CO Hydrogenation on Pd(111): Competition between Fischer–Tropsch

Article pubs.acs.org/JPCC

CO Hydrogenation on Pd(111): Competition between Fischer− Tropsch and Oxygenate Synthesis Pathways Sen Lin,*,† Jianyi Ma,*,‡ Xinxin Ye,† Daiqian Xie,§ and Hua Guo∥ †

Research Institute of Photocatalysis, Fujian Provincial Key Laboratory of Photocatalysis, State Key Laboratory Breeding Base, Fuzhou University, Fuzhou 350002, China ‡ Institute of Atomic and Molecular Physics, Sichuan University, Chengdu, Sichuan 610065, China § Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China ∥ Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States S Supporting Information *

ABSTRACT: The hydrogenation of CO on Pd can lead to methane via the Fischer−Tropsch process and methanol via oxygenate synthesis. Despite the fact that the former is thermodynamically favored, the catalysis is mostly selective to the latter. Given the importance of methanol synthesis in both industry applications and fundamental understanding of heterogeneous catalysis, it is highly desirable to understand the mechanism and selectivity of CO hydrogenation on Pd catalysts. In this work, this process is studied on Pd(111) using periodic plane-wave density functional theory and kinetic Monte Carlo simulations. The barriers and reaction energies for the methanol and methane formation are systematically explored. Our results suggest that methanol is formed via CO* → CHO* → HCOH* → CH2OH* → CH3OH*. The HCOH* and CH2OH* intermediates, which feature a C−O single bond, were found to possess the lowest barriers for C−O bond fission, but they are still higher than those in methanol formation, thus confirming the kinetic origin of the experimentally observed selectivity of the Pd catalysts toward methanol.

1. INTRODUCTION Methanol is considered to be an important alternative fuel for future transportation needs, either for methanol fuel cells or for on-board generation of H2 for proton-exchange membrane fuel cells.1,2 Methanol has several unique advantages as an environmentally friendly energy carrier. For example, this biodegradable liquid is easier and safer to store and transport than hydrogen, and it has a large H/C ratio with no sulfur or nitrogen. In addition, methanol has been an important chemical feedstock for producing higher hydrocarbons and other valueadded chemicals.3 Finally, the conversion of CO2 to methanol represents an attractive and viable means to remove the greenhouse gas from the atmosphere.4 Methanol is typically synthesized from syngas, which is a mixture of CO, CO2 and H2 at elevated temperatures and pressures:5

extremely high, which is peculiar because the Fischer−Tropsch process leading to the formation of CH4: CO + 3H 2 → CH4 + H 2O, ΔH 0 = − 206 kJ/mol

is favored thermodynamically.3 Because of its industrial importance, the mechanism of methanol synthesis on Cu catalysts has been extensively investigated with both experimental6−11 and theoretical methods.11−17 Isotope labeling experiments suggested that reaction 1 is the main pathway on the Cu catalysts.18,19 The latest microkinetic model based on density functional theory (DFT) calculations revealed that both the CO and CO2 hydrogenation pathways (reactions 1 and 2) are operative under reaction conditions, although the latter dominates,17 confirming the established consensus on the reaction mechanism.5 We note in passing that much work has also been done for the reverse reaction for reaction 1 on Cu, namely, methanol steam reforming.20−25 In addition to the traditional Cu catalysts, Pd has also been found to catalyze methanol synthesis from CO + H2 with a high

CO2 + 3H 2 → CH3OH + H 2O, ΔH 0 = − 49.5 kJ/mol (1)

CO + 2H 2 → CH3OH, ΔH 0 = −91 kJ/mol

(2)

Received: May 7, 2013 Revised: June 19, 2013 Published: June 20, 2013

The selectivity of the well-established catalyst made of Cu dispersed on ZnO and Al2O3 support toward methanol is © 2013 American Chemical Society

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dx.doi.org/10.1021/jp404509v | J. Phys. Chem. C 2013, 117, 14667−14676

