Insights into the effect of Pt doping of Cu(110)/H₂O for methanol decomposition: a density functional theory study

Abstract

Density functional theory calculations with the periodic slab model are performed to investigate the mechanism of methanol decomposition with different ratios of Pt doped into Cu(110)/H₂O surfaces. Three catalyst models are constructed, denoted 9Pt–Cu(110)/H₂O, 3Pt–Cu(110)/H₂O and 1Pt–Cu(110)/H₂O. Comparing the results with those from experiments conducted on a Cu(110)/H₂O surface, the interactions between CH₃O and the constructed substrates are weakened, and the adsorption strength of CH₂O, CHO and CO is enhanced. The optimal pathway for methanol decomposition on the 9Pt–Cu(110)/H₂O surface is CH₃OH → CH₂OH → CH₂O → CHO → CO; the favorable pathway for methanol decomposition on 3Pt–Cu(110)/H₂O, 1Pt–Cu(110)/H₂O and Cu(110)/H₂O surfaces is CH₃OH → CH₃O → CH₂O → CHO → CO. Comparing the activation energies and reaction energies of each step, the Pt dopant promotes the C–H bond scission of CH₃O and the dehydrogenation of CH₂O, but slightly hinders the O–H bond breaking of CH₃OH. In general, monatomic Pt doping into the Cu(110) surface (1Pt–Cu(110)/H₂O) can result in excellent catalytic activation considering the price of Pt and the catalyst resistance to CO poisoning. Finally, linear scaling relations for the main elementary steps involved in CH₃OH decomposition on 9Pt–Cu(110)/H₂O, 3Pt–Cu(110)/H₂O and 1Pt–Cu(110)/H₂O surfaces are identified.

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1. Introduction

Due to the gradual exhaustion of fossil fuels and environmental pollution, renewable and environmentally friendly power sources have attracted extensive attention.^1,2 Hydrogen is considered as a promising clean energy carrier, and is widely used in many fields (e.g., fuel cells, green cars and home heating).^1,3 In order to develop hydrogen economy,^4,5 safe, effective and economical hydrogen production is necessary. For small scale hydrogen production, methanol decomposition (CH₃OH → CO + 2H₂) is an attractive and promising method which has been extensively investigated, experimentally and theoretically.^6–12

Currently, Cu-based catalysts are widely used in methanol decomposition due to their excellent catalytic activity and selectivity.^6,11 However, Cu-based catalysts gradually deactivate due to metallic Cu sintering.^13,14 In order to improve the catalytic performance of Cu-based catalysts and accelerate the kinetics of the catalytic reaction, tremendous efforts have been devoted to designing new catalysts.

Binary metal alloy catalysts, such as PtCu, PdZn, CuZn and so on, have received a lot of attention in recent years due to their high catalytic activity.^15–22 De Jongste et al.¹⁹ studied the catalytic activity of Pt–Cu alloy catalysts for hydrocarbon reforming reactions, and it was found that the catalytic mechanism is different due to the “ensemble size effect” with different Pt/Cu molar ratios. Gómez et al.²⁰ investigated carbon monoxide oxidation on Pt–Cu/Al₂O₃ catalysts, and their results showed that reaction activation increases with a small amount of Pt dopant in the Cu/Al₂O₃ catalyst. In addition, catalysts based on the binary metal alloy of Pt–Cu show good catalytic activity and stability because the Pt species hinders Cu segregation. However, few experimental and theoretical studies have investigated methanol decomposition over Pt–Cu based catalysts. The mechanism of methanol decomposition on Pt–Cu bimetallic catalysts still remains unclear at the molecular level.

In the present work, we investigate the effect of binary metal Pt–Cu alloy catalysts for methanol decomposition by using density functional theory (DFT). In the actual methanol decomposition system, the presence of water or water vapor is almost unavoidable. In order to model the real methanol decomposition reaction environment, the presence of water in the catalytic model is necessary. Therefore, methanol adsorption and the sequential decomposition mechanism on a Cu/H₂O surface with different Pt/Cu doping ratios are identified and compared. The results are expected to clarify the mechanism of methanol decomposition on Pt–Cu catalysts in the presence of water, and to illustrate the influence of varying Pt content on methanol decomposition, which is helpful for rationalizing the design of binary metal alloy catalysts.

