Abstract
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).
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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 V...V 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-V=O(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)-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. (C) 2006 Elsevier B.V. All rights reserved.
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