Phase development and morphology during the thermal treatment of VOHPO4·0.5H2O

The thermal activation of VOHPO4·0.5H2O in O2, air, N2 or n-butane-air was studied using in situ X-ray diffraction and ex situ X-ray photoelectron spectroscopy and focussed ion beam microscopy with cross sectioning. In O2 or air, an amorphous solid forms when VOHPO4·0.5H2O is heated above 270 °C and a crystalline V5+ phase develops on heating above 350 °C. By contrast, during activation in n-butane-air or in N2, (VO)2P2O7 is formed at 300 °C without any intermediate amorphous phase. The interiors of VOHPO4·0.5H2O and (VO)2P2O7 particles were identical, with the platelet like morphology of the precursor preserved during activation. By contrast the interiors of particles from the air or O2 activation procedures had become dense and sintered like in appearance. Some oxidation of surface regions was observed during activation in n-butane-air. These results are discussed in terms of platelet type morphology of (VO)2P2O7 generating a resistance to overoxidation during activation and catalytic work.