Syngas to ethanol on MoCu(2 1 1)surface:Effect of promoter Mo on C-O bond breaking and C-C bond formation

Lijuan He, Cuimei Zhi, Lixia Ling, Riguang Zhang, Baojun Wang,*

1 State Key Laboratory of Clean and Efficient Coal Utilization, Taiyuan University of Technology, Taiyuan 030024, China

2 Key Laboratory of Coal Science and Technology (Taiyuan University of Technology), Ministry of Education and Shanxi Province, Taiyuan 030024, China

3 College of Chemistry and Bioengineering, Taiyuan University of Science and Technology, Taiyuan 030024, China

4 College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China

Keywords:Syngas Ethanol DFT Cu catalyst Promoter Mo

ABSTRACT The mechanism of syngas to ethanol on MoCu(2 1 1) surface has been researched by density functional theory (DFT) calculation, and the effects of Mo as a promoter on C-O bond breaking and C-C bond formation have been discussed.Calculations show that Cu-Mo atoms constitute the active sites on MoCu(2 1 1) surface after Mo atom being served as a promoter of Cu catalyst.Compared with Cu(2 1 1),MoCu(2 1 1) has two improvements.Firstly, CH3 is the most advantageous monomer on the MoCu(2 1 1)surface,which provides abundant CH3 intermediate for syngas to ethanol.Secondly,the C-C bond is formed mainly by inserting CHO into the abundant CH3, and the generated CH3CHO through multiple steps of hydrogenation to generate C2H5OH.The key of the promoter Mo on the MoCu(2 1 1)surface also has been verified by the analysis of its electronic properties.Differential charge density shows that the massive electron transfer from Mo to Cu,projected density of states(pDOS)shows that the electron transfer from Mo to Cu makes the d-band center of MoCu(2 1 1)nearer to the Fermi level,these indicate that the MoCu(2 1 1) catalytic capacity increased.The addition of Mo in the Cu-based catalyst not only can effectively solve the problem of C-O bond breaking, but also promote C-C bond formation.About the influence of Mo content on C-O bond breaking and C-C bond formation, compared with MoCu(2 1 1),the DFT results and the d-band center of Mo2Cu(2 1 1) show that the increase of Mo content could not promote the synergistic effect of Cu/Mo on the generation of ethanol more effectively.

1.Introduction

China is a country rich in coal resources,syngas extracted from coal or biomass has attracted more and more attention due to its rich resources[1,2].Among the products produced by syngas,ethanol is a non-toxic clean energy with little harm to the human body,low sulfur content and low ash content,and can be used in chemical, pharmaceutical, fuel and other fields.In terms of energy, the conversion of syngas(CO+H2)to ethanol is a new way to develop new energy [3-6].At present, many researches focus on the synthesis of ethanol from syngas.However, the low activity and low selectivity of syngas to ethanol are still the major problems.In order to solve these problems, it is necessary to exploit a highly active and highly selective catalyst [6].

The mechanism of syngas to ethanol is mainly divided into two key steps.The first is to activate CO and generate CHxthrough the process of hydrogenations and dissociation.The second is to insert CO or CHO into CHx[7,8].Syngas to ethanol is a complicated process because the two processes of C-C bond formation and CO insertion exist synchronously and close to each other, generally happen at disparate active sites.Therefore,the production of ethanol requires the synergy of two active sites, one promoting C-C bond formation and the other promoting CO/CHO insertion.Finding catalysts with synergistic catalysis is key.Chenet al.[9]have found that the interface between Co and CoOxis conducive to form high alcohols,because Co and CoOxhave a synergistic effect,that is,Co facilitates the generation of CHx, while CoOxsupplies nondissociated CO, thereby increasing the selectivity of alcohol.Peiet al.[10]have obtained similar results, the synergistic effect produced by the interface of Co and Co2C is conducive to the generation of high alcohol.

The catalysts commonly used in industry mainly include Cubased [11], Rh-based [12,13], Mo-based [14], and modified Fischer-Tropsch synthesis catalysts[15-17].In the midst of these catalysts, Rh-based catalysts have high selectivity [18,19],nevertheless, in view of their high price and limited practicality,search for some inexpensive active metal catalysts is urgent.

In recent years,Cu-based catalysts have been widely selected as the catalysts for syngas to ethanol reaction due to its cheap,simple preparation,stable performance,flexible structure adjustment,and high CO conversion rate[5].However,the C-O bond is also particularly easy to hydrogenate[20,21].Therefore,methanol generation is relatively easy,which leads to the low CHxselectivity[22-24].In other words,it is difficult to break the C-O bond,resulting in a low CHxconcentration, which further reduces the probability of C-C bond formation [25].In order to promote the breaking of C-O bond, promote the formation of CHxand increase the probability of C-C bond formation, it is imperative to modify the Cu catalyst.

