MAS NMR Studies on the Formation and Structure of Oxygen Vacancies on the CeO<sub>2</sub> {111} Surface Under Reducing Atmosphere
Qingying Yao, Hongxin Ding, Li Zhang, Wenhua Feng, Jinwen Zhang, Yuanhang Ren, Ye Lin, Xueying Chen, Bin Yue, Heyong He
Abstract
The catalytic performance of CeO 2 is closely related to its surface oxygen vacancy, and the formation of surface oxygen vacancies on different facets is expected to be varied significantly. To investigate the generation and properties of oxygen vacancies on the {111} facet with a high formation energy of oxygen vacancies, nanosized CeO 2 polyhedrons with exposed {111} facets were reduced at different temperatures with H 2 . In situ XPS, 1 H MAS NMR, 31 P MAS NMR with trimethylphosphine (TMP) as the probe molecule, 1 H– 31 P CP/MAS NMR, and 17 O MAS NMR with H 2 17 O were performed to characterize the oxygen vacancies on the surface of the reduced CeO 2 -polyhedron. The results indicate that the content of surface hydroxyl groups decreases gradually when the reduction temperature rises. Surface oxygen vacancies are generated at 350 °C and the maximum amount of surface oxygen vacancies are obtained at 400 °C. Further rising of the reduction temperature leads to the structural reconstruction on the surface of CeO 2 -polyhedron. Moreover, the longitudinal relaxation time (T1) of 31 P nuclei of TMP adsorbed on the CeO 2 -polyhedron surface is significantly reduced after the formation of surface oxygen vacancies. The adsorption models of TMP on the CeO 2 -polyhedron surface are established according to characterization results, and the content of surface oxygen vacancies is obtained quantitatively.