ELECTRICAL-PROPERTIES OF PURE VANADIUM PHOSPHATE PHASES AND OF VPO CATALYSTS USED IN THE PARTIAL OXIDATION OF N-BUTANE TO MALEIC-ANHYDRIDE

This work aims to correlate previously observed catalytic behaviour an d electronic conduction properties of various vanadium phosphate phase s active in the mild oxidation of n-butane to maleic anhydride. The pu re VOPO4 vanadium phosphate phases (alpha1, beta, gamma and delta) wer e found to be extrinsic n-type semiconductors with the presence of sub stitutional V4+ ions acting as acceptor centres. Alternate exposure to oxygen and butane indicated redox processes at the surface: the reduc tion of the surface by n-butane creates anionic vacancies which are su bsequently filled by gaseous oxygen. The most active and selective cat alysts, denoted 'VPO', resulting from activation of the precursor unde r the reacting mixture, comprise a (VO)2P2O7 matrix, initially support ing a dispersed V5+ phase. These become more conducting as the time of activation increases due to the progressive dissolution of V5+ ions i nto the bulk of the (VO)2P2O7 matrix by substitution with V4+ ions. Al l the results were in agreement with those obtained with other physico chemical techniques (X-ray diffraction, in situ laser Raman spectrosco py, electron paramagnetic resonance).