Phase development and morphology during the thermal treatment of VOHPO ₄·0.5H ₂O

The thermal activation of VOHPO ₄·0.5H ₂O in O ₂, air, N ₂ 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 O ₂ or air, an amorphous solid forms when VOHPO ₄·0.5H ₂O is heated above 270 °C and a crystalline V ⁵⁺ phase develops on heating above 350 °C. By contrast, during activation in n-butane-air or in N ₂, (VO) ₂P ₂O ₇ is formed at 300 °C without any intermediate amorphous phase. The interiors of VOHPO ₄·0. 5H ₂O and (VO) ₂P ₂O ₇ particles were identical, with the platelet like morphology of the precursor preserved during activation. By contrast the interiors of particles from the air or O ₂ 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) ₂P ₂O ₇ generating a resistance to overoxidation during activation and catalytic work.