(472e) First Principles Insight Into CO2 Hydrogenation to Methanol On Ceria Catalysts

Lo, C. S., Washington University in St. Louis
Cheng, Z., Washington University in St. Louis

CO2 catalytic hydrogenation to Methanol is of considerable importance in the energy industries and greenhouse gas mitigation. Although extensive experimental and theoretical efforts have been carried out based on metal catalysts such as Cu and Pd in the past decades, the metal oxide catalysts have not been well studied yet. The most fundamental questions on COhydrogenation on metal oxide, such as the reaction mechanisms, the key reaction intermediates and rate-determining steps are still in debate. Ceria (CeO2) is commonly used for oxidation and reduction reactions, since Ce ably and reversibly converts between Ce4+ and Ce3+ upon release and storage of oxygen. Our previous study has shown that CO2 molecule can be activated at oxygen vacancies of reduced ceria (110) by transferring a net charge of −0.923|e|, following which the surface is partially re-oxidized. Thus, reduced ceria (110) represents a promising catalyst for CO2 hydrogenation. In this present work, a comprehensive reaction network for CO2 hydrogenation to methanol on ceria (110) is studied using periodic density functional theory calculations and first principles kinetic Monte Carlo simulation. In all possible intermediates, our calculation results show that carbine diol, formic acid and methynol are not feasible due to the high formation energy. Also, direct formyl hydrogenation to formaldehyde (H2CO), the key intermediate for methanol synthesis, is not feasible due to the high activation barrier. Instead, we find that H-formalin to formaldehyde is kinetically more favorable. The formaldehyde then converts into methoxy (H3CO) rather than H2COH, followed by a consecutive hydrogenation step to form methanol. Thus, two reaction channels to methanol are identified (i) COOH pathway via a carboxyl intermediate and (ii) HCOO pathway via a formate intermediate. The rate-limiting step of each pathway and the theoretical selectivity is determined. This study of CO2 hydrogenation to methanol on Ceria (110) provides new insights into methanol synthesis chemistry and ceria catalyst application.