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Mechanistic Study of Pd-Cu Bimetallic Catalysts for Methanol Synthesis from CO Hydrogenation 2

lingna liu, Fei Fan, Zhao Jiang, Xiufeng Gao, Jinjia Wei, and Tao Fang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06166 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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The Journal of Physical Chemistry

Mechanistic Study of Pd-Cu Bimetallic Catalysts for Methanol Synthesis from CO2 Hydrogenation Lingna Liu†, ‡, Fei Fan‡, Zhao Jiang†, Xiufeng Gao†, Jinjia Wei†, Tao Fang†, * †

Department of Chemical Engineering, School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, PR China

‡

Shannxi Key Laboratory of Low Metamorphic Coal Clean Utilization, Yulin University, Yulin 719000, PR China

*

Corresponding author: Prof. Tao Fang, School of Chemical Engineering and

Technology, Xi’an Jiaotong University, No. 28, Xianning West Road, Xi’an 710049, PR China E-mail: [email protected] Phone: (+86)29-82668875

ACS Paragon Plus Environment


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ABSTRACT Density functional theory (DFT) calculations were carried out to explore the adsorptions of reactive species and the reaction mechanisms on Pd-Cu bimetallic catalysts during CO2 hydrogenation to methanol. All the possible preferred adsorption sites, geometries and adsorption energies of the relative intermediates on pure Cu(111) and three PdCu(111) surfaces were determined, revealing that both the adsorption configuration and corresponding adsorption energy are changed by doping with Pd atoms. The strengthened COOH* adsorption and the greatly weakened OH* adsorption change the rate-limiting step from CO2 hydrogenation forming trans-COOH* on Cu(111), Pd3Cu6(111), and Pd6Cu3(111) surfaces to cis-COOH* decomposition forming CO* and OH* on Pd ML surface. Additionally, the highest activation barriers for the overall reaction pathway are reduced in the following trend: Cu(111) > Pd6Cu3(111) > Pd3Cu6(111) > Pd ML (monolayer). Compared to the reaction on clean Cu(111) surface, the complete reaction pathways for CH3OH synthesis on PdCu(111) surfaces, especially on Pd ML, were facilitated and the yield of by-product CO and CH4 are suppressed, which corroborates well with experimental reports showing that Pd-Cu bimetallic catalysts have a strong synergistic effect on CO2 hydrogenation to methanol. The present insights are helpful for the design and optimization of highly efficient Pd-Cu bimetallic catalysts used in CH3OH formation from CO2 hydrogenation.


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1. INTRODUCTION The rising concentration level of carbon dioxide (CO2) in the atmosphere is associated with global climate change and ocean acidification.1-4 Extensive efforts have been made to explore effective ways to convert CO2 into valuable chemicals, e.g., carbon monoxide (CO), methane (CH4), formic acid (HCOOH) and methanol (CH3OH),5-15 to achieve an anthropogenic and sustainable carbon cycle. CO2 hydrogenation to synthesize methanol (CO2 + 3H2 → CH3OH + H2O) is of great industrial importance because CH3OH is an important chemical feedstock as well as a liquid fuel for fuel cells and internal combustion engines.16-17 Industrially, CH3OH is produced from CO2 hydrogenation using a Cu/ZnO/Al2O3 catalyst at 493-573K.18 Although it is an exothermic reaction, the equilibrium yield of CH3OH is kinetically limited to 15-25%.19 Great efforts, both using experimental and theoretical methods, have focused on improving the catalytic performance of Cu-based catalysts and understanding the underlying reaction mechanism. Experimentally, Melián-Cabrera and coworkers20 reported that the performance of a CuO-ZnO catalyst was affected by Pd incorporation for CO2 hydrogenation to CH3OH. The increasing methanol yield was due to a synergistic effect of Pd on the active Cu sites. Jiang and coworkers21 studied novel Pd-Cu bimetallic catalysts on SiO2 and obtained high activity, selectivity and stability for Pd-Cu/SiO2 catalysts. Moreover, examining the CO2 conversion indicated that CO2 was the primary carbon source for CH3OH synthesis at lower CO2 conversions and that a by-product of CO hydrogenation to CH3OH at higher CO2 conversions. Ma et al. investigated the effect of different elemental mixing patterns and compositions of bimetallic Cu-Pd catalysts in CO2 hydrogenation.22 They reported that the phase-separated CuPd catalyst favored the production of C2 compounds, while the ordered phase favored the production of C1 compounds. Similar promotion of CO2 hydrogenation to CH3OH production was also observed on Cu-based surfaces doped with Ni, K, Ga and Co.23-25 Density functional theory (DFT) calculations have become powerful tools to investigate the theoretical chemical reaction mechanisms of methanol synthesis via the hydrogenation of CO2 at the atomic level.11, 26-33 Significant efforts have focused on clarifying the mechanisms on different metal surfaces, such as Cu31, 34, Cu2935, Ni36, Pd/oxides37 and Cu alloys38-40. The mechanism and selectivity of CO hydrogenation on pure Pd surfaces have been studied.41 It confirmed that the selectivity of the pure Pd catalysts toward forming methanol, rather than forming methane. Studies of CO2 hydrogenation on Pd4/In2O3 indicated that the reactivity and selectivity can be optimized by tuning the size and morphology of Pd particles on In2O3 substrates. Only a few theoretical studies have been reported for the CO2 hydrogenation on Pd-based catalysts, although much work has been done for the reverse process.42-44 Nakatsuji and Hu used a Cu7Zn cluster to model the alloy surface of Zn/Cu(100). The results showed that the reaction species are formate, dioxomethylene, formaldehyde, and methoxy. The rate-limiting step is the hydrogenation of H2COO, and the energy barrier is decreased.38 Santiago-Rodríguez et al.39 investigated the effects of Ga, Mg and Ti metal dopants on Cu(111) surface by DFT, suggesting the conversion of CO2 and H2 to CH3OH is facilitated by the addition of Ga and Mg. In contrast, it is not thermodynamically favorable to introduce Ti due to the strong interaction between O and Ti. Yang et al.40 employed DFT calculations and Kinetic Monte Carlo (KMC) simulations to study the promoting effect of Au, Pd, Rh, Pt and Ni dopants on a Cu(111) surface toward CH3OH synthesis. The results showed that the reaction is accelerated by doping Pd, Rh, Pt and Ni, while Au



