Study On The Reaction Mechanism Of Ethanol Synthesis From Syngas On Cu Catalyst

Ethanol, as a clean energy with a broad application prospect, could be synthetized via catalytic conversion of syngas. Cu-based catalysts have become an attractive option because of its low price; however, few studies have been carried out to fully understand the mechanism of ethanol formation on Cu-based catalysts at the fundamental level due to the complexity of the reactions.In recent years, with the rapid development of computing hardware and theoretical calculation methods, the quantum chemical calculations have been widely used to investigate the transition metal catalyzed reaction system. In this paper, based on density functional theory (DFT) together with the periodic slab model, we have investigated the reactioin mechanisms of ethanol formation from syngas on Cu catalysts and discussed the factors affecting the ethanol formation, and provided the necessary condition for ethanol formation Cu-based catalysts, the conclusions obtained in the following:On Cu(111), Cu(110) and Cu(100) surfaces, the direct CO dissociation can not compete with CO hydrogenation, CO hydrogenate mainly to form CHO rather than COH formation both in kinetics and thermodynamics. Among all the CHx(x=1-3), CH2and CH3are the most favorable monomers on Cu(111), and Cu(110) surfaces; CH3is the most favorable monomer on Cu(100) surface.On Cu(111), Cu(110) and Cu(100) surfaces, the most formation mechanism for CH3OH formation is:(1) CHO+Hâ†'CH2O;(2) CH2O+Hâ†'CH3O;(3) CH3O+Hâ†'CH3OH, during the CO hydrogenation to form CRx. They all show good selectivity for CH3OH formation and CHx formation can not compete with CH3OH formation.The research, which is about the formation of C2oxygenate on Cu catalysts, show that on Cu(111) surfaces, C2oxygenate are formed via CO insertion into CH2and CH3to form CH2CO and CH3CO, but CH2coupling and hydrogenation are favorable in kinetically and thermodynamics than CO insertion into CH2to form C2oxygenate to form CH2CO, CH3hydrogenation is favorable in kinetically and thermodynamics than CO insertion into CH3to form CH3CO; on Cu(110) surface, C2oxygenate is formed via CO insertion into CH2to form CH2CO; on Cu(100) surface, C2oxygenate are formed via CO and CHO insertion into CH3to form CH3CO and CH3CHO, but CH3hydrogenation is favorable in kinetically and thermodynamics than CO and CHO insertion into CH3to form CH3CO and CH3CHO.On Cu(111) surface, the formation mechanism for ethanol formation via CH2CO hydrogenation is:(1) CH2CO+Hâ†'CH2CHO;(2) CH2CHO+Hâ†'CH2CHOH;(3) CH2CHOH+Hâ†'CH3CHOH;(4) CH3CHOH+Hâ†'CH3CH2OH, the formation mechanism for ethanol formation via CH3CO hydrogenation is:(1) CH3CO+Hâ†'CH3CHO;(2) CH3CHO+Hâ†'CH3CH2O;(3) CH3CH2OH. On Cu(110) surface, there are two parallel formation mechanism for ethanol formation via CH2CO hydrogenation, One is:(1) CH2CO+Hâ†'CH2CHO;(2) CH2CHO+Hâ†'CH3CHO;(3) CH3CHO+Hâ†'CH3CH2O;(4) CH3CH2O+Hâ†'CH3CH2OH, and the other is:(1) CH2CO+Hâ†'CH2CHO;(2) CH2CHO+Hâ†'CH2CHOH;(4) CH2CHOH+Hâ†'CH3CHOH;(3) CH3CHOH+Hâ†'CH3CH2OH. On Cu(100) surface, the formation mechanism for ethanol formation via CH3CO hydrogenation is:(1) CH3CO+Hâ†'CH3CHO;(2) CH3CHO+Hâ†'CH3CH2O;(3) CH3CH2O+Hâ†'CH3CH2OH, the formation mechanism for ethanol formation via CH3CHO hydrogenation is:(1) CH3CHO+Hâ†'CH3CH2O;(2) CH3CH2O+Hâ†'CH3CH2OH.Cu(111) surface, to achieve ethanol from syngas, must suppress CH3OH formation, CH2coupling and hydrogenation, CH3hydrogenation and/or promote CHx(x=2,3) formation, CO insertion into CHx(x=2,3) to form C2oxygenate; Cu(110) surface, to achieve ethanol from syngas, must suppress CH3OH formation and/or promote the CH2formation; Cu(100) surface, to achieve ethanol from syngas, must suppress CH3OH formation, CH3hydrogenation and/or promote the CH3formation, CO and CHO insertion into CH3to form C2oxygenate.