High temperature preparation of vanadium phosphate catalysts using water as solvent

Abstract

Vanadium phosphate catalysts prepared in aqueous solution at elevated temperature (145 °C) using either H3PO3 or V2O4 as reactants are described and discussed. This methodology produces catalysts with a much higher surface area (ca. 20 m2g−1) compared with those prepared using aqueous routes using HCl as reducing agent (ca. 4 m2g−1). The materials were characterised using a combination of powder XRD, BET surface area measurement, laser Raman spectroscopy, TGA, electron microscopy and 31P spin echo mapping NMR spectroscopy. Refluxing the precursors in water prior to activation was crucial in obtaining high surface area materials, and 31P spin echo mapping NMR together with electron microscopy data indicate that the water reflux step influences the relative amounts of V4+ and V5+ phases present in the catalyst, as well as reducing the size of the crystallites. A correlation between the activity of the catalyst and the surface area is observed. However, a small group of catalysts display a higher activity than that expected from this correlation, and this increased activity is discussed in terms of the interaction of V4+ and V5+ phases.