Yunpeng Guo ,Jie Feng ,Wenying Li,*

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

2 Training Base of State Key Laboratory of Coal Science and Technology Jointly Constructed by Shanxi Province and Ministry of Science and Technology,Taiyuan University of Technology,Taiyuan 030024,China

1.Introduction

The cheap supported Nicatalystshave high activity and selectivity for CH4/CO2reforming,but it is easily deactivated because of coke deposition[1-4].Based on considerable experimental researches,there was a consensus that the carbon formation could be obviously decreasing with the reduction of Ni particle size in this reaction[5-7].When Ni particle size was smaller than the critical size,the carbon deposition would disappear on the Ni catalyst.However,the value of critical size was still under controversy.Lercheret al.[8]indicated the critical size is 2 nm on the Ni/ZrO2catalyst.Tangetal.[9]and Zhangetal.[6]thought 10 nm should be that size on the Ni/γ-Al2O3and the Ni-Co/MgO,respectively.Kimet al.[10]considered the critical ensemble size was 7 nm on the Ni/Al2O3aerogel catalysts.These different critical size values were supposed to be due to their different supporters and reaction conditions.

By far,the metal particle size effect had been widely observed in CH4/CO2reforming.Rostrup-Nielsen[11]inferred the reaction of CH4/CO2reforming need 12 adjacent active sites on Ni particles,and CH4deeply dissociation need 16 adjacent active sites at least.Small Ni particles could not prove a large ensemble for the formation of carbon.Zhanget al.[6]believed CH4dissociation occurred on metal particles and CO2decomposition occurred on the supporter surface,C species from CH4dissociation can be oxidized by O species from CO2decomposition at the interfaces of metal particles with the supporters.The smaller Ni particles,the more interfaces for C species oxidation.Liet al.[12]found a similar mechanism that La2O2CO3from CO2adsorption on La2O3would timely participate to eliminate coke on the Ni/La2O3catalyst with a high metal dispersion.Jeonget al.[13]reported MgO shell could destroy Ni ensemble in the MgO-coated Ni catalyst,and the usual 2-D layered carbon deposition would change into the filamentous carbon species in CH4/CO2reforming on this catalyst.Kozlovet al.[14]used DFT method to investigate the process of CH4decomposition on several nanostructured Pd models,and found out the edges of catalysts surface which were more exposed in small Pd particles had higher activity than the steps position.However,the atoms on the edges and steps had same coordination number(7)in this investigation,which was conflicting with the widely accepted opinions that low-coordinated atoms had high catalytic activity[15-17].Therefore,the particle size effect was caused by multiple factors,and the effects of electronic and structural properties in this phenomenon remained elusive[18].

MgO could form a solid solution structure with NiO by any proportion,so Ni/MgO catalysts were easy to control the Ni particle sizes and widely used in CH4/CO2reforming[19-21].On the other side,the metal-support interaction(MSI)between the Ni particles and MgO supporters was considered to maintain the small sizes and resist the Ni sintering[22,23].However,the Ni electronic structure affected by MgO supporter was rare to discuss in this catalytic system.For nanoparticle supported catalysts,the excellent activity and selectivity derived from its special electronic or/and geometrical structures,which could be in fluence largely by MSI[24-26].Maetal.[27]believed the strengthened interaction between Ni and La in the LaNiAl catalyst could keep the Ni particles in 4-6 nm with high activity and stability in ethanol steam reforming.Valdenetal.[28]found CO was easy to oxidize on the Au particles with only sizes from 2 to 3 nm,and ascribed this to the electronic properties changing of the Au atoms contact with TiO2support.Cargnelloet al.[29]discovered that the turnover frequencies(TOF)of CO oxidation on Ni/CeO2catalysts had an inverse relationship with the perimeter of Ni particles,and the catalytic activities of the Ni atoms in the interface with CeO2were similar to those of noble metals.All of these showed the metal atoms contacting with the supporter performed important roles in the catalytic reaction,especially for the oxidant reactions.These metal atoms were concluded to affect the reaction of CH4/CO2reforming directly,because some researchers believed surface CHxoxidation was the rate-determining step of this reaction[30,31].

