Formation and characterization of nanostructured V—P—O particles in flames: A new route for the formation of catalysts

Abstract

A counterflow diffusion flame burner was used to produce nanophase vanadium-phosphorus oxide powders in a hydrogen-oxygen flame. Liquid precursors, i.e., VOCl3 and PCl3, were used as source materials in a 1:1 ratio. In situ formation processes were investigated at two temperatures by laser light scattering, by emission and absorption spectroscopy, and by collecting particles directly onto carbon-coated TEM grids. At the higher temperature, the collected powders are spherical particles about 30 to 50 nm in diameter. At the lower temperature, the powders collected are chain-like structures composed of particles 5 to 10 nm in diameter. Particles formed in the burner were collected also from the burner's flanges and from two auxiliary strips. Their crystalline phases and surface area were determined by x-ray diffractometry, FT-IR spectroscopy, and BET analysis by nitrogen desorption. These results indicate a strong influence of temperature on the crystalline phases of the powders. At the higher temperature, the powder collected is a mixture of VOPO4 · 2H2O and δ-VOPO4. This mixture forms Λ-VOPO4 upon subsequent reheating at 750 °C. At the lower temperature, the powders collected are a VOHxPO4 · yH2O phase and VO(H2PO4)2, and form β-VOPO4 and V(PO3)3, respectively, upon subsequent reheating at 750 °C.