density functional theory and reaction kinetics studies of the water–gas shift reaction on pt–re...

DOI: 10.1002/cctc.201300365

Density Functional Theory and Reaction Kinetics Studiesof the Water–Gas Shift Reaction on Pt–Re CatalystsRonald Carrasquillo-Flores, Jean Marcel R. Gallo, Konstanze Hahn, James A. Dumesic, andManos Mavrikakis*[a]

Introduction

The water–gas shift (WGS, H2O + CO!CO2 + H2) reaction is tra-ditionally used to produce H2 from synthesis gas,[1] however itcan take place in any system in which CO and H2O are present,such as methanol synthesis,[2] methanol steam reforming,[3]

aqueous-phase reforming,[4] catalytic combustion,[5] and Fisch-er–Tropsch synthesis.[6] The WGS reaction has been studied ex-tensively on Cu-,[7] Fe-,[8] and Ni-based catalysts,[9] because theyare relatively inexpensive, and have high catalytic activities.

For the low-temperature WGS (LT-WGS) reaction, Cu-basedcatalysts have high performance, however they are pyrophoricand undergo deactivation by leaching; this can be attributedto the presence of condensed H2O or the formation of surfacecarbonates.[10] Therefore, noble metals and their alloys appearto be a promising alternative for LT-WGS reactions.[7a, f, 11] Pt-based catalysts have received special attention for LT-WGS re-actions, because they have been shown to be active andstable for fuel processing[11e] (e.g. , aqueous phase reforming ofbiomass-derived oxygenates[4a, 11i, 12]).

Several studies have reported on Pt atoms that are support-ed on metal oxides as catalysts for LT-WGS reactions.[6b, 11e, 13] Inthese studies, supports such as ceria,[14] zirconia,[15] and tita-nia[16] were found to play an important role owing to theirredox properties. Proposed mechanisms assume that the metaloxides oxidize CO, which assumes a reduced form; this is sub-sequently reoxidized by H2O-producing H2. Previously, we have

reported on the reaction mechanism over Pt that is supportedon alumina by using reaction kinetics studies, DFT calculations,and microkinetic modeling.[11i] This previous study revealed thereaction mechanism to be complex, extending beyond thestandard “surface-redox” mechanism, and it suggested that thecarboxyl species, COOH, is a key intermediate, which is respon-sible for the turnover rates, whereas its isomer, formate(HCOO), is a spectator species.[11i] Furthermore, a recent study,which combined experiments with theory, suggested that theactive sites for LT-WGS reactions may be partially oxidized Pt-centers with a single or a few metal atoms that are decoratedwith alkali ions.[11h]

Several recent reports have indicated that the presence ofRe has a beneficial role in the activity of Pt-based catalysts inthe LT-WGS reaction.[17] Although only a few experimental stud-ies have been performed to understand the role of Re, theyagree that Re provides an additional reaction pathway to acti-vate H2O.[17a, e, h] In these studies, however, Pt and Re atoms aresupported on titania and zirconia, which are promoters for Pt-catalyzed LT-WGS reactions themselves, and therefore theirpresence might obscure the isolation of the Re effect on theWGS reactions. In this work, a mechanistic study for the WGSreaction on Pt–Re catalysts supported on an inert support,carbon black Vulcan XC-72, is presented. In addition, DFT calcu-lations, microkinetic modeling, and reaction kinetics experi-mental data are used to probe the reaction mechanism. Pa-rameters from a DFT analysis of elementary steps ona Pt3Re(111) model surface are utilized to construct a compre-hensive mean-field microkinetic model[18] for probing thenature of the active sites on these catalysts.

Periodic, self-consistent density functional theory calculations(DFT-GGA-PW91) on Pt(111) and Pt3Re(111) surfaces, reactionkinetics measurements, and microkinetic modeling are em-ployed to study the mechanism of the water–gas shift (WGS)reaction over Pt and Pt–Re catalysts. The values of the reactionrates and reaction orders predicted by the model are in agree-ment with the ones experimentally determined; the calculatedapparent activation energies are matched to within 6 % of the

experimental values. The primary reaction pathway is predictedto take place through adsorbed carboxyl (COOH) species,whereas formate (HCOO) is predicted to be a spectator spe-cies. We conclude that the clean Pt(111) is a good representa-tion of the active site for the WGS reaction on Pt catalysts,whereas the active sites on the Pt–Re alloy catalyst likely con-tain partially oxidized metal ensembles.

[a] R. Carrasquillo-Flores, Dr. J. M. R. Gallo, Dr. K. Hahn, Prof. J. A. Dumesic,Prof. M. MavrikakisDepartment of Chemical & Biological EngineeringUniversity of Wisconsin-Madison1415 Engineering Dr. , Madison-WI, 53706 (USA)Fax: (+ 1) 608-262-9053E-mail : [email protected]

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cctc.201300365.

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 3690 – 3699 3690

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Results

The proposed reaction mechanism for the WGS reaction con-sists of 15 elementary steps [Steps (2)–(16)] , all of which arestudied thoroughly with DFT calculations on Pt(111) andPt3Re(111) model surfaces. The overall reaction is given byStep (1). These 15 reactions encompass the redox and theCOOH mechanisms:

