Study On Reaction Mechanism For CO Methanation Over Mo-based Catalysts By Density Functional Theory

Natural gas as a kind of clean energy,has led to a sharp decrease in workable reserves due to the greatly increased global demand.Thereout,a new process to prepare substitute nature gas?SNG?from syngas by coal gasification presents a great application prospect.Among the methanation process of coal-to-synthesis gas,direct methanation process has the advantages of low cost and short process,in which the Mo-based catalyst is widely recognized for its high sulfur resistance and good direct methanation performance.The MoS₂ catalyst in pure Mo-S phase and the CoMoS catalyst in Co-Mo-S phase are generally believed to have good catalytic performance through experimental studies.However,there are difficulties in observing the crystal structure and nano-sized crystalline from experiment.In this article,by using of the quantum chemistry simulation,the active species of the MoS₂?010?and?110?surfaces were firstly analyzed by density functional theory.Based on these discussions,the mechanisms of adsorption and reaction were further investigated.For the purpose of prompting Mo-S phase,the adsorptions and reaction mechanisms of CO methanation over CoMoS catalysts were studied in detail.In addition,the activated basal plane?001?surface by applying strains has shown stable S-vacancy,on which the adsorptions and reaction mechanisms of CO methanation were herein investigated.In view of the layer structure of Mo-based catalysts,the active sites are usually located on the unsaturated edge surfaces.There are two kinds of edge sites have been recognized on MoS₂?010?cleaved surface,labeled as S-edge and Mo-edge,while there is only one kind on?110?surface.Since the methanation is conducted in the condition with atmosphere of H₂S,sulfur reconstruction will occur on the unsaturated edge surfaces,and the active sites on both?010?and?110?surfaces should be modified.During methanation reaction,CO is adsorbed on metallic site,while H₂ is dissociative adsorbed on metallic and S sites.The fracture of C-O bond is gradually carried out being facilitated by hydrogenation process.It was deduced that on S-edge,the stable mode of CO adsorption is the bridged adsorbed CO,and CH₂O adsorbate is formed in the second step of hydrogenation.The C-O bond cleavage occurs after CH₂OH being formed.On the Mo-edge,the S-vacancy is unstable and the methanation reaction process is similar with that on S-edge,while the only difference is the formation of CHOH in the second step.On the active site of?110?surface,the complete fracture of C-O bond is achieved by dehydroxylation with H₂O.The rate-controlling step for the whole reaction is CO hydrogenation for?010?surface but CH₂O hydrogenation for?110?surface.The low methanation activity of Mo-edge in Mo-S phase is mainly attributed to the difficulty in creating stable S-vacancy.In view of this,the promotion effect of Co on Mo-edge in CoMoS catalysts is investigated by presenting a group of Co substituted surface models in consideration of four situations:A1 with 0.25 Co/Mo substitution ratio,B1 and C2 with 0.50 Co/Mo substitution ratio in meta-position and ortho-position respectively,and D2 with 0.75 Co/Mo substitution ratio.In terms of adsorption mechanism,CO is most likely adsorbed on four-coordinate Co site in a linear configuration,and H₂ can be dissociated into Co-H and S-H by Co-S pairs.On the surface models of A1,B1 and C2,results from the reaction mechanism study show that the stable CH₃O and CH₃OH adsorbates will be formed.However,in the situation of D2,the reaction pathway will follow the path of CHOH?CH₂OH?CH₂+H₂O.The C-O bond cleavage is followed by the coadsorption of CH₃ and OH on the A1 model,while CH₄ is produced directly on the models of both B1 and C2.According to activation energy of rate-controlling step and reaction heat,a substitute ratio of 0.25?A1?and 0.50 in ortho-position?C2?can exhibit the highest catalytic activities,but a substitute ratio of 0.50 in meta-position?B1?and 0.75?D2?exhibit good stability.A strained two-dimensional nano-MoS₂ could present a coverate of about 11%stable S-vacancies,in which the active sites are included.According to a detailed investigation through adsorptions and reaction mechanism of CO methanation,a weaker C-H bond will be formed between C and adsorbed H in the first state.It was found that the rate controlling step is CO hydrogenation to CHO and the hydrogenation of CH₂ and CH₃ is an easy-to-activate elementary reactions.