Based on previous experiment investigations,in this work,the Nix/MgO(100)system,in which 1,2,or 3 Ni planes(contain 4 atoms per layer)adsorb on the center of the MgO(100)slabs,is used to model the Ni/MgO(100)catalysts with different Ni particle sizes.The most stable adsorbed structures and the favorable reaction pathways in CH4/CO2reforming on different Nix/MgO(100)have been analyzed,and the regularity between the energy barriers of slow elementary steps and the Ni particles sizes has been found out.By the comparison of the electron transfer during these steps on different Nix/MgO(100),the essence of particle size effect has also been inferred.

2.DFT Method and Models

2.1.Computational method

CO2reforming of CH4on Ni x/MgO(100)is simulated by CASTEP(Cambridge Sequential Total Energy Package)program based on DFT method[32,33].The exchange correlation energy is treated with the Perdew-Burke-Ernzerhof(PBE)function that depended on the generalized gradient approximation(GGA)[34,35].Ultra-soft pseudopotential[36]used to describe the ionic cores and a plane wave basis with a cutoff energy of 400 eV,is devoted to expand the Kohn-Sham one-electron states[37-40].Brillouin zone integration is performed using the Monhorst-Pack scheme(2×2×1)[41].Configurations are optimized until the force,energy,and maximum displacement converged on 0.5 eV·nm-1,2×10-5eV per atom,and 2×10-4nm,respectively.All calculations of the energies and geometric optimizations consider the impact of spin polarization.Complete LST/QST method is used to locate the transition states,and the convergence criterion is set to rootmean-square force on atom is below 2.5 eV·nm-1[42].

For adsorption configurations,adsorption energy is defined asEads=E[S-Nix/MgO(100)]-E(S)-E[Nix/MgO(100)],whereE[S-Nix/MgO(100)]is the energy of the Nix/MgO(100)with the adsorbed specie,E(S)is the energy of the free species,and E[Nix/MgO(100)]is the energy of the clean Nix/MgO(100).As a result,a negativeEadsmeans the process of the adsorption is favorable thermodynamically.

2.2.Structure model

The MgO supporter surface is built in software as a three-layer MgO(100)slab with a p(3×3)super-cell and only the bottom layer of the slab is constrained to their crystal lattice positions.The lattice constant of MgO(100)is optimized to be 0.431 nm,which is consistent with the experimental value,0.424 nm[43].The neighboring slabs are separated in the direction perpendicular to the surface by a vacuum region of 2 nm.

Valdenetal.[28]believed that the supported Au cluster on TiO2with the thickness of 2 atoms had a suitable electron structure to catalyze CO oxidation,so it is deduced MSI effecting the metal cluster in a small thickness extent.In this paper,each layer of Ni cluster contains 4 Ni atoms locating on a same plane,and the initialNi4,Ni8,and Ni12 cluster has 1,2,and 3 layers,respectively.These structures are similar as Pacchioniet al.'s work[44,45].The Ni4,Ni8,and Ni12 clusters adsorbed on the center of MgO(100)surfaces separately,the optimized geometries are shown in Fig.1.Liuet al.[46]had calculated the deformation energies of Ni4/MgO in CH dissociation,and the numbers are obviously smaller than adsorption energies.So the atomic coordinates of Nix/MgO(100)are fixed during the process of CH4/CO2reforming in order to reduce calculation time cost.

Fig.1.Structures of Nix Supported on MgO(100)surface.Color-coding:Red,O atoms;Green,Mg atoms;Blue,Ni atoms.

3.Results and Discussion

3.1.MgO(100)supported Nix cluster

As Fig.1 shown,after geometry optimizing,the 4 Ni atoms of one layer are still located on a plane in Ni4/MgO(100)and Ni8/MgO(100).In Ni12/MgO(100),the surface unit has been deformed into a butter fly shape different from the bottom two layers.The structures of adsorbed metalclusters can be influenced by MSI,so it is inferred that the bottom two Ni layers in Ni12/MgO(100)are influenced obviously by MSI,but this interaction becomes weak in the surface Ni layer.

