On the Importance of the Associative Carboxyl Mechanism for the

Article pubs.acs.org/JPCC

On the Importance of the Associative Carboxyl Mechanism for the Water-Gas Shift Reaction at Pt/CeO2 Interface Sites Sara Aranifard, Salai Cheettu Ammal, and Andreas Heyden* Department of Chemical Engineering, University of South Carolina, 301 South Main Street, Columbia, South Carolina 29208, United States S Supporting Information *

ABSTRACT: Periodic density functional theory calculations and microkinetic modeling are used to investigate the associative carboxyl pathways of the water-gas shift (WGS) reaction at the Pt/ CeO2 (111) interface. Analysis of a microkinetic model based on parameters obtained from first principles suggests that the turnover frequencies for the CO-assisted associative carboxyl mechanism are comparable to experimental results. However, this microkinetic model containing various associative carboxyl pathways at interface sites cannot explain the experimentally observed activation barriers and reaction orders of Pt/CeO2 catalysts. Considering furthermore that a model of an associative carboxyl mechanism with redox regeneration, also derived from first principles and recently published by us, accurately predicts all kinetic parameters while displaying a 2 orders of magnitude higher turnover frequency, we conclude that at Pt/CeO2 interface sites, the WGS reaction follows a bifunctional Mars−van Krevelen mechanism in which support oxygen vacancies facilitate water dissociation. reducible oxides,7,22 where spillover and reverse-spillover of active intermediates might occur during the catalytic cycle.23 Next, the support might also change the properties of the metal cluster by charge transfer or it can provide a unique catalytic site for reaction at the interface of the metal and the oxide. Recently, Fu et al. showed very high activity of Au/CeO2 and Pt/CeO2 catalysts for the WGS reaction.21 They suggested that metal nanoparticles are not responsible for the reaction, but small positively charged metal species strongly bonded to the oxide support are the active sites. In another study, Zhai et al. illustrated that it is possible to create a highly active Pt/SiO2 catalyst by stabilizing Pt−OHx species with alkali metal promoters.24 It is worth noticing that while CeO2 is wellknown for its reducibility, SiO2 is basically an irreducible oxide. By adding alkalis to the irreducible oxides, reducible surface oxygen and a large excess of surface hydroxyl groups have been observed, which is very similar to observations for Pt on ceria catalysts. As a result, these observations suggest an active role of the support in the vicinity of the Pt metal species during the WGS reaction, presumably through water activation.24−28 Nonetheless, how the noble metal cluster and the reducible oxide support participate during the reaction is still controversial. By experimental investigation, it has been agreed upon that the surface state and the dominant pathway are strongly dependent on the reaction condition.29 The method of preparation can also strongly affect the surface state. Different pathways have been proposed for the reaction on Pt-ceria

1. INTRODUCTION Clean hydrogen production is a key prerequisite for a future hydrogen economy based on, for example, proton-exchange membrane fuel cells (PEMFC). Carbon monoxide has been identified as a detrimental poison for fuel cell electrode catalysts, especially for Pt-based electrodes.1,2 Even small CO amounts of 10 ppm can irreversibly deactivate Pt electrodes.3 Since hydrogen is for the foreseeable future going to be produced from carbonaceous resources, such as natural gas, the CO content of the feed stream to the PEMFC has to be reduced by the water-gas shift (WGS: CO + H2O → CO2 + H2)4−9 or preferential oxidation (PROX: 2 CO + O2 → 2 CO2)5,10−17 reactions, for example. Since the heterogeneously catalyzed WGS reaction is moderately exothermic (ΔH = −41.1 kJ/mol) and often equilibrium limited, CO conversion is thermodynamically favored at low temperatures. Cu-based catalysts are widely used in industry for the WGS, since they exhibit high activity at low temperatures. However, they are pyrophoric and normally require lengthy and complex activation steps before usage.18,19 As a result, recent studies have suggested supported noble metal catalysts, due to their activity, selectivity, and stability in air, as promising catalysts for the WGS for mobile applications. Generally, noble metals are finely dispersed on a high surface area support for an efficient use of the catalytically active material. As long as the dispersion, activity, and selectivity of the small (precious) metal clusters on the support are maintained, small cluster sizes are preferred over nanometersized particles.20,21 This is even more important if the support plays an active role during the catalytic reaction as it has been proposed for the WGS over noble metal clusters supported on © 2014 American Chemical Society

