The DFT And ReaxFF Reactive Force Field Study On Higher Alcohols Synthesis From Syngas Over Cu/Co Bimetallic Catalysts

Higher alcohols synthesis from syngas could be one of effective way to help ease the energy and environment crisis,among the four kind of catalyst systems,the modified F-T synthesis Cu/Co catalyst is regarded as one of most promising catalyst systems due to mild reaction condition,high yield of C₂+OH.But after years of research,the defects of low selectivity of total alcohol,lots of side reactions and lack of stability are not overcome yet.Based on density functional theory(DFT)and reactive force field theory(Reax FF),this paper circumfuses the conversion of syngas on Cu/Co bimetallic surfaces and systematically investigates the specific role of Cu/Co bimetallic site during the C₂+OH synthesis to provide the theoretical guide of catalyst design.A comprehensive reaction network related to syngas conversion on bimetallic surface is built for the three surfaces considered:Cu9@,Cu8@?1 and Cu6@,there is no evidence of facilitation of bimetallic sites for those elementary reactions,and methanol is main product on those sites.Except for the Cu6@surface,the precursors of C₂+OH could not be synthesized on the bimetallic sites.However,the reaction barriers of insertion of CO and CHO into CH_x are quite low on those surfaces meaning that the precursors of C₂+OH could be generated by CH_x species diffuse from Co-rich sites which may be the mechanism of C₂+OH synthesis on Cu/Co bimetallic catalyst.To apply the reactive molecular dynamics(r MD)method to study the syngas conversion on Cu/Co bimetallic surface,the force field parameters containing Cu/Co/C/H/O elements should be developed with corresponding train set where transient state is the key part.An innovative TS search framework is put forward based on“pathway creation-MD energy scan-energy minimization”idea where a more effective pathway creation method:“energy scan with coordinate control”is proposed,making it possible for Reax FF to search the TS precisely.With the TS searching method,the Cu/Co/C/H/O Reax FF parameters with total interactions are developed.Using annealing r MD method,the configuration transformations of Cu/Co bulks are studied under various temperatures and compositions.The results show that all Cu/Co bulks with Cu/Co composition from 1:3 to 3:1 would experience the process from“shell-core structure”to“bipolar structure”,and the transformation of Cu surface is suppressed by the existence of Co making the Cu surface more stable and part of Cu sites are maintained the same as the origin type as Cu(100).The reactive molecular dynamics are conducted for the CHO and CH₃O conversions on Cu/Co single and bimetallic surfaces.For the pure Cu site,the high H coverage leads to the formation of CH₂O,while low H coverage leads to the dissociation of CHO.The H coverage also influences CH₃O+H process by accelerating the reaction under high H coverage.The existence of Co in the Cu-rich surface could turn the reaction pathway from CH₂O synthesis to CHO dissociation,and the functions of Cu and Co are conformed from molecular level:the Cu-rich site facilitates the hydrogenation of CH_xO,while the Co-rich site facilitates the dissociation of CH_xO.The sublayer Co atoms could stabilize the Cu-rich surface and change the catalysis property to Co-rich site,and the effect of Co increases with the rise of temperature.