Studies have found that changing the electronic structure of metals by forming bimetallic catalysts is an effective way to adjust the reactivity of transition metal catalysts[26,27].A large number of experiments have studied the role of promoters, showing that good promoters help to promote the breaking ability of C-O bond.The results[28]of Sunet al.’s experiments combined with DFT calculations have showed that the additive Cs has higher ethanol synthesis activity for Cu catalyst modification.And previous studies[29-33]have shown that the addition of promoters (Rh, Mn, Co,Ni,Fe)to the Cu-based catalyst could effectively improve the yield of ethanol.Lai [34]have done a quantum chemical study on the nature of the interaction between transition metal promoter ions(V, Ti, Fe, Mn, Mo) in the periodic table and Cu-based catalyst for synthesis of methanol and its effect on CO activation, the results show that Mn and Mo have the best effect on promoting the activation of CO by the Cu-based catalyst.It has been experimentally reported that adding Mo to 5 %Co/50 %CuLa2Zr2O7catalyst can improve the yield of advanced alcohols, while adding 3% Mo can achieve the maximum activity and selectivity of advanced alcohols.However, the VIB group elements Cr and W can not increase the yield of advanced alcohols [5].

As far as we know,many studies on promoters have shown that the addition of promoter can increase ethanol yield.However,there are few experimental and theoretical studies on the promotion of ethanol production by Mo on pure Cu catalysts,let alone the influence of Mo as a promoter.Therefore,the role of Mo in the catalyst and the influence of Mo on syngas to ethanol are unknown.At the same time,the mechanisms of syngas to ethanol on MoCu catalyst have not been studied.

In order to explore the role of the promoter Mo in the MoCu catalyst, syngas to ethanol has been studied on the MoCu(2 1 1) surface by density functional theory (DFT) method.The specific steps in the mechanism are mainly divided into CO activation, CHxgeneration, C-C bond formation, and ethanol generation.Compared with the pure Cu(2 1 1) surface, the influence of the promoter Mo on syngas to ethanol is obtained.In order to further understand the effect of Mo content in the MoCu catalyst,a systematic DFT calculation for syngas to ethanol on Mo2Cu(2 1 1) surface has been performed.In this part, the effect of increasing Mo content on the key steps of syngas to ethanol is obtained.

2.Model and Methods

2.1.MoCu(2 1 1) surface

Convincing evidence have shown that the step surfaces tend to exhibit excellent catalytic activity compared to perfect surfaces[35-42].To be honest, the step (2 1 1) surface composes of(1 1 1) surface and (1 0 0) surface and has good catalytic activity.It plays an important effect during the catalytic reaction[33,41,43-47].To cite a few examples, Tezsevinet al.[48]have found that, unlike other Cu surfaces, the Ho-doped Cu(2 1 1) surface activated CO2better than H2molecules.A detailed microkinetic model study [37]has reported that more open and partially oxygenated surface of Cu(2 1 1)facet is catalytically more efficient than the flat Cu(1 1 1)surface.In addition,Behrenset al.[47]have compared the hydrogenation of CO and CO2to methanol between Cu(1 1 1)and Cu(2 1 1)surface,and the results show that the latter had higher catalytic activity.Based on the above researches, we choose the step surface MoCu(2 1 1)surface as the model to calculate the reaction of syngas to ethanol.

There are two forms of MoCu(2 1 1)surface,the first form is to replace a Cu atom with a Mo atom,the second form is to adsorb a Mo atom on the Cu surface.According to what we have learned,the influence of promoters in the current research mostly focuses on the first form [12,32,49,50].Therefore, we adopt the first form of model, namely the MoCu(2 1 1) surface.

In this work, an eight-layer MoCu(2 1 1) model with periodp(2×3)supercell and vacuum of 1.5 nm is adopted.In all optimization processes, the upper five layers are relaxed together with the adsorbent, the bottom three layers are fixed.For the stepped Modoped Cu(2 1 1) surface, a single Mo atom can be doped at three possible sites of the Cu(2 1 1) surface: a terrace, a step edge, and a step base site[31,33],position 1,2,3(Fig.1(a)).Among the three positions, the model with a Mo atom at position 2 needs only a substitution energy of 4.19 eV, whereas the other two models require substitution energies of 4.91 and 4.97 eV.According to the substitution energy definition [31], a smaller substitution energy denotes easier replacement of a Cu atom by a Mo atom.Therefore, one Cu atom at position 2 is replaced with a Mo atom,and the MoCu(2 1 1) surface is modeled to simulate the MoCu bimetallic catalyst.Similarly, on the basis of MoCu(2 1 1) surface,the second Mo atom may exist in four positions on the MoCu(2 1 1) surface: position 1, 2, 3, 4 (Fig.1(b)).Among the four positions, the model with a Mo atom at position 2 is found to be the most easily formed.This model needs only a substitution energy of 5.04 eV, whereas the other three models require substitution energies of 7.54, 7.55 and 7.57 eV.Therefore, the Cu atom at position 2 is replaced with a Mo atom, named Mo2Cu(2 1 1) surface.

