Enhanced Oxygen Vacancy Formation in CeO2-Based Materials and the Water–Gas Shift Performance

Abstract

The role of dopants (Sm, Tb and Pr) on the water–gas shift performance of CeO2-based materials was studied. Modification of CeO2 with Sm significantly improved the water–gas shift performance. The catalytic activities of doped CeO2 were increased when compared with the catalytic activities of pure ceria (65% conversion at 600 °C for Ce5%SmO and 50% conversion at 600 °C for CeO2). The key factors driving the water–gas shift performance were reduction behavior and oxygen vacancy concentration. In the redox mechanism of the WGS reaction, CeO2 plays a crucial role in transferring oxygen to CO through changes in the oxidation state. Therefore, Sm is effective in catalyzing the water–gas shift activity because the addition of Sm into CeO2 decreases the surface reduction temperature and alters the oxygen transportation ability through the redox mechanism. XRD results suggested that Mn+ (M = Sm, Tb and Pr) incorporate into ceria lattice to form a solid solution resulting in unit cell enlargement. The defect structure inside the CeO2 lattice generates a strain on the oxide lattice and facilitates the generation of oxygen vacancies. XANES analysis revealed that Sm reduced CeO2 easily by transporting its electron into the d-orbitals of Ce, thus giving rise to more Ce3+ at the CeO2 surface. The presence of Ce3+ is a result of oxygen vacancy. Therefore, the high content of Ce3+ provides more oxygen vacancies. The oxygen vacancy formation results in easy oxygen exchange. Thus, reactive oxygen species can be generated and easily reduced by CO reactant, which enhances the WGS activity.