Effect of the Si/Al ratio on the local structure of V oxide/ZSM-5 catalysts prepared by solid-state reaction and their photocatalytic reactivity for the decomposition of NO in the absence and presence of propane

The local structure of V oxide/ZSM-5 catalysts prepared by the solid-state reaction of the ZSM-5 zeolites having different Si/Al ratios have been investigated by means of in situ photoluminescence spectroscopy in combination with other spectroscopic techniques such as UV-Vis, solid-state NMR, and ESR, It was found that vanadyl ions are present as tetrahedrally coordinated V(5+) species bonded to terminal Si-OH groups and square pyramidally coordinated VO(2+) species at the zeolite cationic sites for zeolites having a low Si/Al ratio (Si/Al = 20). On the other hand, for a high Si/Al ratio (Si/Al = 950), only the tetrahedrally coordinated V(5+) species can be observed. UV irradiation of the V oxide/ZSM-5 catalyst in the presence of NO was found to lead to the photocatalytic decomposition of NO into N(2). O(2), and N(2)O. The increase of efficiency was found to be more remarkable on the V oxide/ZSM-5 catalyst with a high Si/Al ratio than with a low Si/Al ratio. The photoluminescence spectrum of the V oxide/ZSM-5 catalyst which can be attributed to the radiative decay process for the charge transfer excited triplet state of the tetrahedrally coordinated VO(4) unit species was quenched by the addition of NO or propane, its extent depending on the pressure of these molecules. These results suggest that the charge transfer excited triplet state of the isolated tetrahedrally coordinated V5+ species plays a significant role not only in the photocatalytic decomposition of NO but also in its enhancement in the presence of propane.