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deactivates the reaction. For bimetallic Cu-based catalysts, it generally has been established that the unique catalytic behavior is governed by a so called strain effect (by changing the lattice distance), ensemble effect (by varying the adsorption sites) and ligand effect (by perturbing the charge polarization of the surface).45-47 Basically, three main reaction pathways have been proposed as illustrated in Figure 1.15 Path 1 is the conventional formate pathway and its revision, where CO2 hydrogenation primarily occurs to form fotmate (HCOO), followed by the formation of dioxomethylene (H2COO), formaldehyde (H2CO), methoxy (H3CO) and the final product (CH3OH).31, 35, 40 Recently, Grabow and Mavrikakis31 reported that HCOO hydrogenation leads to formic acid (HCOOH) rather than H2COO, and that HCOOH further hydrogenates to H2COOH, subsequently generating H2CO (denoted as the revised HCOO mechanism) on pure Cu(111) surface. Path 2 is the reverse water-gas-shift (RWGS) mechanism, where the formation of CO is proposed via the direct decomposition of CO2 of carboxyl (COOH). Formyl (HCO), formaldehyde (H2CO) and methoxy (H3CO) are regarded as the main reaction species. Although the RWGS pathway is considered as a dominant reaction mechanism, it is not feasible on pure Cu surfaces because HCO is favored to dissociate back to CO and H intermediates rather than undergo HCO hydrogenation.35, 40 Path 3 is the trans-COOH mechanism, where H2O is the H atom source for CO2 hydrogenation. Dihydroxycarbene (COHOH), hydroxymethylidyne (COH), hydroxymethylene (HCOH) and hydromethyl (H2COH) intermediates are involved in this reaction process.34 In addition, some researchers have investigated the hydrogenation of CO2 to methanol on Cu-based catalysts, but to the best of our knowledge, there have been few systematic theoretical studies of the reaction mechanism over Pd-Cu bimetallic catalysts. It is important to elucidate the underlying mechanism of the experimentally observed synergistic effect on promoting methanol formation over Pd-Cu bimetallic catalysts.22 To understand the compositions, structures and surface activities of Pd-Cu bimetallic catalysts for methanol formation from CO2 hydrogenation, DFT method was performed to investigate the adsorption and reaction mechanism of CO2 hydrogenation on Cu(111) and three PdCu(111) surfaces. 2. COMPUTATIONAL METHODOLOGY 2.1 Computational Methods All the self-consistent periodic DFT calculations were carried out using the DMol3 code as implemented in the Materials Studio package (Accelrys Inc).48-49 The electron exchange and correlation were described with the generalized gradient approximation (GGA) in the form of the Perdew-Wang-91 (PW91) functional.50-51 The localized double-numerical quality basis set with a polarization d-function (DNP)52 was chosen to expand the wave functions. The core electrons of the metal atoms were treated using the effective core potential (ECP) developed by Berger et al.52 Brillouin-zone integrations were performed on a k-point mesh sampling grid of 3×3×1 by Monkhorst-Pack53 with a thermal smearing of 0.002 Hartree. For the geometry optimization, the convergences of the energy, gradient, and maximum displacement were set as 2×10-5 Hartree, 4×10-3 Hartree/Å and 5×10-3 Å, respectively. All the transition states (TSs) of the elementary reactions are identified using a complete linear synchronous transit and quadratic synchronous transit (LST/QST) approach.54 The convergence criterion was set for the root-mean-square forces on the atoms to be 0.01 eV/Å. In addition, the located transition states were confirmed by imaginary frequency calculations, and the corresponding vibration modes were verified to connect the desired reactants and products.


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2.2 Surface Model As mentioned before, CO2 activation is critical in the methanol synthesis from CO2 hydrogenation on pure Cu(111) surface. Experimental results indicate that synergistic effect promotes the formation on Pd-Cu catalysts. To address this issue, a 3×3 Cu(111) periodic surface slab including four atomic layers was constructed to model the monometallic Cu catalyst, with a vacuum region of 15 Å to separate the repeating slabs. The adsorbates together with the top two layers of the metal atoms were relaxed, and the bottom two layers were fixed during the geometry optimizations. To explore the effect of the Pd atomic distribution on the bimetallic PdCu(111) surface, as shown in Figure 2, four PdCu(111) surfaces were built based on a Cu(111) surface: (1) a monometallic Cu catalyst, denoted as Cu(111) surface; (2) three Cu atoms on the topmost layer replaced by Pd atoms, which approximately describes the formation of Pd dimers over Cu(111), denoted as Pd3Cu6 surface; (3) six Cu atoms on the topmost layer replaced by Pd atoms, which approximately describes the formation of Pd trimers, denoted as Pd6Cu3 surface; (4) a Pd monolayer substituted on the topmost layer of Cu(111), denoted as Pd ML. The adsorption energies of PdCu (111) surfaces, Eads, were calculated using the following equation: Eads = Eadsorbate+slab − (Eadsorbate + Eslab) where Eadsorbate+slab is the total energy of the slab model covered with adsorbates in the equilibrium state, Eadsorbate is the energy of the adsorbate in the gas phase, and Eslab is the energy of the clean slab surface. With these definitions, a more negative Eads reflects a stronger surface adsorption interaction. The activation barrier (Ea) and reaction energy (∆E) were calculated based on the following equations: Ea = ETS − EIS ∆E = EFS − EIS where ETS, EIS and EFS are the total energies of the transition state (TS), initial state (IS) and final state (FS), respectively. 3. RESULTS AND DISCUSSION 3.1 Evaluation of Calculation Method and Model To validate the effectiveness of the selected computational model and method, the bond lengths of a CH3OH molecule in the gas phase were calculated. The corresponding values are RC-H=1.096 Å, RC-O=1.425 Å and RO-H=0.969 Å, which are close to the reported experimental values of 1.09 Å, 1.43 Å and 0.95 Å,55 respectively. The next test was to calculate the lattice constant of bulk Cu. The calculated lattice constant is 3.61 Å, consistent with the experimental value of 3.62 Å.55 3.2 Adsorption of the Reaction Species It is important to characterize the adsorption of the pertinent intermediates to understand the elementary chemical steps in CO2 hydrogenation. In this section, the adsorptions of all reactants, intermediates and products involved in the reverse water-gas shift (RWGS) reaction were considered over different adsorption sites on Cu(111) and three PdCu (111) surfaces. The most favorable configurations on PdCu (111) surfaces are presented in Figure 3 and figure S1 in the Supporting Information, and the preferential adsorption sites and corresponding adsorption energies are listed in Table 1 and Table S1 in the Supporting Information. Methoxy (H3CO*). H3CO* has been observed experimentally as one of the reactive intermediates in methanol decomposition56-57 and methanol synthesis from CO2 hydrogenation



