Structure of Isolated Vanadia and Titania: A Deep UV Raman, UV–Vis, and IR Spectroscopic Study

Abstract

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.

Photocatalytic Methanol Oxidation by Supported Vanadium Oxide Species: Influence of Support and Degree of Oligomerization

“…Supported isolated vanadia species are thus expected to absorb solely at such high energies of <300 nm. Crystalline V 2 O 5 was reported to exhibit three absorption bands, with values given as 236, 334 and 481 nm, or 240, 325 and 477 nm . For fully hydrated silica‐supported surface vanadium oxide species, an additional absorption at about 410 to 440 nm has previously been observed .…”

Understanding the oxidative dehydrogenation of ethyl lactate to ethyl pyruvate over vanadia/titania

“…the similar structures observed for molybdenum 27 ). Increasing the vanadia content to 10 wt% showed crystalline V 2 O 5 species (absorption at 490 nm [28][29] Please do not adjust margins Please do not adjust margins We ran H 2 -TPR experiments to further elucidate the oxidation state of surface vanadia species and the reducibility of V 2 O 5 /TiO 2 catalysts (see Fig. 5).…”

Probing the Catalytic Activity of Sulfate-Derived Pristine and Post-Treated Porous TiO2(101) Anatase Mesocrystals by the Oxidative Desulfurization of Dibenzothiophenes

“…Above 230 nm, bands have been typically associated with either 5- or 6-fold coordinations. 64,70−72 At longer wavelengths, the spectra display the typical broad features of bulk anatase and/or the standard for catalytic studies (titania P25 from Degussa). Thus, UV–vis spectra were recorded for all mesocrystalline samples with titania P25 included for comparison (Figure 4).…”