Characterization and Quantification of Reduced Sites on Supported Vanadium Oxide Catalysts by Using High-Frequency Electron Paramagnetic Resonance

VOx (1.41.7 V?nm-2) supported on SBA-15, Al2O3, or TiO2 was studied before and after exposure to oxidative dehydrogenation of propane (ODP), and pure hydrogen or propane. After treatment, samples were quenched and frozen in quartz vials and characterized by using high-frequency electron paramagnetic resonance (HF-EPR). For SBA-15- and Al2O3-supported vanadia, V4+ sites were the most abundant paramagnetic species, whereas Ti3+ was dominant in TiO2-supported V2O5. For the quantification of paramagnetic reduced sites, Mn2+ was used as reference. The maximum relative numbers of reduced V4+ or Ti3+ sites were found to increase in the sequence SBA-15 (11?% V4+/V)<Al2O3 (16?% V4+/V)<TiO2 (21?% Ti3+/V) if hydrogen was used as a reducing agent. Using propane as a reductant, the percentage ratios were found to be SBA-15 (8?% V4+/V), Al2O3 (18?% V4+/V), and TiO2 (27?% Ti3+/V). The corresponding V4+/V or Ti3+/V percentage ratios for samples exposed to ODP were lower, at 0.08, 9, and 17?%, respectively. The number of paramagnetic non-V4+ or non-Ti3+ sites was negligible for Al2O3 and SBA-15 and in the range of 1237?% for TiO2-supported vanadia. The number of reduced sites after oxidation in air and storage in sealed vials is negligible, which indicates that all reduced sites can generally be reoxidized. After reaction in an oxygen-lean ODP atmosphere, a structureless HF-EPR pattern was observed for SBA-15-supported vanadia, which is tentatively attributed to the formation of two-dimensional polymeric structures of vanadia. For Al2O3-supported vanadia, this loss of spectral resolution was less pronounced, which indicates a more defined environment of V4+ sites. The results are discussed in terms of a two-parameter model, rationalizing the dual influences of reducibility and local structure of redox-active sites on the ODP reaction.