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over oxide-supported Cu-based catalysts.58-60 It was found that H3CO* prefers to occupy the hollow site through its O atom on Cu(111) surface. The corresponding adsorption energy is -2.50 eV, in good agreement with the previous result of -2.45 eV.61 On three PdCu(111) surfaces, the geometry and stability are affected by different Pd distributions on Cu(111) surface. The adsorption energies are -2.28, -1.93 and -1.62 eV for Pd3Cu6, Pd6Cu3 and Pd ML, respectively. The adsorption energies showed the following trend: Cu(111) > Pd3Cu6(111) > Pd6Cu3(111) > Pd ML, indicating that there is a negative correlation between the adsorption energy and the number of doped Pd atoms. Moreover, H3CO* prefers to adsorb at the bridge site through the O atom with neighboring Cu atoms and Pd-Cu atoms on the surfaces of Pd3Cu6(111) and Pd6Cu3(111). In addition, it binds at the hollow site on Pd ML, which is similar to that on clean Cu(111) surface. Formaldehyde (H2CO*). H2CO* is a key intermediate in the decomposition and synthesis of methanol.62-63 It forms a top-bridge configuration (η1-C-η2-O) on both Cu(111) and Pd3Cu6(111) surfaces, and lies above surface with a 2.95 Å C-Pd distance on Pd6Cu3(111) surface, and above surface with a 2.99 Å C-Pd distance on Pd(111) ML surface. The corresponding energies are -0.15, -0.86, -0.30 and -0.32 eV, respectively. These results show that the interaction of H2CO* with Pd3Cu6 (111) surface is much stronger than those on the other three surfaces. Hydroxymethyl (CH2OH*). As a product from hydrogenation of CH2O*, CH2OH* adsorbs at the top site via C-Cu bond on Cu(111) and C-Pd bonds on three PdCu(111) surfaces. The adsorption energies are -1.44(-1.3434), -1.94, -1.96 and -2.22 eV for Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces, respectively. It can be found that the joining of Pd atoms enhances the interaction between O atom and the metal atom. Formyl (HCO*). The configurations on top, bridge and hollow sites were optimized. It was found that HCO* prefers to adsorb at the hollow site on Cu(111) with the η2-C-η1-O mode. On Pd3Cu6(111) and Pd6Cu3(111) surfaces, HCO* binds at the top site with single Pd-O bonds. It favors to adsorb at the bridge on the Pd ML surface through the C atom. The adsorption energies are -1.57(-1.6334, -1.4161), -2.37, -2.50 and -2.53 eV for Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces, respectively. Moreover, the adsorption energy increases gradually with the number of Pd atoms, indicating that the substitution of Pd atoms has a positive effect on the adsorption of HCO*. Hydrocarboxyl (COOH*). COOH* has been identified as a critical reaction intermediate during the water-gas-shift reaction on Cu(111).64 There are two isomers, trans-COOH*, and cis-COOH* depending on the direction of the hydroxyl group. Trans-COOH* and cis-COOH* bind at the bridge position on Cu(111) preferentially via the carbon and ketonic oxygen atoms, defined as η1-C-η1-O. The calculated binding energies for trans-COOH* and cis-COOH* are -2.06 and -2.15 eV, respectively. Trans-COOH* and cis-COOH* can stably absorb at the top sites via C-Pd, C-Cu, and C-Pd bonds on Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces, respectively. The binding energies of trans-COOH* are -2.49, -2.03 and -2.63 eV, while the binding energies of cis-COOH* are -2.52, -1.86 and -2.37 eV, indicating that the introduction of Pd changes the configuration and adsorption. In addition, the calculated results show that cis-COOH* is more stable than trans-COOH* on Cu (111) and Pd3Cu6 (111) surfaces, and the opposite is true on Pd6Cu3 (111) and Pd ML surfaces, in good agreement with previous results.34, 65-66 Carbon monoxide (CO*). A large amount of CO* is observed on Cu during CH3OH synthesis from CO2 hydrogenation experimentally.35 CO* favors to vertically bind at the hollow site through the C atom on Cu(111), the top site via a C-Pd bond on Pd3Cu6(111), and the bridge site (B2Pd ) via


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the C atom on Pd6Cu3(111), and the hollow site through the C atom on Pd ML surface; the corresponding energies are -0.92, -1.35, -1.33 and -1.51 eV, respectively. Hydroxyl (OH*). The optimized geometry for OH* is found to consist of OH* adsorbed perpendicularly onto the surface at the hollow site via its O atom on Cu(111), yielding a binding energy of -3.48 eV. The BPdCu site of Pd3Cu6(111) surface is found to be the most stable adsorption site for OH* with an adsorption energy of -3.09 eV. OH* is observed to bind at the H2PdCu site through its O atom on Pd3Cu6(111) surface, with a binding energy of -2.71 eV. OH* prefers to adsorb at the bridge site on Pd ML surfaces, with a binding energy of -2.67 eV. The doped-Pd atoms weaken the interaction between OH* and PdCu(111) surfaces. Other reaction species. CH3OH*, the final desired product of CO2 hydrogenation interacts very weakly with Cu(111) and three PdCu(111) surfaces. It adsorbs at the top site through the O atom by donating the lone electron pair of oxygen. The corresponding adsorption energies are -0.18, -0.36, -0.27 and -0.30 eV for Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces, respectively. CO2* physisorbs weakly on all adsorption sites through van der Waals interactions and is located far from the surfaces in the optimized configurations, consistent with previous studies.64, 67 The binding energies were calculated to be -0.16, -0.28, -0.17 and -0.10 eV for Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces, respectively. H2O* adsorbs weakly at the top sites on all surfaces, where the H2O* molecule plane is parallel to the surfaces. The adsorption energies were calculated to approximately -0.40 eV for all the surfaces, consistent with previous DFT results of -0.18 to -0.51 eV64-65, 68 and experimental results of -0.40-0.70 eV.69 H* prefers to adsorb at the hollow sites on all surfaces, with calculated adsorption energies of -3.67, -3.56, -3.69 and -3.66 eV for Cu(111), Pd3Cu6 (111), Pd6Cu3 (111) and Pd ML surfaces, respectively. In summary, the adsorption processes of H3CO*, H2COH*, HCO*, CO*, COOH*, H2O*, OH* and * H on Cu(111) and three PdCu(111) surfaces are chemisorption, while CH3OH*, H2CO* and CO2* are considered physisorption due to the weak interactions, except for H2CO* on Pd3Cu6(111). On all surfaces, CH3OH* and CH2OH* bind to the top position, and the corresponding adsorption energies increase with the addition of Pd. Compared to clean Cu(111) surface, the most stable adsorption configurations of H2CO*, HCO* and CO* on PdCu(111) are altered, and the corresponding adsorption energies also increase. For COOH*, cis-COOH* is more stable on Cu(111) and Pd3dCu6(111) than on P6dCu3(111) and Pd ML, and for trans-COOH*, the opposite is true. For H3CO* and OH*, the stable configurations change to some extents. Their binding energies are decreased by the addition of Pd doping. For H2O* and H*, these species prefer to bind at the top and the hollow sites on all the surfaces, and the introduction of Pd has negligible effect on the corresponding energies. Therefore, it can be proposed that the composition and structure of the metal surface not only affects the adsorption configuration but also alters the binding affinity between the binding species and the surface. 3.3 Coadsorption of the Related Intermediates on PdCu(111) Surfaces To explore the CO2 hydrogenation mechanism on the Pd-doped surfaces, the stable coadsorption configurations of H3CO*+H*, H2CO*+H*, H2COH*+H*, HCO*+H*, CO*+H* and CO2*+H* were investigated. All the optimized coadsorption configurations are illustrated in Figure 3, which are selected as the initial states for the elementary reactions. The corresponding



