Theoretical Studies On The Catalytic Performance Of Direct Ethanol Synthesis From Syngas On Cu-based Catalysts

Nowadays, ethanol synthesis from syngas (CO+H2), syngas is obtained from extensive sources, meanwhile, ethanol has several perfect properties:it is clean, producible from renewable sources, and has a high energy density; therefore, it has been widely investigated. Meanwhile, Cu-based catalysts have become a new type of catalyst in ethanol synthesis from syngas. As a result, probing into the mechanism of ethanol synthesis from syngas on Cu-based catalysts, understanding the key step for the reaction, and obtaining the role of second metal promoter for Cu-based catalysts in the catalysis reaction, and proposing method to improve the selectivity to ethanol have become the key problem to solve in ethanol synthesis from syngas.In this study, density functional theory (DFT) method together with periodic slab model are employed to firstly investigate ethanol synthesis from syngas on Cu(211) surface; then, ethanol synthesis from syngas on the second metal promoter doped MCu(211)(M=Ni, Rh, Pd, Pt, Mn, Fe) surface are investigated, and the role of the second metal promoter are illustrated in comparsion with the results on Cu catalyst; further, bansed on the results of ethanol synthesis from syngas on Cu-based catalysts, ethanol synthesis from methane-syngas are proposed; lastly, the catalytic performance for ethanol synthesis from methane-syngas on the second metal promoter M(M=Ni, Rh, Pd, Pt) doped MCu catalysts have been discussed. The main conclusions are as follows:(1) For ethanol synthesis from syngas on Cu(211) surface:CH3is the most favorable monomer among all the CHx(x=1-3), and CH3is formed via CO+3Hâ†'CHO+2Hâ†'CH2O+Hâ†'CH3O+Hâ†'CH3+OH, then, CO insertion into CH3to form CH3CO, followed by successive hydrogenation to produce ethanol. The rate-limiting steps of ethanol synthesis are CH3O+Hâ†'CH3+OH and CO+CH3â†'CH3CO. Further, the hydrogenation, dissociation and coupling of CH3, as well as CO/CHO insertion into CH3show that CO insertion into CH3to CH3CO is the most favorable pathway, which means that Cu(211) surface exhibits a better selectivity to C2oxygenates. Meanwhile, CH3OH is formed by the reaction of CH3O+Hâ†'CH3OH. More importantly, it can be found that Cu(211) surface shows a better catalytic performance for CH3OH formation rather than CH3, thus, ethanol selectivity is low due to less CH3sources and more CH3OH formation. As a result, to achieve high selectivity for ethanol, Cu has to get help from the promoters and/or supports, which should be able to facilitate CH3formation and/or decrease CH3OH formation.(2) For ethanol synthesis from syngas on the second metal promoter M=Ni, Rh, Pd, Pt, Mn, Fe doped Cu-based catalysts:On MCu(211)(M=Ni, Rh, Pd, Mn) surface, CO+3Hâ†'CHO+2Hâ†'CH2O+Hâ†'CH3O is the optimal pathway for the initial CO hydrogenation; on MCu(211)(M=Pt, Fe), CO+3Hâ†'CHO+2Hâ†'CHOH+Hâ†'CH2OH is the optimal pathway for the initial CO hydrogenation. On MCu(211)(M=Ni, Rh, Pd, Pt, Mn), CH3is the most favorable monomer among all the CHx(x=1-3); on FeCu(211), both CH2and CH3are the most favorable monomers among all the CHx(x=1-3). On MCu(211)(M=Ni, Pd, Mn), CH3is formed via CH3Oâ†'CH3+O; on RhCu(211), CH3is formed via CH3O+Hâ†'CH3+OH; on MCu(211)(M=Pt, Fe), CH3is formed via the reaction of CH2OH+Hâ†'CH3+OH; on FeCu(211) surface, CH2is formed via CH2OHâ†'CH2+OH.Starting with CH3as initial state, on MCu(211)(M=Ni, Rh, Pd, Pt), C2oxygenates is dominantly formed via CO insertion into CH3, subsequently, CH3CO is further hydrogenated to ethanol via CH3COH and CH3CHOH intermediates; on MCu(211)(M=Mn, Fe), C2oxygenates is dominantly formed via CHO insertion into CH3, and CH3CHO is further hydrogenated to ethanol via CH3CH2O intermediate. On the other hand, on MCu(211)(M=Ni, Rh, Pd, Mn), CH3OH is formed via CH3O+Hâ†'CH3OH, and on MCu(211)(M=Pt, Fe), CH3OH is formed via CH2OH+Hâ†'CH3OH. Further, on MCu(211)(M=Ni, Rh, Pd, Pt), CH3hydrogenation to CH4is more favorable than CO insertion into CH3to C2oxygenates, namely, the second metal promoter Ni, Rh, Pd, Pt doping cannot enhance the selectivity of ethanol. On MCu(211)(M=Mn, Fe), the rate-limiting steps of ethanol formation are CH3formation and CH3CHO formation. The promoter Mn can decrease the activation barrier of CH3and CH3CHO formation, however, CH3OH formation is still more favorable than CH3formation, therefore, the promoter Mn cannot enhance the selectivity of ethanol. After doping promoter Fe, both CH2and CH3are the most favorable monomers, and they are belong to competitive reactions, once CH2is formed, CH2immediately couples to form C2H4; while CH3is associated with CHO to CH3CHO, namely, C2H4and ethanol are dominant products on FeCu(211) surface, namely, promoter Fe cannot increase the selectivity to ethanol.The study of ethanol synthesis on the second metal promoter M(M=Ni, Rh, Pt, Mn, Fe) doped Cu-based catalysts indicated that although promoter M(M=Ni, Rh, Pt, Mn, Fe) can decrease the activation barrier of partial key steps in ethanol synthesis from syngas, CH4is still easily formed on M(M=Ni, Rh, Pd, Pt) doped Cu-based catalysts, CH3OH and C2H4are easily formed on M(M=Mn, Fe) doped Cu-based catalysts, therefore, the second metal promoter doped Cu-based catalysts cannot well solve the problem of low selectivity in ethanol synthesis form syngas, we must seek alternative method.(3) By analyzing the catalytic performance for ethanol synthesis from syngas on the second metal promoter M(M=Ni, Rh, Pd, Pt, Mn, Fe) doped Cu-based catalysts, it can be found that on MCu(211)(M=Ni, Rh, Pd, Pt) surface, CH4is easily dissociated, thus, we think if CH4can be introduced into syngas, CH3sources can be solved, as a result, an rational design strategy for ethanol formation via methane-syngas route has been proposed.(4) The rate-limiting step for ethanol synthesis from methane-syngas on MCu(211)(M=Ni, Rh, Pd, Pt) surface is CO insertion into CH3; meanwhile, on NiCu(211), the main product is hydrocarbons; on RhCu(211), the main product is ethanol; on PdCu(211), the mian product is methanol, and on PtCu(211), the main product is methanol and ethanol. Most importantly, RhCu(211) surface exhibits a better catalytic activity for ethanol synthesis among all the MCu(211)(M=Ni, Rh, Pd, Pt) surfaces in methane-syngas route, namely, Rh is the excellent promoter in ethanol synthesis from methane-syngas.(5) Most elementary reaction about ethanol synthesis from syngas on Cu(211) and MCu(M=Ni, Rh, Pd, Pt) surface occurs at the step edge of (211) surface, which is consistent with the properties of (211) surface:Containing (100) step, which exhibits better catalytic activity. Meanwhile different type of metal promoter may lead to quite different reaction route, which will regulate the catalytic activity of corresponding reactions.