CO2 Reduction to Methanol on CeO2(110) Surface: Mechanistic Insight

CO₂ reduction to methanol on CeO₂(110) surface: Mechanistic Insight

Neetu Kumari¹, M. Ali Haider¹, Nishant Sinha², Suddhasatwa Basu¹

¹ Chemical Engineering Department, Indian Institute of Technology, New Delhi, Delhi, India

² Dassault Systemes, Bangalore, India.

Numerical basis-set based ab-initio density function theory (DFT) calculations were performed to study the reaction mechanism for CO₂ reduction to methanol on the extended surface of CeO₂(110). Due to high oxygen storage capacity and mixed ionic-electronic conducting property, ceria have been suggested to play an active role in the catalytic and electrocatalytic reduction of CO₂.  In this study, energetics of probable reaction routes involving the formate (HCOO) and carboxyl (COOH) intermediates, have been analyzed to assess the formation of methanol on CeO₂(110) surface. Calculations were performed to determine the most favorable adsorption orientation and corresponding binding energy of reaction intermediates. On the stoichiometric ceria surface, adsorbed formate intermediate species were ascertained to be more stable with higher binding energy (-223 kJ/mole) as compared to the carboxyl (E_binding = -37 kJ/mole) and is likely to be a spectator. In order to produce methanol via formate mediated routes, HCOO is required to be hydrogenated to H₂COOH which subsequently dissociates into H₂CO and OH. The dissociative elementary step of this route is significantly endothermic (DE_rxn=64 kJ/mole). On the contrary, the mechanistic routes involving the carboxyl (COOH) intermediate shows all the way exothermic steps (Figure 1) on the stoichiometric CeO₂ (110) surface to produce methanol. The dissociation step (COOH → CO+OH) is thermoneutral (DE_rxn~5 kJ/mole) on stoichiometric ceria and slightly endothermic (DE_rxn=24 kJ/mole) on reduced ceria. Activation barrier for the dissociation step is estimated to be 127 kJ/mole, which is significantly higher as compared to that on the reduced ceria surface (E_a= 67.2 kJ/mole). The other suggested rate limiting step (CO₂+H→COOH), has lower activation barrier of 37 kJ/mole on ceria surface. Experimentally, electrochemical reduction of CO₂ has been carried out in solid oxide electrolysis cell (SOEC). Ceria with different dopants (Gd, Pr, and Sm) have been incorporated as cathode, for CO₂ reduced to CO and/or methanol.   

Reference: N. Kumari, et al., Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta. 2015.01.153

Figure  SEQ Figure \* ARABIC 1 Reaction energy diagram of CO₂ reduction to methanol on stoichiometric CeO₂(110) surface