Transient responses of the local electronic and geometric structures of vanado-molybdo-phoshate catalysts H_3+nPV_nMo_12-nO₄₀ in selective oxidation

The thermal stability of the series of title compounds was investigated with thermogravimetry (TG), differential thermal analysis (DTA), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) and in situ UV-VIS-NIR difference reflectance spectroscopy (UV-VIS-NIR-DRS). The results were combined with temperature programmed reaction experiments (TPR) of dehydration and catalytic tests involving methanol oxidation. The HPA are fully dehydrated and autoreduced at the temperature of maximum conversion. The presence of gas phase oxygen and of the organic substrate have only a limited influence on the autoreduced state. The oxygen storage function was probed by pulse reduction experiments which gave evidence for the existence of two kinetically different supply processes. The active state of the HPA is a partially reduced oligomer of polyoxoanions bridged by vanadyl groups. During catalytic action a steady-state exists between the precursor monomer and the oligomerised state. This steady-state forms as consequence of the thermal load in water and oxygen atmospheres and is qualitatively not altered upon addition of the organic substrate. Above 673 K the steady-state turns irreversibly into defective oxides associated with the loss of selective oxidation activity. A consistent picture is concluded of the solid state chemistry occurring during response of the precursor HPA to thermal and chemical conditions of selective oxidation catalysis. The HPA solid appears as inherently unstable material during operation as gas phase oxidation catalyst.