Fig.1. (a)Surface morphology of the MoCu(2 1 1)surface with a Mo atom at three different sites marked in a white circle;(b)Surface morphology of the Mo2Cu(2 1 1)surface with the second Mo atom at four different sites marked in a white circle.The cyan and orange balls represent Mo and Cu atoms, respectively.T, T1, T2, B, and H refer to the Mo-Top,Mo-Top1,Mo-Top2,Bridge,and Hollow sites on the edge of the steps, respectively.

2.2.Computational method

All calculations have been carried out using the VASP (Vienna ab initio simulation software package) software [51-53]based on density functional theory [54,55].Perdew-Burke-Ernzerhof (PBE)parameterization method using generalized gradient approximation considers exchange-correlation [56].The k-point grid is 2×3×1,the plane wave cutoff energy is 400 eV.The convergence criteria of electronic self-consistent iteration and force are 10-5eV and 0.3 eV·nm-1,respectively.The Climbing Image Nudged Elastic Band(CI-NEB)method implemented in VASP is used to identify the transition state (TS) [53].The climbing image micro-movement elastic band method is used to get the saddle point from the initial state to the final state [53,57].

The adsorption energyEadsis defined as follows:

whereEslab+adsorbateis the energy of adsorbate with substrate,EslabandEadsorbateare the energies of catalyst and free adsorbate,respectively.

The activation free energyGaand reaction free energy ΔGare calculated as the follows [31]:

whereEIS,ETSandEFSstand for the energy of initial states,transition states and final states, respectively.ΔEZPEaand ΔEZPErefer to the ZPE correction for the reaction barrier and reaction energy,which are determined by the vibrational frequencies of the initial states, transition states and final states.

Experimentally,the temperature for syngas to ethanol over Cubased catalyst is usually 500-600 K [5,16,58].In this work, the temperature correction is based on 500 K.

3.Results and Discussion

In general,there are two key steps in syngas to ethanol.The first step is to activate CO to generate CHxspecies, and the second step is to insert CO/CHO into CHxto form C-C bond.In this process,the selectivity of ethanol is reduced due to the generation of byproducts methanol and hydrocarbons.Therefore, considering the above reactions on the MoCu(2 1 1) surface is helpful to understand the catalytic property of the MoCu(2 1 1)surface in the synthesis of ethanol.Table 1 lists the activation free energy, reaction free energy and TSs image frequency corresponding to the synthesis of ethanol from syngas on the MoCu(2 1 1) surface at 500 K.

3.1.Syngas to ethanol on MoCu(2 1 1) surface

We first conduct a study on the adsorption energies of reactants and intermediate species related to the possible pathways for syngas to ethanol on MoCu(2 1 1) surface, and obtain the results by DFT calculation.Fig.2 correspondingly shows the stable surface adsorption structures; Table 2 lists the adsorption energy and key geometric parameters.

Table 1The activation free energy, reaction free energy and TSs image frequency corresponding to the synthesis of ethanol from syngas on the MoCu(2 1 1) surface at 500 K

Table 2The adsorption energy and key geometric parameters of the stable surface adsorption structures involved in the syngas to ethanol on the MoCu(2 1 1) surface

In the process of adsorption, it is found that reactant small molecules are more inclined to adsorb at Mo atom or Cu-Mo atoms.The results indicate that the Cu-Mo atoms are the primary active sites for syngas to ethanol.We can conclude that Cu-Mo atoms constitute the active sites on MoCu(2 1 1) surface after Mo atombeing served as a promoter of Cu catalyst.The structural feature of the active site on the MoCu(2 1 1)surface is the Mo-Cu point active site centered on Mo [59,60].

Fig.2. The stable surface adsorption structures involved in the syngas to ethanol on the MoCu(2 1 1) surface.

3.1.1.CO activation

During the reaction of syngas to ethanol, three reactions may occur during the activation of CO,namely hydrogenations and dissociation reactions.Fig.3 shows the CO activation potential energy diagram and the initial states(ISs),transition states(TSs)and final states (FSs) structures.