COþ H2O$ CO2 þ H2 ð1Þ

H2Oþ * $ H2O* ð2Þ

COþ * $ CO* ð3Þ

H2O* þ * $ OH* þ H* ð4Þ

OH* þ * $ O* þ H* ð5Þ

2OH* $ H2O* þ O* ð6Þ

CO* þ O* $ CO2* þ * ð7Þ

CO2* $ CO2 þ * ð8Þ

2 H* $ H2 þ 2* ð9Þ

CO* þ OH* $ COOH* þ * ð10Þ

COOH* þ * $ CO2* þ H* ð11Þ

COOH* þ O* $ CO2* þ OH* ð12Þ

COOH* þ OH* $ CO2* þ H2O* ð13Þ

CO2* þ H* $ HCOO* * ð14Þ

HCOO** þ O* $ CO2* þ OH* þ * ð15Þ

HCOO** þ OH* $ CO2* þ H2O* þ * ð16Þ

Previous research has verified the plausibility of additionalspecies such as formyl;[19] however, these species are not ex-pected to play a major role in the reaction mechanism andthey will not be considered further in the present study. Inwhat follows, we start with a more detailed description of theDFT results for the elementary steps [Steps (2)–(16) above] onthe Pt3Re(111) surface, and we compare these results with therespective data on Pt(111). A summary of the results can befound in Table 1. Notably, the preferred binding site of CO,which is determined by DFT calculations (hexaganol closedpack/face-centered cubic)[11i, 20] differs from what is observed(atop) experimentally.[21] This has been discussed extensively inthe literature,[22] in which various possible explanations for thediscrepancy are offered, such as temperature effects and theself-interaction error in DFT. However, the relative stability ofCO on different adsorption sites on Pt(111) surfaces is rathersmall (<0.2 eV), and thereby for the reaction temperaturesconsidered here, the effect is expected to be minimal.

In general, all reaction intermediates are found to be stabi-lized further on the Pt3Re surface, compared to on the Ptsurface.

H2O activation

H2O!OH + H: Adsorption of H2O on the Pt3Re(111) surface isexothermic by E =�0.81 eV and prefers to take place at the Retop-site. The OH and H species prefer to bind atop the Reatom if they are individually absorbed. The most stable stateof coadsorbed OH and H species, formed through H2O dissoci-ation, involves an OH species that is adsorbed on top of theRe atom with the molecular axis tilted towards the surfacenormal, and an H atom that is adsorbed on top of a Pt atom.The total binding energy (BE) for the configuration is �6.24 eV.

The calculated activation energy barrier for the abstractionof the first H atom from H2O is 0.46 eV. The surface reactionstep is exothermic by E =�0.20 eV, and is initiated by a slighttilt of the H2O molecule, with one H atom pointing closer tothe surface. At the transition state, the abstracted H atom is at-tached to the side of a top-Pt site and the HO�H bond lengthis found to be 1.36 �.

OH!O + H: For coadsorbed O and H atoms, the most stableconfiguration is found if the O atom is attached to a top-Resite, and the H atom to a top-Pt site with an overall BE of�8.94 eV. The dissociation of the OH species is even more exo-thermic (E =�0.60 eV) than the abstraction of the first H atomfrom H2O (�0.20 eV). The activation energy barrier for OH dis-sociation is found to be 0.44 eV, which is similar to the valuefor H2O!OH + H (0.46 eV).

On Pt(111) surfaces, the system has to overcome significant-ly higher activation energy barriers for both H-abstractionsteps (0.88 eV and 1.09 eV, respectively[11i]). Furthermore, thefinal decomposition from the OH species to atomic O and H isendothermic on Pt(111) surfaces by E = 0.26 eV. This compari-son indicates that H2O activation is easier on Pt3Re(111) com-pared to Pt(111) surfaces.

OH + OH!H2O + O (Disproportionation reaction): Two OHspecies coadsorbed on the Pt3Re(111) surface (total coverageq= 0.5 ML, in which ML = monolayer) have a total BE of�5.69 eV. At this higher coverage, the OH species is stabilizedby E = 0.13 eV compared to the 1/4 ML coverage case. In themost stable configuration, one OH group is attached to a top-Re site and the other is bound to a bridge-Pt site.

The most stable state for H2O and O coadsorption is foundwith the O atom bound to a top-Re site, and H2O adsorbed on

Table 1. DFT-calculated binding energies (BE) and site preferences forWGS reaction intermediates at q= 1/4 ML coverage. Reference energy isthat of the slab and the gas phase adsorbate at infinite separation fromeach other.

Pt(111) Pt3Re(111)Adsorbate Site BE [eV] Site BE [eV]

H top �2.71 top-Re �2.81O fcc �3.73 top-Re �6.44OH top tilted �2.09 top-Re tilted �3.74H2O top �0.27 top-Re �0.81CO fcc/hcp �1.82 top-Re �2.56CO2 no preference �0.11 top-Re/top-Pt �0.47COOH top, H down �2.44 top-Re/top-Pt �2.87HCOO top–top �2.31 top-Re/top-Pt �3.29


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a top-Pt site with an overall BE of �6.71 eV. The disproportio-nation of OH species is spontaneous and the reaction is exo-thermic by E =�1.63 eV. This behavior is similar to what isfound on Pt(111) surfaces, in which the step is also spontane-ous, but the exothermicity is much lower (E =�0.44 eV).[11i]

Oxidation of CO by atomic O

CO + O!CO2: CO2 is found to be stable on the Pt3Re(111) sur-face in a bent configuration with a BE of �0.47 eV, and it isbound to a Re and a Pt surface atom (Table 1). Similar to the Oatom, the CO molecule by itself is preferentially adsorbed per-pendicular to a top-Re site with a BE of �2.56 eV (Table 1). Co-adsorbed CO and O species are most stable with the CO mole-cule bound to a top-Pt site and the O atom bound to a top-Resite. For this configuration the total BE is �7.67 eV.

The activation energy barrier for CO oxidation is 1.18 eVwith respect to the coadsorbed initial state; this elementarystep is endothermic by E = 1.02 eV. CO oxidation on thePt(111) surface shows entirely different characteristics. The ac-tivation energy on Pt(111) is 0.75 eV with respect to coadsor-bed species, and the reaction itself is exothermic by E =

�0.80 eV,[11i] which suggests that, in contrast to H2O activation,CO oxidation on Pt3Re(111) is more difficult than on Pt(111)surfaces. This difference is reasonable; H2O activation leads tomolecular fragments that bind more strongly to Pt3Re than toPt surfaces, whereas CO oxidation is a bond-making step, inwhich one has to activate strongly-bound species (CO and O)to the transition state for CO2 formation.