The Journal of Physical Chemistry C

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activity.26−28 However, methane was sometimes found in the products. Apparently, this Fischer−Tropsch reaction pathway (reaction 3) leading to CH4 requires C−O bond fission in adsorbed CHxO species somewhere along the CO hydrogenation pathway, thus differing fundamentally from the oxygenate synthesis pathway (reaction 2) in which CH3OH is produced by straightforward hydrogenation without the C−O bond cleavage.3,27,29 Indeed, minor CHx species have been detected in methanol decomposition on Pd surfaces, particularly at high pressures.30−33 There are also some reports suggesting that the selectivity of Pd catalysts depends on the choice of support and promoters,34−36 but others found no such effects.28 An in-depth understanding of the mechanism and particularly the selectivity requires accurate theoretical modeling of different reaction pathways. Thus far, however, only a few theoretical studies have been reported concerning the reaction mechanism of palladium-catalyzed methanol synthesis using Pd clusters as models,13,37 although theoretical studies have been reported for the reverse process.22,23,38−41 C−O bond cleavage of various oxygenates42 and hydrogenation toward CHx43 have also been studied on Pd, but the competition between the two pathways was not explored. This is in sharp contrast with the methanol synthesis on copper-based catalysts where extensive DFT studies have been carried out.11,13,15−17 In this work, we report a DFT study of the reaction mechanism for methanol synthesis on Pd(111), with a focus on the competition between pathways leading to CH3OH and CH4. The Pd(111) surface was selected as our DFT model due to fact that the (111) facet is found to be predominantly exposed in supported palladium catalysts44,45 and thus was a good representation of the real catalysts. This publication is organized as follows. The computational details are described in Section 2. The next section (Section 3) presents the results followed by discussions in the context of methanol synthesis (Section 4). The final section (Section 5) concludes.

convergence criteria. Stationary points were confirmed by normal-mode analysis using a displacement of 0.02 Å and an energy convergence criterion of 10−6 eV. The highest image along the minimum energy path was denoted as transition state. The energy barrier Ea of each elementary reaction was calculated by the energy difference between the transition state and the initial state. The reaction energy ΔE of each elementary reaction was calculated by the energy difference between the final state and the initial state. The vibrational frequencies were used to compute zero-point energy (ZPE) corrections. 2.2. Kinetic Monte Carlo. A simplified kinetic Monte Carlo (kMC) method56−59 was used to study the reactions using the DFT energetics. kMC can be considered as a coarsegrained simulation technique that follows the time evolution of adsorbates on the surface. For the system in which we are interested, all elementary processes are first tabulated. Each process (i) is assigned with a rate constant ki:60 ki =

⎛ E≠ ⎞ ⎛ E≠ ⎞ ⎛ ΔS ⎞ kBT exp⎜ ⎟ exp⎜⎜ − i ⎟⎟ = A 0 exp⎜⎜ − i ⎟⎟ h ⎝ kB ⎠ ⎝ kBT ⎠ ⎝ kBT ⎠

(1)

where kB is the Boltzmann constant, T is the temperature, h is the Planck constant, ΔS is the standard state entropy change between the IS and TS, E≠i is the energy barrier for ith process, and A0 is the frequency factor. The elementary processes treated in our kMC model include desorption, diffusion, and reaction. All of the forward and reverse elementary steps were also included and the barriers, exothermicities, and entropy changes were obtained from the original DFT values. Because of the larger number of species involved in the CO hydrogenation, it is very difficult to perform kMC simulations with microscopic details. Because we are primarily interested in the evolution of the reaction, several simplifications were made. Our model consists of a 30 × 30 lattice mimicking the catalyst surface. Each lattice grid point is treated as a coarsegrained “site”, and all detailed adsorption information of species, such as adsorption site and orientation, is ignored. Adsorbate−adsorbate interactions were not explicitly treated but bundled into the barrier heights. For bimolecular events, only reactions between neighboring species are allowed. At any given time t, the system configuration is denoted as M(t). To advance the system, an event table was first generated that covers all possible processes from M(t). A random number ρ1 was then used to choose the next process from the event table. This process, which is denoted as the jth process, was chosen if it satisfies the following equation:

2. COMPUTATIONAL DETAILS 2.1. Planewave DFT. All periodic DFT calculations were carried out using the Vienna ab initio simulation package (VASP)46−48 with the gradient-corrected PW91 exchangecorrection functional.49 For valence electrons, a plane-wave basis set was employed with a cutoff of 400 eV, while the ionic cores were described with the projector augmented-wave (PAW) method.50,51 To minimize interactions between image species of different unit cells, a large 3 × 3 unit cell of the Pd(111) surface was simulated using a slab supercell approach with periodic boundary conditions. This model consisted of four layers with the top two layers relaxed in all calculations. We also used a vacuum spacing of 14 Å to avoid interactions between adsorbates and slab images in the z direction. A 4 × 4 × 1 Monkhorst-Pack k-point grid52 was employed to sample the Brillouin zone, which was tested for convergence. The Fermi level was smeared using the Methfessel−Paxton method53 with a width of 0.1 eV. The optimized lattice parameter of the bulk Pd crystal was calculated to be 3.952 Å, in good agreement with the previously reported result.41 We have studied the adsorption of various pertinent species on the Pd(111) surface. The adsorption energy was calculated as follows: Eads = E(adsorbate+surface) − E(free molecule) − E(free surface). The climbing image-nudged elastic band (CI-NEB) method54,55 was used to determine transition states with the conventional energy (10−4 eV) and force (0.05 eV/Å)

j−1

j

∑ ki ≤ ρ1K < ∑ ki i=1

i=1

(0 ≤ ρ1 < 1)

