Surface Reduction Mechanism of Cerium-Gallium Mixed Oxides with Enhanced Redox Properties

The doping of CeO2 with different types of cations has been recognized as a significant factor in controlling the oxygen vacancies and improving the oxygen mobility. Thus, the catalytic properties of these materials might be determined by modifying the redox properties of ceria. A combined experimental and theoretical study of the redox properties of gallium doped cerium dioxide is presented. Infrared spectroscopy and time resolved X-ray diffraction were used for temperature programmed reduction (H-2) and oxidation (with, O-2 and H2O) studies. Additionally, X-ray absorption near edge spectroscopy shows that only Ce4+ is reduced to Ce3+ in the ceria-gallia mixed oxides when annealed up to 623 K. The oxygen storage capacity (OSC) measurements show a, pronounced, enhancement on the reduction of ceria by gallium doping. Theoretical calculations by density functional theory (DFT) confirm the higher reducibility of gallium doped ceria oxides and give a molecular description of the stabilization of the doped material. On the basis of infrared spectroscopic measurements, a novel mechanism is proposed for the surface reduction of Ce4+ to Ce3+ where Ga-H species are suggested to be directly involved in the process. In addition, the reoxidation by H2O was precluded in the gallium doped ceria oxide.