Effects Of Cu Catalyst Structure Modified By The Promoter Rh,Co And The Supporter Mo₂C On The Catalytic Performance Of Syngas Conversion To C₂ Oxygenates

Syngas conversion to ethanol is considered as one of the most promising synthetic route due to its wide source of raw materials and high energy utilization.Rh-based catalysts have high selectivity to ethanol with mild reaction conditions,but the CO conversion is low and the price is expensive,which limits it application in industry.Currently,Cu-baesd catalyst is widely used in the process of syngas conversion because of its low cost and availability,but the selectivity of ethanol is low and methanol is easy to be produced.The reason is that the C-O bond cleavage is difficult on Cu catalyst,which causing low concentration of CH_x.In order to improve the selectivity of ethanol,the focus of research were used promoters and surppoter to modify Cu catalysts to promote the formation of CH_x and C-C growth and to obtain highly efficient catalysts for ethanol synthesis.At present,Co,Rh and Mo₂C show better ability of C-C chain growth in syngas conversion and promote the CH_x formation.Therefore,Co and Rh were selected as promoter and Mo₂C as supporter to modify Cu catalysts to modify the catalytic performance of Cu catalyst for synthesis C₂ oxygenates precursor of ethanol from syngas in this paper.The effects of promoter Rh,Co and supporter Mo₂C on the key steps in the reaction mechanism of syngas synthesis of C₂ oxygenates were systematically investigated by DFT studies,including CO activation,CH_x formation,methane formation,methanol formation and C-C chain growth.The main conclusions are obtained as follows:1.The mechanism of synthesis C₂ oxygenates precursor of ethanol from syngas on Co and Rh modified Cu catalysts was clarified.The types of CO initial activation on Co and Rh modified Cu catalysts were CO+H→CHO.On Rh modified Cu catalyst,CH and CH₂ are preferred CH_x species and compete with methanol formation.On Co modified Cu catalyst,CH₂ is preferred CH_x monomer and its formation is easier than methanol.Meanwhile,on Cu catalyst,methanol formation is more favorable than CH_x.Therefore,compared to Cu catalyst,Co modified Cu catalyst can promote the formation of CH_x and inhibit the formation of methanol,while Rh modified Cu catalyst cann’t effectively promote the CH_x formation.For the C₂ oxygenates formation,CHO insertion into CH_x is more favorable route than CO insertion,namely,CHO insertion into CH_x is the main form of C-C bond growth.Meanwhile,on Rh and Co modified Cu catalysts,CHO insertion is the easiest than CH_x hydrogenation,CH_x self-coupling and CO insertion in kinetics.Therefore,Co monolayer modified Cu catalyst has excellent catalytic performance for synthesis of ethanol from syngas.2.The mechanism of synthesis C₂ oxygenates precursor of ethanol from syngas on Cu/Mo₂C catalyst was clarified.The mode of CO initial activation is CO hydrogenation to CHO and CO direct dissociation.Compared to the single Mo₂C catalyst,CO initial activation can be promoted by the addition of Cu to Mo₂C catalyst,especially for CO direct dissociation.CHO dissociation is the easiest than CHO hydrogenation to form CH₂O and CHOH,so CH is the preferred CH_x monomer on Cu/Mo₂C catalyst.Therefore,it can be concluded that CH formation on Cu/Mo₂C catalyst is easier than CH₃OH and the stable adsorption site of CH is on the interface of Cu/Mo₂C.The barriers of CHO insertion into CH is the lowest than CH self-coupling to C₂H₂,CH hydrogenation to CH₂ and CO insertion into CH to form CHCO,and the reaction of CHO insertion into CH occurs at the interface.Therefore,the activity of catalysts and the selectivity of C₂oxygenates can be improved by adding Cu clusters on Mo₂C supporter.