Reduction and Reoxidation of .GAMMA.-Bismuth Molybdate Surface During Ar+-Sputtering, Heating, and the Exposure to O2-Jet.

Abstract

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 O2-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 O2-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.