Emergence of Oxygen Reduction Activity in Partially Oxidized Tantalum Carbonitrides: Roles of Deposited Carbon for Oxygen-Reduction-Reaction-Site Creation and Surface Electron Conduction

摘要

Tantalum-oxide-based oxygen-reduction-reaction (ORR) catalysts have the ORR activity only when they are synthesized from tantalum carbonitrides. Namely pure Ta2O5 does not show ORR activity, and carbon or nitrogen in precursors is inevitable to form ORR active sites and to promote ORR. To clarify this reason, we investigated structural and surface electronic properties of tantalum-oxide-based catalysts, which were synthesized by gradually oxidizing tantalum carbonitride under low partial pressure of oxygen at 1000 °C, by using X-ray absorption spectroscopy, transmission electron microscopy, and Raman spectroscopy. The results indicate that oxidized tantalum carbonitrides with high ORR activity had oxygen vacancies near the surface, indicating that the vacancies could work as an active site for ORR. We also found that carbon was deposited on the oxide's surface, during oxidation of tantalum carbonitrides. The deposited carbon seems to play two important roles in formation of oxygen vacancies (ORR active sites) providing reductive atmosphere, and in producing electron conduction paths on rather insulating oxides' surface.