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.