In order to shed light on the electronic interactions between the Ni clusters and MgO supporters,Hirshfeld charges of Nix are calculated and listed in Table 1.

Table 1Hirshfeld Charges(e)for Separated Layers of Ni x/MgO(100)

As can be seen in Table 1,the total charges of the Ni clusters in Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100)are-0.22,-0.26,and-0.30 e,respectively.As Ni cluster contains several Ni atoms,more charges will transfer from the MgO supporter to the cluster.Nevertheless,the averaged charges of one Ni atom are reducing by the increase of Ni cluster.Herein,4 Ni atoms in one layer are considered as a basic unit,the charges of surface layer in Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100)are-0.22,-0.10,and 0.01 e,respectively.As a result,only the Ni atoms in the two bottom layers can get charges from the supporter.

Moreover,the d-band structures of Nix/MgO(100)show in Fig.2,and the d-band center is calculated with the following equation[47,48]:

Fig.2.The PDOS of d-band for Nix/MgO(100).

where ρdrepresents the density of states projected onto the Ni atoms'd-band andEis the energy of d-band.The d-band centers of Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100)are-1.11,-1.51,and-1.65 eV,respectively.The d-band centers of Nix/MgO(100)downshift to the Fermi level with the increase of Ni cluster.Generally,the catalysts with a closer d-band center to the Fermi level could easily donate electron,which was conducive to catalytic activity for reforming reaction[46].Therefore,Ni4/MgO(100)is surmised to have the best activity,followed by Ni8/MgO(100)and Ni12/MgO(100).However,this inference should be further verified by the investigation of CH4/CO2reforming mechanism.

3.2.CH4/CO2 reforming on Nix/MgO(100)

As Fig.3 shown,CH4/CO2reforming includes the following elementary reactions:1)CH4adsorbs and sequentially dehydrogenate to CHx(x=0-3)species and atomic H;2)CO2chemisorbs and decomposes to adsorbed CO and atomic O;3)CHx(x=0-3)species oxidizes by atomic O to produce CHxO intermediates,then CHxO decomposes into CO and H.In our calculation,the pathways contain CHxOH formation(CO2reacts with surface H to form COOH then decomposes to CO and OH,which can oxidize CHxand form CHxOH)and dehydrogenation are less favorable to the CHxO pathways on different Nix/MgO(100),so they are not involved in this paper.

Fig.3.Simplified reaction process of CH4/CO2 reforming.

The computed adsorption energies of all the reactants,intermediates,and products on Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100)are summarized in Table 2,and the energy barriers and reaction energies of the most favored elementary reactions in CH4/CO2reforming are listed in Table 3.The adsorption configurations and the corresponding transition state structures in CH4/CO2reforming on Nix/MgO(100)are shown in Fig.4.

Table 2Adsorption energies of reactants,intermediates,and products in CH4/CO2 reforming over Ni x/MgO(100)at different Ni particle sizes

Table 3Energetics of the most favored elementary steps in CH4/CO2 reforming over Ni x/MgO(100)at different Ni particle sizes

3.2.1.CH4dissociation at three sizes of Ni particle

Combining with the data of Table 3,CH4dissociated adsorption is a common kinetic block on all three Nix/MgO(100)surfaces.For each corresponding single step in CH4dissociation,the energy barriers will raise by the increase of Ni particle sizes.The energy barrier of CH2decomposition on Ni8/MgO(100)is strangely high,which is caused by C atom transferring from the bridge site of the surface to the hollow site between two layers,but this step on the other two models is only involved the breaking of the CˉˉH bond.The barriers of CH dissociation on Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100)are 0.47,0.91,and 1.39 eV,and reaction energies are-0.01,0.76,and 0.55 eV,respectively.For Ni4/MgO(100),CH dissociation is kinetically favorable and has no thermodynamics block;For Ni8/MgO(100),the similar energy barrier and reaction endothermicity indicates that even if CH dehydrogenation occurs,the generated C species will be hydrogenated easily;For Ni12/MgO(100),the CH dissociation has highest energy barriers and medium reaction endothermicity.

