Potential of High-Frequency EPR for Investigation of Supported Vanadium Oxide Catalysts

Supported vanadium oxide catalysts were studied by high-frequency EPR for determination of the paramagnetic vanadium oxidation states vanadium(IV) and vanadium(III). TiO2-, gamma-Al2O3-, and SiO2 (SBA-15)-supported catalysts were investigated before and after being used for the oxidative dehydrogenation of propane. While the SBA-15-supported catalyst exhibits only one distinct position of vanadium species with the oxidation state IV, Al2O3 reveals a distribution of vanadium(IV) sites. For this support material formation of oxoradicals has been observed, indicating its capability for trapping electrons during the catalytic cycle. The TiO2-supported material apparently does not stabilize vanadium(IV), which can be attributed to either a high mobility of existing oxygen vacancies or the competitive capability of titanium(IV) surface states to trap electrons. Titanium in its oxidation state of vanadium(III) and surface-trapped O-2((-)) species were identified as potential electron sinks in the case of the TiO2-supported catalyst. The increase in catalytic activity in the order of SBA-15 < Al2O3 < TiO2 therefore could be caused by different reduction mechanisms depending on the chosen support material. No vanadium in its oxidation state III was detected in any of the samples, which indicates that such species are either short lived or not existent during the oxidative dehydrogenation of propane.