In Fig.3, the hydrogenation of CO to COH is not easy to take place, on the contrary, CO dissociates to C + O and CO hydrogenation to CHO are more likely.Therefore, the CO adsorbed on the MoCu(2 1 1) surface is mainly hydrogenated to CHO and dissociated to C + O.

The studies on the activation and dissociation results of CO on the surface of Cu(2 1 1) [32], MnCu(2 1 1) [33], CoCu(2 1 1) [31]and FeCu(2 1 1) [30]are 5.56, 4.07, 4.02 and 2.62 eV, respectively.Different from the cases of those surfaces, CO dissociation is the most advantageous on the MoCu(2 1 1)surface.This further shows that the addition of Mo in the Cu-based catalyst can effectively solve the difficult problem of C-O bond breaking.

3.1.2.The hydrogenations process of C atom

From the above research,we know that CO easily dissociates to form C+O on the MoCu(2 1 1)surface.Based on the formation of C,we have studied its continuous hydrogenation reaction processes.Fig.4 shows the potential energy diagram generated by CH3,including the initial states (ISs), transition states (TSs) and final states (FSs) structures.

In Fig.4, provided that CO overcame 1.22 eV activation free energy barrier to generate C+O,the generated C would be continuously hydrogenated to form CH,CH2and CH3.In the processes of continuous hydrogenation of C to CH3, especially CH2+ H to CH3reaction is an exothermic reaction, which shows that the MoCu(2 1 1) surface is not only beneficial to the C-O bond breaking,but also beneficial to the continuous hydrogenation of C to form CH3[25].This provides abundant CHxspecies for C-C bond formation,suggesting that the MoCu(2 1 1)surface promotes the formation of C-C bond in the next step.

3.1.3.The reactions of related species in syngas to ethanol

The above results indicate that CO hydrogenation to CHO and CO dissociates to C+O both are the main paths.Therefore,starting from CHO, we further study the reaction of related species during the synthesis of ethanol from syngas on the MoCu(2 1 1) surface.Fig.5 shows the related species reactions potential energy diagram, including the initial states (ISs), transition states (TSs) and final states (FSs) structures.

Fig.5 clearly shows that CH2generated by direct dissociation of CH2O is the most advantageous; the CH3generated by the direct dissociation of CH3O comes next.And the formation of CH is the most unfavorable,due to the activation free energy of direct dissociation of CHO is significantly higher than the energy barrier for the formation of CH2and CH3.

For the formation of CH2, the total activation free energy of the advantageous path is 1.23 eV.Turning our attention to Cu(2 1 1)[32], MnCu(2 1 1) [33]and FeCu(2 1 1) [30]surface again, it is found that the total activation energies of the optimal path for CH2generation are 2.23, 1.31, 1.67 eV, respectively, higher than that of MoCu(2 1 1) surface.Although the total activation energy of the optimal path for CH2generation on CoCu(2 1 1) [31](0.51 eV) is lower than that of MoCu(2 1 1), the formation of CH3O on the CoCu(2 1 1) surface (-0.19 eV) is more beneficial to the formation of CH2, thus, the formation of CH2on CoCu(2 1 1)surface is disadvantageous.Therefore,the Cu/Mo synergistic effect on MoCu(2 1 1) surface improves the catalytic activity of CH2generation.

Regarding the formation of CH3, On the Cu(2 1 1) [32]surface,CH3is the advantageous monomer, generated from the hydrogen-assisted dissociation of CH3O, and the dissociation energy barrier is 2.22 eV.The most advantageous monomers on RhCu(2 1 1) [32]and MnCu(2 1 1) [33]surfaces are also CH3, and the formation of CH3also come from the hydrogen-assisted dissociation of CH3O, and the dissociation energy barriers are 1.67 and 2.11 eV, respectively.All of them are more difficult than the MoCu(2 1 1) surface.We attribute it to the synergistic effect of Cu/Mo,that is,the synergistic effect of Cu/Mo on MoCu(2 1 1)surface improves the catalytic activity of the CH3generation.

Fig.3. The CO activation potential energy diagram and the initial states (ISs),transition states(TSs) and final states (FSs) structures.The orange, gray, white,red and cyan spheres stand for Cu, C, H, O and Mo atoms, respectively.

Fig.4. The potential energy diagram generated by CH3, and the initial states (ISs),transition states(TSs)and final states(FSs)structures.The orange,gray,white,and cyan spheres stand for Cu, C, H, and Mo atoms, respectively.