2. Computational details

2.1 Surface model

Previous experiments^23–25 have proposed that methanol decomposition into methoxy mainly occurs on the Cu(110) surface. Nakamura et al.²⁶ also found that the activity of a Cu(110) surface is better than that of Cu(111) and Cu(100) surfaces for methanol synthesis via CO₂ hydrogenation. Moreover, Cu(110) has more coordinatively unsaturated sites compared to the other low-index Cu(111) and Cu(100) surfaces, and so may exhibit higher catalytic activity. Therefore, the Cu(110) surface was chosen as the model Cu catalyst. Similar to our previous literature reports,^27,28 four water molecules were added over the Cu(110) surface to model the solid/liquid interface.

The Cu(110) surface is modeled as a four atomic layer with a periodic p(3 × 3) unit cell and nine atoms in each layer. In order to avoid image interaction, a 15 Å thick vacuum gap is employed. Herein, three models are constructed. As shown in Fig. 1, for 9Pt–Cu(110)/H₂O, the upper layer of Cu(110) is replaced by metal Pt atoms; for 3Pt–Cu(110)/H₂O, three neighboring Cu atoms are substituted by Pt atoms; and for 1Pt–Cu(110)/H₂O, one Cu atom is substituted by a Pt atom. During optimization, the water molecules and adsorbates, together with the upper two layers, are allowed to relax and the bottom two layers are fixed. After optimization, the cell parameters of 9Pt–Cu(110)/H₂O are a = 9.54 Å, b = 8.01 Å, and c = 19.23 Å. The cell parameters of un-doped Cu(110)/H₂O are a = 9.98 Å, b = 7.64 Å, and c = 19.03 Å. Meanwhile, dipole corrections are used to reduce the strain from the Pt/Cu alloy formation during the calculation. In addition, the slab thickness was checked for convergence by means of the vacuum potential. We calculated the average electrostatic potential through the slab in the Z axis direction for the 9Pt–Cu(110)/H₂O, 3Pt–Cu(110)/H₂O and 1Pt–Cu(110)/H₂O surfaces, and the associated results are shown in the ESI in Fig. S1.†

Fig. 1 Illustration of Pt–Cu alloy models in the presence of water. Cu, Pt, O and H atoms are shown as the brown, deep blue, red, and white spheres, respectively.

2.2 Calculation methods

DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP)^29–31 using the projector augmented wave (PAW) method³² to describe the electro–ion interactions. Exchange and correlation energies were treated by the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional.^33,34 The cutoff energy was 400 eV. A 3 × 3 × 1 k-point sampling grid with a width of 0.2 eV of Monkhorst–Paxton smearing³⁵ was used in our calculation. Structure optimization was deemed as converged until the minimizing forces of all the atoms were less than 0.03 eV Å⁻¹, and the convergence of energy was set to 1 × 10⁻⁵ eV. The isolated molecules were optimized in a 15 × 15 × 15 Å unit cell. The transition states (TSs) and the minimum energy paths (MEPs) were determined by the climbing-image nudged elastic band (CI-NEB) method.^36–38 Through the vibrational analysis, the TSs were identified with one imaginary frequency.

The calculated adsorption energy (E_ads) is defined as follows:

E_ads = E_{(adsorbate+4H2O)/substrate} − E_{4H2O/substrate} − E_adsorbate

where E_{(adsorbate+4H2O)/substrate}, E_{4H2O/substrate} and E_adsorbate are the total energies of the substrate with the adsorbate and water molecules, the substrate with the water molecules and the adsorbate molecule, respectively. According to the definitions, a negative E_ads value indicates an exothermic adsorption or a stable adsorption on the Cu(110)/H₂O surfaces with different ratios of Pt doping.

3. Results and discussion

For the adsorption of the reactants and possible intermediates, all possible adsorption sites on the 9Pt–Cu(110)/H₂O, 3Pt–Cu(110)/H₂O and 1Pt–Cu(110)/H₂O surfaces were examined. The most favorable adsorption configurations, adsorption energies and corresponding key parameters are presented in Table 1.

