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 0, Co 2+, and Co 3+ 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 0 site is CH 3 CH 2 OH → CH 3 CH 2 O → CH 3 CHO → CH 3 CO → CH 3 +CO, and the final products are CO and H 2 . H 2 is formed by the combination of adsorbed H species. On Co 2+ site, the mechanism is CH 3 CH 2 OH → CH 3 CH 2 O → CH 3 CHO, and the main final product is CH 3 CHO species. On Co 3+ site, the mechanism is CH 3 CH 2 OH → CH 3 CH 2 O → CH 3 CHO → CH 2 CHO → CH 2 CO → CHCO → CCO → COCO → CO → CO 2, and the final products are CO 2 and H 2 O. The rate-limiting step on Co 0, Co 2+, and Co 3+ sites is the form of CH 3 CHO species. The possible reasons for the different catalytic activities may be the following two facts: First, Co 3+ sites density in Co 3 O 4 (110)-A is larger than that of Co 2+ and tends to break the C–C bond to produce CO; second, Co 3+ binds more oxygen atoms that the further oxidation of ethanol requires, which leads to the full oxidation of ethanol to CO 2 on Co 3+ 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 CO 2 which is needed to react with H 2 O in the water–gas shift reaction generating CO 2 and H 2 .