Similar to the formation of CH2and CH3, MoCu(2 1 1) is also more beneficial to the formation of CH than those surfaces.The total activation free energy of the advantageous path for CH generation on the MoCu(2 1 1) surface is 1.60 eV.It is found that the total activation energies of the optimal path of CH formed on pure Cu(2 1 1) [32]and FeCu(2 1 1) [30]surfaces are 2.81 and 1.86 eV,respectively,significantly higher than that of the MoCu(2 1 1)surface.The polarity of C-O bond in CH2O and CH3O increases with the increase of the H atoms in C atom.The promoter of Mo on Cu(2 1 1) increases the breaking ability of C-O bond, and it is easier for CH2O and CH3O to dissociate to CH2and CH3, so CH2and CH3are advantageous monomers.Considering that the activation free energy barrier during the reaction of CHO+H →CHOH is sufficiently high,it is not conducive to the subsequent reaction,so the dissociation reaction of CHOH is not shown in the text.This indicates that the formation of CH on MoCu(2 1 1)surface is easier than pure Cu(2 1 1)and FeCu(2 1 1)surfaces.The same conclusion is that the Cu/Mo synergistic effect on MoCu(2 1 1) surface improves the catalytic activity of CH generation.

Based on Fig.5 and the above analysis, it can be seen that the formation of CHx(x=1-3)mainly come from CHxO(x=1-3)dissociation.Since the rate-determining step (CH2O →CH2+ O) of CH2generation (0.84 eV) is lower in activation free energy than the rate-determining step (CHO → CH + O) of CH generation(0.99 eV)and(CH3O →CH3+O)of CH3generation(1.30 eV),Therefore, CH2is the advantageous monomer on the MoCu(2 1 1) surface.For another, the rate-determining step of CH2generation on the MoCu(2 1 1) surface is lower in activation free energy than the rate-determining step(CH3O+H →CH3+OH)of advantageous monomer CH3generation (2.22 eV) on the Cu(2 1 1) surface [32].Thus, the Cu/Mo synergistic effect improves the catalytic activity of MoCu(2 1 1) surface.

3.1.4.The related reactions of advantageous monomers CH2 and CH3 on MoCu(2 1 1) surface

The related reactions of CHxon Rh-based [7,12,61]and Cubased [29,32]catalysts mainly include CHxhydrogenation, coupling, CO and CHO insertion reactions.Based on the formation of the advantageous monomers CH2and CH3on the MoCu(2 1 1)surface, the hydrogenation, coupling, CO and CHO insertion reactions are studied.Fig.6 shows these reactions potential energy diagrams and the initial states (ISs), transition states (TSs) and final states(FSs) structures.

In Fig.6,CH2hydrogenation to form CH3is the most likely reaction to occur in CH2related reactions, this indicates that CH2is mainly hydrogenated to form CH3.In the related reactions of CH3, it is the easiest reaction for CHO to insert CH3, therefore, the MoCu(2 1 1) surface has higher activity for C2oxygenate CH3CHO than CH4and C2H6.In short, CH3is the most advantageous monomer, CH3CHO is the most advantageous ethanol precursor.

As mentioned above, the ethanol precursor C2oxygenate is mainly generated by the insertion of CHO into CH3,and the generated CH3CHO through multiple steps of hydrogenation to generate C2H5OH.In the processes of CH3CHO hydrogenation,there are two possible routes.One is the reaction of H atom with the α-C atom to CH3CH2O, the activation free energy is 0.45 eV; the other is the reaction of H atom with the O atom to CH3CHOH, the activation free energy is 1.46 eV,higher than the formation of CH3CH2O.Thus,the formation of CH3CHOH is merely listed in Table 1.Fig.7 shows these reactions potential energy diagram and the initial states(ISs),transition states (TSs) and final states (FSs) structures.

Fig.7 clearly tells us that the whole process of syngas to ethanol.Firstly,CO activation generates the most advantageous monomer CH3.From above we know that the formation of CH3has two pathways,CO+H →C+O+H···CH3and CO+H →CHO···CH3,the total activation free energies of the formation of CH3are 1.22 and 1.23 eV, respectively.Which provide abundant CH3intermediate for syngas to ethanol.Then CHO inserts CH3to generate the most advantageous C2oxygenate CH3CHO.Next the CH3CHO is subsequently hydrogenated to ethanol through the intermediate product CH3CH2O.

CH3CHO is an important intermediate.If the CH3CHO desorbed as a byproduct, the selectivity of the catalyst would be decreased.Considering this situation, I have compared the desorption energy of CH3CHO and the activation free energies of hydrogenation of CH3CHO to CH3CH2O and CH3CHOH.The desorption energy of CH3-CHO is 2.26 eV, the activation free energies of hydrogenation of CH3CHO to CH3CH2O and CH3CHOH are 0.45 and 1.46 eV, respectively.The results show that the hydrogenation of CH3CHO is more advantageous than its desorption.Thus,CH3CHO will not desorb as a byproduct.