CO oxidation via the COOH intermediate

CO + OH!COOH: Adsorbing CO and OH species simultane-ously on the Pt3Re(111) surface results in a total BE of�5.23 eV for a configuration in which the OH species is boundperpendicularly to a top-Re site and the CO molecule is boundto a face-centered cubic-mixed site. The CO molecule is slightlytilted with the C atom closer to the Re atom. Adsorbed individ-ually, both molecules prefer the top-Re site with BE values of�2.56 and �3.74 eV (Table 1) for the CO and OH species,respectively.

In its most stable configuration, the COOH intermediateforms bonds to a top-Re and a top-Pt site with the H atompointing towards the surface. However, direct formation of thisstructure from CO and OH species is not feasible. First, the COand OH species react to give COOHu, with only the C atombound to the surface and its H atom pointing upwards awayfrom the surface. Subsequently, the COOH group realigns tothe most stable COOHd configuration, with the C and O atomsbound to a top-Re site, and the H atom pointing downwardsto the surface. The first reaction step requires an activationenergy of 1.21 eV and is endothermic by E = 0.80 eV. At thetransition state, the OC�OH bond length is 1.44 � and the Hatom points out of the OCO plane. The subsequent reorienta-tion of the COOHu to the COOHd configuration is exothermicby E =�0.25 eV and has an activation energy barrier of 0.37 eV.Overall, the reaction is endothermic by E = 0.55 eV, in contrast

to the formation of the COOH intermediate on the Pt(111) sur-face, which is exothermic by E =�0.45 eV.[11i] In addition, theenergy barriers for the described reaction steps on the Pt(111)surface are significantly lower than on the Pt3Re(111) surface(0.46 eV and 0.32 eV, respectively).[11i] Again, as discussed forthe aforementioned CO + O!CO2 reaction, the formation ofthe COOH intermediate is a bond-making step, which takesplace with greater difficulty on the Pt–Re alloy surface than onthe Pt surface, for the same reasons outlined earlier.

COOH decomposition

COOH!CO2 + H: In the most stable configuration of the coad-sorbed CO2 and H species on the Pt3Re(111) surface, the Hatom is bound to a hexagonal close packed-mixed site, andthe CO2 molecule is bound to both a top-Pt and a top-Re sitein a bent configuration. This state has a total BE of �2.94 eV.

The dissociation of the COOH intermediate is simulatedstarting from the most stable configuration with the H atompointing towards the surface (COOHd). The activation energybarrier for dissociation of COOHd to CO2 and H is 1.23 eV andthe reaction step is endothermic by E = 0.28 eV. Dissociation ofCOOH is initiated by bending of the O and H atoms towardsthe surface followed by the abstraction of the H atom. In thetransition state, the H atom is attached to a top-Pt site and theCOO�H bond length increases up to 1.45 � (from 0.98 � in theinitial configuration). Enhanced activation has been reportedon Pt(111) in which the dissociation of COOHd has an activa-tion energy barrier of 0.78 eV and the reaction step itself isthermoneutral.[11i] The COO�H bond length of the transitionstate on the Pt(111) surface is slightly longer (1.49 �).[11i]

COOH + O!CO2 + OH: In the most stable coadsorbed stateof COOHd and O, the COOH species is bound to a top-Pt siteand the oxygen atom is bound to a top-Re site. The calculatedtotal BE of the COOH + O initial state is �8.43 eV. Simultaneousadsorption of CO2 and OH leads to a similar arrangement tothat of CO2 and H coadsorption, in which the OH species isbound to a top-Re site, and the CO2 molecule is physisorbedapproximately 4 � above the surface. The total BE of the finalstate is �3.99 eV.

The reaction of COOH and O to CO2 and OH has an activa-tion energy barrier of 1.11 eV and is endothermic by E =

0.22 eV. At the transition state, the distance between the Catom of the COOH intermediate and the surface is 2.39 �. TheO*�H bond length at the transition state (*O�H�OCO) is1.63 �. Again, this reaction step is facilitated on pure Pt(111)sites with an activation energy barrier of 0.35 eV and an exo-thermicity of E =�0.27 eV.[11i]

COOH + OH!CO2 + H2O: Most investigated configurationsof coadsorbed COOH and OH species spontaneously react togive CO2 and H2O, which demonstrate the absence of an acti-vation energy barrier. However, a stable configuration is foundwith the OH species bound to a top-Re site and tilted, and theCOOHd intermediate adsorbed on a top-Pt site with the Hatom pointing towards the surface. The total BE for this initialstate is �5.91 eV. Compared to infinite separation of the ad-sorbed species on the corresponding site, coadsorbed CO2 and



H2O molecules (in the most stable final state) are stabilized byE = 0.12 eV, with the H2O molecule attached to a top-Re siteand the CO2 molecule located approximately 4 � abovea bridge-Pt site. This configuration has a total BE of �1.02 eV.

To calculate the minimum energy path for the reaction ofCOOH and OH to CO2 and H2O, it is assumed that CO2 isformed on the surface first and desorbs subsequently. There-fore, an energy minimization calculation is performed for coad-sorption of CO2 and H2O, in which the C atom of the CO2 mole-cule is fixed in the z-direction. The calculated state has a BE of�0.86 eV and is E = 0.16 eV less stable than the most stableconfiguration for coadsorbed H2O and CO2. The climbingimage-nudged elastic band (CI-NEB) calculation yields anenergy barrier of 0.17 eV for the COOH molecule to rotate toan intermediate state, which is E = 0.06 eV less stable than theinitial state. The following transition of the H atom from theCOOH molecule to the OH species is endothermic by E =

0.11 eV without an additional energy barrier to activate the re-action. This reaction on Pt(111) active sites is shown to occurquasi-spontaneously with a small activation barrier and with-out the formation of a stable intermediate species. No explicittransition state is identified and the reaction is exothermic byE =�0.71 eV.