As a result,CH4will be easier to dissociate on Nix/MgO(100)with small Ni particle.On Ni4/MgO(100),CH4is favorable to decompose into surface C.If produced C species cannot react with O species timely,it will accumulate rapidly and lead to catalyst deactivation.Comparing with Ni4/MgO(100),the dehydrogenation of CH4on Ni8/MgO(100)is unfavorable.The low energy barrier(0.15 eV)of the reverse reaction of CH dissociation means CH will be the main product of CH4dissociation on Ni8/MgO(100).The CH4dissociated adsorption has highest energy barrier on Ni12/MgO(100),so big Ni cluster will have lower catalytic activity for this reaction.

Fig.4.Geometries of adsorption configurations and transition states structures in CH4/CO2 on Nix/MgO(100).Color-coding:Red,Oatoms;Green,Mg atoms;Blue,Niatoms;Grey,C atoms;White,H atoms.

3.2.2.CO2decomposition

When a CˉˉO bond of CO2is parallel the surface NiˉˉNi bond,the CO2molecule will become bent configurations with theEadsof-1.35,-0.57,and-0.76 eV on Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100),respectively.On all three Nix/MgO(100),the processes of CO2chemisorption are exothermic and have low energy barriers,so these steps are easy to occur.CO2decomposition on Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100)need to overcome barriers of 0.04,1.45,and 0.43 eV,with reaction energies-0.19,0.14,and-0.03 eV,respectively.Comparing with CH4dissociations,CO2adsorptions and decompositions on Nix/MgO(100)are more favorable both kinetically and thermodynamically.On the other side,theEadsof O species are from-2.42 to-3.05 eV,indicating produced O have larger coverage on Nix/MgO(100).As a result,the O species production is not the bottleneck for CH4/CO2reforming.

3.2.3.Oxidation of CH or C and dissociation of CHO

As mentioned in Section 3.2.1,CH or C should be the products of CH4dissociation on Nix/MgO(100)models.Therefore,both CH and surface C are taken as the possible species to be oxidized to produce CO at last.

As shown in Table 3,the energy barriers of CH+O formation into CHO on Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100)are 2.06,1.39,and 3.28 eV,with reaction energies-0.46,-1.04,and 0.44 eV,respectively.Although the formation of CHO on Ni8/MgO(100)is most favorable kinetically,theEadsof CH+O is-8.57 eV,which is 1.10 eV less negative than the value of theEadsof separately adsorbed CH and O(-9.67 eV).It means the formation of CH+O on Ni8/MgO(100)is obviously unfavorable thermodynamically.Comparing with CH dissociation,the oxidant of CH is not favorable kinetically on all three surface.

Different from other two models,CO produced by surface C oxidation on Ni8/MgO(100)is adsorbed on the bridge site between two layers,and be denoted as CO′.C+O translation into CO(which is CO′on Ni8/MgO(100))need to overcome energy barriers of 1.47,1.66,and 2.24 eV on Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100),respectively.The energy barrier of CO′translation into CO on Ni8/MgO(100)is 0.53 eV,with reaction energy of-0.13 eV,so it is very easy to happen.

As competitive reactions,the formations of CHO are unfavorable than CH decomposition on Nix/MgO.Surface C reacting with O species are the main pathways of CHxoxidation on all three Nix/MgO(100)models.Similar to CHxdissociation,surface carbon oxidation are easier to run on smaller Ni particles.

3.2.4.Potential energy surface of CH4/CO2reforming on Nix/MgO(100)

Based on previous calculation results,the favorable reaction pathways of CH4/CO2reforming on Nix/MgO(100)are shown in Fig.5.The energy barriers of rate determining steps of CO2decomposition on Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100)are 0.52,1.45,and 1.15 eV,respectively.Comparing with the processes of CH4sequential dissociation or CHxoxidation,CO2can be easier to adsorb and decompose on Nix/MgO(100).As a result,the generation of surface O would not be the block of CH4/CO2reforming.

