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Active Site Dependent Reaction Mechanism over Ru/CeO2 Catalyst toward CO2 Methanation
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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 2th, 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 CO2d-, (2) hydrogenation of CO2d- 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 CO2d- 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.

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