Received: January 3, 2014 Revised: March 2, 2014 Published: March 6, 2014 6314

dx.doi.org/10.1021/jp5000649 | J. Phys. Chem. C 2014, 118, 6314−6323

The Journal of Physical Chemistry C

Article

Figure 1. (a) Pt/CeO2 (111) catalyst model used to study the WGS reaction mechanism. The highlighted areas correspond to the active site (*Pt− Oint−*Ce). (b) CO adsorbed at the Pt/CeO2 interface. The highlighted areas correspond to the atoms relaxed during vibrational frequency calculations. Cream, red, and navy balls represent Ce, O, and Pt atoms, respectively, while white and gray balls represent H and C atoms.

sites and supported on a partially hydroxylated ceria (111) surface in the presence of two vacancies beneath the Pt cluster. We assumed a 0.375 ML hydrogen coverage for the bare ceria surface. The CeO2 (111) slab is modeled by a p(4 × 4) unit cell, 15 Å vacuum gap, and nine atomic layers with the bottom three layers fixed in their bulk positions. All calculations presented in this work have been performed using the periodic plane wave DFT implementation in the VASP code.43−46 We chose the PBE functional47 within the generalized gradient approximation to describe exchange and correlation effects. The core−electronic states were described by potentials constructed with the projector augmented-wave (PAW) method.48,49 Spinpolarized calculations have been carried out at the Γ-point using the Gaussian smearing method (σ = 0.1 eV) with an energy cut off for the plane waves of 500 eV. A Hubbard, U−J, parameter of 5 eV has been employed to describe the strongly correlated 4f electrons of ceria. The number of valence electrons considered for Ce, O, Pt, C, and H are 12, 6, 10, 4, and 1, respectively. Dipole and quadrupole corrections to the energy are taken into account using a modified version of the Markov and Payne method.50 Harris−Foulke-type corrections51 have been included for the forces. The TS structures are optimized using the climbing image-NEB52 and Dimer methods.53,54 We used harmonic transition state theory for elementary surface reactions and collision theory for adsorption processes. The ZPE is obtained as ∑i(1/2)hvi from calculated vibrational frequencies, vi. The gas phase molecules and only a few oxygen atoms on the CeO2 surface that are directly involved in the reaction mechanism are included in the frequency calculations (refer to Figure 1b). Considering that the harmonic approximation is least accurate for small vibrational frequencies, we shifted all (real) frequencies below 200 cm−1 to 200 cm−1. This procedure ensures that for surface reactions, low frequency modes have no effect on reaction energies and rate constants (these frequencies in effect cancel out). After calculating the forward and reverse rate constants for each elementary reaction, we constructed a Master equation and solved for the mean-field steady-state solution of probability densities for the system to occupy each discrete state. In the following, we call these probability densities interface coverages,

catalytic surfaces. The debate is basically between a redox pathway and an associative (formate/carboxyl) pathway.23,26−31 Recently, it has been suggested that at some specific reaction conditions the redox pathway might combine with a formate pathway for the regeneration of the active OH groups required for formate formation. Also, carboxyl species have theoretically been suggested to be actively involved during the WGS reaction on Pt and Cu metal sites and also on Au/ceria (111) interface sites.32−34 Recently, we investigated redox pathways of the WGS reaction at Pt/TiO2 (110)35 and Pt/CeO2 (111)36 interface sites. Our analysis suggested that the redox pathway is possibly the dominant pathway at the Pt/TiO2 interface and both the redox pathway and associative pathway with redox regeneration could operate on Pt/CeO2 catalysts. We attributed the high reactivity of both catalysts to the presence of an increased number of oxygen vacancies at the three-phase boundary (TPB) which facilitates water activation. The associative pathway, in which the active formate or carboxyl intermediate is formed from adsorbed CO and −OH from dissociated H2O, has also been proposed in the literature for Au- and Ptsupported CeO2 catalysts.27,37−41 Thus, in the following, we study the associative carboxyl pathway that does not involve the creation of oxygen vacancies during the catalytic cycle at interfacial Pt/CeO2 (111) sites. We used the Pt10/CeO2 (111) catalyst model identified in our previous constrained thermodynamic analysis42 for this study and developed a mean-field microkinetic model to investigate the associative carboxyl route. All parameters in this microkinetic model have been determined from first principles without any fitting parameters. We find that the associative pathways have higher activation barriers and lower rates compared to the associative pathway with redox regeneration.

