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The effect of hydration on the molecular structure of silica-supported vanadium oxide catalysts with loadings of 1–16 wt.% V has been systematically investigated by infrared, Raman, UV–vis and EXAFS spectroscopy. IR and Raman spectra recorded during hydration revealed the formation of V–OH groups, characterized by a band at 3660 cm−1. Hydroxylation was found to start instantaneously upon exposure to traces of water, reflecting a very high sensitivity of the supported vanadium oxide catalysts for H2O. Further hydration resulted in the appearance of a V–O–V vibration band located around 700 cm−1 pointing to the formation of di- or polymeric species. EXAFS analysis at 77 K indicated structural changes as the oxygen coordination changed from four to five. Moreover, a VV contribution was detected for the hydrated species. The IR, Raman and UV–vis data suggested a pyramidal anchoring of the dehydrated VOx species, whereas, the EXAFS data pointed to the presence of single V–O–Si bonded VOx species. This difference is attributed to water condensation effects at 77 K during EXAFS acquisition, resulting in a partial re-hydroxylation of the dehydrated samples, as confirmed by complementary IR and Raman analysis. Combining the results of this study with data from our previous studies [D.E. Keller, F.M.F. de Groot, D.C. Koningsberger, B.M. Weckhuysen, J. Phys. Chem. B 109 (2005) 10223; D.E. Keller, D.C. Koningsberger, B.M. Weckhuysen, J. Phys. Chem. B 110 (2006) 14313] as well as literature led to a reaction scheme in which a monomeric VOx species anchored by three Si–O–V bonds to the silica support (pyramidal-type structure) is transformed into a monomeric VOx species anchored by one Si–O–V bond (umbrella-type structure) by partial hydration of the catalyst material. This results in the formation of both V–O–H and Si–O–H bonds. At higher water pressures, larger vanadium oxide clusters are formed due to full hydration of the catalyst surface and a de-attachment of the vanadium oxide from the support surface. The results of this study provide evidence, that an umbrella-type structure (i.e., Si–O–VO(OH)2) could be present under catalytic conditions where H2O is a reaction product (e.g., partial oxidation of methanol to formaldehyde and oxidative dehydrogenation of alkanes). In other words, both the pyramidal ((Si–O)3–VO) and the umbrella (Si–O–VO(OH)2) model can exist at a support surface, their relative ratio depending on the hydration degree of the catalyst material. This study also illustrates that a corroborative characterization requires the use of multiple spectroscopic techniques applied at the same samples under almost identical measuring conditions.

The influence of changes in coordination number, interatomic distances, and oxidation state on the intensity and centroid position of the Fourier transform (FT) of the atomic X-ray absorption fine structure (AXAFS) peak of vanadium oxide bulk model compounds and alumina-supported vanadium oxide clusters has been investigated. Using Na3VO4 and V2O5 as model compounds, it has been shown that the nearest neighbour shells have a pronounced influence on the AXAFS intensity; specifically, a 40 % decrease in intensity was observed between these two compounds. Secondly, the influence of partial reduction of the vanadium oxide species has been determined; this led to a 50 % decrease in the AXAFS intensity and to an increase in the centroid position. Furthermore, the influence of the vanadium oxide loading has been assessed. A non-linear relationship between the vanadium oxide loading and the AXAFS intensity has been found, indicating that the AXAFS intensity is sensitive to the formation of VOV bridging bonds between the vanadium VO4 clusters. The results show that AXAFS can be used to probe the relative energy level of the vanadium valence orbitals.

The potential of atomic XAFS (AXAFS) to directly probe the catalytic performances of a set of supported metal oxide catalysts has been explored for the first time. For this purpose, a series of 1 wt % supported vanadium oxide catalysts have been prepared differing in their oxidic support material (SiO2, Al2O3, Nb2O5, and ZrO2). Previous characterization results have shown that these catalysts contain the same molecular structure on all supports, i.e., a monomeric VO4 species. It was found that the catalytic activity for the selective oxidation of methanol to formaldehyde and the oxidative dehydrogenation of propane to propene increases in the order SiO2

