Mechanistic Insights of CO2 Reduction to Methanol: DFT Study

Abstract

Theoretical calculations were performed, using density functional theory (DFT), to estimate the energetics of the elementary steps involved in the conversion of CO2 to methanol on the CeO2 (110) surface. Due to high oxygen storage capacity and mixed ionic electronic conductive nature, ceria based materials have been suggested to enhance the electrocatalytic activities in solid oxide electrolysis cell. However, no study so far has highlighted the role of extended ceria surface alone in carrying out the CO2 reduction reaction. The energetics of suggested mechanisms, via formate (HCOO) and carboxyl (COOH) intermediates, for CO2 hydrogenation to methanol were studied on stoichiometric and reduced ceria surface. Calculations were performed to determine the most favorable adsorption orientation and corresponding binding energy of reaction intermediates. The adsorbed formate intermediate species on the stoichiometric ceria surface, were observed to be stable with higher binding energy (-223 kJ/mole) as compared to the carboxyl (Ebinding = -37 kJ/mole), and is likely to be a spectator. In order to produce methanol via formate mediated routes, HCOO requires to be subsequently hydrogenated to H2COOH which ultimately needs to dissociate into H2CO and OH. The dissociative elementary step of this route is significantly endothermic (DErxn=64 kJ/mol). On the other hand, the mechanistic routes involving carboxyl (COOH) shows all the way exothermic steps on the CeO2 (110) surface, except only to the dissociation of COOH into CO and OH, to produce methanol. The dissociation step is thermoneutral (DErxn~5 kJ/mol) on stoichiometric ceria and slightly endothermic (DErxn=24 kJ/mol) on reduced ceria. Activation barrier for the dissociation step (COOH → CO+OH) is estimated to be 127 kJ/mole, which is significantly higher as compared to the other expected rate limiting, CO2 hydrogenation (CO2+H→COOH) step (Ea=37 kJ/mole). We will extend this work to analyze the activation barriers on reduced ceria surface which will ultimately give an inference of the effect of oxygen vacancy on the reduction of CO2 to hydrocarbons.