In general,CH4dissociated adsorption is always the RDS of CH4/CO2reforming on Ni catalysts[49,50].The energy barriers of CH4dissociated adsorption on Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100)are 1.96,2.24,and 3.45 eV,respectively.On Ni4/MgO(100)and Ni12/MgO(100),this step is the RDS.On Ni8/MgO(100),although the energy barrier of CH2dissociation is a little higher(0.07 eV),CH4dissociation is still a main obstruction of the whole reaction pathway.As shown in Fig.5,the energy barriers of CH4dissociation raise with the growth of Ni clusters on Nix/MgO(100),so it indicates that the smaller Ni particles are more efficient on supported Ni catalysts for CH4/CO2reforming.

As the precursor of carbon deposition,the generation of surface C from CH dissociation relates directly to the stability of Ni catalysts.Similar to CH4dissociation,the energy barriers of CH dissociation will raise with the growth of Ni cluster on Nix/MgO(100).On Ni8/MgO(100)and Ni12/MgO(100),this step is obviously endothermic,so the corresponding reverse reaction is more favorable kinetically.On Ni4/MgO(100),even the reaction energy is close to 0 eV,theEadsof C+O is-9.87 eV,which is 1.29 eV more negative than the value of theEadsof separately adsorbed C and H(-8.56 eV),indicating the attractive interaction of the co-adsorb surface C and H.This would be helpful for inhibiting the polymerization of carbon on Ni4/MgO(100).

It was reported that CHxoxidation to CHxO was also very important for CH4/CO2reforming,which was helpful to reduce the carbon deposition[30,31].Comparing with CH oxidations,surface C oxidations are more favorable on all three Nix/MgO(100).The energy barriers of surface C oxidation Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100)are 1.47,1.66,and 2.24 eV,respectively.Surface C will be easier to react with surface O on small Ni cluster,so supported Ni catalysts with small Ni particles can have better anti-carbon ability.

3.3.Discussion of charges transfer during key steps

As shown in Section 3.2,CH4dissociated adsorption has direct relationship with catalytic activity,and CH dissociation and C oxidation jointly decide the Ni catalyst stability,so these three elementary reactions are considered as the key steps of CH4/CO2reforming on Nix/MgO(100).Generally speaking,all three reactions will be easier to occur on Nix/MgO(100)with a smaller Ni cluster.In order to expound the reason of this phenomenon,Hirshfeld charges of Ni clusters and species of reactants and transition states in these three reactions are calculated and listed in Table 4.

For CH4dissociated adsorption,molecule CH4as reactants nearly do not get any charges from Ni clusters.Hirshfeld charges of CH3+H of transition states are-0.12,-0.20,and-0.14 e on Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100),respectively.At the same time,comparing with the clean Nix/MgO(100),the surface layers of Ni clusters lose 0.09,0.15,and 0.12 e on Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100),respectively.In the process of CH4activation,the Ni atoms in surface layer contact with the species directly and prove most electrons in this charge transfer.The rules of the charge transfer in CH dissociation and surface C oxidant are similar to CH4dissociated adsorption.

Fig.6 shows the relationship between the reduction of Nicharges on the surface layer and the increment of species charges.Because CH adsorbs on the hollow site between two layers on Ni8/MgO(100),it can get charges from both two layers'Ni during its activation.Besides this step,the slop of the fitting line is 0.99 and the corresponding intercept is 0.02.This means the charges transfer during the reactant activation just occurs between adsorbed species and their directly contacted Ni atoms,which are main in the Ni clusters'top-layer.

Fig.5.Potential energy surfaces of CH4/CO2 reforming over Ni x/MgO(100).

