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
S. Higashimoto et al., 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, J PHYS CH B, 104(44), 2000, pp. 10288-10292
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 inve
stigated by means of in situ photoluminescence spectroscopy in combination
with other spectroscopic techniques such as UV-Vis, solid-state NMR, and ES
R, It was found that vanadyl ions are present as tetrahedrally coordinated
V5+ species bonded to terminal Si-OH groups and square pyramidally coordina
ted VO2+ 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 V5+ species can be observed. UV ir
radiation 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 N2O. 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 photolum
inescence 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 VO4 unit species was quenched by the addit
ion of NO or propane, its extent depending on the pressure of these molecul
es. These results suggest that the charge transfer excited triplet state of
the isolated tetrahedrally coordinated V5+ species plays a significant rol
e not only in the photocatalytic decomposition of NO but also in its enhanc
ement in the presence of propane.