3.2.The role of promoter Mo in syngas to ethanol

Based on the above research, we found that the addition of Mo as a promoter in the MoCu(2 1 1)surface significantly improves the activity and selectivity of ethanol generation, which is mainly reflected in two aspects: one is C-O bond breaking, the other is C-C bond formation.

3.2.1.The role of promoter Mo in C-O bond breaking

Regarding C-O bond breaking,we have compared MoCu(2 1 1)surface with Cu(2 1 1)[32],MnCu(2 1 1)[33],CoCu(2 1 1)[31]and FeCu(2 1 1) [30]surfaces for the CO →C + O reaction in Section 3.1.1, it is found that the CO dissociation energy barrier on the MoCu(2 1 1) surface is significantly lower than the above surfaces.This shows that the addition of Mo as a promoter in MoCu(2 1 1) surface can effectively solve the problem of C-O bond breaking.

Fig.5. The potential energy diagram of the related species reactions and the initial states(ISs),transition states(TSs)and final states(FSs)structures.The orange,gray,white,red and cyan spheres stand for Cu, C, H, O and Mo atoms, respectively.

From above we know that CH3is the most advantageous monomer, and the total activation free energies of the formation of CH3are 1.22 and 1.23 eV, respectively.The CH3OH formation path as shown by the pink line in Fig.5 is 1.77 eV,significantly higher than CH3.For the Cu(2 1 1)[32],the formation of the most advantageous monomer CH3with the total activation energy barrier 1.98 eV,and the formation of CH3OH is 1.45 eV, significantly lower than CH3.The differences between the Cu(2 1 1) and MoCu(2 1 1) surfaces show that MoCu(2 1 1)surface exhibits higher activity for the most advantageous monomer CH3.On the other hand, it indicates that once the C-O bond breaks on the MoCu(2 1 1) surface, abundant CHxwill be generated.It will provide the CHxneeded for syngas to ethanol.

What is more,we have calculated the rate constants of the most advantageous monomer CH3and CH3OH on the MoCu(2 1 1) surface at different temperatures, and have compared them with the Cu(2 1 1), RhCu(2 1 1), CoCu(2 1 1) and FeCu(2 1 1) surfaces in the literature [30-32], listed in Table 3.The rate constant can be obtained as follows:

Fig.6. Advantageous monomer CH2 and CH3 hydrogenation, coupling, CO/CHO insertion reactions and the initial states(ISs),transition states(TSs)and final states(FSs)structures.The orange,gray,white,red and cyan spheres stand for Cu,C,H,O and Mo atoms, respectively.

Fig.7. The potential energy diagram generated by C2H5OH and the initial states(ISs),transition states(TSs)and final states(FSs)structures.The orange,gray,white,red and cyan spheres stand for Cu, C, H, O and Mo atoms, respectively.

whereEais the activation energy;his Planck’s constant,kBis Boltzmann’s constant;qIsis the initial state partition functions,qTsis the transition state partition functions;Tis the reaction temperature;viis vibrational frequency.

In Table 3, the rate constants of the same catalyst increase as the temperature increase.At the same temperature, the rate constants of CH3generation on Cu(2 1 1) and RhCu(2 1 1) are much smaller than that of CH3OH, indicating that CH3OH generation is more advantageous.The rate constants of the advantageous monomer CHxformation on the CoCu(2 1 1), FeCu(2 1 1) and MoCu(2 1 1) surfaces are greater than that of CH3OH, indicating that the formation of the advantageous monomer CHxare easier.

Table 3The rate constants k for the most advantageous monomer CHx and CH3OH formation on the Cu(2 1 1), RhCu(2 1 1), CoCu(2 1 1), FeCu(2 1 1) and MoCu(2 1 1) surfaces at the different temperatures

From the other aspect, at the same temperature, the rate constant of CH3generation on MoCu(2 1 1) is much greater than that of Cu(2 1 1), RhCu(2 1 1), CoCu(2 1 1), FeCu(2 1 1) surfaces, and CH3OH generation is lower than those surfaces, which means that the formation of CH3on the MoCu(2 1 1) surface is easier,and the formation of CH3OH is more difficult.Therefore,we come to a conclusion that the Cu(2 1 1)surface with Mo promoter facilitates the formation of CH3and minimizes the formation of CH3OH, agreed with the thermodynamics results.In short, the promoter Mo promotes the C-O bond breaking.

