Electrical properties of pure vanadium phosphate phases and of VPO catalysts used in the partial oxidation of n-butane to maleic anhydride

Abstract

This work aims to correlate previously observed catalytic behaviour and electronic conduction properties of various vanadium phosphate phases active in the mild oxidation of n-butane to maleic anhydride. The pure VOPO₄ vanadium phosphate phases (α^II, β, γ and δ) were found to be extrinsic n-type semiconductors with the presense of substitutional V⁴⁺ ions acting as acceptor centres. Alternate exposure to oxygen and butane indicated redox processes at the surface: the reduction of the surface by n-butane creates anionic vacancies which are subsequently filled by gaseous oxygen. The most active and selective catalysts, denoted ‘VPO’, resulting from activation of the precursor under the reacting mixture, comprise a (VO)₂P₂O₇ matrix, initially supporting a dispersed V⁵⁺ phase. These become more conducting as the time of activation increases due to the progressive dissolution of V⁵⁺ ions into the bulk of the (VO)₂P₂O₇ matrix by the subsitution with V⁴⁺ ions. All the results were in agreement with those obtained with other physicochemical techniques (X-ray diffraction, in situ laser Raman spectroscopy, electron paramagnetic resonance).