Table 4Hirshfeld charges for reactants and transition states of CH4 dissociation,CH dissociation and C oxidant over Ni x/MgO(100)

Based on potential energy surfaces and Hirshfeld charges analyses,the conceptual explanation of different catalytic performances on Nix/MgO(100)with different Ni particle size are shown in Fig.7.In this figure,the border colors of Ni atoms means different electron abundance,red means Ni atoms have most electrons,followed by yellow and light blue.As shown in Fig.7,only Ni atoms in bottom two layers can obtain electrons from the MgO(100)slab.When Ni cluster is smaller,the Niatomsin surface layer will have more electrons.During the reactants and intermediates activation,the charges transfers are occurring between adsorbed species and their directly contacted Ni atoms,which are main in surface layer of the Ni cluster.CH4/CO2reforming will occur easier when these Ni atoms have more negative charges.Surface layer's Ni atoms in the small particle capture more electrons from MgO slab,so Nix/MgO(100)with small Ni particle has better catalytic performance.

Nowadays,both experimental and theoretical studies paid attention on the special activities of the interface between metal particles and supporters.Liet al.[18]believed a bifunctional mechanism,which oxygenated hydrocarbons activated and transformed on metal surface and CO2or water activated on metal oxide site,prevailed in most reforming reactions.Hence,they deduced that the interface between the two sites was where the reforming reaction could occur.Liet al.[51]reported the Ceria-promoted Ni/SBA-15 catalyst with high Ni-CeO2interface could obviously enhance its catalytic performance and anti-coking ability in ethanol steam reforming.Valdenet al.[34]found that Au cluster on TiO2with the thickness of 2 atoms had high activity for CO oxidant,and it was similar to Ni cluster on MgO(100)in our work.In other words,MSI,which can change metals'electronic structures,is effective just in a distance with a few atoms.It is reported the Ni particle adsorbed on MgO(100)surface as a hemisphere,so only Ni atoms in the perimeters of bottom two layers are exposed and electron-rich,which will have high activity and stability for CH4/CO2reforming[52].If Ni particles are smaller,the proportion of these Ni atoms will be higher,which explains the high catalytic performance of the Ni/MgO catalysts with small Ni metal sizes.

Fig.6.Relationship between the reduction of the surface layer charges and the increment of the species charges during reactants activation in key steps.Color-coding:Red,CH4 dissociated adsorption;Blue,CH dissociation;Green:C oxidant.Shape coding:△,on Ni4/MgO(100);□,on Ni8/MgO(100);○,on Ni12/MgO(100).

On the other hand,the shape of noble metal particle will become flat by strong MSI interaction when it adsorbs on reducible supporters,so it can expose more atoms of bottom two layers[53,54].This might be one reason of high activity and stability of supported noble metal catalysts.If Ni particles can be prepared as a flat shape,this supported Ni/MgO catalysts will have better catalytic performance.

4.Conclusions

In this work,Ni4/MgO(100),Ni8/MgO(100),and Ni12/MgO(100),in which supports 1,2,and 3 layers of Ni4 plane on the centers of the MgO(100)slabs,respectively,are used to model the Ni/MgO catalysts with different Ni particle sizes.DFT calculations have been used to investigate the mechanism of CH4/CO2reforming on Nix/MgO(100)models.Comparing with the reaction mechanisms of CH4/CO2reforming on different Nix/MgO(100)models,it is found that the energy barriers of CH4dissociated adsorption,CH dissociation,and C oxidation will decrease by fewer layers of Ni cluster in Nix/MgO(100).By the analyses of Hirshfeld charges,electrons can only transfer from MgO supporter to Ni atoms in the bottom two layers,and the electrons transfer during elementary steps mainly occurs between adsorbed species and their directly contacted Ni atoms in the surface layer.The mechanism investigations on different Nix/MgO(100)indicate that CH4/CO2reforming is easier to happen on the electron-rich Ni sites.If Ni particles are small or become flat,electron-rich Ni atoms in the bottom two layers will be easy to expose on the particle surface,this catalyst will have great performance in CH4/CO2reforming.

Fig.7.Conceptual model of electron transfer between MgOslab,Niatoms and reactants in CH4/CO2 reforming on Ni x/MgO(100).The blue circle means Ni atom.

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