Studies On The Newly Assisted Catalytic Mechanism Of Key Intermediates CH_x(x=1-3) And OH In The Syngas Conversion Reaction Over The Co And Ni Catalysts

For the reaction mechanism of syngas conversion to generate C₂₊hydrocarbons and alcohols,it is generally accepted that the key intermediates CH_x（x=1-3）are mainly formed through two paths:one is the direct dissociation of CO into C and O,and followed by the continuous hydrogenation of C to produce CH_x（x=1-3）;the other is the CO hydrogenation to produce CH_xO（x=1-3）or CH_xOH（x=1,2）,then the C-O bond scission of CH_xO（x=1-3）or CH_xOH（x=1,2）to generate CH_x（x=1-3）and OH intermediates.Above two paths,as CH_xspecies are formed,O and OH intermediates are simultaneously generated.Since syngas conversion always takes place under hydrogen-rich conditions,O or OH species are easily hydrogenated to produce OH or H₂O.In recent years,numerous studies have shown that the key intermediates of the reaction not only participate in the reaction as reactant species,but also can serve as co-adsorption promoter to influence the catalytic performance.Therefore,the exploration of the micro mechanism of key intermediates on catalytic performance is of great significance for understanding the syngas conversion reaction on metal catalysts.In this work,the Ni-catalyzed CO methanation and Co-based FTS to C₂hydrocarbons as research objects,which serve as the typical reaction systems in the syngas conversion to hydrocarbons.The new assisted catalytic mechanism of the key intermediates OH and CH_x（x=1-3）in syngas conversion reaction has been elucidated by density functional theory,which enriches the catalytic basic theory of syngas conversion reaction,and provides a new method and a new theoretical basis for regulating the catalytic performance of syngas conversion reaction.The main conclusions obtained are as follows:1.The effect of two action modes of surface OH intermediate（co-adsorption promoter or hydrogenating species）on the catalytic performance of the syngas conversion to hydrocarbons has been exploried,and the promotion mechanism of surface OH intermediate in the Ni-catalyzed CO methanation and Co-based FTS has been clarified.（1）The different action modes of surface OH intermediate on Co and Ni catalysts（co-adsorption promoters or hydrogenating species）affect the activity of CO activation to form CH_x（x=1-3）intermediates.The main action mode of surface OH intermediate depends on the type of metal catalysts:the surface OH intermediate on Ni catalyst is mainly served as co-adsorption promoter to show high catalytic performance for the CO methanation reaction;the surface OH intermediate on Co catalyst is mainly served as hydrogenating species to show high catalytic performance.（2）The surface OH intermediate will affect the activity and selectivity of the CO methanation reaction on Ni catalyst.Surface OH intermediate could promote methane production which served as co-adsorption promoter and hydrogenating species.Surface OH intermediate served as co-adsorption promoter could promote the formation of methanol,but served as hydrogenating species would suppresses methanol production.Namely,the different effects of surface OH intermediate can improve methane formation activity and/or selectivity.（3）The surface OH intermediate served as co-adsorption promoter to affect the activity and selectivity of C₂ species formation on Co catalyst.Starting from the reaction of the main monomer CH to form C-C bond,the main monomer CH on OH/Co catalyst is prone to generate C₂ hydrocarbons（C₂H₂）rather than hydrogenation to generate methane or C₂ oxygenates.Namely,the surface OH intermediate could improve the activity and selectivity of C₂ hydrocarbon generation.（4）The analysis of the electronic properties about the Co and Ni catalysts which under the function of surface OH intermediate,including Bader charge,differential charge,density of state,d-band center and HOMO-LUMO orbital,the results confirm that the surface OH intermediate can make the d-band center of the Co and Ni catalysts far away from the Fermi level,reduce the adsorption energy of reactive species,improve the ability of the metal atoms on the catalyst surface to excite electrons,and promote CO activation,as well as accelerate key intermediate CH_x generation with high catalytic performance.2.The effect of the surface CH_x（x=1-3）as co-adsorption promoter on the catalytic performance in the syngas conversion to hydrocarbons has been explored,and the promotion mechanism of surface OH intermediate in the Ni-catalyzed CO methanation and Co-based FTS has been clarified.（1）The adsorption of CH_x（x=1-3）intermediates on Co catalyst affect the selectivity and activity of CO conversion to CH_x intermediates.The CH_x（x=1-3）intermediates as co-adsorption promoter cannot change the main CH_x monomer type（CH）and improve the formation activity of the main monomer CH and CH_x（x=2,3）.Among them,CH/Co and CH₃/Co catalysts can improve the activity and selectivity of CH generation.（2）The adsorption of CH_x（x=1-3）intermediates on Co catalyst affect the selectivity and activity of the C₂ hydrocarbon generation.Starting from the main monomer CH to form the C-C bond reaction,surface CH_x（x=1-3）intermediates as co-adsorption promoter can improve the activity of C₂ hydrocarbon generation,especially CH/Co and CH₂/Co catalysts.At the same time,the CH₂/Co catalyst can improve the selectivity of the C₂ hydrocarbon generation.（3）The adsorption of CH_x（x=1-3）intermediates on Ni catalyst affect the selectivity and activity of the CO methanation reaction.Compared with Ni catalyst,CH₃/Ni catalyst can change the main path of methane generation,and it has the highest methane generation activity.The CH/Ni catalyst will suppresse the production of methanol which can improve the selectivity and yield of methane.（4）The analysis of the electronic properties about the Co and Ni catalysts which under the function of surface CH_x（x=1-3）intermediates,including Bader charge,differential charge,density of state,d-band center and HOMO-LUMO orbital,the results confirm that the surface CH_x（x=1-3）intermediates improve the ability of the metal atoms on the catalyst surface to excite electrons,and promote CO activation,as well as accelerate key intermediate CH_x generation with high catalytic performance.