(2)

where K = ∑Ni=1 ki is the total rate for all events with N as the number of all possible events. The time step can also be evaluated, assuming it follows the Poisson statistics. Specifically, Δt is determined by a second random number ρ2 according to Δt = − ln(1 − ρ2 )/K

(3)

The configuration of the system is updated accordingly and denoted by M(t + Δt). Starting with a mixture of CO* and H*, the kMC model monitors the evolution of intermediate species 14668



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as well as the final products, namely, CH4, H2O, and CH3OH, in the gas phase as a function of time. The rapid diffusion of species on the surface requires excess of computing resources in kMC, which is unnecessary because it simply leads to equilibration. In this work, we adapted strategy similar to that of Liu and Evans.61 Specifically, a Metropolis Monte Carlo procedure with importance sampling was used to redistribute the diffusing species on the lattice instead of following eq 2. This algorithm assumes instantaneous equilibration with negligible diffusing time.

study.62 The adsorption energy of H* is −2.72 eV, and the distances between H* and three Pd atoms are equal to 1.81, 1.81, and 1.82 Å, respectively. Hydroxyl (OH*). The OH* species is found to be adsorbed perpendicularly to the surface at the fcc site with its oxygen binding to three Pd atoms. The calculated O−Pd distances are 2.16, 2.16, and 2.17 Å, and the binding energy is found to be −2.49 eV. Water (H2O*). As displayed in Figure 1, H2O* adsorbed weakly with the lone pair of oxygen interacting with the surface Pd atom. The adsorption energy is calculated about −0.23 eV, which agrees well with the value (−0.29 eV) reported by Gu and Li.41 Carbon Monoxide (CO*). CO* prefers to adsorb at the fcc hollow site in a vertical configuration with an adsorption energy of −1.98 eV, with the Pd−C distances of 2.07, 2.07, and 2.08 Å. The adsorption of CO* at the hcp site also yields a similar binding energy of −1.97 eV. Methane (CH4*). Methane is found to be above the Pd (111) with a small positive adsorption energy (0.25 eV), as shown in Table S-1 in the Supporting Information. The positive value, which has also been previously found,63 is most likely spurious, as it is well known that the adsorption well of CH4 is very shallow on transition-metal surfaces.64 Methyl (CH3*). The CH3* species adsorbs with the optimal configuration on the top of a Pd atom with the Pd−C distance of 2.06 Å, in good agreement with the value (2.07 Å) found in a recent theoretical study by Herron et al.65 This adsorption state yields a binding energy of −1.76 eV. Methylene (CH2*). The bridge site of the Pd (111) surface is found to be the most stable adsorption site for CH2*, and the adsorption energy is computed to be −3.80 eV, consistent with the value (−3.97) reported by Yudanov et al.42 Methylidyne (CH*). The most stable adsorption site for CH* is the fcc hollow site through its C atom, with an elongated C−H bond. The molecule is found to be perpendicular to the surface with the three C−Pd bond lengths of 1.96 Å. Methanol (CH3OH*). By donating the lone pair of oxygen, methanol adsorbs at the top site with the C−O surface angle of 117.39° and the O−Pd distance of 2.36 Å. Our calculated adsorption energy of −0.25 eV is consistent with previous DFT results of −0.24 to −0.35 eV37,40−42 but smaller than the experimental value of −0.51 eV.66 Hydroxymethyl (CH2OH*). As a product from hydrogenation of HCOH* or CH2O*, CH2OH* prefers to adsorb atop a Pd atom with the C−Pd distance of 2.07 Å. The molecule is almost perpendicular to the surface normal with an angle between the O−C axis and the surface normal of 112.25°. The C−O bond length of 1.38 Å, which is much larger than that in CO* (1.19 Å), suggests a weaker bond. This configuration affords an adsorption energy of −1.81 eV. Hydroxycarbene (HCOH*). HCOH* adsorbs perpendicularly at the bridge site through its C atom. The distances of Pd−C, C−O, C−H, and H−O are found to be 2.04, 1.35, 1.10, and 1.00 Å, respectively. Like CH2OH*, the C−O bond length of 1.35 Å indicates a weaker bond. The adsorption energy is ∼1.33 eV larger than that of CH2OH*, which might be caused by the more unsaturated carbon atom. Formyl (CHO*). Formyl formed by the hydrogenation of CO* adsorbs at the fcc site through O anchoring on the top of Pd atom and C sitting at the bridge site. The C−O bond length

3. RESULTS 3.1. Adsorption. It is important to characterize the adsorption of pertinent species to understand the elementary chemical steps in CO hydrogenation. In Table S-1 (Supporting Information), the geometric and energetic information for various species involved in CO hydrogenation on Pd(111) is presented. The side and top views of the adsorption geometries are also included in Figure 1. The adsorption for some species has been investigated before, and our results are in general agreement with the available literature values.

Figure 1. Side and top views of the most stable adsorption configurations of relevant species. Atom colors are Pd (blue), O (red), C (black), and H (white).

