Study by XPS and TPD of the interaction of n-pentane and n-butane with the surface of `non-equilibrated' and `equilibrated' V–P–O catalysts

Abstract

Surface characterization of V–P–O catalysts used in the oxidation of n-butane and n-pentane to maleic and phthalic anhydrides is reported. Two different states for the same catalyst are selected: after a short time under reaction conditions (100 h), the `non-equilibrated' system, and after a much longer time-on-stream (1000 h), the `equilibrated' catalyst. Characterization has been carried out by following the desorption of molecules with a mass spectrometer after catalysts had been in contact with the hydrocarbons and/or O 2 mixtures at 613 K. Valence state of vanadium under environmental conditions similar to those used in the desorption studies was obtained by XPS. It was concluded that both V V and V IV are capable of interacting with the hydrocarbon and that not all the V IV present at the (VO) 2P 2O 7 surface can interact. On the other hand, desorption of molecules resulting from such interaction occurs at lower temperatures from V IV-rich surfaces (`equilibrated' catalyst), than from the V V richer surface (`non-equilibrated' catalyst).