Xueyan Zou,Xiaodong Li,Xiaoyu Gao,Zhihua Gao,Zhijun Zuo,,Wei Huang,

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

2 College of Chemistry and Chemical Engineering,Jinzhong University,Jinzhong 030619,China

Keywords:DRM MgO Ni α-MoC SMSI

ABSTRACT Oxide supports modify electronic structures of supported metal nanoparticles,and then affect the catalytic activity associated with the so-called strong metal–support interaction(SMSI).We herein report the strong influence of SMSI employing Ni4 /α-MoC(111)and defective Ni4 /MgO(100)catalysts used for dry reforming of methane(DRM,CO2 +CH4 →2CO+2H2 )by using density functional theory(DFT)and kinetic Monte Carlo simulation(KMC).The results show that α-MoC(111)and MgO(100)surface have converse electron and structural effect for Ni4 cluster.The electrons transfer from α-MoC(111)surface to Ni atoms,but electrons transfer from Ni atoms to MgO(100)surface;an extensive tensile strain is greatly released in the Ni lattice by MgO,but the extensive tensile strain is introduced in the Ni lattice by α-MoC.As a result,although both catalysts show good stability,H2 /CO ratio on Ni4 /α-MoC(111)is obviously larger than that on Ni4 /MgO(100).The result shows that Ni/α-MoC is a good catalyst for DRM reaction comparing with Ni/MgO catalyst.

1.Introduction

Dry reforming of methane(DRM,CO2+CH4→2CO+2H2)is one of the effective ways for utilizing two of the greenhouse gases,carbon dioxide(CO2)and methane(CH4)to produce syngas[1–5],which can be used as the feedstock for catalytic processes,such as Fischer–Tropsch and methanol synthesis[6–8].However,Ni based catalysts for the DRM deactivate quickly because of the sintering of the active metal phase and carbon deposition[3,9–12].To solve the problem,many efforts have been made,such as varying oxide supports (MgO,SBA-15,γ-Al2O3,etc.),tuning the size and distribution of Ni particles or adding promoters[1–4,9,10,12–21].

Transition-metal carbides are less ionic than the metal oxides,which show the similar chemical behavior of noble metal(Pt,Pd,Rh,etc.)and seem to be excellent supports for the dispersion of metals[22,23].In addition,transition-metal carbides also show good catalytic electivity or resistance to poisoning for the conversion of carbon-containing molecules[22,24,25].Recently,Ma groups found that Pt1/α-MoC and Au/α-MoC catalysts show the better catalytic activity for steam reforming of methanol and water gas shift reaction[26,27].

In the paper,we investigate the reaction details of DRM on Ni/α-MoC,where Ni4clusters supported on α-MoC(111)is employed.The reaction pathways of DRM are studied by using density functional theory(DFT)and kinetic Monte Carlo simulations(KMC),and the influence of pre-covered C,H,OH and O for the reaction pathways is also considered.In addition,in order to find the differences between α-MoC and MgO supports,the electron structure of Ni4/α-MoC(111)is compared with defective Ni4/MgO(100)[28].Such fundamental insight can help guiding the further optimization of Ni-based catalysts for the DRM reaction.

2.Computational Method

Total-energy and electronic structure calculations have been performed using the Vienna Ab-initio Simulation Package(VASP)[29–31].The calculations were conducted with the generalized gradient approximation with the PW91 exchange–correlation functional[32].A planewave energy cutoff of 400 eV was used for all geometry optimizations.K point was sampled using a 3 × 3 × 1 Monkhorst–Pack mesh [33].The transition state(TS)was derived by using the nudged elastic band method[34],and all the TSs were verified to exhibit only one imaginary frequency by frequency calculations.

The equilibrium lattice constant of α-MoC was aα-MoC=0.4370 nm,which was consistent with the experimental value of aα-MoC=0.4278 nm [35,36].α-MoC(111) surface with both C and Mo terminations were considered using a seven-layered slab with p(3 × 3) in each layer.During the calculations,the bottom two layers were fixed at bulk positions,whereas the other layers with the adsorbates were allowed to relax,and volume was maintained constant.With respect to Ni bulk and bare α-MoC(111) surface,the adsorption energies of Ni4on C-and Mo-terminated α-MoC(111) surfaces were 5.85 and?7.49 eV respectively,indicating that Ni preferred to adsorb on α-MoC(111) with Mo-termination.Therefore,in the work,only Moterminated α-MoC(111)surface was considered.For Ni4/α-MoC(111)(Fig.1),the surface free Mo sites were occupied by O due to the water or CO2dissociation on Mo sites according to previous study of Mo2C surfaces[37].