HCOO decomposition

HCOO!CO2 + H: Besides the most stable state for CO2 and Hcoadsorption, which has been described above (BE =

�2.94 eV), another state with a BE of �2.72 eV is found. In thisconfiguration, the CO2 molecule is nonlinearly adsorbed ona top-Re site and the H atom is bound to a hexagonal closepacked-mix site. CI-NEB calculations are performed with thelatter configuration as the final state, instead of with the moststable state in which the CO2 molecule is bound to both a top-Re and a top-Pt site. The activation energy barrier for this stepis 1.38 eV, and the reaction is endothermic by E = 0.66 eV. Ifone takes into account the subsequent diffusion of CO2 to themost stable configuration of CO2 and H; this results in an en-dothermicity of E = 0.44 eV for the entire step. The reverse re-action, that is, HCOO formation from a CO2 molecule and an Hatom, is exothermic (E =�0.44 eV) and has an energy barrierof 0.94 eV. In contrast to Pt(111) surfaces, in which HCOO for-mation is modestly endothermic (by E = 0.35 eV) and has anenergy barrier of 1.39 eV,[11i] on Pt3Re surfaces, the exothermici-ty and lower energy barrier facilitate the formation of HCOO.

HCOO + O!CO2 + OH: Coadsorption of an HCOO moleculeand an O atom leads to a slight stabilization by E = 0.11 eV,compared to their adsorption at infinite separation. In themost stable configuration, the O atom is located on a top-Resite and the HCOO molecule is bound through its O atoms totwo top-Pt sites. The total BE is found to be �8.62 eV. By usingCI-NEB calculations, this step is found to be hindered by a sig-nificant activation energy of 1.83 eV. Furthermore, it is endo-thermic by E = 0.11 eV. This behavior differs remarkably fromwhat has been found on the Pt(111) surface, for which en-hanced activation has been reported with an activation energy

of 1.17 eV, and in which the step is found to be exothermic byE =�0.60 eV.[11i]

HCOO + OH!CO2 + H2O: The most stable state of coadsor-bed HCOO and OH species is similar to coadsorption of HCOOwith O. The OH species is found to be tilted on a top-Re site,whereas the HCOO molecule is bound through its O atoms totwo top-Pt sites. The overall BE is �6.12 eV and is more stableby E = 0.29 eV than adsorption at infinite separation, which in-dicates a relatively strong attraction between coadsorbedHCOO and OH species. The activation energy barrier for thisstep is 1.34 eV, and it is quasi-thermoneutral (DE =�0.05 eV). Acomparable activation energy barrier is found on Pt(111) surfa-ces (1.23 eV); however, on Pt(111) surfaces, this step is thermo-dynamically favorable with an exothermicity of E =�1.04 eV.[11i]

Coverage dependence of CO binding energy

The binding energy of various adsorbed species can dependsignificantly on the surface coverage of the same or other co-adsorbed species. In particular, the binding energy of CO mole-cules on transition-metal surfaces strongly depends on its owncoverage.[19] Grabow et al.[11i] have found that the BE of CO onPt(111) surfaces, in electron volts, depends on the coverage ina way that is captured by Equation (17) below:

BECOðqCOÞ ¼ �1:78þ0:0065 � expð4:79 � qCOÞþ0:031135 � qCO � expð4:79 � qCOÞ

ð17Þ

This equation is used for the microkinetic modeling of Ptcatalysts in this study. The use of coverage-dependent BE forH, O and other species on Pt(111) surfaces is not utilized be-cause these effects are not as significant.

To a first approximation, the microkinetic model discussed insubsequent sections for Pt–Re catalysts does not utilize an ex-plicit CO-coverage dependence correlation for the BE values.Instead, the initial estimation for BECO is defined as the valueobtained from the fitted Pt(111) microkinetic model by substi-tuting into Equation (17) the value of the CO coverage predict-ed by the model for the Pt catalyst under WGS reaction condi-tions. The sensitivity to this parameter for the Pt–Re catalyst isdetermined and further optimizations of this parameter (BECO)are performed in the framework of the Pt–Re microkineticmodel.

Microkinetic models

Microkinetic models for three catalysts, Pt, Pt–Re(2:1) and Pt–Re(1:2), are constructed based on the parameters obtainedfrom DFT calculations on Pt(111) and Pt3Re(111) surfaces. Thecalculated turnover frequencies (TOF) from these models arecompared with the respective experimental TOF values. BEvalues, pre-exponential factors and forward activation energybarriers obtained from DFT calculations are input to the micro-kinetic code as initial estimations for the model parameters.For the case of the Pt–Re microkinetic models, two initial esti-mations are utilized:



1) The most favorable adsorption sites with the highest abso-lute BE, on the Pt3Re(111) alloy.

2) The BE of species on the Pt-sites in the Pt3Re(111) alloy(i.e. , Re is modeled as a promoter).

The results for each set of initial estimations are discussed inthe following sections. Based on the results of Grabow et al. ,[11i]

only the coverage dependence for BECO [Eq. (17)] is included inthe Pt model. Accordingly, thefinal BECO in this model isa result of the behavior predict-ed by the equation. In contrast,the Pt–Re surface models havetheir binding energies adjustedonly through the microkineticmodeling optimization proce-dure, in which the final BECO isequal to the converged valuefrom the fit. No assumptions aremade on the identity of the rate-limiting step.

