A DFT Study Of The Reaction Mechanism Of CH₄/CO₂ Reforming Affected By The Interface Features Of Ni/MgO Catalysts

CO₂ reforming of CH₄ could not only produce syngas with low H2/CO ratio（1/1）for F-T synthesis,but also be instead of the water-gas-shift reaction to control the component of syngas in the dual-gas poly-generation system.So this reaction is very important to both natural gas and coal chemical industries.The lack of suitable catalyst is the main obstacle with respect to the commercialization of this reaction.The Ni/MgO catalyst using NiO-MgO as the precursor has shown great catalytic activity and stability with CH₄/CO₂ reforming,so it is regarded as one of the most attractive catalysts for this process.In this study,several Ni/MgO models are built,and density functional theory（DFT）method is used to analyze the structures and energy barriers of CH₄/CO₂ reforming on those models systematically,and the mechanisms has been proposed on the basis of the computed kinetic parameters of the elementary steps.The relationship between the catalyst structures and catalytic performance on electron level is also investigated.The main research contents and conclusions in this paper are described as follows:1)Based on the experiment results,the Ni8 cluster supported on the MgO（100）slab has been used to the model Ni/MgO catalyst.The reaction mechanism of CH₄/CO₂ reforming on Ni8/MgO（100）shows CO₂ can decompose on a Ni8 cluster and produce O species whether through direct dissociation or hydrogenated dissociation.As a key intermediate produced by CH₄ dissociation,CH₂ can be oxygenated by O species（atomic O or OH）,generating CH₂ O or CH₂ OH,respectively;or dehydrogenate into CH,which is then oxygenated by atomic O,resulting in CHO dissociating.These three pathways can occur in parallel because of the similar energy barriers of their rate-determining steps.Atomic H from CH_x decomposition has the best reaction activity with surface C,followed by O and OH species produced by CO₂ dissociation.2)Ni clusters,Ni4,Ni8 and Ni₁2 supported on the MgO（100）slabs,have been adapted to simulate Ni/MgO catalysts.By using DFT calculation,the reaction pathway of CH₄/CO₂ reforming on Ni_x/MgO（100）models are used to investigate the particle size that might affect the reaction pathway.The reforming mechanisms of CH₄/CO₂ on different Ni_x/MgO（100）indicate the energy barriers of CH₄ dissociated adsorption,CH dissociation and C oxidation three factors are all declining with the decrease of the Ni cluster size.The Hirshfeld charges analyses of those three steps show only atoms of Ni_x cluster on bottom or second bottom layer can obtain electrons from the MgO supporter,and the main electrons transferring are occurring between adsorbed species and the Ni atoms in surface layer.The Ni atoms of surface layer in small Ni cluster will have more electrons and have better catalytic activity.As a result,the NiO/MgO catalysts with small Ni particle would expose more Ni atoms in contact with the MgO supporter,which might be the reason of metal particle size effect.3)Several Ni8/Ni_xMg_yO（100）are used to investigate the effect of supporter’s basicity to CH₄/CO₂ reforming.The reforming mechanisms of CH₄/CO₂ on different models indicate the energy barriers of CH₄ dissociated adsorption and CH_x oxidation are both going up with the increase of supporter’s Ni:Mg ratio,with the pyrolytic carbon formation easily.The electron structure analysis shows the direction of electron transferring would change to “metal to supporter” by the increasing of supporter’s Ni:Mg ratio but not “supporter to metal” on pure Ni/MgO.When Ni cluster is negative,electron transferring occurs between adsorbed species and Ni atom in surface layer of Ni cluster,and CH₂ is the main species for CH_x oxidation.When Ni cluster is positive,its electronic structure would be stable and electron transferring occurs between adsorbed species and Ni atom in surface layer of NiO-MgO solid solution,and CH is the main oxidative species.4)CO₂ can chemisorb on the clean MgO（1 1 0）,MgO（2 1 0）,MgO（2 1 1）and Mg atoms polarized MgO（1 1 1）[MgO（1 1 1）Mg] surfaces,and it can direct dissociate merely on MgO（1 1 1）Mg,but the surface would poisoned by strong adsorbed atomic O.CO₂ can easily adsorbed and hydrogenated dissociation on MgO（1 1 1）Mg surface,follows by MgO（1 1 0）,（2 1 1）and（2 1 0）surface.The DRIFTS experiment shows CO₂ can only dissociate by atomic H on 400 °C.In conclusion,the Ni atoms on the interface between the Ni particles and the MgO supporter can get electrons from MgO supporter,and these Ni atoms have high catalytic activity and stability.For Ni atoms in the interface,their electron structures are decided by the basicity of the supporter,and the amount is closely related by the Ni particle size.These two factors together have determined the catalysis’ performance.