The reactions involved the reaction for the DRM on Ni4/α-MoC(111),adsorption and desorption of the reactants and possible productions were considered in the KMC simulations.For the adsorbed species on the surface,the prefactor of kBT/h and adsorption coefficient(kads)was used[28,38–41].For the adsorbed and desorbed species,the contribution from the entropy and was considered in the KMC simulations[42].Both forward and backward reactions were included in the KMC model,and the other information sees the our previous paper[28].

3.Results and Discussion

3.1.DFT result

Previous studies show that*H,*O,*OH and*C is easily covered on the active site[28,43,44],so the influence of these species is considered in there.

3.1.1.CO2scission

CO2prefers to reside on bridge site seeing Fig.2,and the bond lengths of C-O are 0.125 and 0.123 nm,which is larger than that of gas CO2(0.118 nm),but slightly smaller than that on defective Ni4/MgO(100) (0.127 nm) [28].The bond strength of CO2on Ni4/α-MoC(111)surface(Eads=?0.91 eV,Table 1)is also stronger than that of Ni(111)surface(Eads=?0.02 eV),but is smaller than that of Ni4/MgO(100) (Eads=?0.99～ ?1.35 eV) [28,45].The energy barrier of CO2scission(*CO2+* →*O +*CO) is 0.70 eV(Table 2),which is higher than that on defective Ni4/MgO(100) (Ea=0.35 eV)[28]and Ni(111)surface(Ea=0.55 eV)[46].The result indicates that α-MoC support refrains CO2scission comparing with the MgO support.

On the*C or*O pre-covered surface,the energy barriers of*C+*CO2→*C+*O+*CO and*O+*CO2→*O+*O+*CO are 0.88 and 1.02 eV,which are larger than that of*CO2+*→*O+*CO(0.70 eV).The result shows that the pre-covered*C and*O are disadvantageous of*CO2scission.In the case of*H pre-covered surface,the energy barriers of*CHOO(Ea=2.78 eV)or*COOH(Ea=1.75 eV)formation are far larger of*CO2direct dissociation (Ea=0.70 eV).That is,CO2hydrogenation is unlikely,but preferring dissociation.

In addition,some authors propose that CO2is the reactant to remove*C,named as reverse Boudouard reaction[3,9–12],so the reaction is also studied in there.The energy barriers of*C+*CO2→*CCOO+*,*CCOO+*→*CO+*CO and*C+*CO2→*CO+*CO are 1.19,0.72 and 1.82 eV.For reverse Boudouard reaction,the energy barriers of two-step reaction (*C+*CO2→*CCOO+* →*CO+*CO) are lower than that of the direct reaction(*C+*CO2→*CO+*CO),indicating that the direct reaction is unlikely.

For the production of CO from CO2scission,it prefers to adsorb at the hollow site,which the adsorption energy is ?1.92 eV.There are three possible pathways:hydrogenation,CO scission,desorption or CCOO formation.The energy barriers of*C formation from C--O bond scission(Ea=3.06 eV),*CCOO formation from two*CO(Ea=3.27 eV)and*COH from *CO hydrogenation (Ea=2.01 eV) are far larger than that of*CHO formation(Ea=0.83 eV),so*HCO formation is possible.However,the desorption energy of*CO is only～0.3 eV at 700 K because of the entropic contribution(see supplement material).Therefore,neither process can compete with*CO desorption under the reaction conditions.