2. COMPUTATIONAL MODEL AND METHODS The catalyst model used to investigate various associative pathways of the WGS reaction is shown in Figure 1a. We chose this model based on our previous constrained ab initio thermodynamic analysis on the stability of the Pt/CeO2 (111) surface under WGS reaction conditions.42 This model consists of a Pt10 cluster covered by CO molecules on the atop 6315



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θ. A detailed description of microkinetic modeling approach used in this study can be found in our recent report.36 The BzzMath library developed by G. Buzzi-Ferraris55 is used to solve the steady-state equations. For clarity, some of the reactant or product states in the elementary reactions are represented as having multiple species; in the Master equation each reactant or product state constitute one discrete state. The interface coverages are used to find the reaction rate (turnover frequency) of each pathway. Campbell’s degree of rate control and degree of thermodynamic rate control analyses were used to identify the rate-limiting steps and the most important reaction intermediates.56

activity, such as Pt/CeO2, the formates seen by IR spectroscopy typically account for less than about 10−15% of the total WGS reaction products. Thus, the contribution from the associative mechanism to the overall rate might be non-negligible, however, not dominant. Recent computational studies on Cu and Pt metal surfaces suggested that the WGS reaction on these surfaces proceeds through the associative mechanism. However, these studies revealed that the active reaction intermediate is a carboxyl (−COOH) species rather than the formate (−HCOO) species because of the large activation barrier for the formation and decomposition of formate intermediates.32,63,64 Thus, we investigated the associative mechanism at the Pt/CeO2 interface and considered the carboxyl intermediate as an active species. We will show in the latter part of this article that a formate mechanism is energetically not viable. The reaction steps we considered for this pathway are depicted in equations (R1) to (R8):

3. RESULTS AND DISCUSSION 3.1. Classical Associative Pathway. The associative pathway proposed for the water-gas shift reaction at the metal/oxide interface is described in Figure 2. In this

*Pt − Oint − *Ce (IM1) + CO(g) → COPt − Oint − *Ce (IM2)

(R1)

COPt − Oint − *Ce (IM2) + H 2O(g) → COPt − Oint − H 2OCe (IM3)

(R2)

COPt − Oint − H 2OCe (IM3) → COPt − OH int − OHCe(IM4)

(R3)

COPt − OH int − OHCe(IM4)

Figure 2. Schematic representation of the associative carboxyl pathway at an interfacial site of a metal cluster supported on a reducible metal oxide.

→ COOHPt − OH int − *Ce (IM5)

(R4)

COOHPt − OH int − *Ce (IM5) + *Pt

mechanism, CO adsorbs on the interfacial metal atom and H2O adsorbs and dissociates on the oxide support. Then, an associative carboxyl/formate intermediate forms that can subsequently dissociate to produce CO2 and H2. The oxygen from the support oxide is not removed during the catalytic cycle. Surface formate species have often been proposed to be a key reaction intermediate in the WGS reaction mechanism.29,57 For ceria-based catalysts, the associative reaction mechanism has been proposed mainly based on Fourier transform-infrared (FT-IR) studies. Nearly 2 decades ago, Shido and Iwasawa proposed an associative mechanism based on their FT-IR studies on CeO2 and Rh-doped CeO2.58,59 A decade later, Jacobs et al. proposed a similar mechanism over Pt/CeO2 based on their isotope switching experiments and DRIFTS measurements.40,41 In another study, Meunier et al. reported that a switchover from a nonformate to a formate-based mechanism could take place over a narrow temperature range (as low as 60 K) on a 2% Pt/CeO2 catalyst.30 Azzam et al. proposed the associative formate mechanism over Pt/CeO2 catalyst based on FT-IR spectra and also observing simultaneous formation of H2 and CO2 during H2O pulses.27 However, recent experiments challenged the importance of the associative formate mechanism and suggested that the formate species observed by infrared are mainly spectator species.60−62 Burch and coworkers31 reviewed the literature based on the associative formate mechanism and suggested that formates observed by IR were potentially the main reaction intermediates only in the case of catalysts with very low activity. For catalysts with high

→ CO2(Pt) − HPt − OH int − *Ce (IM6)

(R5)

CO2(Pt) − HPt − OH int − *Ce (IM6) → HPt − OH int − *Ce (IM7) + CO2(g) + *Pt

(R6)

HPt − OH int − *Ce (IM7) + *Pt → 2HPt − Oint − *Ce (IM8)

(R7)

2HPt − Oint − *Ce (IM8) → *Pt − Oint − *Ce (IM1) + H 2(g) + *Pt

(R8)

The calculated energy profile for this pathway starting from our catalyst model described in the computational method section is provided in Figure 3. The intermediate structures are shown in the inset, and the transition state structures are provided in Figure S1 of the Supporting Information. The TS structures are numbered with reference to the elementary reaction steps throughout this paper. All reported reaction and activation energies are ZPE corrected. In this pathway, CO adsorbs on an empty interfacial Pt site in bridge position with a binding energy of −2.17 eV (R1). H2O adsorbs on a Ce atom of the oxide support with a binding energy of −0.26 eV (R2) and dissociates by transferring one H atom to the interfacial oxygen atom (R3). The H2O dissociation process is 0.34 eV endothermic and has an activation barrier of 0.35 eV (TS3). In the next step, CO adsorbed on Pt reacts with an adsorbed 6316



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Figure 3. (Free) energy profile (eV) for the classical associative mechanism of the WGS reaction at an Pt10/CeO2 (111) interface site (T = 500 K; Pi(gas) = 1 atm). All energies are with reference to the sum of the energies of the initial state (*Pt−Oint−*Ce, IM1) and the gas phase molecules.

