X-ray-induced transformations on the surface of MoVNbTe mixed oxide catalyst: An XPS study

MoVNbTe mixed oxide, which is used as a catalyst for the oxidative dehydrogenation of ethane, was investigated by the XPS technique during prolonged treatment with X-rays. This treatment caused a modification of the MoVNbTe oxide surface related to the reduction of V, Mo, and Te species. This behavior is supposed to be related to the interaction of oxide with secondary electrons emitted during X-ray absorption. Firstly, the reduction of Te4+ into Te0 species, accompanied by the removal of no more than ~ 5% of overall oxygen from the surface/subsurface region, was found during ~ 8.5 h under X-rays. After ~ 28 h of exposure, the significant surface restructuring resulted in a change in the relative amounts of V and Te. This was accompanied by a high degree of surface reduction and the appearance of V3+ species. Under X-rays, an abnormal broadening of Mo 3d spectral lines was observed in contrast to other photoelectron regions. The possible reasons for this phenomenon were discussed. The limit for the amount of lattice oxygen, which might be extracted with the preservation of the pristine surface structure of MoVNbTe oxide, was in the range from 5 to 7%.