3.1.2.CH4scission

CH4prefers to adsorb on Ni top site with C atom (Fig.2),which the bond length of C--Ni is about 0.2078 nm.Comparing with the previous results,the bond strength of CH4on Ni4/α-MoC(111) surface (Eads=?0.39 eV,Table 1) is clearly larger than that of CH4adsorption on Ni(111) surface (Eads=?0.02 eV) [46–48],and is also larger that on perfect Ni4/MgO(100) (Eads=0.05 eV) [48],defective Ni4/MgO(100) (Eads=?0.18 eV) [28]and Ni4/γ-Al2O3(Eads=?0.18 eV) [49].The energy barrier of *CH4dehydrogenation (*CH4+→*CH3+H,Ea=0.45 eV) is also far lower than that of on Ni(111) surface (Ea=0.91～ 1.18 eV) [46,48,50],and lower than that of on perfect Ni4/MgO (Ea=1.52～ 1.96 eV)[45,48],defective Ni4/MgO (Ea=0.53 eV) [28]and Ni4/Al2EO3(Ea=0.63～ 0.71 eV) [49,51].In general,the energy barrier of CH4dehydrogenation decreases on Ni4/α-MoC(111) surface,the result shows that the support effect of α-MoC is also benefit of CH4dehydrogenation.It is also found that the energy barriers of*CH4+*O →*CH3+*OH (Ea=0.49 eV) and *CH4+*O →*CH3+*H+*O (Ea=0.41 eV) are similar with the energy barrier of *CH4+* →*CH3+*H (Ea=0.45 eV).However,the energy barriers of *CH4+*C →*CH3+E*CH (Ea=0.62 eV) and *CH4+*C →*CH3+*HE+*C (Ea=0.67 eV) are higher than that of*CH4+* →*CH3+*H.The result shows that the deposition C is disadvantage of CH4decomposition,but pre-covered *O is not obviously effect for its decomposition.The trend of *O and *C for CH4decomposition on Ni4/α-MoC(111) surface is same as on defective Ni4/MgO surface [28].

Fig.1.The top(left)and side(right)view of Ni4 /α-MoC(111).top,bri(bridge)and hol(hollow)stand for the adsorption sites of Ni4 .

Fig.2.The most stable configurations of the possible intermediates involved in DRM on Ni4 /α-MoC(111).

CH3binds to the bridge site with the adsorption energy of ?2.90 eV,and the energy barrier of *CH3+→*CH2+* is 0.58 eV.On *O precovered surface,the energy barriers of *CH3+*O →*CH2+*OH,*CH3+*O →*CH2+*H+*O and*CH3+*O →*CH3O+*are 1.09,1.24 and 1.41 eV,which is larger than that of*CH3+→*CH2+*.On*C pre-covered surface,the energy barriers of *CH3+*C →*CH +*CH2(Ea=1.13 eV)and*CH3+*C →*C+*H+*CH2(Ea=0.79 eV)are also larger than that of its dehydrogenation on clean surface.On*OH pre-covered surface,the energy barriers of*CH3+*OH →*CH2+*OH+*H and*CH3+*OH →*CH3OH+*are 0.60 and 1.68 eV.Even if the surface is covered by*O,*OH or*C species,the energy barriers of*CH3O species and*CH3OH formation are higher than that of*CH2formation,indicating that the process of*CH3O or*CH3OH formation can compete with*CH2formation.In addition,*C and*O are disadvantageous of*CH3dehydrogenation,but*OH is not obviously affect for its dehydrogenation.

Table 1 Adsorption energies(Eads )and corresponding geometric parameters(d)of possible intermediates involved in DRM on Ni4 /α-MoC(111)

CH2adsorbs on the hollow site,which the adsorption energy is.Similar with *CH2dehydrogenation on Ni(111),Ni4/MgO(100) and Ni4/Al2O3surface[28,46,48–50],the reaction of*CH2dehydrogenation also easily occur on Ni4/α-MoC(111)among of CHx(x=1,2,3 and 4)dehydrogenation,which the energy barrier is 0.37 eV.The energy barriers of*CH2dehydrogenation on*O,*OH and*C pre-covered are larger than that on clean surface (0.69,0.86 and 0.79 vs.0.37 eV),showing that*O,*OH and*C are disadvantage of*CH2further reaction.In addition,the energy barriers of*CH2+*O →*CH2O+*(Ea=1.64 eV)and*CH2+*OH →*CH2OH+*(Ea=1.69 eV)are higher that of*CH2dehydrogenation,so the probability of*CH formation is larger than that of*CH2O and*CH2OH formation.

CH also prefers to adsorb on hollow site with the adsorption energiey is ?6.85 eV,which the energy barrier of*CH dehydrogenation on clean surface is 1.13 eV.Table 2 shows that the probability of*CHO formation is larger than that of*C formation(0.93 vs.1.80 eV)on*O pre-covered surface.On*OH pre-covered surface,*CHOH formation is unlikely,because the energy barrier of*CHOH formation is far larger than that of*C formation(2.95 vs.1.73 eV).In general,*OH and*C are disadvantageous of*CH dehydrogenation,but*O is benefit of*CH further reaction and formation of*CHO(0.93 vs.1.13 eV).Therefore,*CH prefers to be oxidized to*CHO by*O,rather than being hydroxylated to*CHOH or dissociated to*C.The energy barrier of*CO from*CHO dehydrogenation (Ea=0.20 eV) is lower than that of *CH2O (Ea=0.65 eV)or*CHOH formation(Ea=1.18 eV).Therefore,*CHO hydrogenation is unlikely.

