X-ray photoelectron spectroscopic study on γ-Bismuth molybdate surfaces on exposure to propene and oxygen

γ-Bismuth molybdate (γ-Bi2MoO6) surfaces reduced by C3H6, reoxidized by oxygen, on exposure to C3H6-jet and subsequently O2-jet were time- or temperature-dependently measured by X-ray photoelectron spectroscopy (XPS). The C3H6 molecule interacted simultaneously with two different lattice oxide ions to result in almost equivalent amounts of Mo5+ and Mo4+, but reduced species of bismuth were not found. The Mo5+ appeared likely to be formed by oxygen abstraction from Bi3+-O-Mo6+ and the Mo4+ from those of O = Mo6+ along with the elimination of two oxide ions bound on both sides of the [MoO2]2+ layer. A part of Mo5+ was reoxidized faster than Mo4+, below 373 K, while above 473 K Mo4+ was reoxidized preferentially. On exposure to C3H6-jet and O2-jet under ultrahigh vacuum (UHV), only Mo5+ was detected on the surface and the core spectrum intensity of Mo5+ oscillated with respect to exposure time. The oscillation phenomena were interpreted by the time lags between the surface reduction rate with C3H6 and the diffusion rate of oxygen from the oxide bulk, and by the difference between the surface reoxidation (oxygen incorporation) rate and the diffusion rate of newly incorporated oxygen into the bulk anion vacancies. It was also clarified that incorporation of oxygen occurred on the lattice oxide ion vacancies bridging Bi and adjacent Mo atom.