Activation of carbon dioxide with surface oxygen vacancy of ceria catalyst: An insight from in-situ X-ray absorption near edge structure analysis

The concentrations of oxygen vacancy (O_v) and trivalent Ce ions (Ce³⁺) have been widely recognized as key reactivity descriptors to the CeO₂-based catalysts. Nonetheless, the O_v and Ce³⁺ concentrations of CeO₂-based catalysts are often determined by ex-situ characterizations without considering their interactions with reactants, a situation that is difficult to reflect their effective amounts participated in the catalytic process. In this study, a series of CeO₂ catalysts with varied O_v and Ce³⁺ concentrations were prepared by a hydrothermal method followed by calcination at different temperatures. These CeO₂ catalysts were characterized and assessed by the synthesis of dimethyl carbonate (DMC) from the reaction of CO₂ and methanol. Using in-situ X-ray absorption spectroscopy without and with CO₂ adsorption, the quantitative analysis of effective Ce³⁺ (associated with surface O_v) present in CeO₂ catalysts for DMC synthesis is achieved. These results shed light on the critical role of surface O_v in CeO₂ catalysts for non-reductive conversion of CO₂ as well as the importance of in-situ characterizations for effective O_v and Ce³⁺ concentrations in CeO₂ catalysts.