Carbon (C*). C atom can be formed after cleaving the C−O bond of CO*. As shown in Table S-1 in the Supporting Information, it preferentially adsorbs at the hcp site. The distances between C and three surface Pd atoms are 1.87, 1.88, and 1.88 Å, respectively. The calculated adsorption energy is −6.92 eV. Oxygen (O*). The scission of C−O bond also produces the oxygen species. The atomic O* species binds strongly at the fcc hollow site of Pd(111) with the distance of the three Pd−O bond computed to be 2.00 Å. A large adsorption energy of −4.50 eV (relative to O(g)) is obtained. Hydrogen (H*). For H atom, the most stable adsorption sites is the fcc hollow site, consistent with the previous LEED 14669



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Table 1. Calculated Activation Energies, Reaction Energies and Pre-Exponential Factors Using kMC Simulations for the Reactions on Pd(111) Surface Studied in This Worka reaction type

no.

elementary reaction

methanol formation

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19

CO* + H* → COH* CO* + H* → CHO* CHO* + H* → CH2O* CHO* + H* → HCOH* CH2O* + H* → CH3O* CH2O* + H* → CH2OH* HCOH* + H* → CH2OH* CH2OH* + H* → CH3OH* CH3O* + H* → CH3OH* CO* → C* + O* CHO* → CH* + O* CH2O* → CH2* + O* HCOH* → CH* + OH* CH2OH* → CH2* + OH* CH3OH* → CH3* + OH* CH* + H* → CH2* CH2* + H* → CH3* CH3* + H* → CH4* OH* + H* → H2O*

C−O bond cleavage

methane formation

a

Ea (eV) 1.80 1.44 1.04 0.88 0.97 0.72 1.02 0.82 0.79 3.91 2.00 1.89 1.43 1.42 1.85 0.77 0.60 0.68 0.70

(1.75) (1.48) (1.03) (0.83) (0.95) (0.67) (0.96) (0.78) (0.71) (3.85) (1.96) (1.80) (1.35) (1.30) (1.70) (0.74) (0.57) (0.63) (0.62)

ΔE (eV) 0.94 (1.06) 1.28 (1.38) 0.68 (0.80) 0.24 (0.38) 0.36 (0.49) −0.05 (0.12) 0.28 (0.42) 0.15 (0.29) −0.45(−0.31) 2.65 (2.63) 0.83 (0.81) 0.62 (0.57) 0.36 (0.30) 0.72 (0.62) 0.34 (0.23) 0.44 (0.54) −0.25 (−0.12) −0.13 (−0.04) −0.37 (−0.25)

A0f 4.2 9.1 2.2 4.4 1.8 3.0 6.0 1.2

× × × × × × × ×

1012 1012 1013 1012 1013 1013 1013 1014

1.4 × 1013 2.6 × 1013 1.3 2.2 1.4 1.1

× × × ×

1013 1012 1012 1013

A0r 5.5 5.7 1.4 4.8 1.5 2.6 1.8 2.7

× × × × × × × ×

1012 1012 1013 1012 1013 1013 1011 1011

1.2 × 1013 4.0 × 1012 1.9 1.2 1.5 1.0

× × × ×

1012 1011 1011 1013

Entries in the parentheses are the ZPE-corrected values.

is 1.26 Å, only 0.07 Å longer than that in CO*. The calculated adsorption energy is found to be −2.32 eV. Formaldehyde (CH2O*). The formaldehyde species adsorbs in a bidentate fashion on Pt(111), with both O and C interacting with the substrate, yielding a binding energy of −0.48 eV. The distances of O−Pd and C−Pd are very close and are found to be 2.25/2.23 and 2.10 Å, respectively. Methoxyl (CH3O*). As shown in Figure 1, the fcc site is the most stable adsorption site for CH3O*. The three distances between O atom and surface Pd atoms are equal to 2.17 Å. The adsorption energy is −1.76 eV, making it relatively stable on the Pd(111) surface. Hydroxymethylidyne (COH*). The COH* species prefers to adsorb at the fcc site with three Pd−C bond lengths of 1.99, 1.99, and 1.97 Å, respectively. This adsorption configuration leads to a very large adsorption energy of −4.48 eV. 3.2. Reactions. To study the reaction mechanism for methanol synthesis through CO hydrogenation on Pd (111) surface and understand when and how the C−O bond cleavage to form methane species, we here calculated the activation energy barriers and exothermicities for all important elementary steps. The hydrogenation and C−O cleavage of COH* is not considered due to the high-energy barrier for the generation of COH* from CO* and H*. We also did not consider the reaction CH3O*→ CH3* + H* because the energy barrier for the formation of CH3O* from CH2O + H* is ∼0.30 eV higher than that for the generation of CH2OH*. The calculation results are listed in Table 1, and the configurations of initial state, transition state, and final state for each elementary step are displayed in Figures 2−4. 3.2.1. Methanol Formation. R1. CO* + H* → COH* + *. As shown in Table 1, CO* hydrogenation to COH* must overcome an energy barrier as high as 1.75 eV, and the reaction is found to be very endothermic (1.06 eV). In the initial state, the reactants CO* and H* are both located at their most stable sites. The transition state resembles a quadrilateral species locating at a fcc site, in which the angles of C−O−H, O−H−Pd, H−Pd−C, and

Figure 2. Side and top views of the initial state (IS), transition state (TS), and final state (FS) for each step of methanol formation listed in Table 1.

