MAS NMR Studies on the Formation and Structure of Oxygen Vacancies on the CeO₂ {111} Surface Under Reducing Atmosphere

The catalytic performance of CeO₂ 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 CeO₂ polyhedrons with exposed {111} facets were reduced at different temperatures with H₂. In situ XPS, ¹H MAS NMR, ³¹P MAS NMR with trimethylphosphine (TMP) as the probe molecule, ¹H–³¹P CP/MAS NMR, and ¹⁷O MAS NMR with H₂¹⁷O were performed to characterize the oxygen vacancies on the surface of the reduced CeO₂-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 CeO₂-polyhedron. Moreover, the longitudinal relaxation time (T1) of ³¹P nuclei of TMP adsorbed on the CeO₂-polyhedron surface is significantly reduced after the formation of surface oxygen vacancies. The adsorption models of TMP on the CeO₂-polyhedron surface are established according to characterization results, and the content of surface oxygen vacancies is obtained quantitatively.