Supported vanadium oxide catalysts have been the subject of detailed investigations for many decades and a relatively large amount of information is available on their structure, however, the exact molecular structure and the way these surface species are anchored on the support oxide has not yet been unambiguously determined. It has been attempted in this work to determine the exact molecular structure of supported vanadium oxides on several support oxides under various experimental conditions by using a wide variety of characterization techniques, although the main emphasis has been on the application of X-ray absorption spectroscopy. Furthermore, we have explored the potential of X-ray absorption spectroscopy to probe metal oxide-support effects in a quantitative manner. It is found that the support ionicity can be related to the catalytic performances of supported vanadium oxide catalysts. The molecular structure of low-loaded vanadium oxide catalysts has been investigated on four different supports, i.e. Al2O3, Nb2O5, SiO2 and ZrO2. Raman and UV-VIS diffuse reflectance spectroscopy in combination with EXAFS and structural models showed that the majority of the vanadium oxide species are present as a monomeric VO4 cluster. Under the measurement conditions applied the VO4 cluster was found to have only one V-O-Msupport bond. It has to be stressed that the overall vanadium oxide structure is the same on all supports under investigation: one V=O bond, one V-O-Msupport bond and two V-O bonds. The nature of the V-O groups has been investigated for a series of silica-supported vanadium oxide catalysts varying in their vanadium oxide loading. No evidence was found for the presence of a peroxo species or V-O-V bridges on the silica surface for low-loaded vanadium oxide catalysts. It was found that trace amounts of water resulted in the presence of V-OH groups on the vanadium oxide cluster. Total hydration of low-loaded supported vanadium oxide catalysts resulted in the formation of larger vanadium oxide clusters. The exact size and geometric structure of these clusters depend on the PZC of the support material. A novel characterization tool to study the influence of the support oxide material on the structural, electronic and catalytic properties of supported vanadium oxide species has been explored. It has been shown that Atomic XAFS (AXAFS) spectroscopy is able to probe variations in the vanadium valence orbitals as a function of nearest and next nearest neighbours. It has been illustrated that, although the overall molecular structure of the supported vanadium oxide is the same on all supports, the AXAFS intensity is a function of the support oxide. The support effect is a next nearest neighbour (Msupport) effect that acts through the V-Ob-Msupport bond. The trend in AXAFS intensity showed a linear relation with the catalytic performance of the supported vanadium oxide catalysts in both selective oxidation and dehydrogenation reactions.

Vanadium oxide (1 wt %) supported on gamma-Al(2)O(3) was used to investigate the interface between the catalytically active species and the support oxide. Raman, UV-vis-NIR DRS, ESR, XANES, and EXAFS were used to characterize the sample in great detail. All techniques showed that an isolated VO(4) species was present at the catalyst surface, which implies that no V-O-V moiety is present. Surprisingly, a Raman band was present at 900 cm(-1), which is commonly assigned to a V-O-V vibration. This observation contradicts the current literature assignment. To further elucidate on potential other Raman assignments, the exact molecular structure of the VO(4) entity (1 V=O bond of 1.58 A and 3 V-O bonds of 1.72 A) together with its position relative to the support O anions and Al cation of the Al(2)O(3) support has been investigated with EXAFS. In combination with a structural model of the alumina surface, the arrangement of the support atoms in the proximity of the VO(4) entity could be clarified, leading to a new molecular structure of the interface between VO(4) and Al(2)O(3). It was found that VO(4) is anchored to the support oxide surface, with only one V-O support bond instead of three, which is commonly accepted in the literature. The structural model suggested in this paper leaves three possible assignments for the 900 cm(-1) band: a V-O-Al vibration, a V-O-H vibration, and a V-(O-O) vibration. The pros and cons of these different options will be discussed.

Vanadium oxide (1 wt %) supported on gamma-Al2O3 was used to investigate the interface between the catalytically active species and the support oxide. Raman, UV-vis-NIR DRS, ESR, XANES, and EXAFS were used to characterize the sample in great detail. All techniques showed that an isolated VO4 species was present at the catalyst surface, which implies that no V-O-V moiety is present. Surprisingly, a Raman band was present at 900 cm(-1), which is commonly assigned to a V-O-V vibration. This observation contradicts the current literature assignment. To further elucidate on potential other Raman assignments, the exact molecular structure of the VO4 entity (1 V=O bond of 1.58 angstrom and 3 V-O bonds of 1.72 angstrom) together with its position relative to the support 0 anions and Al cation of the Al2O3 support has been investigated with EXAFS. In combination with a structural model of the alumina surface, the arrangement of the support atoms in the proximity of the VO4 entity could be clarified, leading to a new molecular structure of the interface between VO4 and Al2O3. It was found that VO4 is anchored to the support oxide surface, with only one V-O support bond instead of three, which is commonly accepted in the literature. The structural model suggested in this paper leaves three possible assignments for the 900 cm(-1) band: a V-O-Al vibration, a V-O-H vibration, and a V-(O-O) vibration. The pros and cons of these different options will be discussed.