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

Surface characterization of V-P-O catalysts used in the oxidation of n -butane and n-pentane to maleic and phthalic anhydrides is reported. T wo different states for the same catalyst are selected: after a shea t ime under reaction conditions (100 h), the 'non-equilibrated' system, and after a much longer time-on-stream (1000 h), the 'equilibrated' ca talyst. Characterization has been carried out by following the desorpt ion of molecules with a mass spectrometer after catalysts had been in contact with the hydrocarbons and/or O-2 mixtures at 613 K. Valence st ate of vanadium under environmental conditions similar to those used i n the desorption studies was obtained by XPS. It was concluded that bo th V-V and V-IV are capable of interacting with the hydrocarbon and th at not all the V-IV present at the (VO)(2)P2O7 surface can interact. O n the other hand, desorption of molecules resulting from such interact ion occurs at lower temperatures from V-IV-rich surfaces('equilibrated ' catalyst), than from the V-V richer surface ('non-equilibrated' cata lyst). (C) 1998 Elsevier Science B.V.