*H is formation from CHxdehydrogenation,which it prefers to absorb on hollow site with the adsorption energy of ?2.75 eV.H2occupies on the top site with the adsorption energy of ?0.48 eV,which the energy barrier of*H+*H →*H2+*is 0.55 eV.

3.2.KMC simulations

The section explores the reaction pathway,the conversion and activity of DRM reaction under the reaction conditions on Ni4/α-MoC(111).Then,the differences of DRM on Ni4/α-MoC(111)and defective Ni4/MgO(100)are compared.All the reactions identified in the DFT calculations were included in the KMC simulations(Table 2).

Table 2 The forward energy barriers(Eaf ),reverse energy barriers(Ere )and reaction energies(ΔE)of elementary reaction involved in DRM on Ni4 /α-MoC(111)

Fig.3.The reaction pathways of DRM at temperatures ranging from 873 K to 1173 K at 0.1 MPa with a CO2 :CH4 ratio of 1.

Fig.3 is the reaction pathways of DRM at temperatures ranging from 873 K to 1173 K at 0.1 MPa with a CO2:CH4ratio of 1.KMC results show that*CO and*O are formation from CO2scission.Then,*CO is easily escaped from the active sites at the reaction condition,and*O occupies a hollow site seeing Fig.2.As a result,there are two reaction pathways for*CH4dehydrogenation.One is*CH4+*→*CH3+*H;the other is*CH4+*O →*CH3+*OH.The further dehydrogenation of*CH3produces*CH2and*H2O with*OH assistance.At the meantime,the reaction of*CH3+*→*CH2+*H occurs.Due to low energy barrier of*O+*H+*CH →*CH2+*O(0.03 eV),there is only one pathway of*CH2further dehydrogenation (*CH2→*H+*CH).For *CH further reaction,*CH nonoxidative dehydrogenation to *C(*CH+* →*H+*C) occurs.It should be noted that some*CHO is formation from*CH oxidation,but the amount of*CHO decreases at high temperature(Fig.S1).The result is accordance with the surface coverage,which there are some*O at low temperature,but*O is disappearance(Fig.S2).Finally,some*CO is observed from*CHO dehydrogenation at low reaction temperature.

For*C elimination,the reverse Boudouard reaction is the main pathway of*C elimination on Ni4/α-MoC(111),of which 80%*C is eliminated as shown in Fig.4;the other*C is eliminated by carbon oxidation;carbon hydroxylation has no effect on*C elimination.In addition,increasing reaction temperature slightly improves the percent of reverse Boudouard reaction for*C elimination,but the percent of carbon oxidation for*C elimination slightly decreases.Previous studies on Ni(hkl)surfaces proposed that the carbon deposition was from*CH dissociation or Boudouard reaction[46,47].However,no*C2is observed,indicating that there is no carbon deposition on Ni4/α-MoC(111).The result is in accordance with defective Ni4/MgO(100)[28].

Fig.4.The percent of*C elimination of DRM at temperatures ranging from 873 K to 1173 K at 0.1 MPa with a CO2 :CH4 ratio of 1.

According to the KMC simulations,CO,H2,and H2O are detected at 0.1 MPa with a CH4:CO2ratio of 0.1 MPa and ranging from 873 K to 1173 K;no methanol and other product are observed.Increasing temperatures from 873 K to 1173 K is able to enhance the overall TOF of CO,H2and H2O,where TOF of CO and H2increase drastically seen in Fig.5.CO and H2are the major product.In addition,increasing temperatures also benefit of H2/CO ratio.It should be noted that both CH4andCO2are sources for CO formation,and CH4provides the only hydrogen source to produce both H2and H2O.Because of H2O formation,H2/CO ratio is less than 1 in the range of reaction temperature.Comparing with our previous results[28],TOF of H2O on Ni4/α-MoC(111)is obviously larger than that on Ni4/MgO(100)especially at high temperature.The reason is that*O coverage on defective Ni4/MgO(100)is far larger on Ni4/α-MoC(111),so TOF of*H2O on defective Ni4/MgO(100)is far larger on Ni4/α-MoC(111).As a result,H2/CO ratio on Ni4/α-MoC(111)is obviously larger than that on defective Ni4/MgO(100)(0.6～0.8 vs.0.34～ 0.36).The result shows that α-MoC(111) is benefit of high H2/CO ratio.