Table 1 The adsorption energies (E_ads, eV), configurations and key parameters (d, Å) of reactants, possible intermediates and products involved in methanol decomposition on Cu(110)/H₂O surfaces with different Pt/Cu doping ratios

Species	Eads (eV)	Configurations	d (Å)
9Pt–Cu(110)/H2O surface
CH3OH	−0.59	Top-Pt via O	dPt–O: 2.416
CH2OH	−2.70	SB via C and O	dPt–C: 2.053; dPt–O: 2.376
CH3O	−2.93	SB via O	dPt–O: 2.118, 2.143
CHOH	−3.67	SB via C	dPt–C: 2.047, 2.058
CH2O	−1.00	SB via C and O	dPt–C: 2.109; dPt–O: 2.057
CHO	−3.04	SB via C	dPt–C: 2.102, 2.105
CO	−2.12	SB via C	dPt–C: 1.996, 1.997
3Pt–Cu(110)/H2O surface
CH3OH	−0.46	Top-Pt via O	dPt–O: 3.065
CH2OH	−2.31	SB via C and O	dPt–C: 2.070; dPt–O: 2.716
CH3O	−2.56	Top-Pt via O	dPt–O: 2.074
CHOH	−3.74	SB via C	dPt–C: 2.052, 2.053
CH2O	−1.31	SB via C and O	dPt–C: 2.094; dPt–O: 2.129
CHO	−3.18	SB via C	dPt–C: 2.134, 2.095
CO	−2.41	SB via C	dPt–C: 1.994, 1.992
1Pt–Cu(110)/H2O surface
CH3OH	−0.40	Top-Pt via O	dPt–O: 2.659
CH2OH	−1.72	SB via O–Pt, C–Cu	dPt–O: 2.659; dCu–C: 2.009
CH3O	−2.88	SB via O–Pt, O–Cu	dPt–O: 2.190; dCu–C: 2.005
CH2O	−0.89	SB via C–Pt, O–Cu	dPt–O: 2.126; dCu–C: 1.972
CHO	−2.97	Top-Pt via C	dPt–C: 1.986
CO	−1.76	Top-Pt via C	dPt–C: 1.867
Cu(110)/H2O surface
CH3OH	−0.49	Top-Cu via O	dCu–O: 2.237
CH2OH	−1.97	SB via O–Cu, C–Cu	dCu–O: 2.124; dCu–C: 2.014
CH3O	−3.19	SB via O	dCu–O: 1.981, 1.984
CH2O	−0.61	H:C–SB,O–SB	dCu–O: 2.025, 2.040; dCu–C: 2.225, 2.193
CHO	−2.33	Top-Cu via C	dCu–C: 1.931
CO	−1.21	Top-Cu via C	dCu–C: 1.840

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3.1 Reaction mechanism

Our previous DFT calculation results^27,28 demonstrated that H₂O solvent molecules on a Cu(110) surface and a secondary metal (Pt, Pd, Ni) doped into the Cu(110) surface in the presence of H₂O are conducive to methanol decomposition and enhancing the catalytic activity. As is well-known, methanol decomposition involves many elementary reactions. In Fig. 2 all the possible elementary steps for CH₃OH decomposition are presented. Two main pathways are considered in detail, which involve the C–H and O–H bond scission pathways of CH₃OH. Meanwhile, all the possible crossover steps that occur in the two pathways are also calculated.

Fig. 2 The possible pathways of CH₃OH decomposition on Cu(110)/H₂O surfaces with different Pt/Cu doping ratios.

3.1.1 Methanol decomposition on the 9Pt–Cu(110)/H₂O surface. The potential energy diagram for CH₃OH dehydrogenation on the 9Pt–Cu(110)/H₂O surface together with the structures of the initial state (IS), TS and final state (FS) of the corresponding elementary steps involved in the optimal pathway is shown in Fig. 3. Some other corresponding structures of the IS, TS and FS are presented in the ESI in Fig. S2 and S3.†

Fig. 3 Potential energy profile of methanol decomposition on the 9Pt–Cu(110)/H₂O surface together with the structures of the initial state (IS), TS and final state (FS) of the corresponding elementary steps of the optimal pathway. Cu, Pt, C, O and H atoms are shown as brown, blue, gray, red and white spheres, respectively.

C–H bond scission pathway. This dissociation reaction pathway starts from the C–H bond activation of CH₃OH, followed by sequential dehydrogenation steps to form the final products of CO and H.

It is widely believed that CH₃OH binds to metallic surfaces through the oxygen,^39–43 which is also observed here. In the IS, the stable configuration of CH₃OH adsorbs on the top site of Pt through O with an adsorption energy of −0.59 eV. The binding strength is slightly higher than that with the Cu(110)/H₂O surface of −0.49 eV.²⁷ This step of CH₂OH formation through C–H bond scission of CH₃OH is exothermic by −0.11 eV with an activation energy of 0.77 eV.

