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 Co0, 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 Co0 site is CH3CH2OH → CH3CH2O → CH3CHO → CH3CO → CH3+CO, and the final products are CO and H2. H2 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 Co0, 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 H2 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 H2.