γ-ビスマスモリブデート表面のAr⁺スパッタリングによる還元挙動と加熱およびO₂-Jetによる再酸化挙動

抄録

In situ XPS measurements of Ar⁺-sputtered γ-bismuth molybdate surfaces during heating under ultra high vacuum (UHV) and/or on exposure to oxygen stream (O₂-Jet) at 5×10^-6 Pa were dynamically performed. By Ar⁺-sputtering, Bi³⁺ or Mo⁶⁺ was reduced partially to Bi⁰ or Mo⁵⁺, respectively, and Mo⁶⁺ was reduced in preference to Bi³⁺. When the reduced surfaces were heated under UHV, Bi⁰ and Mo⁵⁺ began to be converted to the original states, and at 423 K they were completely oxidized. The reoxidation of Bi⁰ to Bi³⁺ proceeded smoothly for a long time. The reoxidation rate of Mo⁵⁺ decreased rapidly with time, although it was larger than that of Bi⁰ in the initial stage. Exposure of the reduced species to O_2-Jet prompted the reoxidation. The initial reoxidation rate for Bi⁰ or Mo⁵⁺ was 1.4 or 2.4 times that obtained during heating under UHV, respectively. Apparent activation energies for the reoxidation of Bi⁰ and Mo⁵⁺, which were calculated from the Arrhenius plots of the initial rates, were 24 and 30 kJ mol^-1 under UHV, whereas during the exposure to O_2-Jet they were 20 and 33 kJ mol^-1, respectively. These results were able to be explained in terms of the diffusible migration of lattice oxygen anions to anion vacancies.