, with regard to the tran-sition region between the oxygen and CO rich reactionregimes, it is demonstrated that all the features—includingthe transient CO2 production peak—are semiquantitativelyreproduced by the homogeneous surface model.

A more quantitative agreement would require a refine-ment of the simulations, e.g.,~a! keeping the ‘‘mean-field’’approach, to improve the model, by using, e.g., coverage-dependent energy barriers for the CO desorption and theCO1O elementary reaction and more realistic sticking coef-ficients of CO and O2, or ~b! using a more local approach, toaccount for the fact that the surface of a supported modelcatalyst is heterogeneous and that the adsorbates are notequally distributed over the sample surface. As a basis, forsuch an attempt, a detailed knowledge of the energetics andkinetics of the surface–gas interaction and structural charac-teristics of the catalytic system is required, which is, how-ever, only partially available at present.

H. Discussion

In the previous section we have shown that the transientand steady-state kinetics can at least be semiquantitativelyunderstood assuming a simple homogeneous surface. How-ever, certain deviations remain, in particular with respect tothe transient CO2 peaks. In this final section we will end witha brief discussion, considering how the transient behaviormay be further modified by the heterogeneity of the modelsystem.

In general we have to distinguish between intraparticleheterogeneity~different sites or facets on a particle! and in-

terparticle heterogeneity~structural differences between indi-vidual particles!. In the first case, strong coupling betweenthese sites by surface diffusion has to be taken into account.5

In the latter case every metal particle represents a largelyisolated reaction system. Due to the weak adsorption of thereactants on the alumina support inter-particle diffusion isstrongly suppressed. Yet, individual adsorption and the reac-tion rates on the individual particles may differ considerably,e.g., due to differences in particle size, shape or structure. Asa consequence, different steady-state coverages may coexistunder given conditions on the individual particles. This ef-fect will be particularly important close to the transition re-gion between the two reaction regimes, where the surfacecoverages sensitively depend on the reaction parameters. Fi-nally, the total macroscopic reaction rate will be a superpo-sition of the individual particle rates, i.e., a superposition oftransients ranging from type 1~oxygen rich, Fig. 3! to type 2~CO rich, Fig. 3!. As a consequence, we expect that in agree-ment with the experimental data the characteristic transientCO2 production in the transition region will occur over awider range of CO fluxesxCO and may appear more pro-nounced.

On a microscopic level, there are several pathways viawhich the heterogeneity of the system may have an influenceon the adsorption and reaction rates:

~a! Variations of the local effective pressure:At low reac-tion temperatures a large fraction of the adsorbing re-actants is captured via trapping on the support and dif-fusion to the metal particles. This ‘‘capture zone’’effect has been early discussed by Gilletet al.65 andHenryet al.66 It has been quantitatively investigated inseveral studies~see, e.g., Refs. 34, 38, 67, 68!, recentlyalso on the model system used in this work.69 This newadsorption channel gives rise to the effect that the re-actant flux per Pd surface atom depends on both theparticle size and its surrounding. Due to overlapping ofthe capture zones, on a particle in close proximity toother particles the effective local pressure will be sig-nificantly lower than on an isolated particle. This effectis schematically illustrated in Fig. 15. Experimentally,the effect of the total effective pressurepeff at constantxCO is demonstrated in Fig. 13. With increasing totalpressure the transient behavior changes in a way simi-lar to what is found for an increasing CO flux fractionat constant total pressure, i.e., with increasing pressurethe transition region in a reactivity diagram shifts to-wards lower CO flux fraction.

~b! Variations in the sticking coefficientdue to variationsin the surface and defect structure.

~c! Variations in the desorption rateon different adsorp-tion sites~facets, edges, defects!.

~d! Variations in the reaction rate:Every particle repre-sents an ensemble of active sites with locally varyingactivation barriers. Thus the total reaction rate on theparticle depends on the ensemble of individual reactionsites.

It remains the question in how far these contributionscan be included in further microkinetic simulations to finally

FIG. 14. Simulated steady-state CO2 production rate as a function of the COfraction in the impinging gas fluxxCO (ptotal51.03106 mbar).

