| Popis: |
V2O5-TiO2(anatase) catalysts have been studied under oxidizing and reducing conditions using in situ laser Raman spectroscopy (LRS) and temperature-programmed reduction (TPR) and oxidation (TPO). Quantitative Raman and TPO analysis of the oxidized samples show that these materials are comprised of a distribution of monomeric vanadyls, polymeric vanadates, and crystallites of V2O5. At low loadings, the predominant species are monomeric vanadyls, with the remaining vanadia being present in the form of polymeric vanadates. As the surface concentration of V2O5 increases, a maximum in the concentration of the polymeric vanadates is detected. Crystallites of V2O5 form at the expense of the polymeric vanadates as the loading is raised above the dispersive capacity of the TiO2(anatase) support. An equilibrium polymerization model is proposed to account for the observed concentration of vanadia species, which leads to an initial polymer size of ∼2 at low loadings, consistent with the formation of dimeric species. Raman and TPR/TPO studies of the reduction process indicate that the terminal V = 0 groups of the monomeric and polymeric vanadia species are removed preferentially to the bridging oxygen atoms of the polymeric species. The maximum loss of oxygen upon reduction is one oxygen atom per vanadium atom. |