Subsequently, CH₂OH dissociation also involves two possible routes which are the O–H and C–H bond rupture pathways. The adsorption energy of CH₂OH is −2.70 eV, which is higher than that on the Cu(110)/H₂O surface.²⁷ The reaction energy of CH₂O formation is −0.03 eV with an activation energy of 0.67 eV. Alternatively, the C–H bond scission of CH₂OH can lead to the formation of hydroxymethylene (CHOH) and H. The activation energy and reaction energy are 0.44 and −0.60 eV. Although the activation energy of CHOH formation is smaller than that of CH₂O formation on the 9Pt–Cu(110)/H₂O surface, the lower activation energy shows that CH₂O formation is possible under the reaction conditions. Therefore, further reactions of CH₂O and CHOH were studied as follows.

In the case of the C–H bond scission of CH₂O, the adsorbed CH₂O locates at the short bridge (SB) of Pt through C and O with an adsorption energy of −1.00 eV. This step needs to overcome an activation energy of 0.36 eV with an exothermicity of −0.64 eV. Furthermore, CHO dehydrogenation to CO and H has a low activation energy of 0.09 eV with an exothermicity of −0.95 eV, indicating that this step can easily occur.

For further CHOH decomposition, two possible routes are also considered here, which are the C–H and O–H bond scissions of CHOH producing CHO and hydroxymethylidyne (COH). As shown in Fig. S2,† CHO formation needs to overcome an activation barrier of 0.78 eV with a reaction energy of −0.01 eV. Alternatively, COH formation via the C–H bond scission of CHOH has an activation energy of 1.71 eV, which is obviously higher than that of CHO formation (0.78 eV). Therefore, the C–H bond cleavage of CHOH is unlikely. Finally, CO forms via CHO dehydrogenation.

O–H bond scission pathway. The sequential dissociation steps for the O–H bond scission pathway of CH₃OH decomposition are presented in Fig. S3,† and the stable adsorption geometries of all the possible intermediates and products are identified.

The O–H bond scission of CH₃OH produces methoxy (CH₃O) and H, and CH₃O adsorbs onto the SB of Pt through O with an adsorption energy of −2.93 eV. The reaction energy is 0.01 eV with an activation energy of 0.79 eV. Subsequently, the adsorbed CH₃O yields CH₂O and H through methyl H abstraction. The activation energy and reaction energy of this step are 1.10 and −0.25 eV, respectively. Then, CO is formed through CHO from CH₂O dehydrogenation.

3.1.2 Methanol decomposition on the 3Pt–Cu(110)/H₂O surface. In this section, we also investigated C–H and O–H bond scission pathways of CH₃OH decomposition on the 3Pt–Cu(110)/H₂O surface. The potential energy diagram together with the structures of the IS, TS and FS of the corresponding elementary steps involved in the optimal pathway are shown in Fig. 4. Some other corresponding structures of the IS, TS and FS are depicted in Fig. S4 and S5.†

Fig. 4 Potential energy profile of methanol decomposition on the 3Pt–Cu(110)/H₂O surface together with the structures of the initial state (IS), TS and final state (FS) of the corresponding elementary steps of the optimal pathway. See Fig. 3 for color coding.

C–H bond scission pathway. CH₃OH adsorbs on the top site of Pt via O with an adsorption energy of −0.46 eV, which is slightly less than that on the 9Pt–Cu(110)/H₂O surface, and close to the value of −0.49 eV on the Cu(110)/H₂O surface. The reaction has an activation energy of 0.57 eV with an exothermicity of −0.27 eV. In the following, two possible dissociation pathways for the further decomposition of CH₂OH are also considered in detail.

In the case of C–H bond scission of CH₂OH, the activation energy of CHOH formation is 0.34 eV, and the reaction energy is exothermic by −0.68 eV. The formation of CH₂O and H via O–H bond scission of CH₂OH needs to overcome an activation barrier of 1.06 eV and be exothermic by −0.49 eV. Therefore, CHOH formation from CH₂OH dissociation is more favorable than CH₂O formation (0.34 vs. 1.06 eV). The formation of CHO and H via O–H bond scission of CHOH has an activation energy of 1.12 eV with an exothermicity of −0.33 eV. The C–H bond cleavage of CHOH to yield COH and H needs to overcome a high activation energy of 2.0 eV, and the reaction energy is endothermic by 0.40 eV. Therefore the results indicate that COH formation is unlikely. Subsequently, CO forms from CHO dehydrogenation. The activation energy and reaction energy of this step are 0.20 and −1.08 eV.