We start by performing sensitivity analysis on the activationenergy barriers and BE values. The results show that for allthree catalysts studied, the BECO and the activation energy bar-rier for Step (10), COOH formation, are sensitive parameters.For both Pt–Re models, the activation energy barrier ofStep (11), COOH decomposition to CO2 and H*, is also found tobe a sensitive parameter. Decomposition of the COOH species,which includes the reaction with surface O* atoms, and theformation of CO2 and *OH species, could have been expectedto facilitate COOH decomposition. However, as a result of thestrong binding of O* atoms to Re sites, the activation energybarrier is decreased by only 0.24 eV compared with direct de-composition (E = 0.99 eV vs. 1.23 eV for the direct COOH de-composition reaction). These parameters are optimized in therespective models and the predicted TOF values are comparedwith experimental TOF values, as shown in Figure 1. ThePt(111) model shows excellent agreement with experiments,after adjusting only the DFT value for the activation energy forStep (10). For the Pt–Re models, fitting of the experimental

TOF values leads to larger deviations from the DFT results forthe activation energy barriers of Steps (10) and (11), the impli-cations of which will be discussed later.

Table 2 shows the predicted and experimental values of thereaction orders and apparent activation energies. The reactionorders obtained through the microkinetic models are in agree-ment with experiments; significant differences are observedonly for H2. In general, our models tend to overestimate the in-

hibiting effect of H2. In all cases, CO is the most abundant sur-face species, qCO�2/3 ML, with negligible amounts of othersurface species, except on the Pt–Re surfaces in which approxi-mately qH�0.15 ML is calculated. Still, the obtained apparentactivation energies are in excellent agreement withexperiments.

Campbell’s degree of rate control is used to assess the exis-tence of rate-determining steps [Eq. (18)]:[23]

XRC;i ¼ki

r@r@ki

� �Ki;eq ;kj

ð18Þ

Table 3 shows the results of this analysis for Pt—Re models.Elementary Step (10) shows the highest degree of rate controlfor all catalysts studied. In the Pt–Re models, the reaction ofStep (11) follows (10) in the degree of rate control, but witha much lower value for Step (11). We also studied the reactionrate (flux) for each elementary step in our microkinetic model;we find that the mechanism proceeds predominantly throughH2O activation (4), COOH formation (10), and direct COOH de-composition (11) in all three microkinetic models, and for theconditions studied.

Discussion

Density functional theory

A potential energy surface (PES), Figure 2, shows the results ofthe DFT calculations on the Pt3Re(111) surface as implementedin the microkinetic model (i.e. , adsorbate binding energies ofthe Pt(111) surface and activation energies from thePt3Re(111) surface). The PES shows two paths that occurduring the redox mechanism; OH dissociation and OH + OHdisproportionation, which both generate O* atoms on the sur-face, whereas the other two pathways occur via a COOH inter-mediate; one pathway consists of a direct-decomposition reac-

Figure 1. Calculated (model) versus experimental turnover frequency (TOF).The parameters used to calculate the model TOF values are those obtainedfrom fitting the experimental TOF values.

Table 2. Reaction orders and apparent activation energies, Eapp, which are measured in kJ mol�1. Elementarystep activation energies and binding energy values can be found in the Supporting Information.

Pt Pt:Re(2:1) Pt:Re(1:2)Species Experiment Model Experiment Model Experiment Model

CO �0.20�0.03 �0.20 �0.17�0.02 �0.22 �0.34�0.01 �0.36CO2 �0.03�0.01 0.00 0.01�0.02 0.00 �0.02�0.01 �0.01H2 �0.37�0.01 �0.48 �0.35�0.01 �0.63 �0.36�0.04 �0.45H2O 0.55�0.06 0.68 0.73�0.04 0.68 0.78�0.07 0.77Eapp 74.7�2.94 74.6 76.1�2.33 76.2 77.6�1.27 82.2



tion [Step (11)] , and the other consists of an OH-mediated de-composition reaction [Step (13)] . Inspection allows us to identi-fy the COOH direct-decomposition path as the one with theleast deviation from the average potential energy between re-actants and products. The OH-dissociation route and the OH +

OH disproportionation route are the least plausible in this reac-

tion system. Still, the OH dissoci-ation reaction is favored overthe disproportionation step togive surface O* atoms. Notably,the previous DFT calculations onthe WGS reaction mechanismon various TiC nanoparticleshave shown that O2 is alsostrongly bound on these surfa-ces, similar to what we are find-ing here.[24] HCOO, which is notshown in any of the pathwaysleading from reactants to prod-ucts, has been determinedthrough the microkineticmodels to be a spectator spe-cies, which is formed througha reaction between surface H2

and CO2. A compact representa-tion of the WGS reaction path-ways is shown in Figure 3. Theminimum energy route hasbeen highlighted with solidarrows; the reaction proceedsthrough the COOH intermediatewhich, through subsequent ele-mentary steps, leads to CO2 andH2 formation.

Microkinetic models

Figure 1 and Table 4 show howour microkinetic models areable to capture the respectiveexperimental results (additionaldata for the remaining catalystsstudied can be found in theSupporting Information). The re-action orders with respect to re-actants and products are pre-dicted well, with only a few out-lying cases in the H2 orders. Im-portantly, the apparent activa-tion energies, are predicted towithin 6 % across a wide rangeof working conditions (Table 2).