Pd−C−O are found to be 85.98, 117.15, 55.24, and 101.63°, respectively. The product COH* is generated after the oxygen catching the H atom and sits at the fcc site with the C atom on the top of Pd atom. As far as we know, only the reverse process over Pd(111) has been studied theoretically.40,41 For example, Jiang et al. reported a barrier of 1.86 eV and a reaction energy of 1.09 eV, consistent with our calculation results.40 R2. CO* + H* → CHO* + *. CHO* is formed from the initial hydrogenation of CO*. As presented in Table 1, this step is endothermic by 1.38 eV and has an energy barrier of 1.48 eV. To date, there exist only two theoretical studies of this process, one was carried out by Neurock with a Pd19 cluster37 and the 14670



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calculated by Au et al.13 and those (0.80/0.34 eV) obtained by Jiang et al. on Pd(111) surface with periodic DFT methods.40 Compared with R3, the barrier is only 0.20 eV lower and that for the reverse reaction is 0.22 eV higher. R5. CH2O* + H* → CH3O* + *. The addition of another H to the carbon atom of the adsorbed CH2O* leads to the formation of methoxyl with a moderate reaction barrier of 0.95 eV and reaction energy of 0.49 eV, very similar to the previous calculation values.40,41 As shown in Figure 2, at the transition state, the carbon is close to the hydrogen atom with the C−H distance of 1.58 Å. In the final state, CH3O* species binds through its oxygen atom on the bridge of two Pd atoms. R6. CH2O* + H* → CH2OH* + *. The hydrogenation of the oxygen moiety in formaldehyde results in CH2OH* with an energy barrier of 0.67 eV, lower than that of CH3O* formation. This is also an endothermic process with ΔE equal to 0.12 eV. Our calculated barrier is slightly lower than those from previous theories.40,41 Comparing with R5, which generates CH3O*, CH2OH* is the more likely product of hydrogenation from CH2O* because the energy barrier is ∼0.28 eV lower. R7. HCOH* + H* → CH2OH* + *. The only possible product for hydrogenation of HCOH* is CH2OH*. The reaction has an energy barrier of 0.96 eV, with ΔE equal to 0.42 eV. The cluster model used by Au et al. gave a much lower barrier (0.13 eV) than ours and a large exothermicity (−0.78 eV).13 Our calculated barrier is in excellent agreement with that reported by Jiang et al., while the reaction energy is ∼0.37 eV higher than their value, which might be caused by the interaction between the products coadsorbed.40 In the reactant side, the hydrogen atom at the hcp site is near the HCOH* species with a C−H distance of 3.12 Å. At the transition state, HCOH* has to leave its bridge site and move to the Pd top to meet with the attacking H atom. R8. CH2OH* + H* → CH3OH* + *. The results for R5, R6, and R7 suggest that the formation of CH2OH* is the preferred and this species is considered for the final hydrogenation. In the transition state, the bond length of the forming C−H bond is ∼1.54 Å, and at the final state the CH3OH* weakly adsorbs at the surface through its hydroxyl oxygen atom. The reaction is endothermic (ΔE = 0.29 eV) with a moderate barrier of 0.78 eV, in agreement with previous calculations (0.79/0.24 eV).41 R9. CH3O* + H* → CH3OH* + *. Besides CH2OH* intermediate, CH3O* can also transform to methanol through hydrogenation. In the initial state, CH3O* and H* coadsorb at the adjacent hcp hollow sites. In the transition state, the distance between carbon atom and the reactant H atom is ∼1.65 Å. After reaction, CH3OH* is produced. R9 also possesses a moderate energy barrier with a value of 0.71 eV similar to that of R8 and consistent with the previous calculations.40,41 In addition, different from R8, this step is exothermic (ΔE = −0.31 eV). 3.2.2. C−O Bond Cleavage. R10. CO* + * → C* + O*. The decomposition of CO* into C and O is very unfavorable. Our calculated barrier of 3.85 eV and a large endothermicity (ΔE = 2.63 eV) agree well with the previous values of Yudanov et al. (4.29/2.50 eV).42 However, our barrier is significantly larger than that found in a previous theoretical study with a cluster model (2.21 eV).37 All theoretical results indicated that the C−O bond breaking is unlikely, consistent with the experimental observations that adsorbed CO does not decompose on Pd catalysts.67 R11. CHO* + * → CH* + O*. As mentioned above, the formation of COH* requires a higher barrier than that for the

Figure 3. Side and top views of the IS, TS, and FS for each C−O bond cleavage reaction listed in Table 1.

