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Abstract

In situ X-ray absorption spectroscopy (XAS) and in situ X-ray photoelectron spectroscopy (XPS) have been applied to study the active surface of vanadium phosphorus oxide (VPO) catalysts in the course of the oxidation of n-butane to maleic anhydride (MA). The V L3 near edge X-ray absorption fine structure (NEXAFS) of VPO is related to the details of the bonding between the central vanadium atom and the surrounding oxygen atoms. Reversible changes of the NEXAFS were observed when going from room temperature to the reaction conditions. These changes are interpreted as dynamic rearrangements of the VPO surface, and the structural rearrangements are related to the catalytic activity of the material that was verified by proton transfer reaction mass spectrometry (PTR-MS). The physical origin of the variation of the NEXAFS is discussed and a tentative assignment to specific V-O bonds in the VPO structure is given. In situ XPS investigations were used to elucidate the surface electronic conductivity and to probe the ground state of the NEXAFS spectra.