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adsorption sites, coadsorption energies and the sums of separated energies are listed in Table 2. The coadsorption energy between the related species and each surface is defined as follows: Ecoads = E(A+B)/slab - (EA + EB + Eslab) where E(A+B)/slab is the total energy of Cu(111) or three PdCu(111) surfaces covered with the coadsorbed A and B species in the optimized geometries, EA and EB are the energies of the free A and B species, and Eslab is the energy of clean Cu(111) or three PdCu(111) slabs. In all the calculations, the initial coadsorption species were placed at the adjacent and the most stable binding sites on Cu(111) and PdCu(111) surfaces.70-72 After the geometry optimizations, it is revealed that most of the groups in the coadsorption configurations maintain their initial states. However, for H3CO*+H* on Pd6Cu3(111) surface, H3CO* in the initial state moves to the adjacent hollow site. For H2CO+H and CO*+H* on Pd3Cu6(111) surface, H2CO* and CO* in the initial states move to the adjacent bridge and hollow sites; for HCO+H on Cu (111) and Pd ML surfaces, HCO* in the initial state moves to the adjacent bridge and top sites. This may be due to varying degrees of repulsion between those groups. The differences between the coadsorption energies and the sums of the separated adsorption energies are in the range of 0.02-0.18, 0-0.45, 0.01-0.22 and 0.09-0.48 eV for Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces, respectively, indicating that interaction energies between the coadsorption groups on Cu(111) and PdCu(111) surfaces exist, which are influenced by the addition of Pd atoms on Cu(111) surface.70-74 3.4 Mechanisms of Methanol Synthesis from CO2 Hydrogenation on the PdCu(111) Surfaces As mentioned above, CO2 hydrogenation to CH3OH occurs via three pathways consisting of the formate mechanism, trans-COOH mechanism and reverse water-gas shift mechanism. Yang et al.40 have found that RWGS pathway is the the more effective way for CH3OH formation than that path 1 on Pd-doped Cu(111) suface. However, only one Cu atom of the surface was substituted by a metal dopant. The reverse water-gas-shift pathway was chosen to explore CH3OH synthesis on Cu(111) and three PdCu(111) surfaces based on previous reports35, 40 following reactions R1-R10. In this section, we calculated the activation energy barriers and thermodynamics of the elementary steps, as list in Table 3, and the potential energy profiles are presented in Figures 5-11. R1: CO2* + H* → trans-COOH* + * R2: trans-COOH* → cis-COOH* R3: cis-COOH* + * → CO* + OH* R4: OH* + H* → H2O* + * R5: CO* +H* → HCO* + * R6: HCO* + H* → H2CO* + * R7: H2CO* + H* → H3CO* + * R8: H2CO* + H* → H2COH* + * R9: H2COH* + H* → CH3OH* + * R10: H3CO* + H* → CH3OH* + * 3.4.1 CO2* + H* → trans-COOH* + * The adsorption of CO2 is very weak on Cu(111) and PdCu(111) surfaces. As the initial state, the most stable configuration of the coadsorbed CO2 and H is presented in Figure 3 (CO2*+H*), in which H* prefer to the hollow site and CO2 is far away from the surfaces. The distance between H and O is ~3.00 Å. Therefore, the formation of trans-COOH* from coadsorbed CO2 and H* is most likely activated by free CO2 reacting directly with with H at the hollow site on Cu(111) and


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PdCu(111) surfaces via ER mechanism.34-35 In the transition states (Figure 5), the distances between the adsorbed H and the O atom of CO2* decreases dramatically. After TS1, trans-COOH* adsorbs at its most stable site on all the surfaces. This elementary reaction has relatively high energy barriers of 1.97(1.80 eV40), 1.43 and 1.52 eV on Cu(111), Pd3Cu6(111) and Pd6Cu3(111) surfaces, respectively, the values of which are much higher than those (1.02 eV) on Pd ML. The corresponding reaction energies are 0.57, 0.26, 0.87 and 0.16 eV, indicating that Pd ML surface is more favorable for trans-COOH formation than Cu(111) surface. 3.4.2 trans-COOH* → cis-COOH* → CO* + OH* As described earlier in section 3.2, COOH* has two isomers, trans- and cis-COOH*. CO* formation is a two-step process in which trans-COOH* first undergo a structural transformation to a cis-COOH* with hydroxyl H atom pointing away from the surface, This transformation has similar activation barriers of 0.59 (0.53 eV34), 0.54, 0.58, and 0.58 eV on Cu(111), Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces, respectively. This step is slightly endothermic by 0.017-0.24 eV, as displayed in Figure 6. Then, scission of the C-OH bond in cis-COOH* occurs via TS2b and forms CO* and OH* species. In TS2b, CO* prefers to adsorb at the top site, and the C-OH distance lengthens to 1.826-2.154 Å. After the transition state, CO* and OH* further settle into their stable adsorption sites. On Cu(111) surface, H3Cu-site-adsorbed CO* and B2Cu-site-adsorbed OH* form with an energy barrier of 0.68 eV and reaction energy of -0.15 eV, which are close to the previously reported barrier of 0.59 eV and reaction energy of -0.02 eV.40 On Pd3Cu6 (111) surface, a TPd-site-adsorbed CO* and B2Cu-site-adsorbed OH* form with an energy barrier of 0.62 eV, which is slightly endothermic (0.15 eV). On Pd6Cu3(111) surfaces, the CO* and OH* groups shift to the B2Pd-site and H2CuPd-site, respectively, with an energy barrier of 1.00 eV, which is endothermic by 0.35 eV. However, H3Pd-site-adsorbed CO* and B2Pd-site-adsorbed OH* form on Pd ML surface. This C-OH bond cleavage process of cis-COOH* that occurs on Pd ML surface is endothermic by 0.40 eV with a relatively high activation barrier of 1.41eV, indicating that the C-OH bond scission is more difficult on Pd ML than on the other three surfaces. 3.4.3 OH* + H* → H2O* + * The formed OH* from cis-COOH* decomposition further reacts with a H* atom to generate H2O* molecule. The OH*+H* coadsorption structure occupies two adjacent hollow sites as the initial state on Cu(111). At TS3 (Figure 7), OH* and H* adsorb only to the Cu atom shared by these species in the initial state, forming an O-H bond of 1.591 Å. Then, the top-site weakly adsorbed H2O species is utilized as the final state. This step has an activation barrier of 1.31 eV and exothermic by 0.42 eV. On Pd3Cu6(111), a B2Cu adsorbed OH and HPd2Cu adsorbed H comprise the initial state. TS3 and the final state are similar to those on Cu(111). The distance between the O atom and reactant H is 1.702 Å. Due to the weakening of the OH adsorption by the addition of Pd, the activation barrier is reduced to 0.92 eV, and this step is more exothermic (0.62 eV) than that on Cu(111). On Pd6Cu3(111), the coadsorption of OH at the BPdCu site and H at the H2PdCu site comprises the initial state. TS3 and the final state are also similar to those on Cu (111), with an O-H bond of 1.827 Å. Similarly, on Pd ML, OH* hydrogenation process is remarkably exothermic (-1.14 eV) and needs to overcome only a relatively small energy barrier of 0.77 eV. These large differences in the configurations, activation barriers and reaction energies between Cu(111) and PdCu(111) surfaces are mainly attributed to the weakened adsorption of OH by doping of Pd. 3.4.4 CO* + H* → HCO* + * For hydrogenation of CO* on all the surfaces, a CO*+H* coadsorption structure was selected as the initial state, After this initial state, CO* is hydrogenated to HCO* via a C-H bond formation. At the transition state (Figure 8), CO* moves to the bridge sites on Cu(111) and Pd ML surfaces and