O–H bond scission pathway. In the case of the O–H bond scission pathway of CH₃OH decomposition, as shown in Fig. 4, the activation energy of CH₃O formation is 0.56 eV and is endothermic by 0.23 eV. CH₃O adsorbs on the top site of Pt with an adsorption energy of −2.56 eV. The reaction of CH₂O formation has an activation energy of 0.39 eV and is exothermic by −0.71 eV. The calculated adsorption energy of CH₂O is −1.31 eV. CHO formation needs to overcome an activation energy of 0.61 eV, and the reaction energy is exothermic by −0.39 eV. Then, CO is formed from CHO dehydrogenation.

3.1.3 Methanol decomposition on the 1Pt–Cu(110)/H₂O surface. Similarly to CH₃OH decomposition on the 9Pt–Cu(110)/H₂O and 3Pt–Cu(110)/H₂O surfaces, CH₃OH decomposition on the 1Pt–Cu(110)/H₂O surface was studied. The potential energy diagram for CH₃OH dehydrogenation together with the structures of the IS, TS and FS of the corresponding elementary steps involved in the optimal pathway is shown in Fig. 5. The structures of the IS, TS and FS involved in the step of C–H bond scission of CH₃OH are described in Fig. S6.†

Fig. 5 Potential energy profile of methanol decomposition on the 1Pt–Cu(110)/H₂O surface together with the structures of the initial state (IS), TS and final state (FS) of the corresponding elementary steps of the optimal pathway. See Fig. 3 for color coding.

O–H bond scission pathway. As shown in Fig. 5, CH₃OH binds to the top site of Pt through O. The calculated adsorption energy of CH₃OH is −0.40 eV, which is close to the value of −0.46 eV on the 3Pt–Cu(110)/H₂O surface. The activation energy of CH₃O formation is 0.45 eV, and the reaction energy is exothermic by −0.28 eV. The activation energy of CH₂O and H formation from CH₃O dehydrogenation is 0.51 eV with an endothermicity of 0.16 eV. The formed CH₂O binds to the Pt–Cu SB site with C and O adsorbed onto the top sites of Pt and Cu. The activation energy and reaction energy of CHO formation are 0.52 and −0.32 eV. Ultimately, CHO undergoes dehydrogenation to CO and H. The activation energy of this step is 0.63 eV, and the reaction energy is exothermic by −0.75 eV.
C–H bond scission pathway. The C–H bond cleavage of CH₃OH to CH₂OH needs to overcome a high activation barrier of 1.95 eV with a strong endothermicity of 1.00 eV. The activation energy of CH₂OH formation is obviously larger than that of CH₃O formation, indicating that CH₂OH formation is unlikely. Therefore, the pathways of CH₂OH further dehydrogenation are not considered in the following.

3.2 General discussion

According to the systemically calculated activation energies and reaction energies of each step of CH₃OH decomposition on the 9Pt–Cu(110)/H₂O, 3Pt–Cu(110)/H₂O and 1Pt–Cu(110)/H₂O surfaces, CH₃OH decomposition mechanisms were identified. In order to verify the influence of the Pt dopant, CH₃OH decomposition on a Cu(110)/H₂O surface is also presented here.²⁷ The adsorption energies, configurations and key parameters of the reactants, possible intermediates and products involved in CH₃OH decomposition on a Cu(110)/H₂O surface are also shown in Table 1. The optimal pathway is CH₃OH → CH₃O → CH₂O → CHO → CO, and the corresponding activation energies are 0.43, 1.17, 0.95 and 0.12 eV, respectively. The calculated reaction energies of each step are −0.15, 0.95, 0.20 and −0.46 eV, respectively. The rate limiting step is CH₂O formation from CH₃O dehydrogenation.

