Reaction Mechanism of Ethanol on Model Cobalt Catalysts: DFT Calculations

In the present work, the density functional theory calculations analysis are performed to study the reaction mechanisms and catalytic activity of ethanol reactions over Co degrees, Co2+, and Co3+ sites. Adsorption situations and the reaction cycles for ethanol reactions on cobalt catalysts were clarified. The mechanisms include the dehydrogenation steps of ethanol and the C-C cleavage step. The present calculation results show that the mechanism of ethanol reaction on Co degrees site is CH3CH2OH -> CH3CH2O -> CH3CHO -> CH3CO -> CH3+CO, and the final products are CO and H-2. H-2 is formed by the combination of adsorbed H species. On Co2+ site, the mechanism is CH3CH2OH -> CH3CH2O -> CH3CHO, and the main final product is CH3CHO species. On Co3+ site, the mechanism is CH3CH2OH -> CH3CH2O -> CH3CHO -> CH2CHO -> CH2CO -> CHCO -> CCO -> COCO -> CO -> CO2, and the final products are CO2 and H2O. The rate-limiting step on Co degrees, Co2+, and Co3+ sites is the form of CH3CHO species. The possible reasons for the different catalytic activities may be the following two facts: First, Co3+ sites density in Co3O4 (110)-A is larger than that of Co2+ and tends to break the C-C bond to produce CO; second, Co3+ binds more oxygen atoms that the further oxidation of ethanol requires, which leads to the full oxidation of ethanol to CO2 on Co3+ sites. The present result may help people to design an ESR (ethanol stream reaction) catalyst by controlling its oxidation state, and the catalyst with modest oxidation state is benefit for the H-2 formation. The proper catalyst should own the ability to break C-C to form CO but avoid the full oxidation of CO into CO2 which is needed to react with H2O in the water-gas shift reaction generating CO2 and H-2.