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the TPd sites on Pd3Cu6(111) and Pd6Cu3(111) surfaces, indicating that the Pd sites are reactive centers on PdCu(111) surfaces. The newly formed C-O bond distance is 1.406-1.521 Å. After TS4, HCO* binds at the hollow, TPd, TPd and bridge sites on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces, respectively. This elementary reaction needs to overcome energy barriers of 1.27 (0.99 eV31), 1.24, 1.41 and 1.03 eV, and the steps are endothermic by 0.83 (0.78 eV31), 0.52, 0.50 and 0.60 eV. This process was found to be highly endothermic on the Cu(111) surface. The formation of COH* was also calculated, as shown in Figure S3 and Table S2, CO* hydrogenation to COH* must overcome an energy barrier as high as 1.7 eV on Pd ML surface and it is even higher than that on other three surfaces. Accordingly, the CO hydrogenation starts with HCO formation. 3.4.5 HCO* + H* → H2CO* + * H2CO* can be formed from HCO* hydrogenation. On Cu(111) surface, the initial state comprises the most stable HCO*+H* coadsorption configuration, where HCO* binds at the bridge site, and H* binds at the hollow site. At the transition state (Figure 9), HCO* is located at the top site, and the forming C-H bond distance is 1.620 Å. Then, H2CO* adsorbs at the hollow site via an η1-C-η2-O mode. The energy barrier for this hydrogenation step is 0.67 eV, which is exothermic by 0.29 eV. The activation barrier is 0.21 eV higher than the previous reported value.40 On Pd3Cu6(111) surface, the HCO*+H* coadsorption structure comprises the initial state, where HCO* binds at the top site via a C-Pd bond, and H* binds at the hollow site. In TS5, the distance of the formed C-H bond is 1.527 Å. After TS5, H2CO* approaches the HPd2Cu site via η1-C-η2-O mode. This reaction needs to overcome an energy barrier of 0.57 eV, with a slightly exothermic value of 0.08 eV. On Pd6Cu3(111) and Pd ML surfaces, the initial and transition states are similar to those on the Pd3Cu6(111) surface. However, in the final state, H2CO* is physisorbed on the two surfaces. The calculated energy barriers are 0.76 and 0.64 eV for Pd6Cu3(111) and Pd ML surfaces, respectively and the corresponding reaction energies are endothermic by 0.16 and 0.10 eV. 3.4.6 H2CO* + H* → H3CO* (H2COH) + * The addition of H* to the adsorbed H2CO* species can lead to the formation of H3CO* and H2COH*. As presented in Table 3, it is easy to form C-H bond on Cu(111) and Pd3Cu6(111) surfaces. On Cu(111) surface, H2CO*+H* coadsorption structure comprises the initial state, where the species occupy two adjacent hollow sites. At TS6a (Figure 10), H2CO* binds at the top site through an O atom, and the H atom moves to the C atom of H2CO*. The distance between the O atom and the surface H atom is shortened from 3.601 to 1.778 Å. After TS6a, H3CO* binds through its O atom onto the hollow site. The energy barrier to form the C-H bond is 0.47 eV, with a high exothermic value of 0.87 eV. This step has been extensively explored using DFT calculations. The value of energy barrier has been calculated to be 0.14-0.69 eV, with an exothermic energy of 0.92-1.20 eV.35, 61, 75 On Pd3Cu6(111) surface, H2CO* and a H* atom coadsorb at the most stable sites, where H2CO* prefers to adsorb at the bridge site via C-Pd and O-Cu bonds in the initial state. At TS6a, H2CO* binds at the top site through an O-Cu bond, and the distance of the formed C-H bond is 1.652 Å. In the final state, H3CO* situates at the Cu-Cu bridge site via its O atom. This step is endothermic by 0.35 eV with an energy barrier of 0.69 eV. While on Pd6Cu3(111) and Pd ML surfaces, the reaction prefers to proceed through O-H bond formation rather than C-H bond formation. On Pd6Cu3(111) surface, in the initial state, H2CO* binds weakly onto the surfaces and H* atom binds at the adjacent hollow site. At TS6b, H2CO* moves on the surface forming a new C-Pd bond, and the distance of the forming O-H bond is


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1.627 Å. Then, H2COH* binds through its C atom at the top site. This step is exothermic (-0.47 eV) with an energy barrier of 0.66 eV. On Pd ML surface, it is similar to that on Pd6Cu3(111) surface. The reaction is exothermic (-0.60 eV) with the energy barrier of 0.62 eV, which is 0.10 eV lower than the previously reported value.41 3.4.7 H3CO* (H2COH*) + H* → CH3OH* + * The final step is the hydrogenation of H3CO* and H2COH* species leading to the formation of CH3OH* via O-H and C-H bond-making processes on different metal surfaces. In the initial state, the H3CO*+H* coadsorption structure is located at the adjacent hollow site on Cu(111) surface and H3CO* prefers to the bridge site via C-Pd and O-Cu bonds on Pd3Cu6(111) surface. In TS7a (Figure 11), H3CO* prefers to situate at the Cu top site through its O atom. The distance between the O atom and the H reactant is ~1.50 Å. After the reaction, CH3OH* adsorbed weakly at the top site via a Cu-O bond. On Pd6Cu3(111) and Pd ML surfaces, H2COH* binds at the top site and an H atom binds at the adjacent hollow site in the initial state. At TS7b, the distance of the formed O-H bond is ~2.00 Å. Then, H3COH* binds through its O atom at the top site. This elementary reaction have activation barriers of 1.38 (1.25 eV40), 1.32, 0.86 and 0.94 eV (0.82 eV41) for Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces, respectively. The corresponding reaction energies are -0.22, -0.67, -0.56 and -0.32 eV, respectively. 3.5 General discussion Figure 2 and Table 1 display the structural parameters and adsorption energies for all the intermediates on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Previous studies have shown that the influence of adsorption properties on Cu-Pd bimetallic surfaces can be described in terms of ensemble effect, ligand effect and strain effect.46 The most stable adsorption configurations of H2COH*, H2CO*, HCO* and CO* prefer to bind on the surfaces through C atom and the adsorption energies are increased after the addition of Pd. On Pd3Cu6(111) and Pd6Cu3(111) surfaces, the adsorption configurations of HCO* and CO* are altered due to the nature of the ensemble of metal atoms (ensemble effect). On Pd ML surface, it is almost the same as that on Cu(111) and the strain effect become dominant. For H2COH*, the adsorption configurations is not changed on all the surfaces due to the ligand effect and strain effect. The intermediates of H3CO* and OH* favor to bind through O atom and are gradually weakened by an increasing number of Pd atoms indicating that O atom prefers to occupy the Cu sites. This means the adsorption energy is effected by the ensemble effect. In addition, most of these effects are combined in these bimetallic surfaces, acting in a cooperative, synergic way as normal. Figure 12 shows the potential energy profiles for the complete methanol synthesis from CO2 hydrogenation on Cu(111) and three PdCu(111) surfaces. On Cu(111) surface, the activation barrier of CO2 hydrogenation is much higher than the other elementary steps. Hence, CO2 hydrogenation can be considered the rate-limiting step for the complete reaction indicating the producing of methanol is difficult to proceed due to the unfavorable kinetics and energetics. By comparing the results for Cu(111) and PdCu(111) surfaces, the lower reaction barriers of CO2 hydrogenation on PdCu(111) surfaces indicate that the existence of Pd can easily form C-H bond of trans-COOH*. For Pd3Cu6(111) and Pd6Cu3(111), the rate-limiting step is the same as that on Cu(111), whereas cis-COOH* decomposition becomes the rate-determining step on Pd ML surface. Pd monolayer on Cu under a compressive strain because of a smaller lattice constant of Cu than that of Pd, moreover the charge is accumulated in Pd overlayer.76 Both of the effects lead to enhance the adsorption of trans-COOH*, the reaction is dramatically facilitated. Although the