3.3.Electron structure

Table 3 shows the bader charge of Ni4/α-MoC(111)and defective Ni4/MgO(100)surfaces(the detail model seeing Ref.[28]).The bader charge of all Ni atoms on α-MoC(111)surface are negative,indicating that electrons transfer from the surface to four Ni atoms.However,for defective Ni4/MgO(100)surface,three Ni atoms are positive;only the Nidatom is negative.The total bader charges of Ni4cluster are positive,indicating that some electrons transfer from four Ni atoms to the surface.The result shows that α-MoC(111)and defective MgO(100)surface have converse electron effect for Ni4cluster(electronic effect).

Then,the d-band structures of Ni4cluster on defective MgO(100)and α-MoC(111)surface are studied.As shown in Fig.6,the d-band of Ni4on α-MoC(111) surface is slightly wider than that on defective MgO(100),but the intensity of Ni4on α-MoC(111) is slightly lower than that on defective MgO(100).The d-band centers of Ni4on α-MoC(111)and defective MgO(100)are ?1.02 and ?1.12 eV,which is obviously closer to the Fermi level than that of metal Ni(?1.42 eV)[48].

Fig.5.Turnover frequency(TOF)of(a)CO,H2 ,and H2O and(b)the H2 /CO ratio for the DRM reaction on Ni4 /α-MoC(111)from 873 K to 1173 K at 0.1 MPa with a CO2 :CH4 ratio of 1.

Table 3 The bader charge of Ni4 cluster on α-MoC(111)and defective MgO(100)surface

Compared to defective Ni4/MgO(100) surface,the use of α-MoC(111)support has little effect on the favorable pathway.Big differences are observed for the stability of reaction intermediates.Ni4/α-MoC(111)is able to provide stronger binding to the reaction intermediates of *CH3,*CH2and *CH compared to defective Ni4/MgO(100)(Table S1),but it is able to provide weaker binding to the reaction intermediates of*C,*Hand*O,which is associated with the so-called strong metal–support interaction(SMSI)[52–56].As a result,an extensive tensile strain is greatly released in the Ni4(?5.79%)by MgO,but the extensive tensile strain is introduced in the Ni4(1.29%)due to the adoption of α-MoC lattice (structural effect),which are accompanied by the upshifted Ni d band center from ?1.42 eV for Ni(111)to ?1.02 and?1.12 eV for Ni4/α-MoC(111)and defective Ni4/MgO(100).This is in agreement with extensive previous studies,showing that the compressed strain or the shortened metal–metal bond length is likely to decrease the binding ability of metal sites by shifting the corresponding dband away from the Fermi level[57,58].

Fig.6.The PDOS of Ni4 cluster on α-MoC(111)(a)and MgO(100)(b)surface.

4.Conclusions

The reaction of DRM over Ni/α-MoC catalyst was systemically studied by combining DFT and KMC simulations,then the results are compared with Ni/MgO catalyst.The results show that SMSI of α-MoC and MgO have converse electron and structural effect for Ni cluster.As a result,Ni/α-MoC is able to provide stronger binding to the reaction intermediates of*CH3,*CH2and*CH compared to Ni/MgO,but it is able to provide weaker binding to the reaction intermediates of*C,*Hand*O.

CO,H2and H2O are the main product.Compared with Ni/MgO catalyst,O coverage on Ni/α-MoC decreases.As a result,TOF of H2O decreases,on the contrary,TOF of H2increases.Therefore,H2/CO ratio on Ni/α-MoC catalyst is obviously larger than that of on Ni/MgO catalyst,meanwhile,TOF of CO on Ni/α-MoC catalyst is slightly larger than that of on Ni/MgO catalyst.There are no obvious difference of reaction pathways on Ni/α-MoC and Ni/MgO catalysts except for *CH oxidation,which some*CH is oxidizated and formed as*CHO at low reaction temperature.In general,DFT and KMC shows that the Ni/α-MoC is good catalyst for DRM reaction compared with Ni/MgO catalyst.

Acknowledgements

The authors would like to thank the National Natural Science Foundation of China (21776197 and 21776195),Shanxi Province Science Foundation for Youths(201701D211003)and Key Research and Development Program of Shanxi Province (International Cooperation,201903D421074)for their financial support.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.05.009.