Distinguished Roles with Various Vanadium Loadings Of CoCr<sub>2–<i>x</i></sub>V<sub><i>x</i></sub>O<sub>4</sub> (<i>x</i> = 0–0.20) for Methane Combustion

A series of vanadium-doped CoCr<sub>2–<i>x</i></sub>V<sub><i>x</i></sub>O<sub>4</sub> (<i>x</i> = 0–0.20) catalysts were prepared via the citric acid method and investigated for methane combustion. A significant improvement of the catalytic activity was observed over a CoCr<sub>1.95</sub>V<sub>0.05</sub>O<sub>4</sub> catalyst, and the catalytic activity reached 90% of methane conversion at 438 °C, which was 76 °C lower than that of the undoped catalyst. Vanadium doping caused disorder in the spinel structure and various changes in physicochemical properties. The higher concentration of suprafacial, weakly chemisorbed oxygen and the active cobalt species, especially Co<sup>3+</sup>, could play a major role in the oxidation reaction. Results of XRD, Raman, and XPS suggested that excess vanadium probably formed separate catalytically inactive VO<sub><i>x</i></sub> species, which would explain the decline of catalytic activity for samples with a higher substitution degree.