−OH on Ce forming a −COOH intermediate (R4). This process has an activation barrier of 0.76 eV (TS4) and is 0.66 eV endothermic. We note that this step also involves an −OH rotation (not shown here) that corresponds to the cis−trans isomerization of the adsorbed −COOH intermediate. The trans−COOH intermediate (IM5) then dissociates on Pt, leading to the formation of adsorbed CO2 and H species on Pt (IM6). This process (R5) is exothermic by −0.20 eV with a barrier of 0.30 eV. CO2 then desorbs from Pt in another slightly exothermic process (R6; ΔE = −0.11 eV), leaving two H atoms adsorbed on Pt and an interfacial oxygen. The transfer of H from the interfacial oxygen to Pt followed by H2 desorption takes place in the following two steps (R7 and R8) that complete the catalytic cycle. The activation barrier calculated for the H-transfer process (R7; Eact = 0.62 eV) is slightly smaller than the COOH formation process. The Gibbs free energy profile calculated at 500 K for this associative carboxyl pathway is provided in Figure 3 together with the energy diagram. The partial pressures of the gas phase molecules were assumed to be 1 atm. The free energy profile indicates that the CO-adsorbed intermediate (IM2) is the most stable intermediate in this catalytic cycle, and the TS corresponding to COOH dissociation (TS5) is the highest energy state in the free energy profile.

Next, the DFT-derived reaction free energies and activation free energies are used to build an entirely DFT-based microkinetic model of the WGS reaction, as explained in the computational section and in our recent work.36 In order to compare our results to previous experimental reports, we chose two different experimental studies that report a detailed kinetic analysis of the WGS reaction over Pt/CeO2 catalysts.22,65 The Pt contents of the catalysts and the reaction conditions at which the kinetic measurements were carried out are slightly different in these two experimental studies. For example, Phatak and coworkers22 used a 1 wt % Pt/CeO2 catalyst and investigated the WGS reaction kinetics by changing the gas composition in the ranges of 5−25% CO, 5−30% CO2, 25−60% H2, and 10−46% H2O. The apparent activation energy and the turnover frequency were measured at 200 °C. The reported reaction orders, apparent activation energy (Eapp), turnover frequency (TOF), and the results of our current model are presented in Table 1 under experimental condition no. 1. On the other hand, Thinon and co-workers65 used a 1.2 wt % Pt/CeO2 catalyst and conducted the kinetic study in the temperature range of 150−400 °C at a total pressure of 1 atm. The apparent activation barrier and the turnover frequency have been measured at 300 °C and 10% CO, 10% CO2, 20% H2O, and 40% H2. The results of these studies along with our carboxyl 6317



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Table 1. Reaction Orders (ni), Apparent Activation Energies (Eapp), and Turnover Frequencies (TOF) for the Associative Carboxyl Pathway at Different Experimental Conditions

nCO nH2O nCO2 nH2 TOF (s‑1) Eapp (eV) a

experimental condition no. 1

experimental condition no. 2

(PCO = 0.07, PCO2 = 0.085, PH2O = 0.22, PH2 = 0.37 atm; T = 473 K)

(PCO = 0.1, PCO2 = 0.1, PH2O = 0.2, PH2 = 0.4 atm; T = 573 K)

model

experimenta

model

experimentb

0.00 1.00 0.00 0.00 9.51 × 10−8 0.99

−0.03 0.44 −0.09 −0.38 4.14 × 10−2 0.78

0.00 1.00 0.00 0.00 6.17 × 10−6 1.00

0.14 0.66 −0.08 −0.54 0.2 0.94

See ref 22. bSee ref 65. Figure 4. Pt/CeO2 (111) catalyst model used to investigate COassisted associative pathways of the WGS reaction. Cream, red, and navy balls represent Ce, O, and Pt atoms, respectively; while white and gray balls represent H and C atoms. The highlighted areas correspond to the active sites (COPt−*Pt−Oint−*Ce).

