Dispersed
vanadia and titania are of great interest due to their
(photo)catalytic properties. The structure of isolated (highly dispersed)
vanadia and titania has been studied using a combination of deep UV
Raman, UV–vis, and IR spectroscopy. Highly dispersed vanadia
and titania were prepared by incipient wetness impregnation of the
corresponding isopropoxide precursors onto silica SBA-15. Using resonance
Raman spectroscopy, a wide range of vanadium (0.00001–0.7 V/nm2) and titanium (0.001–0.7 Ti/nm2) loadings
can be analyzed. At very low loadings (<0.05 M/nm2)
the structure of both highly dispersed vanadia and titania is characterized
by tetrahedral coordination of the central atom and anchoring to the
support by one or more M–O–S linkages. At higher loadings,
titania partly forms oligomeric surface structures according to UV–vis
results. In the case of highly dispersed vanadia samples (<0.1
V/nm2), Raman spectroscopy reveals distinct differences
in the vanadia surface structures under ambient and dehydrated conditions.
Corresponding UV–vis spectra indicate the formation of isolated
and oligomeric vanadia surface structures. A smaller dependence of
the titania surface structure on the environmental conditions (ambient/dehydrated)
was observed. For the 0.7 M/nm2 samples, the presence of
hydroxylated vanadia and titania structures in the dehydrated state
was verified by IR spectroscopy. The assignment of the vibrational
bands was facilitated by the results of a normal-mode analysis using
models based on polyhedral oligomeric silsesquioxane (POSS). Our study
clearly demonstrates the potential of resonance Raman spectroscopy
using 217.5 nm deep UV excitation to study the surface structure of
transition metal oxide species even in isolated conditions.
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