As shown in Fig. 3, three pathways are presented for CH₃OH decomposition on the 9Pt–Cu(110)/H₂O surface. The first pathway is CH₃OH → CH₂OH → CH₂O → CHO → CO, the second pathway is CH₃OH → CH₂OH → CHOH → CHO → CO, and the third pathway is CH₃OH → CH₃O → CH₂O → CHO → CO. Comparing the activation energies of these elementary steps, it is found that the most favorable reaction route is CH₃OH → CH₂OH → CH₂O → CHO → CO, and the corresponding activation energies are 0.77, 0.67, 0.36 and 0.09 eV, respectively. The rate limiting step is the first step, the formation of CH₂OH. Obviously, the Pt dopant promotes the C–H bond scission of CH₃OH (0.77 vs. 1.74 eV),²⁷ but hinders the O–H bond cleavage of CH₃OH (0.79 vs. 0.43 eV). As a result, the reaction mechanism for CH₃OH decomposition on the 9Pt–Cu(110)/H₂O surface is different to that on Cu(110)/H₂O, and is similar to that on a Pt(111) surface.^44,45

On the 3Pt–Cu(110)/H₂O surface, three possible pathways for CH₃OH decomposition are identified, as shown in Fig. 4. The first route is CH₃OH → CH₂OH → CH₂O → CHO → CO, the second route is CH₃OH → CH₂OH → CHOH → CHO → CO, and the third route is CH₃OH → CH₃O → CH₂O → CHO → CO. From the potential energy diagram shown in Fig. 4, it is found that the O–H bond scission pathway of CH₃OH is more favorable than the C–H bond scission route, both thermodynamically and kinetically. Therefore, the optimal reaction route is CH₃OH → CH₃O →CH₂O → CHO → CO and the corresponding activation energies of each step are 0.56, 0.39, 0.61 and 0.20 eV, respectively. The reaction mechanism of methanol dehydrogenation on the 3Pt–Cu(110)/H₂O surface is the same as on Cu(110)/H₂O and clean Cu(110) surfaces.^27,28,40 The results reveal that the Pt dopant effectively reduces the activation energies of C–H bond scission for CH₃O (0.39 vs. 1.17 eV) and CH₂O (0.61 vs. 0.95 eV).

From Fig. 5, it is found that the feasible reaction pathway for CH₃OH decomposition on the 1Pt–Cu(110)/H₂O surface is CH₃OH → CH₃O → CH₂O → CHO → CO, and the activation energies are 0.45, 0.51, 0.52 and 0.63 eV, respectively. The results show that the most favorable pathway on the 1Pt–Cu(110)/H₂O surface is the same as that on Cu(110)/H₂O and clean Cu(110) surfaces.^27,28,40 The Pt dopant on the 1Pt–Cu(110)/H₂O surface significantly reduces the activation energies of C–H bond scission of CH₃O (0.51 vs. 1.17 eV) and C–H bond cleavage of CH₂O (0.52 vs. 0.95 eV) compared to that on a Cu(110)/H₂O surface. However, we also note that CO formation needs to overcome an activation energy of 0.63 eV, which is much higher than that on a Cu(110)/H₂O surface (0.63 vs. 0.12 eV).

In summary, for the possible intermediates of CH₃OH decomposition on the 9Pt–Cu(110)/H₂O, 3Pt–Cu(110)/H₂O and 1Pt–Cu(110)/H₂O surfaces, the interactions between CH₃O and the substrates are weakened, and the adsorption strengths of CH₂O, CHO and CO are enhanced compared to that on a Cu(110)/H₂O surface. The Pt dopant promotes the C–H bond scission of CH₃O and the dehydrogenation of CH₂O, but slightly hinders the O–H bond breaking of CH₃OH. The results are similar to the results of Chen et al.⁴⁶ They found that the activation energy of CH₃O dehydrogenation on a PdZn(111) surface is higher than that on a Pd(111) surface due to the fact that CH₃O interacts more weakly with Pd than with PdZn. In addition, from Table 1, it is clear that the adsorption strength of CO upon Pt doping a Cu(110)/H₂O surface, especially on the 9Pt–Cu(110)/H₂O and 3Pt–Cu(110)/H₂O surfaces, is high, which may lead to catalyst CO poisoning. Overall, the methanol decomposition mechanism on the 9Pt–Cu(110)/H₂O surface is consistent with that on a clean Pt(111) surface,^44,45 and the final product of CO from the decomposition of methanol is in good agreement with the experimental findings on (2 × 1) Pt(110) when using TPD and EELS.⁴⁷ For methanol decomposition on the 3Pt–Cu(110)/H₂O and 1Pt–Cu(110)/H₂O surfaces, the mechanism is in good agreement with that on a Cu(110) surface.^27,40