model are presented in Table 1 under experimental condition no. 2. The microkinetic analysis of the classical associative carboxyl pathway under these two experimental conditions predicts an apparent activation barrier of about 1.00 eV, which is close to the value predicted for experimental condition no. 2 (Table 1). However, the calculated TOFs are about 5−6 orders of magnitude lower than the experimental values. Furthermore, computations predict a much higher reaction order with respect to H2O and zero orders with respect to CO, CO2, and H2. Campbell’s degree of rate control analysis suggests that the COOH dissociation process is rate-limiting, which agrees with an earlier report on the WGS over Pt (111).32 The low TOF predicted by our microkinetic model is due to the strong adsorption of CO at the interfacial Pt, which is the dominant intermediate throughout this temperature range (interface coverage, θIM2 ≈ 1.0). A similar observation was made in our recent work on the redox pathways at the Pt/CeO2 interface.36 Our previous investigation suggested that this CO is not reactive at this low temperature range (423−673 K); instead, this strongly adsorbed CO assists the WGS reaction at the neighboring interface site by reducing the CO adsorption strength. Thus, we chose the CO pt −O int −* Ce (IM2) intermediate as our initial reactant structure and investigated CO-assisted associative pathways at the neighboring site. 3.2. CO-Assisted Associative Pathways. The catalyst model used to investigate the CO-assisted associative pathways (COpt−*Pt−Oint−*Ce, IM2) and the active sites considered are illustrated in Figure 4. In this model, we considered the empty Pt site neighboring the adsorbed CO and the Ce site next to this empty Pt site as an active site. The interfacial oxygen site (Oint) that is involved in the catalytic process is the same as that in the classical associative pathway. The eight reaction steps (R9−R16) considered for the COassisted associative carboxyl pathway starting from IM2 are very similar to the steps described in the previous section. A graphical representation of this catalytic cycle is depicted in Figure 5, and the free energy profile calculated at 500 K is provided in Figure 6 together with the intermediate structures. The corresponding TS structures can be found in Figure S2 of the Supporting Information. Compared to the classical associative pathway, the CO adsorption free energy in the presence of CO at a neighboring site is greatly reduced (−0.17 eV vs −1.22 eV). The product of water dissociation, 2COPt− OHint−OHCe (IM11), is slightly more stable than the COadsorbed intermediate, IM9. Both, the −COOH formation

Figure 5. Network of possible CO-assisted associative carboxyl pathways of the WGS reaction at a Pt10/CeO2 (111) interface site.

(R12) and dissociation (R13) processes are endergonic with activation barriers of 0.92 and 0.63 eV, respectively. The desorption of CO2 leads to another stable intermediate (IM14) in the free energy profile. The second H transfer from the interface oxygen to Pt has an even higher activation barrier (0.96 eV) in the presence of adsorbed CO compared to the classical associative pathway. H2 desorbs easily from IM15, which completes the catalytic cycle. COPt − *Pt − Oint − *Ce (IM2) + CO(g) → 2COPt − Oint − *Ce (IM9)

(R9)

2COPt − Oint − *Ce (IM9) + H 2O(g) → 2COPt − Oint − H 2OCe (IM10)

(R10)

2COPt − Oint − H 2OCe (IM10) → 2COPt − OH int − OHCe(IM11)

(R11)

2COPt − OH int − OHCe(IM11) → COPt − COOHPt − OH int − *Ce (IM12) 6318

(R12)



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Figure 6. Free energy profile (eV) for the CO-assisted associative carboxyl pathways at a Pt10/CeO2 (111) interface site (T = 500 K; P(gas) = 1 atm). All energies are with reference to the sum of the energies of the initial state (COPt−*Pt−Oint−*Ce, IM2) and the gas phase molecules.

(R17−R19) and the corresponding energetics are provided in Figure 6.

COPt − COOHPt − OH int − *Ce (IM12) + *Pt → COPt − CO2(Pt) − HPt − OH int − *Ce (IM13)

COPt − HPt − OH int − *Ce (IM14) + CO(g) + *Pt

(R13)

→ 2COPt − HPt − OH int − *Ce (IM16)

COPt − CO2(Pt) − HPt − OH int − *Ce (IM13)

2COPt − HPt − OH int − *Ce (IM16) + *Pt

→ COPt − HPt − OH int − *Ce (IM14) + CO2(g) + *Pt (R14)

→ 2COPt − 2HPt − Oint − *Ce (IM17)

COPt − HPt − OH int − *Ce (IM14) + *Pt → COPt − 2HPt − Oint − *Ce (IM15)

(R17)

(R18)

2COPt − 2HPt − Oint − *Ce (IM17) → 2COPt − Oint − *Ce (IM9) + H 2(g) + 2*Pt

(R15)

(R19)

The second CO molecule adsorbs even stronger in the presence of interfacial −OH (R17, ΔG = −0.66 eV) compared to (R9). The H transfer process from the interfacial OH to the Pt cluster in the presence of two adsorbed CO molecules is endergonic with a high barrier (R18, Gact = 1.16 eV). The catalytic cycle is completed with the desorption of H2 from the Pt cluster and formation of 2COPt−Oint−*Ce (IM9). Although the H transfer processes, both in the presence of one and two CO molecules (R15 and R18), have high activation barriers, the overall free-energy profile suggests that the TS corresponding to the −COOH dissociation process (TS13) is the highest energy state in this catalytic cycle.