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adsorption of CO* is strengthened, the increased adsorption strength is smaller than the decreased binding energy of OH*. Consequently, the energy barrier of cis-COOH* decomposition rises. In the overall synthesis of methanol from CO2 hydrogenation, the energy barriers of the rate-limiting step are 1.97, 1.43, 1.52 and 1.41 eV on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces, respectively. In addition, the changes of the activation barriers for all the elementary steps are caused by Pd doping (Figure 12). The activation barriers of R2, R5 and R6 are slightly affected by Pd doping. For R3 and R7, the activation barriers are considerably altered. These results may originate from the greatly strengthened adsorptions of cis-COOH* and H2CO* after doping with Pd. However, the activation barriers of R1, R4 and R10 are reduced by Pd doping. The cause for this may be the reduction of the adsorption energies of OH* and H3CO* as well as the increased adsorption energies of trans-COOH* and CH3OH* when Cu is substituted by Pd. Moreover the reaction steps are more favorable kinetically on Pd3Cu6(111) and Pd ML surfaces than on the other two PdCu surfaces. The activation barriers of R7 and R8 show that the C-H bond formation of H3CO* is easier than the O-H bond formation of H2COH* on Cu(111) and Pd3Cu6(111) surfaces, while it is opposite on the Pd6Cu3(111) and Pd ML surfaces. Therefore, producing H3CO* or H2COH* via H2CO* hydrogenation obviously depends on the Pd atomic ensemble. The above results show that, compared to pure Cu(111) surface, an appropriate number of doped-Pd atoms on Cu(111) surface not only alter the rate-limiting step but also change the activation barriers of all the reaction steps. Furthermore, compared to the desorption barriers of CO*, CO* hydrogenation is easier than desorption on Pd3Cu6 (111) and Pd ML surfaces indicating the producing of CO is inhibited; however, the opposite is true on Cu(111) and Pd6Cu3(111) surfaces. This is in good agreement with the prior results by Zhao34 and Liu et al.35, 40, reporting that CO favors to desorb from Cu(111) rather than to form HCO. In addition, CO2 reduction on Cu(111) and Pd–Mg/SiO2 can produce methane.77-78 Therefore, the possible reaction route of producing CH4 was taken into consideration. The sequence of elementary steps involved were: CH2OH* → CH2* → CH3* → CH4*79 (figure S1, Table S3). To produce CH4, the CH2OH* adsorbed on the surfaces needs to dissociated into CH2* and OH* with a high activation barrier of ~2.00 eV, which is 1.50 eV higher than that for CH2OH* hydrogenation on Pd3Cu6(111) and Pd ML surfaces, as listed in Table S3, and the corresponding potential energy profiles are showed in Figure S5-S9. Therefore, it ensures a higher selectivity to CH3OH rather than CH4. Such enhanced catalytic activity and selectivity of bimetallic Pd-Cu catalysts accelerate the reaction for producing methanol and suppress the forming of CO and CH4. 4. CONCLUSIONS This study systematically investigated the effect of the surface composition of Cu(111) catalysts and the role of metal dopants on the catalytic activity of CH3OH synthesis via CO2 hydrogenation on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces by DFT. It was found that the addition of Pd atoms on the Cu(111) surface not only affect on the adsorption configuration but also alter the interactions between the adsorbed species and the metal surfaces. More importantly, the energy barriers and reaction energies of the reverse water-gas-shift (RWGS) pathway were determined. The rate-limiting step was testified to be the formation of trans-COOH* from CO2 hydrogenation on Pd3Cu6(111) and Pd6Cu3(111) surfaces which is the same as that on pure Cu(111) surface, however, it changes to cis-COOH* decomposition to CO* and OH* on Pd ML surface. The change of the rate-limiting step is mainly due to the strengthened adsorption of COOH*, while the adsorption of OH* is greatly weakened by the added monolayer of Pd atoms. The calculated


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results showed that the overall reaction process of CH3OH synthesis is facilitated and the yields of CO and CH4 are inhibited on PdCu(111) surfaces, especially on Pd ML surface. These findings reveal that the ensemble effect and strain effect acting in synergetic play crucial role to tune the reactivity and selectivity for producing CH3OH, providing fundamental and useful information for designing and optimizing Pd-Cu bimetallic catalysts. ACKNOWLEDGMENTS The authors greatly appreciate Prof. B. J. Wang (Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology), who help us get access to the software of Materials Studio. In addition, the authors would like to acknowledge the following financial supports: National Natural Science Foundation of China (No. 21376186), Fundamental Research Funds for the Central Universities (Creative Team Plan No. cxtd2017004, in Xi’an Jiaotong University) Supporting Information Additional adsorption configurations and energies of intermediates involved in methanol and methane. Potential energy diagram of COH*, HCOH, CH, CH2, CH3 and CH4 producing with corresponding depictions of transition states structures. The corresponding reaction barriers and energies of the related reaction elements steps. REFERENCES (1) Aresta, M.; Dibenedetto, A., Utilisation of CO2 as a Chemical Feedstock: Opportunities and Challenges. Dalton T. 2007, 2975-2992. (2) Appel, A. M.; Bercaw, J. E.; Bocarsly, A. B.; Dobbek, H.; DuBois, D. L.; Dupuis, M.; Ferry, J. G.; Fujita, E.; Hille, R.; Kenis, P. J., Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation. Chem. Rev. 2013, 113, 6621-6658. (3) Aresta, M.; Dibenedetto, A.; Angelini, A., Catalysis for the Valorization of Exhaust Carbon: from CO2 to Chemicals, Materials, and Fuels. Technological Use of CO2. Chem. Rev. 2013, 114, 1709-1742. (4) Wang, W.-H.; Himeda, Y.; Muckerman, J. T.; Manbeck, G. F.; Fujita, E., CO2 Hydrogenation to Formate and Methanol as an Alternative to Photo-and Electrochemical CO2 Reduction. Chem. Rev. 2015, 115, 12936-12973. (5) Yoshihara, J.; Parker, S.; Schafer, A.; Campbell, C. T., Methanol Synthesis and Reverse Water-Gas Shift Kinetics over Clean Polycrystalline Copper. Catal. lett. 1995, 31, 313-324. (6) Song, C., Global Challenges and Strategies for Control, Conversion and Utilization of CO2 for Sustainable Development Involving Energy, Catalysis, Adsorption and Chemical Processing. Catal. Today 2006, 115, 2-32. (7) Olah, G. A.; Goeppert, A.; Prakash, G. S., Chemical Recycling of Carbon Dioxide to Methanol and Dimethyl Ether: From Greenhouse Gas to Renewable, Environmentally Carbon Neutral Fuels and Synthetic Hydrocarbons. J. Org. Chem. 2008, 74, 487-498. (8) Mikkelsen, M.; Jørgensen, M.; Krebs, F. C., The Teraton Challenge. A Review of Fixation and Transformation of Carbon Dioxide. Energy Environ. Sci. 2010, 3, 43-81. (9) Darensbourg, D. J., Chemistry of Carbon Dioxide Relevant to Its Utilization: A Personal Perspective. Inorg. Chem. 2010, 49, 10765-10780.



