In-situ electron spin resonance study of vanadium phosphate catalysts during the selective oxidation of n-butane to maleic anhydride

Differently disordered (VO)(2)P2O7 phases and an amorphous V3+PO catalyst were studied by electron spin resonance (ESR) spectroscopy at various temperatures and in various atmospheres, as well as under catalytic conditions, using an in-situ ESR flow reactor operating in the X band. Exchange integrals and the second and fourth moments of the ESR signals were used to characterize spin-spin exchange in (VO)(2)P2O7, which depends on the degree of structural disorder. The catalytic activity of (VO)(2)P2O7 and the maleic anhydride selectivity are enhanced with rising disorder. During butane oxidation a significant reversible alteration of the ESR line shape became evident. This unique effect can only be observed under working conditions and is discussed in terms oi a perturbation of exchange interactions between neighbouring vanadyl centres near the surface, the oxidation state of which is assumed to fluctuate between +4 and +5 during the catalytic reaction. In contrast, heating in pure air improves the spin-spin exchange and lowers the disorder by healing lattice defects. The activity of the V3+PO catalyst in the title reaction ''awoke'' after about 20 min time on stream, simultaneous with the formation of a (VO)(2)P2O7-like local arrangement of V4+ centres on the catalyst surface.