COPt − 2HPt − Oint − *Ce (IM15) → COPt − *Pt − Oint − *Ce (IM2) + H 2(g) + *Pt (R16)

Our investigation of the redox pathways of the WGS reaction at the Pt/CeO2 interface sites suggested that it is always favorable to add a second CO molecule on Pt whenever there is an oxygen vacancy or −OH present at the interface.36 Thus, we have also considered here the possibility of adsorbing a second CO molecule in IM14 before the H transfer (R15). The reaction steps considered for this possibility are described in 6319



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Table 2. Reaction Orders, Apparent Activation Energies (Eapp), and Turnover Frequencies (TOF) for the CO-Assisted Associative Carboxyl Pathway at Different Experimental Reaction Conditions reaction order reaction condition PCO = 0.07, PCO2 = 0.085, PH2O = 0.22, PH2 = 0.37 atm; T = 473 K

experiment

associative carboxyl with redox regenerationc experimentd associative carboxylb associative carboxyl with redox regenerationc

a

CO

H2O

CO2

H2

4.14 × 10−2

0.78

−0.03

0.44

−0.09

−0.38

1.66 × 10−2 (2.90 × 10−3)e 7.48 (1.31)e

1.18

0.00

1.00

0.00

−1.00

0.93

0.00

0.76

0.00

−0.78

0.2

0.94

0.14

0.66

−0.08

−0.54

2.24 (0.39)e 4.27 × 102 (7.49 × 101)e

1.17 0.92

0.00 0.00

1.00 0.93

0.00 0.00

−1.00 −0.81

TOF (s−1)

method a

associative carboxylb

PCO = 0.1, PCO2 = 0.1, PH2O = 0.2, PH2 = 0.4 atm; T = 573 K

Eapp (eV)

See ref 22. bCurrent work. cSee ref 36. dSee ref 65. eTOFs extrapolated to match experimental particle size.

clearly indicate that the associative formate pathway is highly unlikely at Pt/CeO2 interface sites, and the associative pathway could possibly only occur via carboxyl intermediates. Thus, we did not include the formate steps in our microkinetic model. 3.3. Insights from Microkinetic Modeling. The free energy profiles calculated at 500 K for the classical and COassisted carboxyl pathways (Figures 3 and 6) suggest that the CO-assisted pathway is likely the more favorable pathway. In order to estimate the effect of experimental reaction conditions such as temperature and partial pressure of the gas phase molecules on experimental observables, we developed a microkinetic model, including all eleven reaction steps (R9− R19) of the CO-assisted associative carboxyl pathways that are described in Figure 5. A detailed explanation of the experimental conditions is provided in Classical Associative Pathway. The kinetic parameters such as zero-point energy corrected reaction energies, activation barriers, forward rate constants, and equilibrium constants for the 11 elementary steps are provided in Tables S1 and S2 of the Supporting Information. Table 2 summarizes the key properties obtained from our microkinetic analysis, such as reaction orders, turnover frequencies, and apparent activation energies calculated at two different experimental conditions. With a comparison of the CO-assisted carboxyl pathway with the classical associative pathway (see Table 1), it becomes apparent that although the calculated apparent activation barriers are slightly larger in the CO-assisted pathway, the calculated TOFs are for the COassisted carboxyl pathways 6 orders of magnitude higher than in the classical associative pathway and are overall in good agreement with the experimental values. This improvement in TOFs could be achieved only after including coadsorbed CO in our model. Our microkinetic analysis also suggests that a second CO adsorption is still highly favorable during the catalytic cycle when the interfacial oxygen is reduced to −OH. After CO2 desorption (R14) from IM13 (Figures 5 and 6), the reaction proceeds favorably through the second CO adsorption (R17) rather than the second H transfer process (R15), leading to a positive reaction order with respect to H2O and a negative reaction order with respect to H2. In order to compare these results with our recent work on various redox pathways, we also list in Table 2 the results obtained for the associative carboxyl pathway with redox regeneration, which involves an oxygen vacancy formation during the catalytic cycle.36 A schematic representation of this