The microkinetic model basedon the Pt(111) surface showsonly small changes in the pa-rameter that was chosen to beoptimized, that is, the activation

energy of the reaction of Step (10). The BECO is not adjustedfrom the results of DFT calculations, that is, it is only a functionof the CO coverage from Equation (17). After optimization, theactivation barrier for Step (10) changed to 0.20 from 0.47 eV.According to Equation (17), the BECO is equal to �1.26 eV atthe typical value of CO coverage under WGS conditions, which

Table 3. Kinetic parameters in the PtRe(2:1) microkinetic model obtained from Pt3Re(111) DFT calculations atq= 1/4 ML coverage for each of the adsorbed reactants. DFT and fitted binding energies for this model are pro-vided in the Supporting Information.

Derived from the microkinetic modelStep Reaction Ef,DFT

[a] [eV] Ef[b] [eV] XRC

[c]

(2) H2O + *$H2O* 0.0 0.0 0(3) CO + *$CO* 0.0 0.0 0(4) H2O* + *$OH* + H* 0.46 0.46 �3 � 10�4

(5) OH* + *$O* + H* 0.44 0.44 0(6) 2 OH*$H2O* + O* 0.0 0.0 0(7) CO* + O*$CO2* + * 1.18 1.18 0(8) CO2*$CO2 + * 0.0 0.0 0(9) 2 H*$H2 + 2* 0.0 0.0 0(10) CO* + OH*$COOH* + * 0.37 0.03 0.95(11) COOH* + *$CO2* + H* 1.23 0.69 9 � 10�3

(12) COOH* + O*$CO2* + OH* 0.99 0.99 0(13) COOH* + OH*$CO2* + H2O* 0.33 0.33 0(14) CO2* + H*$HCOO** 0.94 0.94 0(15) HCOO** + O*$CO2* + OH* + * 1.83 1.83 0(16) HCOO** + OH*$CO2* + H2O* + * 1.34 1.34 0

[a] Forward reaction barrier, obtained from DFT calculations. [b] Fitted reaction barrier from the microkineticmodel shown for the experimental conditions in Table 4. [c] Campbell’s degree of rate control, calculated as de-tailed in Equation (18).

Figure 2. Potential energy surfaces for four possible reaction paths for the WGS reaction on the Pt3Re(111) sur-face, which is based on DFT-derived parameters. Surface species are denoted with the subscript ads, whereas tran-sition-state complexes are marked as TS. Additional OH species are added or removed if necessary because it isregenerated throughout the reaction.



is predicted by the microkinetic model (i.e. , 2/3 ML coverageby CO). These results agree well with experimental findingsand the work of Grabow et al.[11i]

The approach taken for the Pt3Re(111) models, however, ismore elaborate. By modeling our surface with the most favora-ble sites for adsorption, in most cases Re sites, we find that thesurface coverage by all species is negligible except for theatomic O coverage, which is approximately 0.98 ML in mostcases. Prior work has suggested that the oxidation state of Recan affect its interaction with the support and with the otheralloyed metal ; in some cases Re can exist as Re+ 4 or Re+ 7.[17h, 25]

The high O coverage observed in our models is in accord withthese claims. Therefore, the emerging picture is that Re atoms

under reaction conditions arepermanently decorated by Oatoms, which in turn induce de-stabilization of other speciesthat adsorb on neighboring Ptsites; this leads to enhanced re-activity compared to pure Pt. Inthat sense, one can argue thatRe promotes the WGS reactivityof Pt.

The majority of our efforts inthe Pt3Re(111) microkineticmodels deal with the behaviorof species adsorbed on pure Pt-sites, with Re sites serving asa promoter. The Pt3Re(111)models, even though similar insome aspects to the Pt(111)model, show distinct differencesin the BECO and the activationbarriers for elementary Steps (10)and (11). The final BECO for thetwo Pt–Re catalysts, obtainedfrom the microkinetic model op-timization, is approximately�1.11 eV (notably, Equation (17)was not used for the Pt–Re cata-lyst, and this binding energy cor-responds to the value at the COcoverage on the catalyst underWGS reaction conditions). Forthese catalysts, CO is destabi-lized from the surface comparedto the Pt(111) model, in whichthe BECO is found to be �1.26 eVat the CO coverage on the cata-lyst under WGS reaction condi-tions. The barriers for Steps (10)and (11) are significantly loweredfrom the calculated DFT values,with the former step occurringalmost spontaneously (calculat-ed DFT value of BEco = 0.37 eV)and the latter step having a barri-

er of 0.69 eV after optimization (calculated DFT value of BEco =

1.23 eV) in both Pt–Re models. The coverage of CO is approxi-mately 2/3 ML with the coverage of H atoms on the surfaceranging from 0.15 ML to 0.2 ML.

The results obtained can be rationalized in the followingmanner. First, all surfaces are highly covered by CO molecules.Ojeda et al.[6a, 26] have shown that the energetics of chemical re-actions in highly-saturated environments are markedly differ-ent from their low-coverage counterparts and that, in general,bond-making reactions such as (10) and (11) are facilitated inthese environments. Notably, although the reaction ofStep (11) has its barrier decreased considerably in the microki-netic model optimization procedure, the final fitted value is

Figure 3. WGS reaction pathway on the Pt3Re(111) surface, based on a reaction flux analysis through the microki-netic model. For each elementary step, the activation energy barrier, Ea, and the reaction energy, DE, values areshown in electron volts, which are based on the calculated DFT values. The COOH-mediated pathway, which ishighlighted by solid arrows, is found to be prevalent through a flux-analysis from the microkinetic models, notfrom the DFT results. HCOO is a spectator species, which is only produced after the main reaction products (CO2

and H2) have been formed.

