The oxygen vacancy on the surface of CeO₂ is one of the most interesting catalytic structures in the field of heterogeneous catalysis. In a variety of reactions (e.g. CO oxidation, water gas shift reaction, hydrogenation of CO/CO₂), the oxygen vacancies participate in the catalytic process via two key routes: (1) storing and releasing oxygen, and (2) promoting the dispersion degree and activity of supported noble-metal. Great efforts have been made to develop novel catalysts with abundant oxygen vacancies in CeO₂ by various methods. However, detailed understanding on the critical role of oxygen vacancies in the reaction mechanism (e.g., reaction pathway and rate-determining step) is still deficient. A team from Beijing University of Chemical Technology has gained insight into the defect structure of the support and corresponding catalytic mechanism. Their research has been published on May 2^th, 2016 in Journal of the American Chemical Society.

Scheme 1. Schematic illustration of the formate route for CO₂ methanation over the Ru/CeO₂ catalyst: (1) conversion of CO₂ to CO₂^d-, (2) hydrogenation of CO₂^d- to formate, (3) dissociation of formate to methanol, (4) hydrogenation of methanol to CH₄.

   Based on the Ru/CeO₂-NCs catalyst with preferably exposed (100) CeO₂ facet, the team used series of in situ techniques to study the oxygen vacancy and related structures on the support and their structural evolvements under real reaction conditions. The steady-state isotope transient kinetic analysis (SSITKA)-type in situ DRIFT infrared spectroscopy confirmed the catalytic mechanism of CO₂ methanation over oxygen vacancy at the molecular scale level.

   By using synchrotron radiation at BSRF, the detailed structural information of Ce³⁺ and corresponding structural changes during the catalytic reaction was studied. In the reduction process, the percentage of Ce³⁺ gradually increases, indicating the transformation from Ce⁴⁺ to Ce³⁺. However, in the reaction process, the percentage of Ce³⁺ declines significantly after the introduction of reaction gas. Combined with the result of in situ DRIFT, Ce³⁺ acts as Lewis base to adsorb CO₂, leading to the conversion from CO₂ to CO₂^d- and the resulting Ce³⁺ to Ce⁴⁺.

   This work reported an active site-dependent catalytic mechanism toward CO₂ methanation. By rationally using a series of in situ techniques, including XANES, IR, and Raman, it provides a feasible strategy to uncover the intrinsic structure-activity correlation for the exploration of heterogeneous catalysts. 

Article: Fei Wang, Shan He, Hao Chen, Bin Wang,^* Lirong Zheng, Min Wei,^* David G. Evans, Xue Duan, Active Site Dependent Reaction Mechanism over Ru/CeO₂ Catalyst toward CO₂ Methanation, J. Am. Chem. Soc. 2016, 138, 6298-6305.