Experimental studies suggesting that the associative mechanism is of importance generally also observe the formation of formate (−HCOO) intermediates. However, theoretical studies on metal surfaces and on Au/CeO2 interface models have shown that the carboxyl pathway is clearly preferred over the formate pathway.32,34,63,64 In our recent study on the redox pathways at Pt/CeO2 interface sites, we have also observed that the formation of formate intermediates requires overcoming very high activation barriers compared to the carboxyl intermediates.36 Here again, we tested the possibility of forming a formate intermediate in the associative pathway. In contrast to the single step formation of −COOH from Ptadsorbed CO and Ce-adsorbed OH (R12), the −HCOO formation is a two-step process (R20 and R21). In the first step (R20), CO abstracts H from the −OHCe group forming intermediate (IM18) with −CHO adsorbed on Pt and an oxygen atom bridge-bonded to Ce and Pt. The formyl group (−CHO) then reacts with the adsorbed oxygen atom in the following step (R21) to form the −HCOO intermediate (IM19). Further dissociation of this formate intermediate leads to the formation of intermediate (IM13) with adsorbed CO2 and H on the Pt cluster (R22). 2COPt − OH int − OHCe(IM11) → COPt − CHOPt − OH int − OCe (IM18)

(R20)

COPt − CHOPt − OH int − OCe (IM18) → COPt − HCOOPt − OH int − *Ce (IM19)

(R21)

COPt − HCOOPt − OH int − *Ce (IM19) + *Pt → COPt − CO2(Pt) − HPt − OH int − *Ce (IM13) (R22)

The structures and their relative energies of these steps are provided in Figure S3 of the Supporting Information. Our calculations suggested that the formate intermediate (IM19) possesses a very similar energy to the carboxyl intermediate (IM12). However, the formyl intermediate (IM18) is highly unstable (R20, ΔE = 2.42 eV), suggesting that the formation of the −HCOO intermediate requires overcoming very high activation barriers compared to −COOH formation. Further, dissociation of the formate intermediate (R22) also has a high activation barrier (1.52 eV) compared to the dissociation barrier of the −COOH intermediate (0.63 eV). These results 6320



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Table 3. Campbell’s Degree of Rate Control (XRC), Degree of Thermodynamic Rate Control (XTRC), and Interface Coverage (θ) of Key Intermediates in the Associative Carboxyl Pathway of WGS Reaction at a Pt/CeO2 (111) Interface Site rate control analysis

intermediate properties

XRC

473 K

573 K

reaction

473 K

573 K

intermediate

θ

XTRC

θ

XTRC

R12 R13

0.07 0.93

0.08 0.92

2COPt−HPt−*Ce−OHint (IM16)

1.00

−1.00

1.00

−1.00

H2O dissociation on the CeO2 surface is exergonic with a small activation barrier (Figure 6). However, the rate-controlling COOH formation and dissociation processes take place solely on the metal cluster and, thus, the calculated rates are quite similar to the rates over the metal surfaces. In the associative carboxyl pathway with redox regeneration,36 the −COOH group and its dissociated product (−CO2) are stabilized by an interface oxygen vacancy, which in turn decreases the dissociation barrier and increases the rate. We note here that in our previous work we included additional H atoms on the CeO2 surface while investigating the associative carboxyl pathway with redox regeneration, which facilitated the O−H bond-breaking process as well as the −COOH dissociation process. Since the O−H bond breaking process is not ratecontrolling in the associative carboxyl pathway studied here and the −COOH dissociation is preferred on the Pt metal rather than on surface oxygen, adding more H atoms to the CeO2 surface did not change/improve the overall rate. To conclude, the investigated associative carboxyl pathway at Pt/CeO2 interface sites illustrates characteristics (such as TOFs and reaction orders) very similar to those of the pure Pt surfaces or experimental reports over irreducible Al2O3-supported Pt catalysts that are significantly less active than the CeO2 supported Pt catalysts.