3.2.1 Brønsted–Evans–Polanyi (BEP) correlations. The kinetic–thermodynamic parameters relationship was put forward by Brønsted⁴⁸ and developed in depth by Evans and Polanyi (BEP).⁴⁹ BEP correlations are widely used in many reactions, for example, methanol steam reforming on Cu(111) and Pd(111)⁵⁰ and methanol decomposition on Cu₄ and Co₄ clusters.⁵¹ Interestingly, in the previous literature it has been found that BEP correlations can be better applied to dissociation reactions rather than synthesis reactions in many case.⁵²

As shown in Fig. 6, BEP plots for the main elementary reaction steps of CH₃OH decomposition on the 9Pt–Cu(110)/H₂O, 3Pt–Cu(110)/H₂O and 1Pt–Cu(110)/H₂O surfaces are presented. According to the relationship between the transition state energy and the final state energy, the corresponding linear scaling relations were identified as follows: (a) Y = 0.84X + 0.70 with R² = 0.98, (b) Y = 0.61X + 0.45 with R² = 0.87, and (c) Y = 0.88X + 0.57 with R² = 0.91. Remarkably, the slopes of (a) and (c) are 0.84 and 0.88, much larger than that of (b) of 0.61, indicating that the TSs of (a) and (c) are final state-like or belong to the “late barrier”.^50,51,53 Through the identified linear scaling relation of BEP correlations, we can easily estimate activation energies from the reaction energy changes for the elementary steps in the heterogeneous catalysis process instead of calculating the corresponding transition states, which saves computational costs effectively.

Fig. 6 Brønsted–Evans–Polanyi plots of the calculated transition state energy (E_TS) versus the final state energy (E_FS) for the main route of CH₃OH decomposition on the 9Pt–Cu(110)/H₂O, 3Pt–Cu(110)/H₂O and 1Pt–Cu(110)/H₂O surfaces. The reactant energy in a vacuum is used as a reference to calculate the transition and final state energies of each elementary reaction.

4. Conclusions

In the current work, DFT studies using the periodic slab model were performed to systematically investigate CH₃OH decomposition on Cu(110)/H₂O surfaces with different Pt/Cu doping ratios. To understand the effect of different ratios of Pt dopant for this catalytic process three catalytic models were constructed, denoted 9Pt–Cu(110)/H₂O, 3Pt–Cu(110)/H₂O and 1Pt–Cu(110)/H₂O. All the possible elementary steps for CH₃OH decomposition were calculated in detail. Comparing the results with those of experiments conducted on a Cu(110)/H₂O surface, the interactions between CH₃O and the substrates are weakened, and the adsorption strengths of CH₂O, CHO and CO are enhanced. The most favorable reaction pathway for CH₃OH decomposition on 9Pt–Cu(110)/H₂O is CH₃OH → CH₂OH → CH₂O → CHO → CO, differing from that on the 3Pt–Cu(110)/H₂O, 1Pt–Cu(110)/H₂O and Cu(110)/H₂O surfaces, which have the main reaction route of CH₃OH → CH₃O → CH₂O → CHO → CO. On the basis of the calculated activation energies and reaction energies of each step, we also found that the Pt dopant promotes the C–H bond scission of CH₃O and the dehydrogenation of CH₂O, but slightly hinders the O–H bond breaking of CH₃OH. Moreover, on the 9Pt–Cu(110)/H₂O and 3Pt–Cu(110)/H₂O surfaces, the O–H bond rupture and C–H bond scission of CH₃OH are competitive due to their similar activation energies. Overall, different ratios of the Pt dopant in a Cu(110)/H₂O surface can effectively reduce the activation energies of some main elementary reactions, and the introduction of monatomic Pt into a Cu(110)/H₂O surface (1Pt–Cu(110)/H₂O) is enough to obtain better catalytic activation for CH₃OH decomposition. This may be attributed to the synergistic effect of Pt and Cu when compared with a pure Cu catalyst.

Finally, the linear scaling relations for the main elementary steps involved in CH₃OH decomposition on the 9Pt–Cu(110)/H₂O, 3Pt–Cu(110)/H₂O and 1Pt–Cu(110)/H₂O surfaces were identified. The calculated results are useful for understanding the mechanism of CH₃OH decomposition on the binary metal alloy solid/liquid interface at the molecular level.

Acknowledgements

The authors gratefully acknowledge the financial support of this study by the National Natural Science Foundation of China (21406154), Natural Science Foundation of Shanxi (2013021007-5), Special/Youth Foundation of Taiyuan University of Technology (2012L041 and 2013T092). The authors especially thank the anonymous reviewers for their helpful suggestions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09395g

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