Theoretical Studies About The Effect Of Crystal Facet On The Product Selectivity Over Co₂C Catalyst In The FTS Reaction

Co₂C formation was seemed as a sign of the catalyst deactivation or non-active species in FTS.Recently,the FTS experimental studies have shown that Co₂C catalyst is considered to be an active phase,one is for the formation of lower olefin,the other is for the formation of higher alcohol,and the last is for the synergy effect between Co and Co₂C.However,it is not clear what effect of crystal facet structure on the catalyst performance of Co₂C catalysts.Therefore,the density functional theory?DFT?were employed to systematacially investigate the selective conversion of syngas over Co₂C catalysts at the electronic-molecular level,which include CO activation to form CH as well as FTS reaction to form C₂ oxygenates and hydrocarbons over Co₂C catalysts.This work provides a newly physical insight for the design of Co₂C catalysts to improve the selectivity of desired products in FTS reaction.The main conclusions in this paper are obtained as follows:1.The effect of crystal facet structure of Co₂C catalysts for the CO activation to form CH was clarified.These results indicate that Co₂C?011?facet prefers to be CO direct dissociation;Co₂C?101?facet prefers to be CO direct dissociation and H-assisted CO dissociation via CHO intermediate,which are parallel and favorable pathways over Co₂C?101?facet;Co₂C?010?facet prefers to be H-assisted CO dissociation via COH intermediate;Co₂C?110?and?111?facets prefer to be H-assisted CO dissociation via CHO intermediate.Aiming at providing a physical insight to illustrate the relationship between the sensitivity of CO activation and Co₂C crystal facets,the structural properties of different Co₂C crystal facets and the TSs structures for CO direct dissociation over different Co₂C crystal facets are examined.when Co₂C crystal facet formed the active unit which is suitable for four coordinations of C and three coordinations of O,simultaneously C and O atoms are in the dissociation state adsorbing at adjacent 3-fold sites in the TSs,this crystal facet is easy to reduce the reaction barrier of CO direct dissociation,and is beneficial to CO activation.Therefore,the crystal facet structure of Co₂C catalysts has sensitivity for CO activation.2.The effect of crystal facet structure of Co₂C catalysts for the selectivity of FTS reaction to form C₂ species was clarified.On the Co₂C?101?facet,the dominant monomer CH self-coupling to C₂H₂ is the dominant product;on the Co₂C?011?facet,CH₃ monomer is the dominant CH_x species,and CO insertion into CH₃ contributes to CH₃CO;on the Co₂C?010?and?110?facets,CH and CH₂ species are the dominant form of CH_x species,CH₂ coupling with CH to CH₂CH is the dominant product;on the Co₂C?111?facet,CO insertion into the dominant monomer CH can form CHCO.The effective barrier is furether employed to quantitatively evaluate the selectivity,these results shown that Co₂C?011?and?111?facets have the high selectivity toward C₂ oxygenates;whereas Co₂C?101?,?010?and?110?facets exhibit high selectivity toward C₂hydrocarbons.Simultaneously,Co₂C crystal facet with the B₅-type active unit have high selectivity toward C₂ oxygenates;Co₂C crystal facets without the special B₅-type active unit exhibit high selectivity toward C₂hydrocarbons.Thus,the sensitivity of Co₂C crystal facet is a cause to affect the selectivity of desired products in FTS reaction.3.It was clarified that Co₂C catalyst is the multi-functional catalyst with multi-active centers and sites.On the Co₂C catalyst,part of the crystal facets are responsible for the CO direct dissociation or H-assisted dissociation to form CH_x species,which further react to form C₁ species or C₂ hydrocarbons;another part of the crystal facets are responsible for CO non-dissociated adsorption;when CH_x species is formed on the crystal facet of Co₂C catalyst,adsorbed state CO would further insert into the CH_x species to form C₂ oxygenates or even alcohols.These results are clarified that Co₂C catalyst is active species in FTS:one is for the formation of lower olefin,and the other is for the formation of C₂ alcohol.The results revealed that the microcosmic reasons is clarified that the crystal facet structure of Co₂C catalysts has sensitivity for selective conversion of syngas.The results described the structure-activity relationship between the crystal facet structure and the properties of catalysts at the electron-molecular level.This work provides a newly physical insight for the design of Co₂C catalysts to improve the selectivity of desired products in FTS reaction.