Figure 4. Side and top views of the IS, TS, and FS for each step of methane formation listed in Table 1.

other is performed by Au et al. with a Pd10 cluster.13 Our barrier height is between the two previous theoretical values (1.2940 and 1.68 eV41). Compared with the formation of COH*, this reaction seems more likely to take place due to its lower energy barrier. Below, the CHO* species is hence selected as the species for further hydrogenation. R3. CHO* + H* → CH2O* + *. The addition of a hydrogen to the carbon in CHO* leads to CH2O*. The energy barrier is calculated to be 1.03 eV with the endothermicity of 0.80 eV, consistent with the previous results (0.92/0.4740 and 1.17/0.81 eV41). In the initial state, the hydrogen atom and formyl coadsorb at the hcp and fcc sites, respectively. After the reaction, CH2O* is found to be adsorbed on the surface through its C and O atoms. R4. CHO* + H* → HCOH* + *. CHO* may also evolve by adding a hydrogen onto the oxygen atom. In the transition state, H is found to be almost atop a Pd (111) atom. The energy barrier of this step is 0.83 eV and the reaction is endothermic by 0.38 eV. These values are in reasonably good agreement with previous theoretical values (0.92/0.61 eV) 14671



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Figure 5. Energetics of preferred pathways for the CO hydrogenation on Pd(111) leading to methane and methanol. The barrier heights in the Figure include both the intrinsic barriers listed in Table 1 and the energy needed to bring two reactants to the IS configuration from infinite separation. In addition, the adsorption energy for CH4 is assumed to be zero.

R16. CH* + H* → CH2* + *. The product CH* from the C−O bond cleavage of HCOH* or CHO* serves as a precursor for the methane formation. R16 has a moderate barrier of 0.74 eV with endothermicity of 0.54 eV. In the transition state, the C−H distance is ∼1.32 Å and the angle of H−C−H is shown to be 105.21°. The CH2* product is found to stay at a bridge site. There is no previous calculation about the barrier for R16, although it is known to be an endothermic process.68 R17. CH2* + H* → CH3* + *. The further hydrogenation of CH2* leads to CH3*. In the initial state, CH2* and H* coadsorb with their most stable configurations. The TS is presented in Figure 2, and the distance between the carbon and H atoms is found to be 1.65 Å. As shown in Table 1, this reaction has a relatively low barrier of 0.57 eV. While no previous theoretical value has been reported, our calculated reaction energy (−0.25 eV with no ZPE correction) is consistent with that (−0.23 eV) of a previous DFT work.68 R18. CH3* + H* → CH4* + *. The CH3* species can be further hydrogenated to the final methane product. As shown in Figure 2, the reactants CH3* and H* are adsorbed at the top and fcc sites, respectively, with a C−H distance of 3.51 Å. The transition state shortens this distance to 1.56 Å, consistent with the value (1.54 Å) obtained by Liu et al.69 In the final product, CH4* was found to be substantially away from the Pd surface because of its weak adsorption. Compared with R17, the energy barrier for this step is slightly higher (0.63 eV), while the reaction becomes less exothermic with a reaction heat of −0.04 eV. Our calculation results are in excellent agreement with the previous theoretical work carried out by Liu et al.,69 in which the barrier and energy difference for this reaction are found to be 0.68 eV and almost 0 eV, respectively. R19. OH* + H* → H2O* + *. After the generation of methane, OH* and H* remain adsorbing on Pd (111). The combination of these two species produces a water molecule with its oxygen atom on the top of Pd, yielding an energy barrier of 0.62 eV and an exothermicity of −0.25 eV, respectively, consistent with those reported by Gu and Li.41 3.3. kMC Simulations. In our KMC model, the adsorption energies of the pertinent species were adapted from Table S-1 in the Supporting Information. All of the elementary processes shown in Figure 5 are included, and the reaction barriers and the pre-exponential factors of these processes are listed in Table

