MAS NMR Studies on the Formation and Structure of Oxygen Vacancy on the CeO2 {110} Surface under a Reducing Atmosphere

The surface oxygen vacancy of metal oxide is a type of important catalytic active site. To investigate the origin and nature of oxygen vacancy, CeO2 nanorods with exposed {110} facets were treated at different temperatures under H2 atmosphere. 1H MAS NMR, 31P MAS NMR with trimethylphosphine (TMP) as the probe molecule, in situ XPS, and in situ EPR were employed to characterize the surface oxygen vacancy of the reduced samples. The results show that the decrease of the hydroxyl group content and the increase of Ce3+ concentration occur gradually with the increase of reduction temperature. When the temperature reaches 350 °C, the surface and bulk oxygen vacancies of CeO2 are generated. The longitudinal relaxation time (T 1) and the motion of 31P nuclei from TMP molecules adsorbed on the CeO2 surface were found to be significantly reduced with the formation of surface oxygen vacancies. Based on the characterization results, the adsorption models of probe molecules on the surface of the samples are established and the amount of surface oxygen vacancies can be estimated quantitatively.