4682 J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 Libuda et al.

yield a more complete picture of the reaction kinetics on themodel system. On complex catalytic surfaces, on whichnaturally a certain degree of heterogeneity exists, such simu-lations will finally be necessary in order to identify andquantify the kinetic effects under discussion. As a basis, suchsimulations, however, require a detailed knowledge of theenergetic and structural properties of the model systems.Here, the available data is still rather limited. Whereas somedetailed and representative structural information is availablefrom STM and TEM~transmission electron microscopy!, thedata basis on gas–surface interaction and surface adsorption/reactions is largely restricted to single-component systemsand to ideal single crystal surfaces. Starting from thesesimple cases, studies on supported model catalysts with areduced and controllable level of complexity may provideway to address these questions.

IV. CONCLUSIONS

In conclusion, we have applied a combined molecularbeam/in situ IR reflection absorption spectroscopy approachto study the kinetics of the CO oxidation reaction understeady state and transient conditions. We employ a supportedPd model catalyst, based on an ordered Al2O3 film grown onNiAl ~110!. Previously, this systems has been characterizedwith respect to its structural and adsorption properties. Thesurface of the Pd islands is to a large extend dominated by~111!-facets. The particles are characterized by an averagesize of 5.5 nm and an island density of 1012cm22.

~i! In the temperature range~415–465 K! and pressurerange (<1026 mbar) investigated, we find no indica-tions for a significant deactivation of the model cata-lyst on the time scale of the experiment (;103 s).

~ii ! The steady-state reaction rate was systematically de-termined as a function of the CO flux fraction and thesurface temperature. The different reaction regimesare discussed.

~iii ! We have employedin situ IRA spectroscopy to followchanges in the occupation of adsorption sites underreaction conditions. Starting from O-rich reactionconditions, the linear increase in CO coverage withthe CO flux is accompanied by a increased occupationof bridge sites versus hollow sites, which are pre-dominantly occupied at low CO coverage. The vibra-tional frequencies are shifted due to the influence of

coadsorbed oxygen. In the transition region to theCO-rich regime, the sudden increase in the CO cov-erage is due to a strongly increased occupation ofbridge sites. Additionally, in the high CO-flux region,a small fraction of on-top sites is occupied which maytentatively be assigned to defects.

~iv! We have systematically investigated the transient be-havior of the system under conditions of continuousO2 flux and modulated CO flux. The different types oftransients are discussed. In particular we have inves-tigated the appearance of a transient CO2 peak afterswitching off the CO beam. Using time-resolved IRabsorption spectroscopy we demonstrate that the ob-served transient behavior close to the transitionregion—at least for the model system used in thisstudy—is not related to defect sites, as was proposedpreviously. Instead we suggest complementary expla-nations, which are related to~a! the CO induced inhi-bition of O2 adsorption and~b! the intrinsic heteroge-neity of the model system allowing coexistence ofdifferent steady states.

~v! With a simple kinetic model based on the Langmuir-Hinshelwood mechanism and on previous experimen-tal data available on Pd~111! and on the Pd modelsystem used here, we show that a large part of thereaction kinetics can be understood by considering ahomogeneous surface. The good semiquantitativeagreement between experiments and simulations isobtained over the range of flux and temperature con-ditions covered in this study. A close investigation ofthe CO2 production after closing of the CO beam sug-gests, that the transient behavior can be largely ex-plained by these simulations, taking into account theinterplay between CO desorption/reaction and the in-hibition of O2 adsorption by CO. Additionally, theeffect is expected to be enhanced by the surface het-erogeneity of the supported cluster system leading tocoexistence of a multitude of differing steady statesunder given reaction conditions.

ACKNOWLEDGMENTS

This project has been funded by the Max-Planck-Societyand the Deutsche Forschungsgemeinschaft. We are particu-larly grateful to O. Frank whose work related to solution ofnumerous technical difficulties with respect to the molecular

FIG. 15. Schematic representation ofthe surface coverages close to the tran-sition region between the oxygen-richand the CO-region. Depending ontheir structure and surrounding, someparticles may remain in the oxygen-rich state, whereas others switch to thehigh CO coverage region.

4683J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 CO oxidation kinetics on supported Pd

beam system was extremely valuable. We would like tothank the service groups for mechanics and electronics at theFHI which have been substantially involved in the realiza-tion of the molecular beam project. H.-J. F. is grateful for hissupport from CNRS as chercheur associe´ at CRMC2-CNRSin Luminy.

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