Arne Dinse

VO_x (1.4–1.7 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)2O₃ (16 % V⁴⁺/V)2 (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 12–37 % 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.