production of CHO*. Therefore, we here only considered the C−O bond scission of CHO*. As shown in Table 1, the C−O bond cleavage requires a high-energy barrier of ∼1.96 eV and a reaction energy of 0.81 eV, which are in reasonable accordance with the values (2.07/0.54 eV) reported by Yudanov et al.42 In the transition state, the bond of C−O has a length of 1.94 Å. After decomposition, the products are found to locate together at two fcc sites nearby. R12. CH2O* + * → CH2* + O*. To better understand when and how the C−O bond breaks, the other intermediates containing a C−O bond have also been considered. As shown in Table 1, the scission of the C−O bond in CH2O* has the energy barrier of 1.80 eV and is endothermic (ΔE = 0.57 eV), in good agreement with previous theory.42 R13. HCOH* + * → CH* + OH*. The C−O bond scission in HCOH has an energy barrier (1.35 eV) with ΔE = 0.30 eV. To the best of our knowledge, there is neither theoretical nor experimental data about this reaction step. In Figure 2, the length of the cleaving C−O bond is calculated to be 1.94 Å in the transition state. In the final state, CH* and OH* located on the adjacent fcc sites through C and O, respectively. R14. CH2OH* + * → CH2* + OH*. The C−O bond breaking in CH2OH* leads to the production of CH2* and OH*. This step has an energy barrier and endothermicity of 1.30 and 0.62 eV, respectively. These values can be compared with the values (1.35 and 0.48 eV) obtained by Yudanov et al.42 using a Pd79 cluster. Although it is the lowest among those for C−O bond cleavage of relevant intermediates, the barrier is still too high to compete with the hydrogenation way. The distance of the scissile C−O bond is 2.02 Å in the transition state, while after dissociation this value change to be 2.94 Å and CH2/OH coadsorb at their preferred sites. R15. CH3OH* + * → CH3* + OH*. The physisorbed CH3OH* species also has a high (1.70 eV) barrier for C−O bond cleavage, which is endothermic by 0.23 eV. Our values are consistent with other theoretical results for this process (1.90/ 0.3039 and 1.92/0.16 eV,42 respectively). It is also found that this value is comparable to that of R12 but much higher than those of R13 and R14, suggesting that the cleavage of the C−O bond is highly unfavorable for this species on Pd (111). After decomposition, CH3* locates on the top site of Pd atom and OH* adsorbs on the bridge site. 3.2.3. Methane Formation. 14672



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share the first few steps in CO hydrogenation, in which the barriers for C−O bond cleavage of the intermediates, such as CO* and CHO*, are exceedingly large. This observation is consistent with the experimental evidence31,32,67 and recent computational work of Rösch and coworkers on the C−O bond cleavage on Pd clusters.42 The barriers for C−O bond cleavage in HCOH* and CH2OH* are lower because of the weaker C− O bond, but they are still much higher than those leading to the formation of methanol. As a result, methanol is kinetically favored, despite the fact that the formation of methane is thermodynamically favored by >100 kJ/mol. Different from Pd, copper is more selective toward methanol synthesis. The reason can be understood from the recent DFT results that the barriers for the hydrogenation steps along the methanol synthesis pathway are found to be much lower than those for the C−O cleavage steps.70 Interestingly, on some traditional Fischer−Tropsch synthesis catalysts such as Fe or Co, the barriers for the C−O cleavage are significantly reduced and lower than those for the hydrogenation steps. For example, on Fe, the energy barrier (0.91 eV) of CHO* → CH* + O* is lower than that (1.10 eV) of CHO* + H* → CH2O*, preferring the production of hydrocarbon species.71 On Co, the barriers for the C−O cleavage of CHO* and CH2O* are found to be 0.93 and 0.72 eV, respectively, much lower than those on Pd catalyst.72 The preferred pathway for methanol synthesis differs somewhat from that proposed by Neurock based on a Pd cluster model,37 in which the CH2O* and CH3O* were proposed the precursors to CH3OH*. Unfortunately, the HCOH/CH2OH route was not examined in that pioneer study. From Figure 5, it is clear that the conversion of CHO* to CH2O* requires much higher barrier than the conversion to HCOH*. Furthermore, on Pd(111), the reaction barrier of CHOH* + H* → CH2OH* is comparable to that of CHOH*→ CH* + OH*, making it the key step that determines the selectivity. If the barrier for the C−O cleavage of CHOH* is reduced, then the reaction might shift the selectivity to the generation of methane. Our results are consistent with more recent DFT studies of methanol decomposition on Pd39−41 and closely related Pt surfaces.73−75 It is also worth noting that the reaction pathway for methanol synthesis on Pd(111) differs from that on Cu(111), on which the preferred pathway is CO* → CHO* → CH2O* → CH3O* → CH3OH*.17 This is mainly because the barrier for CHO* + H → CH2O* on Cu is about half of that for CHO* + H* → CHOH*, whereas on Pd the energy barrier of the latter reaction is ∼0.20 eV lower than that of the former reaction. The methoxyl (CH3O*) is the preferred intermediate before the formation of CH3OH* on copper,11,17 whereas hydroxymethyl (CH2OH*) is the likely precursor on palladium. This difference has been noted in previous DFT studies of methanol decomposition on Pd(111).39−41 The reaction condition is also an important factor that could alter the predicted selectivity on Pd. From the previous experiment,26 it is found that hydrogenation of CO over supported Pd catalysts selectively yields methanol within the temperature−pressure regime for which methanol formation is a thermodynamically allowed process. Only when the reaction conditions are changed, namely, outside such temperature− pressure regime, the production of methane will be significant. The kMC results reported here are consistent with experimental observations that methanol is the favored product on Pd-catalyzed CO hydrogenation. However, it should be