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7756-7764. (50) Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (51) Perdew, J. P.; Wang, Y., Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Phys. Rev. B 1992, 45, 13244. (52) Bergner, A.; Dolg, M.; Küchle, W.; Stoll, H.; Preuß, H., Ab Initio Energy-Adjusted Pseudopotentials for Elements of Groups 13–17. Mol. Phys. 1993, 80, 1431-1441. (53) Monkhorst, H. J.; Pack, J. D., Special Points for Mrillouin-Zone Integrations. Phys. rev. B 1976, 13, 5188. (54) Halgren, T. A.; Lipscomb, W. N., The Synchronous-Transit Method for Determining Reaction Pathways and Locating Molecular Transition States. Chem. Phys. Lett. 1977, 49, 225-232. (55) Lide, D. R.; Baysinger, G.; Berger, L. I., CRC Handbook of Chemistry and Physics, 87th ed Editor-in-Chief; CRC Press, Inc, 2007, p 1438. (56) Hofmann, P.; Schindler, K.-M.; Bao, S.; Fritzsche, V.; Ricken, D.; Bradshaw, A.; Woodruff, D., The Geometric Structure of the Surface Methoxy Species on Cu(111). Surf. Sci. 1994, 304, 74-84. (57) Johnston, S. M.; Mulligan, A.; Dhanak, V.; Kadodwala, M., The Structure of Methanol and Methoxy on Cu(111). Surf. Sci. 2003, 530, 111-119. (58) Bowker, M.; Hadden, R.; Houghton, H.; Hyland, J.; Waugh, K., The Mechanism of Methanol Synthesis on Copper/Zinc Oxide/Alumina Catalysts. J.Catal. 1988, 109, 263-273. (59) Fisher, I. A.; Bell, A. T., A Mechanistic Study of Methanol Decomposition over Cu/SiO2, ZrO2/SiO2, and Cu/ZrO2/SiO2. J. Catal. 1999, 184, 357-376. (60) Clarke, D. B.; Bell, A. T., An Infrared Study of Methanol Synthesis from CO2 on Clean and Potassium-Promoted Cu/SiO2. J.Catal. 1995, 154, 314-328. (61) Gu, X.-K.; Li, W.-X., First-Principles Study on the Origin of the Different Selectivities for Methanol Steam Reforming on Cu(111) and Pd(111). J. Phys. Chem. C 2010, 114, 21539-21547. (62) Weigel, J.; Fröhlich, C.; Baiker, A.; Wokaun, A., Vibrational Spectroscopic Study of Ib Metal/Zirconia Catalysts for the Synthesis of Methanol. Appl. Catal. A-Gen. 1996, 140, 29-45. (63) Shekhar, R.; Barteau, M. A.; Plank, R. V.; Vohs, J. M., Adsorption and Reaction of Aldehydes on Pd Surfaces. J. Phys. Chem. B 1997, 101, 7939-7951. (64) Gokhale, A. A.; Dumesic, J. A.; Mavrikakis, M., On the Mechanism of Low-Temperature Water Gas Shift Reaction on Copper. J. Am. Chem. Soc. 2008, 130, 1402-1414. (65) Zhang, R.-G.; Liu, H.-Y.; Wang, B.-J.; Ling, L.-X., Insights into the Preference of CO2 Formation from HCOOH Decomposition on Pd Surface: A Theoretical Study. J. Phys. Chem. C 2012, 116, 22266-22280. (66) Zhang, R.-G.; Song, L.-Z.; Liu, H.-Y.; Wang, B.-J., The Interaction Mechanism of CO2 with CH3 and H on Cu(111) Surface in Synthesis of Acetic Acid from CH4/CO2: A DFT Study. Appl. Catal. A-Gen. 2012, 443, 50-58. (67) Columbia, M.; Thiel, P., The Interaction of Formic Acid with Transition Metal Surfaces, Studied in Ultrahigh Vacuum. J. Electroanal. Chem. 1994, 369, 1-14. (68) Phatak, A. A.; Delgass, W. N.; Ribeiro, F. H.; Schneider, W. F., Density Functional Theory Comparison of Water Dissociation Steps on Cu, Au, Ni, Pd, and Pt. J. Phys. Chem. C 2009, 113, 7269-7276. (69) Thiel, P. A.; Madey, T. E., The Interaction of Water with Solid Surfaces: Fundamental Aspects. Surf. Sci. Rep. 1987, 7, 211-385.


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(70) Wang, B-J.; Song, L-Z.; Zhang, R-G., The Dehydrogenation of CH4 on Rh(111), Rh(110) and Rh(100) surfaces: A Density Functional Theory Study. Appl. Surf. Sci. 2012, 258, 3714-3722. (71) Wang, B.-J.; Yan, R.-X.; Liu, H.-Y., Effects of Interactions Between NiM(M= Mn, Fe, Co and Cu) Bimetals with MgO(100) on the Adsorption of CO2. Appl. Surf. Sci. 2012, 258, 8831-8836. (72) Jiang, Z.; Guo, S.; Fang, T., Theoretical Investigation on the Dehydrogenation Mechanism of CH3OH on Cu(100) Surface. J. Alloy. Compd. 2017, 698, 617-625. (73) Bo, J.-Y.; Zhang, S.; Lim, K. H., Steam Reforming of Formaldehyde on Cu(100) Surface: A Density Functional Study. Catal. Lett. 2009, 129, 444-448. (74) Zuo, Z.-J.; Wang, L.; Han, P.-D.; Huang, W., Insights into the Reaction Mechanisms of Methanol Decomposition, Methanol Oxidation and Steam Reforming of Methanol on Cu(111): A Density Functional Theory Study. Int. J. Hydrogen Energ. 2014, 39, 1664-1679. (75) Greeley, J.; Mavrikakis, M., Methanol Decomposition on Cu(111): A DFT Study. J. Catal. 2002, 208, 291-300. (76) Cho, J.; Lee, S.; Yoon, S. P.; Han, J. H.; Nam, S. W.; Lee, K. Y.; Ham, H. C., Role of Heteronuclear Interactions in Selective H2 Formation from HCOOH Decomposition on Bimetallic Pd/M (M=Late Transition Fcc Metals) Catalysts. Acs Catal. 2017, 7, 2553-2562 (77) Nie, X.; Luo, W.; Janik, M. J.; Asthagiri, A., Reaction Mechanisms of CO2 Electrochemical Reduction on Cu(111) Determined with Density Functional Theory. J. Catal. 2014, 312, 108-122. (78) Park, J. N.; Mcfarland, E. W., A Highly Dispersed Pd-Mg/SiO2 Catalyst Active for Methanation of CO2. J. Catal. 2009, 266, 92-97. (79) Kattel, S.; Yan, B.; Chen, J. G.; Liu, P., Co 2 Hydrogenation on Pt, Pt/SiO2 and Pt/TiO2: Importance of Synergy between Pt and Oxide Support. J. Catal. 2016, 343, 115-126.



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Table 1. Adsorption Sites and Energies (in eV) for Intermediate Involved in Methanol Synthesis from CO2 Hydrogenation on Cu(111) and Three PdCu(111) Surfaces. Pd3Cu6(111)

Cu(111) species

sites

CH3OH

Cu

Eads

T (O) 3Cu

H3CO

H

CH2OH

TCu(C)

H2CO HCO

3Cu

H

3Cu

H

3Cu

(O)

1

2

2

1

(η -C-η -O) (η -C-η -O) (C)

sites

Pd6Cu3(111) Eads

Pd

sites

Eads

Pd

-0.18

T (O)

-0.36

T (O)

-2.52

2Cu

B

-2.28

PdCu

B

-1.44

TPd(C)

-1.94

TPd(C)

-0.15

Pd2Cu

-0.86

above surface

-1.57

(O)

H

1

2

(η -C-η -O)

Pd

T (C) Pd

Pd ML

(O)

Pd

-2.37

T (C) 2Pd

Pd

Eads

-0.40

T (O)

-0.30

-1.93

3Pd

H

-1.62

-1.96

TPd(C)

-2.22

-0.30

above surface

-0.32

-2.50

2Pd

B

(O)

(C)

3Pd

CO

H

-0.92

T (C)

-1.35

B

-1.33

H

B2Cu(η1-C-η1-O)

-2.06

TPd(C)

-2.49

TCu(C)

-2.03

TPd(C)

-2.63

cis-COOH

B2Cu(η1-C-η1-O)