Table 4. Experimental and microkinetic model turnover frequencies (TOF) for the PtRe(2:1) catalyst ; see alsoFigure 2. Inlet mole fractions are provided (y). The total pressure was kept at 1 atm; He served as an inert carri-er gas.[a]

Experiment T y(CO) y(H2O) y(H2) y(CO2) Experimental TOF[b] Model TOFnumber [K] [min�1] [min�1]

1 548 0.15 0.25 0 0 5.70 5.772 548 0.2 0.25 0 0 5.26 5.333 548 0.1 0.25 0 0 6.03 6.054 548 0.25 0.25 0 0 5.11 4.915 548 0.15 0.15 0 0 3.59 4.066 548 0.15 0.2 0 0 4.74 4.957 548 0.15 0.35 0 0 7.21 7.258 548 0.15 0.25 0.15 0 2.07 2.129 548 0.15 0.25 0.1 0 2.37 2.66

10 548 0.15 0.25 0.25 0 1.71 1.5311 548 0.15 0.25 0.35 0 1.55 1.2012 548 0.15 0.25 0 0.15 5.44 5.7613 548 0.15 0.25 0 0.25 5.48 5.7514 548 0.15 0.25 0 0.35 5.51 5.7415 573 0.15 0.25 0 0 11.48 9.2516 533 0.15 0.25 0 0 3.32 3.3817 518 0.15 0.25 0 0 2.15 1.67

[a] All experiments were performed at a total flow rate of 100 cm3 min�1. [b] Reported values are the average ofseveral experiments for each condition.



the same as for the Pt(111) surface, E = 0.69 eV. This similaritycould indicate that this reaction proceeds mostly over Pt surfa-ces and is only marginally affected by the presence of Re, if atall. Second, the fitted BECO values for both Pt–Re models is ap-proximately 0.15 eV lower than on Pt(111) models. As wasmentioned previously, there is a high probability for Re atomsto be bonded to surface O atoms, and that the presence ofthese Re�O species could destabilize the binding of CO mole-cules attached to nearby Pt sites. In turn, this weaker bindingof CO molecules would tend to increase their reactivity and ac-cordingly enhance the measured reaction rates. Zhang et al.[27]

have documented a similar effect on the electrocatalyticoxygen reduction reaction. Given the above insights, Figure 4gives a graphical representation of a possible structure for theactive site on the Pt3Re(111) surface.

Rate-determining step

The results of our Campbell’s degree of rate control analysis in-dicate that the formation of COOH from CO and OH species isthe step with the highest effect on the overall reaction ratesfor all studied catalysts. Notably, this reaction is dependent onthe availability of both CO and OH species on the catalyst sur-face. We have previously mentioned that the CO coverage inall models is approximately 2/3 ML, and therefore we do notconsider CO coverage to be a limiting factor. On the otherhand, OH coverage is negligible in all models. In our proposedreaction scheme, OH species are produced on the surfacethrough H2O activation, Step (4). The reaction rates (flux) forH2O activation [Step (4)] , COOH formation [Step (10)] and thedirect COOH decomposition reaction [Step (11)] are dominantthroughout the reaction network. Putting all the observationstogether we infer that H2O activation, Step (4), is the underly-ing rate-controlling step. After COOH is formed in Step (10), itprefers to directly decompose through Step (11). Increasingamounts of H2O in the reactor inlet promote higher coverageof OH species and increase overall reaction rates. This behavioris in agreement with the positive reaction order for H2O, whichis observed in our experiments and in our microkinetic models,and also with previous studies.[17g, 28]

Conclusions

DFT calculations, experimental reaction kinetics measurementsand microkinetic models are utilized to study and understand

the mechanism of the water—gas shift (WGS) reaction over Ptand Pt–Re alloys. DFT-derived pre-exponential factors, bindingenergies and activation energies are used to construct a micro-kinetic model, which is capable of simulating the observed ex-perimental reaction rates and orders for the low-temperatureWGS reaction. The Pt(111) model shows remarkable agreementbetween model, experiments and DFT results, which indicatesthat the Pt(111) model is a reasonable representation of theactive site for this reaction. On the other hand, the Pt–Remodels diverge significantly from the DFT-derived parameterset for the Pt3Re(111) surface, which suggests that the activesite is quite different than that provided by the Pt3Re(111)model surface. Steps, defects or more complicated structuressuch as mixed metal–metal oxides might be better representa-tions of the active site. In all cases, the overall apparent activa-tion energies are predicted to within 6 % of the experimentalvalues. CO is determined to be the most abundant surface in-termediate with a coverage of approximately 2/3 ML. Formate,HCOO, is identified as a spectator byproduct species producedfrom a recombination of the main reaction products, CO2 andH2. Carboxyl, COOH, is found to be a key intermediate, whichis responsible for turning the reactants into products. Based onthe DFT results alone, the OH-assisted decomposition of COOHinto CO2 and H2 is highly preferred; however, this pathway iskinetically hindered by a low OH coverage. Instead, the reac-tion proceeds through the direct decomposition of COOH,which, at the conditions studied, produces the majority of CO2.The redox mechanism is found to have minimal contributionto all the conditions studied.

Experimental Section

Density functional theory (DFT)

All DFT calculations performed for the Pt(111) surface are reportedin the work of Grabow et al.[11i] The DFT calculations for thePt3Re(111) surface were performed by using DACAPO, a totalenergy code.[29] A 2 � 2 � 4 unit cell with 6 equivalent layers ofvacuum was used to represent the surface as a periodic supercell(Figure 5). The slab consisted of four layers of metal atoms. Adsorp-tion was allowed on only one of the exposed surfaces in which thetop two surface layers were allowed to relax, whereas the bottomtwo layers were fixed at their bulk coordinates. Ultrasoft Vanderbiltpseudopotentials[30] were utilized to describe core-electron interac-tions, and the Kohn–Sham one-electron valence states were ex-panded in a basis of plane waves with kinetic energy below 25 Ry.The surface Brillouin zone was sampled at 18 special Chadi-Cohen

Figure 4. Illustration of the proposed active site on the Pt3Re(111) surface.Re atoms (*) exist in an oxidized form, whereas Pt atoms (*) serve as thesite for CO (*: C, *: O atoms) adsorption in this reaction.

