In situ study of redox and of p-type semiconducting properties of vanadyl pyrophosphate and of V-P-O catalysts during the partial oxidation of n-butane to maleic anhydride
- Ecole Centrale de Lyon, Ecully (France)
- Institut de Recherches sur la Catalyse, Villeurbanne (France); and others
In situ electrical conductivity measurements have been performed on vanadyl pyrophosphate samples as a function of their preparation temperature and on V-P-O catalysts as a function of their activation time. In agreement with X-ray diffraction results, electrical conductivity has shown that heat treatment of vanadyl pyrophosphates between 450 and 800{degrees}C results in a less conducting state corresponding to a very crystalline form. Such a stabilized form can already be reached at a temperature between 600 and 700{degrees}C. For V-P-O catalysts, electrical conductivity decreased as a function of the activation time, tact, under the reaction mixture, reaching a steady value for tact > 84 h. Both types of solids were found to be p- type semiconductors with positive holes as the main charge carriers. During subsequent exposures to n-butane and to oxygen at reaction temperature (400{degrees}C), it was confirmed that vanadyl pyrophosphate constitutes the main phase of efficient V-P-O catalysts working with some excess of surface oxygen probably in relation with the presence of V{sup 5+} species. The recorded changes in conductivity upon exposure to reactants provide illustrative evidence of a redox (Mars and van Krevelen) mechanism. The long activation period necessary to produce a high yield of maleic anhydride corresponds to structural and chemical changes of the surface which accommodates the reaction mixture and gives rise to active and selective sites able to convert n-butane to maleic anhydride. 45 refs., 10 figs., 3 tabs.
- OSTI ID:
- 563503
- Journal Information:
- Journal of Catalysis, Vol. 167, Issue 1; Other Information: PBD: 1 Apr 1997
- Country of Publication:
- United States
- Language:
- English
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