Supported vanadium oxide catalysts are active in a wide range of applications. In this review, an overview is given of the current knowledge available about vanadium oxide-based catalysts. The review starts with the importance of vanadium in heterogeneous catalysis, a discussion of the molecular structure of vanadium in water and in the solid state and an overview of the spectroscopic techniques enabling to study the chemistry of supported vanadium oxides. In the second part, it will be shown that advanced spectroscopic tools can be used to obtain detailed information about the coordination environment and oxidation state of vanadium oxides during each stage of the life-span of a heterogeneous catalyst. Three topics will be discussed: (1) the molecular structure of supported vanadium oxide catalysts under hydrated, dehydrated and reduced conditions, including the parameters, which influence the molecular structures formed at the surface of the support oxide; (2) elucidation of the active surface vanadium oxide during the oxidation of methanol to formaldehyde, the reaction mechanism and the vanadium oxide-support effect; and (3) deactivation of fluid catalytic cracking (FCC) catalysts by migration of vanadium oxides and the development of a method preventing the structural breakdown of zeolites by trapping the mobile vanadium oxides in an aluminum oxide coating.

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 À1. Hydroxylation was found to start instantaneously upon exposure to traces of water, reflecting a very high sensitivity of the supported vanadium oxide catalysts for H 2 O. Further hydration resulted in the appearance of a V-O-V vibration band located around 700 cm À1 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 VÁ Á ÁV contribution was detected for the hydrated species. The IR, Raman and UV-vis data suggested a pyramidal anchoring of the dehydrated VO x species, whereas, the EXAFS data pointed to the presence of single V-O-Si bonded VO x 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. as well as literature led to a reaction scheme in which a monomeric VO x species anchored by three Si-O-V bonds to the silica support (pyramidal-type structure) is transformed into a monomeric VO x 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-V O(OH) 2 ) could be present under catalytic conditions where H 2 O 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 -V O) and the umbrella (Si-O-V O(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. #

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Vanadium oxide (1 wt %) supported on γ-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 VdO bond of 1.58 Å and 3 V-O bonds of 1.72 Å) 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.

The influence of the support oxide on the molecular structure of a VO 4 cluster and its interfacial geometry has been determined for SiO 2 , Nb 2 O 5 , and ZrO 2 as supports. Raman, IR, UV-vis-NIR diffuse reflectance, electron spin resonance, and extended X-ray absorption fine structure (EXAFS) spectroscopies were used to characterize the supported vanadium oxide clusters after dehydration. It has been found that for all supports under investigation the vanadium ion is tetrahedral coordinated and consists of one VdO and three V-O bonds.

Raman spectroscopy experiments found the V@O stretching frequency for the supported VO 4 species to decrease with increasing catalyst temperature. Calculations on the vibrational frequencies of several models using density functional theory show that a consistent description of the experimental data can be obtained if we assume that the VO 4 species are anchored to the oxidic surface by one V-O bond only, in contrast to the traditional pyramidal model, which assumes three V-O support bonds and one V@O. The proposed VO 3 structure points away from the surface and consists of one V@O unit and an active oxygen ÔmoleculeÕ loosely bound to the vanadium atom, a peroxide species.

Abstract: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 SiO 2 < Al2O3 < Nb2O5 < ZrO2. The opposite trend was observed for the dehydrogenation of propane to propene in the absence of oxygen. Interestingly, the intensity of the Fourier transform AXAFS peak decreases in the same order. This can be interpreted by an increase in the binding energy of the vanadium valence orbitals when the ionicity of the support (increasing electron charge on the support oxygen atoms) increases. Moreover, detailed EXAFS analysis shows a systematic decrease of the V-O b (-Msupport) and an increase of a the V-O(H) bond length, when going from SiO2 to ZrO2. This implies a more reactive OH group for ZrO2, in line with the catalytic data. These results show that the electronic structure and consequently the catalytic behavior of the VO4 cluster depend on the ionicity of the support oxide. These results demonstrate that AXAFS spectroscopy can be used to understand and predict the catalytic performances of supported metal oxide catalysts. Furthermore, it enables the user to gather quantitative insight in metal oxide support interactions.

The effect of the point of zero charge (PZC) of the support oxide (Al 2 O 3 , Nb 2 O 5 , SiO 2 and ZrO 2 ) on the molecular structure of hydrated vanadium oxide species has been investigated with EXAFS spectroscopy for low-loaded vanadium oxide catalysts. It was found that the degree of clustering (i.e., the V---V coordination number) and the V---V distance increase with decreasing PZC of the support oxide;i.e., Al 2 O 3 (8.7) < ZrO 2 (7) < Nb 2 O 5 (3.3) < SiO 2 (2). Upon hydration the silicasupported vanadium oxide exhibited a clear alteration in the position of the oxygen atoms surrounding the central vanadium atom and thenumber of oxygen atoms around vanadium increased to five. In contrast, only minor changes in the molecular structure were detected for the alumina-, niobia-and zirconia-supported vanadium oxide catalysts. Based on a detailed analysis of the EXAFS data a semi-quantitative distribution of vanadium oxide species present on the surface of the different support oxides can be obtained, which is in good agreement with earlier characterization studies making primarily use of Raman spectroscopy.