Carbon dioxide adsorption and activation on Ceria (110): A density functional theory study

Abstract

Ceria (CeO2) is a promising catalyst for the reduction of carbon dioxide (CO2) to liquid fuels and commodity chemicals, in part because of its high oxygen storage capacity, yet the fundamentals of CO2 adsorption and initial activation on CeO2 surfaces remain largely unknown. We use density functional theory, corrected for onsite Coulombic interactions (DFT+U), to explore various adsorption sites and configurations for CO2 on stoichiometric and reduced CeO2 (110). Our model of reduced CeO2 (110) contains oxygen vacancies at the topmost atomic layer and undergoes surface reconstruction upon introduction of these vacancies. We find that CO2 adsorption on reduced CeO2 (110) is thermodynamically favored over the corresponding adsorption on stoichiometric CeO2 (110). The most stable adsorption configuration consists of CO2 adsorbed parallel to the reduced CeO2 (110) surface, with the molecule situated near the site of the oxygen vacancy. Structural changes in the CO2 molecule are also observed upon adsorption, so that the resulting O-C-O angle is 136.9 degrees and the C-O bonds are 1.198 Angstroms and 1.311 Angstroms in length, respectively. The molecule bends out of plane to form a unidentate carbonate, as opposed to the bidentate carbonate found by other researchers for CO adsorption to stoichiometric CeO2. We deduce that charge transfer from reduced surface Ce3+ ions to the adsorbate to form the carbonate anion is the first step in the activation and reduction process and cleavage of the C-O bond.