1. (A0f and A0r denote the pre-exponential factor for forward and reverse reaction, respectively, in Table 1.) Additionally, the desorption processes of H2O, CH3OH, and CH4 were considered as irreversible processes in the kMC model, and their pre-exponential factors are set as 1012. In the simulations, 177 processes, including desorption, reactions, and diffusion, were involved. The surface was preadsorbed with 1.0 ML H* and CO* species and set with the ratio of H/CO = 6. At 500 K, only CH3OH species was observed, as shown in the upper panel of Figure 6. It was found that methanol

Figure 6. Desorption probabilities for CH3OH, CH4, and H2O as a function of time obtained from kMC simulations at 500 and 1000 K.

synthesis proceeds via the following sequences: CO* → CHO* → HCOH*/CH2O* → CH2OH* → CH3OH*, and the pathway going through HCOH* is the dominant one. The species of CHO, HCOH, CH2O, and CH2OH will decompose or hydrogenate once they appear on the surface, and the decomposition processes are always faster than the hydrogenation processes by many times for these species. In the simulations, CH3OH desorbs immediately after its formation, due apparently to the small adsorption energy. When the simulation temperature is increased to 1000 K, CH3OH, CH4,and H2O were observed, as shown in the lower panel of Figure 6. CH3OH is still the dominant product, and its principal reaction path remains the same as at 500 K. The methane formation proceeds via CO* → CHO* → HCOH* → CH* → CH2* → CH3* → CH4*. Although the reaction barrier of HCOH* + H* → CH2OH* + * is lower than the barrier of HCOH* + * → CH* + OH*, there is no CH4 species formation via the pathway of HCOH* + H* → CH2OH*. This is because the competitive reaction of CH2OH* + H* → CH3OH* is favored over the CH2OH* → CH2* + OH*. This counterintuitive observation underscores the importance of global simulation of the reaction network for complex catalytic processes.

4. DISCUSSION The pathways of CO hydrogenation on Pd(111) are displayed in Figure 5 for both methanol synthesis and methane formation. The kMC simulations suggested that the former is likely to proceed via CO* → CHO* → HCOH* → CH2OH* → CH3OH*, while the latter probably follows the following sequence: CO* → CHO* → HCOH* → CH* → CH2* → CH3* → CH4*. It is important to note that both pathways 14673



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noted that the model used in the current work ignored many important factors, such as defects, promoters, and support, which can significantly affect the reaction. For example, the presence of defects is known to affect the C−O bond dissociation of CHxO* species on Pd surfaces.67 Surface defects have been demonstrated to significant affect the adsorption energy and reaction barriers for methanol/methane decomposition on Pd and Pt surfaces.11,69,75 The study of promoters and support effects are more involved but should be pursued in future studies.

ASSOCIATED CONTENT

S Supporting Information *

Adsorption energies and geometric parameters for various pertinent species on Pd(111). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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5. CONCLUSIONS In this work we have examined the competitive paths for methanol and methane formation on the Pd(111) surface using a plane-wave DFT method. The prevailing mechanism based on DFT calculations suggests that the reaction from the CO and H2 synthesis gas to methanol is more favorable than methane generation due to kinetic factors. This conclusion is consistent with most experimental observations. The synthesis of methanol on Pd surfaces prefers the pathway: CO* → CHO* → HCOH* → CH2OH* → CH3OH*. Our calculations suggest that the C−O scission is all but impossible for CO* and CHO* due to exceedingly high barriers. The C−O bond cleavage of the HCOH* is viable due to the fact that this intermediate has a C−O single bond. However, even for these species, the barrier is still too high compared with those leading to the methanol formation. These details of the reaction network obtained from our theoretical studies on Pd will help us to better understand the competition between the Fischer−Tropsch process and oxygenate synthesis on a microscopic scale.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.L.), [email protected] (J.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of National Natural Science Foundation of China (21203026 to S.L., 21133006 and 91221301 to D.X.), Specialized Research Fund for the Doctoral Program of Higher Education (20123514120001 to S.L.), Natural Science Foundation of Fujian Province, China (2012J05022 to S.L.), Faculty startup Fund of Sichuan University (2082204154001 to J.M.), the Ministry of Science and Technology (2013CB834601 to D.X.), a New Direction grant from the Petroleum Research Fund administered by the American Chemical Society (48797-ND6 to H.G.), and US National Science Foundation (CHE-0910828 to H.G.). We are also grateful to the High Performance Computing Center of Nanjing University for the award of CPU hours to accomplish this work. 14674



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CO Hydrogenation on Pd(111): Competition between Fischer–Tropsch

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