X-ray photoelectron spectroscopic study on α- and γ-bismuth molybdate surfaces exposed to hydrogen, propene and oxygen

Abstract

In order to shed light on the redox behavior of γ- and α-bismuth molybdates (γ- and α-BiMos), the oxide surfaces time-dependently reduced by hydrogen and propene at 723 K and subsequently reoxidized by oxygen at 373–723 K were measured by using XPS. On the oxide surfaces reduced either by hydrogen or by propene, simultaneous generation of Mo 5+ and Mo 4+ was observed. At the initial stage, the generation of Mo 5+ occurred faster than Mo 4+. With exposure time, each of the cations increased up to almost equivalent amounts: 34–37 and 7–14% of total Mo atom for the γ- and α-BiMos, respectively, while no reduced bismuth species were found except for the formation of metallic bismuth on the γ-BiMo reduced by hydrogen for a long time at 723 K. In reoxidation of the reduced surfaces by oxygen, a part of the Mo 5+ was oxidized to Mo 6+ faster than the Mo 4+ below 373 K, while above 473 K, the Mo 4+ was oxidized preferentially. The results indicated that the lattice oxide ion bridging Bi 3+ and adjacent Mo 6+ was more active and mobile than the oxide ion doubly bonded to Mo 6+. The surface reduction accompanied the diffusion of lattice oxide ions from the bulk to the surface, while in the reoxidation, the incorporated lattice oxide ions migrated into bulk oxide ion vacancies and to the doubly bonded oxide ion vacancies on the Mo 4+ through the bulk.