Theoretical Study On The Effect Of Hydroxyl On C₂ Species Synthesis From Syngas Over Cu-based Catalysts

In recent years,Cu-based catalysts have been widely used in syngas conversion owing to the low cost,available preparation,flexible structure and easy adjustment.At present,the reaction mechanism of syngas-to-C₂ species on Cu-based catalysts has been extensively studied,which mainly includes two key steps:（1）The cracking of C-O bond:CO activation toCH_x（x=1-3）intermediates along with the formation of OH intermediates;（2）The formation of C-C bond:CO/CHO insertion intoCH_x（x=1-3）toC₂ oxygenates or its self-coupling toC₂ hydrocarbons.Our recent study has confirmed that CH_x（x=1-3）intermediates can be used as surface co-adsorbed promoter to faciliate syngas conversion toC₂ oxygenates,which is called as CH_x assisted catalytic mechanism;while it is generally believed that OH intermediate is mainly used as hydrogenation specie.Thus,for OH intermediate formed along with CH_x（x=1-3）formation,it is important to explore whether OH intermediate adsorbed on the surface of catalyst in syngas conversion still has the similar assisted catalytic mechanism with CH_x（x=1-3）intermediate.Based on the above issue,the density functional theory（DFT）calculation methods were employed to investigate the reaction mechanism of syngas-to-C₂species on Cu-based catalysts in this paper.The new mechanism in syngas-to-C₂species catalyzed by adsorbed OH intermediate as co-adsorbed promoter was elucidated.This new mechanism is called as OH-assisted catalytic mechanism,which opens a new vision for the assisted catalytic function of OH intermediate adsorbed on the metal catalysts that may be general in the reaction systems related to OH intermediate generation.The main conclusions are as follows:1.The formation mechanism and existence form of CH_x（x=1-3）intermediates on theCu-based catalysts modified by Co,Fe and Rh,as well as Cu-based catalysts with co-adsorbed OH intermediates were clarified,and the influence of OH intermediates on the catalytic performance of CH_x（x=1-3）formation from syngas was revealed.（1）In syngas conversion,the key intermediateCH_x（x=1-3）is produced by theC-O（H）bond cracking of CH_xO（x=1-3）and CH_xOH（x=1,2）,accompanied by the formation of O and OH intermediates.In the hydrogen-rich atmosphere,O is easily hydrogenated to OH;OH intermediate can exist stably on MCu catalyst and maintain dynamic equilibrium during the catalytic reaction.Thus,OH species can be stably adsorbed on the surface of MCu catalysts and form OH/MCu catalysts with special local microstructure and electronic properties.（2）OH intermediate adsorbed on the surface of MCu（M=Co,Fe,Rh）as co-adsorbed promoter and hydrogenation specie can promoteCO initial activation,especially as co-adsorbed promoter can effectively promoteCO hydrogenation toCHO.On OH/CoCu,OH can improve the activity of CHO formation as co-adsorbed promoter,and increase the activity of COH formation as hydrogenation specie,but OH acts as co-adsorbed promoter to promoteCHO formation is more favorable in kinetics and thermodynamics;On OH/FeCu,CHO formation activity is improved by OH intermediate as the co-adsorbed promoter and hydrogenation specie,while OH acts as co-adsorbed promoter is more favorable in kinetics and thermodynamics;On OH/RhCu,OH intermediate acts as co-adsorbed promoter to improveCHO formation activity.（3）OH intermediates adsorbed on MCu（M=Co,Fe,Rh）catalysts are accepted as co-adsorbed promoters to affect the activity and selectivity of syngas conversion toCH_x（x=1-3）.On CoCu,FeCu,RhCu and OH/CoC,OH/FeCu,OH/RhCu catalysts,the dominant CH_x（x=1-3）monomers areCH₃,CH₃,CH₂and CH,CH₂,CH₂,respectively.Compared with MCu catalysts,OH intermediates on OH/MCu catalysts can change the optimal formation paths and existence forms of the dominant CH_x（x=1-3）monomers,and improve the activity of syngas conversion toCH_x（x=1-3）.Compared with MCu catalysts,CH_x formation rates on OH/MCu catalysts increase sharply,while methanol formation rates increase slightly,especially the methanol formation rate decreases on OH/RhCu catalyst.At the same time,the energy barrier differences between CH_x and methanol decrease,and OH/MCu improve the selectivity of CH_x and inhibit methanol formation to a certain extent.2.The formation mechanism and existence form of C₂ species on theCu-based catalysts modified by Co,Fe and Rh,as well as Cu-based catalysts with co-adsorbed OH intermediates were clarified,the influence of OH on the catalytic performance of C₂ species formation was revealed.（1）OH species adsorbed on the MCu（M=Co,Fe,Rh）catalysts are accepeded as co-adsorbed promoter to affect the activity and selectivity of CH_x（x=1-3）.OnceCH_x（x=1-3）intermediate is formed,the subsequent C₂ species can be generated with high activity and high selectivity.Thus,CO activation to form CH_x（x=1-3）intermediate is the key step of the reaction.（2）The selectivity of C₂ species is also different on the OH/MCu catalysts.Co and Fe catalysts are commonly used for Fischer-Tropsch synthesis.On OH/CoCu and OH/FeCu,C₂ hydrocarbons and C₂ oxygenates are favorable products;Rh catalyst is mainly used for ethanol formation;on OH/RhCu catalyst,C₂ oxygenates are the main products.3.The analysis of electronic properties clarifies the influence of OH intermediate as co-adsorbed promoter on the catalytic performance of CO activation process.The presence of OH intermediate on the MCu（M=Co,Fe,Rh）catalyst alters the microscopic properties of CO activation process:the presence of OH species changes electron distribution of the second metal M in the MCu（M=Co,Fe,Rh）catalyst,and forms a unique electronic micro-environment,which promotes the electron transition of the metal d orbital to the 2π*anti-bonding orbital of CO molecule,and enhance the activity of CO activation to form CH_x（x=1-3） intermediate.