Figure 5. Illustration of the Pt3Re(111) surface, which is used for the DFT cal-culations. *: Pt, *: Re atoms.



k-points. Convergence was confirmed with respect to the k-pointset and the number of metal layers used in the slab. The PW91generalized gradient approximation (GGA-PW91)[31] was used self-consistently for describing the exchange-correlation energy andpotential. The electron density was determined by iterative diago-nalization of the Kohn–Sham Hamiltonian, the Fermi-population ofthe Kohn–Sham states (kBT = 0.1 eV), and Pulay mixing of the re-sulting electron density.[32] The calculated equilibrium lattice con-stant for the bulk Pt3Re alloy was 3.97 �, which is in agreementwith the experimental value of 3.90 �.[33]

Activation energy barriers for the elementary steps were calculatedby using the CI-NEB method.[34] For determining the minimumenergy pathways, a total of seven images were utilized, which in-cluded the initial and final state for each elementary step that wasconsidered. To verify the validity of the identified transition states,vibrational frequency analysis was performed, which yieldeda single negative curvature mode.

Microkinetic models

Our mean-field microkinetic models accounted for a total of 15 ele-mentary steps (see the Results section). These steps include thesurface-redox mechanism and the COOH-mechanism. Relevant ki-netic and thermodynamic parameters (e.g. , pre-exponential factors,activation energies, reaction energies, and so on) were obtainedfrom DFT calculations by following the methodology utilized byGrabow et al.[11i] and Gokhale et al.[35] Spontaneous reactions, inwhich no transition state was found, were assigned pre-exponen-tial factors of 1013 s�1. All gas-phase enthalpies and entropies wereobtained from the National Institute of Standards and Technology(NIST) archives.[36] The entropy of adsorbed species was calculatedas the gas-phase entropy minus the three-dimensional translationalcontribution Strans,3D. To account for the vibrational and rotationalcontributions to the entropy of adsorbed species, a fitting factor,Floc, was utilized.[18b]

Our reactor was modeled as a continuously-stirred tank reactor(CSTR) with the integration and parameter optimization performedby using MATLAB (2010A, The MathWorks, Natick, MA).

Experiments

Catalysts for the study included 5 wt % Pt, 5 wt % Re, 10 wt % Pt—Re (2:1), and 10 wt % Pt—Re (1:2), which were supported on theinert carbon black Vulcan XC-72 (CABOT). These catalysts were pre-pared as reported by Kunkes et al.[17d] by incipient wetness impreg-nation with a chloroplatinic(IV) acid hexahydrate (Aldrich) and per-rhenic acid (aqueous solution, 50–54 % Re, Strem) dissolved in de-ionized water. The catalysts were reduced at T = 573 K (with a heat-ing rate of 0.5 K min�1) under a pure hydrogen flow at standardtemperature and pressure (100 cm3 min�1). CO chemisorption wasperformed in a Micromeritics ASAP 2020, and was used to deter-mine the number of catalytically active sites.

WGS reaction studies were conducted in a fixed-bed down-flow re-actor that contained the catalyst (200 mg) mixed with silica chips(6 g) in a 1/4-inch outer-diameter stainless steel tube. The tempera-ture was measured by using a K-type thermocouple attached tothe outside of the reactor. The temperature of the reactor was ad-justed by using a furnace connected to a variable autotransformerpower source, which was controlled with a temperature controller.The total pressure in the reactor was maintained at 1 atm, and thepartial pressures of the gases were controlled by adjusting the

flow-rates at the reactor inlet. The flow-rates of all gases were fixedby using calibrated mass-flow meters.

An inlet composition with between 10–25 % of CO, 0–35 % of H2,0–35 % of CO2, and 15–35 % of H2O was used, in which the remain-ing balance consisted of He. The gases were used as provided,with a purity of 99.99 %. Steam was fed to the reactor by vaporiz-ing Millipore-filtered, deionized liquid H2O at T = 433 K; this wasdelivered by using a syringe pump (Harvard Apparatus). The feedand effluent gases were analyzed by using gas chromatographywith a thermal conductivity detector (TCD). Steam was removedfrom the reactor effluent stream by using a water trap (trap im-mersed in an ice/H2O bath). CO conversions were maintainedbelow 30 % to achieve differential-reactor operation. All reactionrates are reported in turnover frequency (TOF), which is defined asthe number of moles of a product species formed per mole of cat-alytically active sites per unit time; this is given in units of inverseminutes (min�1).

Acknowledgements

This material is based upon work that is supported as part of theInstitute for Atom-efficient Chemical Transformations (IACT), anEnergy Frontier Research Center funded by the U.S. Departmentof Energy, Office of Science, Office of Basic Energy Sciences. Thecomputational work was performed in part by using supercom-puting resources from the following institutions : the Environmen-tal Molecular Sciences Laboratory (EMSL), a national scientificuser facility at the Pacific Northwest National Laboratory (PNNL);the Center for Nanoscale Materials at Argonne National Labora-tory (CNM); the National Center for Computational Sciences atOak Ridge National Laboratory (NCCS); and the National EnergyResearch Scientific Computing Center (NERSC). EMSL is sponsoredby the Department of Energy’s Office of Biological and Environ-mental Research located at PNNL. CNM, NCCS, and NERSC aresupported by the U.S. Department of Energy, Office of Science,under contracts DE-AC02-06CH11357, DEAC05-00OR22725, andDE-AC02-05CH11231, respectively.

Keywords: density functional calculations · kinetics ·platinum · reaction mechanisms · rhenium

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Received: May 12, 2013

Published online on November 5, 2013



density functional theory and reaction kinetics studies of the water–gas shift reaction on pt–re...

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