3.2.2.The role of promoter Mo in C-C bond formation

Regarding C-C bond formation on the Cu(2 1 1) surface, Wanget al.[29]have studied the related reactions of CHx(x= 1-3), the results show that Cu(2 1 1)surface has higher activity to C2hydrocarbons, and the corresponding reaction is CH2coupling to form C2H4.CH3CO formed by the insertion of CO into CH3is the main C2oxygenate, the total activation energy barrier during the reaction is 1.10 eV.Different from Cu(2 1 1) surface, MoCu(2 1 1) sur-face has higher activity to CH3CHO.CH3CHO formed by the insertion of CHO into CH3is the main C2oxygenate, and the total activation free energy during the reaction is 0.05 eV.Therefore,MoCu(2 1 1)surface is easier to generate ethanol precursor C2oxygenate than Cu(2 1 1) surface.We have reached a conclusion that the promoter Mo promotes C-C bond formation on Cu-based catalysts.

On the basis of transition-state theory [32], the rate constants of C2oxygenates generation on MoCu(2 1 1) atT= 550, 575 and 600 K have been calculated and compared with the calculated results of the Cu(2 1 1) and MnCu(2 1 1) [33], listed in Table 4.It can be understood from Table 4 that the rate constants increase with the temperature increase on the Cu(2 1 1), MnCu(2 1 1) and MoCu(2 1 1) surfaces.At the same temperature, the C2oxygenate generation rate constant on MoCu(2 1 1) surface is more higher than that on the other two surfaces, indicating that C2oxygenate is more easily generated on the MoCu(2 1 1)surface.It can be seen that,compared with pure Cu(2 1 1)and MnCu(2 1 1),MoCu(2 1 1)is more conducive to the generation of C2oxygenate.In short, the promoter Mo promotes the C-C bond formation.

Table 4The elementary reaction rate constants k of C2 oxygenates generation on the Cu(2 1 1), MnCu(2 1 1) and MoCu(2 1 1) surfaces at different temperatures

3.2.3.Analysis of electronic properties

In order to understand the catalytic performance of the catalyst used in this work from a microscopic point of view, we have performed differential charge analysis on the Cu(2 1 1) and MoCu(2 1 1)surfaces,as shown in Fig.8(a).According to the differential charge density obtained, it is found that electrons from other nearby Cu atoms gather at the Cu(2 1 1)step site and form an electron region.On the MoCu(2 1 1) surface, a large amount of electrons are transferred from Mo to Cu, forming a large electron cluster in the Mo atom and nearby Cu atoms region.The Bader charge on the MoCu(2 1 1) surface has been calculated, the result shows that the total amount of charge transferred by Mo is 0.57e.

In order to enrich the differential charge density results, projected density of states (pDOS) [62]analysis on Cu(2 1 1) and MoCu(2 1 1) surfaces are applied, as shown in Fig.8(b).The dband center of Cu(2 1 1) and MoCu(2 1 1) are -2.56 and-2.42 eV, respectively, the d-band center of MoCu(2 1 12 1 1) is closer to the Fermi level.PDOS results show that the electron transfer from Mo to Cu makes the d-band center of MoCu(2 1 1)nearer to the Fermi level.The comprehensive analysis of electronic properties indicate that the MoCu(2 1 1) catalytic capacity increased.

We have systematically analyzed the structural properties of the adsorption sites and reaction sites for synthesis of ethanol from syngas on the MoCu(2 1 1)surface.CO is preferentially adsorbed at Mo atom of MoCu(2 1 1)surface,which promotes the direct dissociation of CO.The formation of CHO and CH3, CHO inserting CH3process all occur at the Cu-Mo active sites,these show that the promoter Mo on MoCu(2 1 1) surface promotes the formation of CHxand C2oxygenate.CHO hydrogenation to form CH2O, CH3O and CH3OH also occur at Cu-Mo active sites, but the formation of methanol is more difficult in this process.In contrast, the formation of methanol on Cu(2 1 1)surface is easier,which indicates that the promoter Mo on MoCu(2 1 1)surface inhibits the formation of methanol.

In summary,on the one hand,the promoter Mo on MoCu(2 1 1)surface promotes the formation of the most advantageous monomer CH3and C2oxygenate CH3CHO; on the other hand, it inhibits the formation of methanol.Therefore, the promoter Mo makes MoCu(2 1 1) surface have higher activity in syngas to ethanol.