pathway is provided in Figure S4 of the Supporting Information. The calculated apparent activation barriers are about 0.2 eV higher and the TOFs are about 2 orders of magnitude lower for the “CO-assisted associative carboxyl pathway” considered in the present work compared to the “associative carboxyl pathway with redox regeneration” considered in our previous work. Although the TOFs predicted by the present calculations seem to be in good agreement with the experimental values, one should expect that the computed TOFs be higher because they are based on interfacial Pt atoms, while the experimental TOFs are normalized by the total number of surface Pt atoms. In order to make a more direct comparison between our calculated TOFs and experimental TOFs, we used the following procedure: we build (half) cuboctahedral Pt particles with increasing size and identified a Pt cluster model with a dispersion that agrees with the experimental dispersion (28−34%).22,65 A Pt particle containing 792 atoms (Ø ≈ 3.9 nm) has about 34% of its atoms exposed to the gas phase. For this cluster, the ratio of interface to total surface Pt atoms is 15.7%. With the use of this model and assuming that only interface atoms are catalytically active, Table 2 also shows the extrapolated TOFs (numbers in brackets). The extrapolated TOFs for the associative carboxyl pathway with redox regeneration are still about 2 orders of magnitude larger than the experimental TOFs, which we contribute to GGA-DFT underestimating activation barriers. Nevertheless, it appears most likely that the associative pathway with redox regeneration is the dominant pathway for the WGS at Pt/CeO2 interface sites and the associative carboxyl pathway contributes only to a small extent to the overall rate. Campbell’s degree of rate control analysis for the associative carboxyl pathway suggests that the reaction step involving −COOH dissociation (R13) has the largest effect on the overall rate at both temperatures (Table 3) and the carboxyl formation process (R12) also has a small effect on the rate. Thermodynamic rate control analysis (XTRC) suggests that the intermediate 2COPt−HPt−*Ce−OHint (IM16) is the dominant species throughout the temperature range considered in this study, and the stability of this intermediate has a significant effect on the overall rate of reaction. Overall, the predicted results for the associative carboxyl pathway at Pt/CeO2 (111) interface sites seem to be in agreement with the computational predictions for the WGS over Pt (111) and the stepped Pt (211) and Pt (322) surfaces. The TOF predicted for the associative carboxyl pathway at 573 K (2.24 s−1) in the present work is similar to the computed TOFs for the WGS reaction on Pt (111) (0.22 s−1 at 573 K)32 and on the stepped Pt (211) and Pt (322) surfaces (on the order of 10° s−1 at 575 K).63 The previous reports suggested that the associative carboxyl pathway is the dominant pathway on the metal surfaces and either carboxyl formation or dissociation is rate controlling. On Pt (111) and Pt (211), H2O dissociation was reported to be endergonic with highactivation barriers of 0.64−0.90 eV.32,63 In our current model,

4. CONCLUSIONS We investigated the associative carboxyl pathway of the WGS reaction at Pt/CeO2 (111) interface sites that does not involve the creation of an oxygen vacancy during the catalytic cycle. Our microkinetic analysis based on parameters obtained from first principles (without any fitting parameters) predicts a nonnegligible rate for the CO-assisted associative carboxyl pathway. However, the computed rates at different experimental conditions are about 2 orders of magnitude lower and the activation barriers are about 0.2 eV higher than the associative carboxyl pathway with redox regeneration we previously investigated. For the associative carboxyl pathway computations predicts a positive reaction order of 1.0 with respect to H2O and a negative reaction order of −1.0 with respect to H2. Campbell’s degree of rate control analysis suggests that the −COOH dissociation is rate-controlling. The calculated parameters are in agreement with those predicted for the pure Pt metal surfaces and Al2O3-supported Pt catalysts. Nevertheless, the unique activity of Pt/CeO2 catalysts can only be explained by the associative carboxyl pathway with redox regeneration, which involves the creation of an oxygen vacancy at the interface during the catalytic cycle. In this pathway, the oxygen vacancy facilitates H2O dissociation as well as stabilizes intermediates such as −COOH and −CO2. Hence, the stability of the adsorbed intermediates continuously increases along the catalytic cycle as demanded for very efficient catalytic cycles. In contrast, in the associative carboxyl pathway, the −COOH and −CO2 intermediates adsorbed on the metal cluster are less 6321



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stable, which in turn increases the activation barrier and decreases the rate. To conclude, by comparing the two pathways, we are able to illustrate the importance of the ceria support and the bifunctional mechanism operating for the WGS reaction over reducible oxide-supported metal catalysts.

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ASSOCIATED CONTENT

* Supporting Information S

Reaction energies, activation barriers, and kinetic parameters for the elementary steps considered in the CO assisted pathways, calculated transition state structures for the associative carboxyl and formate pathways, and schematic representation of the associative carboxyl pathway with redox regeneration. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation under Grant CBET-0932991 and in part by CBET-1254352 and XSEDE resources provided by the National Institute for Computational Sciences (NICS), Texas advanced Computing Center (TACC), and the Purdue University under Grant TGCTS090100. Furthermore, a portion of this research was performed at the U.S. Department of Energy facilities located at the National Energy Research Scientific Computing Center (NERSC) and at EMSL, located at Pacific Northwest National Laboratory (Grant Proposal 47447). Finally, computing resources from the USC NanoCenter and USC’s High Performance Computing Group are gratefully acknowledged.

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REFERENCES

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