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In-situ UV−visible spectroscopy was used to measure the extent of reduction of active centers in VOx/γ-Al2O3 during oxidative dehydrogenation (ODH) of propane. Prevalent extents of reduction (0.062 to 0.30 e-/V) are much smaller than required for the formation of stoichiometric V3+ or V4+ suboxides. Surface oxygen atoms are the most abundant reactive intermediates during propane ODH, as previously suggested by kinetic and isotopic studies. These measurements involved the rigorous calibration of UV−visible intensities in the pre-edge region using quantitative reoxidation of a small number of centers reduced in H2. Transients observed during changes in C3H8 and O2 concentrations indicate that only a fraction of the prevalent reduced centers (∼30−40%) are active in catalytic turnovers, while the rest are reoxidized in time scales much longer than turnover times. The number of catalytically relevant reduced centers depends only on C3H8/O2 ratios, and not on individual reactant concentrations, indicating that ...

Extent of Reduction of Vanadium Oxides during Catalytic Oxidation of Alkanes Measured by in-Situ UV−Visible Spectroscopy