3.3.The effect of two Mo atoms on the key steps of ethanol synthesis on Mo2Cu(2 1 1) surface

3.3.1.The effect of increasing Mo content on key steps

In order to further understand the effect of Mo content on C-O bond breaking and C-C bond formation,we have performed a systematic DFT calculation for syngas to ethanol on Mo2Cu(2 1 1)surface.In this part, we study the effect of increasing Mo content on the key steps of syngas to ethanol.The specific details are that the total energy barriers of the eight key steps on the MoCu(2 1 1)and Mo2Cu(2 1 1)surfaces are compared,as shown in Fig.9.

Fig.9. (a) The total energy barriers of the four key steps on the MoCu(2 1 1) and Mo2Cu(2 1 1)surfaces(b)The total energy barriers of the other four key steps on the MoCu(2 1 1) and Mo2Cu(2 1 1) surfaces.The black and red lines in the (a) and (b)represent the total energy barriers of key steps on the MoCu(2 1 1) and Mo2Cu(2 1 1) surfaces, respectively.

From Fig.9, we know that the total energy barriers of the key steps on the Mo2Cu(2 1 1) surface represented by red line are higher than the corresponding reactions on the MoCu(2 1 1) surface represented by black line, this indicates that increasing Mo content is not beneficial to improve the catalytic performance.

Fig.8. (a) Differential charge density of Cu(2 1 1) and MoCu(2 1 1) surfaces.The yellow and blue shaded areas represent the gain and loss of charge,respectively.(b)The projected density of states curves(pDOS)of Cu(2 1 1)and MoCu(2 1 1)surfaces,where the vertical dashed line represents the Fermi level.

3.3.2.The related reactions of advantageous monomers CH2 and CH3 on Mo2Cu(2 1 1) surface

Through systematic DFT calculation of the key reactions of syngas to ethanol on Mo2Cu(2 1 1) surface, it is found that the most advantageous monomers are CH2and CH3.Therefore, the related reactions of CH2and CH3are studied in this section, as shown in Fig.10.

Fig.10. Advantageous monomers CH2 and CH3 hydrogenation, coupling, CO/CHO insertion reaction on Mo2Cu(2 1 1) surface.

As can be seen from Fig.10(a), the activation free energy of C2hydrocarbons C2H4generated by CH2coupling is the lowest in the related reaction of CH2.The insertion of CHO into CH3is the most advantageous in CH3related reactions in Fig.10(b).Comparing the total energy barriers of the two reactions, it is found that the formation of C2hydrocarbons is the easiest.Therefore, we can draw a conclusion that Mo2Cu(2 1 1) surface has low activity and low selectivity for ethanol production.This further shows that the increase of Mo content on the MoCu(2 1 1) surface is not beneficial to improve the catalytic performance.

3.4.The effect of Mo content on syngas to ethanol

Throughout the full text,there are two key problems to solve in this article:the first is C-O bond breaking,the second is C-C bond formation.The solution of these two key problems mainly depends on the modulation of the electronic properties of Cu by Mo on Cu-based catalysts.In fact, the DFT results prove that the modulation of the electronic properties of Cu by Mo has achieved this goal,what’s more, the increase of Mo content could not promote the synergistic effect of Cu/Mo on the generation of ethanol more effectively.

In order to enrich the DFT results,pDOS [62]analysis on two surfaces are applied, as shown in Fig.11.The d-band center of MoCu(2 1 1) and Mo2Cu(2 1 1) are -2.42 and -2.50 eV, respectively.The d-band center of MoCu(2 1 1) is nearer to the Fermi level, this shows that the MoCu(2 1 1) catalytic capacity is better than Mo2Cu(2 1 1).The electrical property analysis confirms the DFT result, namely, the increase of Mo content could not promote the synergistic effect of Cu/Mo on the generation of ethanol more effectively.

Fig.11. The projected density of states curves (pDOS) of MoCu(2 1 1) and Mo2Cu(2 1 1) surfaces, where the vertical dashed line represents the Fermi level.

4.Conclusions

The mechanism of synthesis of ethanol from syngas on MoCu(2 1 1)surface is described,and the optimal reaction path of syngas to ethanol is obtained.The effects of Mo as a promoter on C-O bond breaking and C-C bond formation have been discussed.It is clear that the modulation of the electronic properties of Cu by Mo on the Cu-based catalyst can effectively solve the two key problems of C-O bond breaking and C-C bond forming.It is understood that the increase of Mo content could not promote the synergistic effect of Cu/Mo on the generation of ethanol more effectively.And it can be used to predict the relationship between Mo content and the performance of Cu-based catalyst.The research results provide a definite theoretical clue for the design of high activity catalyst for synthesis of ethanol from syngas.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is financially supported by the Key Projects of National Natural Science Foundation of China (21736007) and the National Natural Science Foundation of China (21776193,21476155).