-2.15

TPd(C)

-2.52

TCu(C)

-1.86

TPd(C)

-2.37

CO2

above surface

-0.16

above surface

-0.28

above surface

-0.17

above surface

-0.10

Cu

Pd

(C)

-2.53

trans-COOH

Cu

(C)

sites

Pd

-1.51

H2O

T (O)

-0.40

T (O)

-0.40

T (O)

-0.41

T (O)

-0.43

OH

H3Cu(O)

-3.48

BPdCu(O)

-3.09

H2PdCu(O)

-2.71

B2Pd(O)

-2.67

H

H3Cu(H)

-3.67

HPd2Cu(H)

-3.56

H2PdCu(H)

-3.69

H3Pd(H)

-3.66


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Table 2. Coadsorption Sites and Energies (in eV) on Cu(111) and Three PdCu(111) Surfaces. Cu(111)

Pd3Cu6(111)

Pd6Cu3(111)

Pd ML

species

sites

Ecoads

Esu-ads

sites

Ecoads

Esu-ads

sites

Ecoads

Esu-ads

sites

Ecoads

Esu-ads

H3CO+H

H+H

-6.28

-6.19

B+H

-5.93

-5.93

H+H

-5.60

-5.62

H+H

-5.47

-5.28

H2CO+H

H+H

-3.80

-3.82

B+H

-3.97

-4.42

as+H

-3.91

-3.99

as+H

-4.07

-3.98

H2COH+H

T+H

-4.93

-5.11

T+H

-5.47

-5.50

T+H

-5.43

-5.65

T+H

-5.77

-5.88

HCO+H

B+H

-5.42

-5.24

T+H

-5.90

-5.93

T+H

-6.04

-6.19

T+H

-6.33

-6.19

CO+H

H+H

-4.54

-4.59

H+H

-5.03

-4.91

B+H

-4.99

-5.02

H+H

-5.27

-5.17

CO2+H

as+H

-3.73

-3.83

as+H

-3.84

-3.84

as+H

-3.84

-3.86

as+H

-3.90

-3.76

OH+H

H+H

-7.12

-7.03

B+H

-6.77

-6.65

B+H

-6.77

-6.40

B+H

-6.81

-6.33



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Table 3. Elementary Reactions Involved in Methanol Synthesis from CO2 Hydrogenation on Cu(111) and Three PdCu(111) Surfaces Together with the Activation Energies (Ea/eV) and Reaction Energies (∆E/eV) Cu(111) elementary steps

Ea

*

*

∆E

Ea

∆E

Pd6Cu3(111) Ea

∆E

Pd ML Ea

∆E

R1

CO2* +

1.97

0.57

1.43

0.26

1.52

0.87

1.02

0.16

R2

trans-COOH* → cis-COOH*

0.59

0.02

0.54

0.07

0.58

0.12

0.58

0.24

R3

cis-COOH* + * → CO* + OH*

0.68

-0.15

0.62

0.15

1.00

0.35

1.41

0.40

H → trans-COOH +

*

*

*

*

R4

OH + H → H2O +

R5

CO* + H* → HCO* + *

R6

*

*

*

HCO + H → H2CO + *

*

*

* *

R7

H2CO + H → H3CO +

R8

H2CO* + H* → H2COH* + *

R9

H2COH* + H* → CH3OH* + *

*

Pd3Cu6(111)

1.31

-0.42

0.92

-0.72

0.66

-0.91

0.81

-1.05

1.27

0.83

1.24

0.52

1.41

0.50

1.03

0.60

0.67

-0.29

0.57

-0.08

0.76

0.16

0.64

0.10

0.47

-0.87

0.69

0.35

0.85

0.07

1.01

0.30

1.02

-0.08

1.22

-0.27

0.66

-0.47

0.62

-0.60

0.85

-1.11

0.82

-0.59

0.86

-0.56

0.94

-0.32

R10

H3CO + H → CH3OH +

*

1.38

-0.22

1.32

-0.62

0.98

-0.99

0.77

-1.14

R11

H2COH* + * → CH2* + OH*

1.55

0.29

1.89

0.87

2.16

1.20

2.30

1.09

*

*

*


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Figure captions Figure 1. Reaction scheme for CO2 hydrogenation to CH3OH. (X)* indicates adsorbed species. Figure 2. Top view of Pd-Cu bimetallic metal surfaces. Figure 3. Most stable adsorption configurations of key intermediates involved in methanol synthesis from CO2 hydrogenation on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Figure 4. Most stable coadsorption configurations on Cu (111) and three PdCu (111) surfaces. Figure 5. Potential energy diagram of CO2* hydrogenation with corresponding depictions of transition states (TS) structures on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Figure 6. Potential energy diagram of COOH* decomposition with corresponding depictions of transition states (TS) structures on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Figure 7. Potential energy diagram of H2O* dissociation with corresponding depictions of transition states (TS) structures on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Figure 8. Potential energy diagram of CO* hydrogenation with corresponding depictions of transition states (TS) structures on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Figure 9. Potential energy diagram of HCO* hydrogenation with corresponding depictions of transition states (TS) structures on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Figure 10. Potential energy diagram of H2CO* hydrogenation with corresponding depictions of transition states (TS) structures on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Figure 11. Potential energy diagram of CH3OH* producing with corresponding depictions of transition states (TS) structures on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Figure 12. Potential energy profiles for the complete methanol synthesis from CO2 hydrogenation on Cu(111) and three PdCu(111) surfaces. Table of Contents (TOC) Image



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Figure 1. Reaction scheme for CO2 hydrogenation to CH3OH. (X)* indicates adsorbed species.


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Figure 2. Top view of Pd-Cu bimetallic metal surfaces (Cu: yellow, Pd: blue).



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Figure 3. Most stable adsorption configurations of key intermediates involved in methanol synthesis from CO2 hydrogenation on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Cu, Pd, C, O and H atoms are represented as yellow, blue, gray, red and white spheres, respectively.


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Figure 4. Most stable coadsorption configurations on Cu(111) and three PdCu(111) surfaces.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. Potential energy diagram of CO2* hydrogenation with corresponding depictions of transition states (TS) structures on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Bond lengths are in Å. See Figure 3 for color coding.


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Figure 6. Potential energy diagram of COOH* decomposition with corresponding depictions of transition states (TS) structures on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Bond lengths are in Å. See Figure 3 for color coding.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Potential energy diagram of H2O* dissociation with corresponding depictions of transition states (TS) structures on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Bond lengths are in Å. See Figure 3 for color coding.


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Figure 8. Potential energy diagram of CO* hydrogenation with corresponding depictions of transition states (TS) structures on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Bond lengths are in Å. See Figure 3 for color coding.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9. Potential energy diagram of HCO* hydrogenation with corresponding depictions of transition states (TS) structures on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Bond lengths are in Å. See Figure 3 for color coding.


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Figure 10. Potential energy diagram of H2CO* hydrogenation with corresponding depictions of transition states (TS) structures on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Bond lengths are in Å. See Figure 3 for color coding.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 11. Potential energy diagram of CH3OH* producing with corresponding depictions of transition states (TS) structures on Cu(111), Pd3Cu6(111), Pd6Cu3(111) and Pd ML surfaces. Bond lengths are in Å. See Figure 3 for color coding.


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Figure 12. Potential energy profiles for the complete methanol synthesis from CO2 hydrogenation on Cu(111) and three PdCu(111) surfaces.



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Table of Contents (TOC) Image


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