Roles of Li+ and Zr4+ Cations in the Catalytic Performances of Co1-xMxCr2O4 (M = Li, Zr; x=0-0.2) for Methane Combustion

Co1-xMxCr2O4 (M = Li, Zr; x = 0-0.2) catalysts were prepared via the citric acid method and investigated for catalytic combustion of methane. Substitution at tetrahedral (A) sites with monovalent (Li) or tetravalent (Zr) metal ions led to a decrease or increase of the catalytic activity, respectively. The Co0.95Zr0.05Cr2O4 catalyst proved to be the most active and its catalytic activity reached 90% of methane conversion at 448 degrees C, which dropped by 66 degrees C compared with that of the undoped CoCr2O4 catalyst. XRD and Raman results indicated that lithium or zirconium substitution could modify the spinel structure and electronic properties. For lithium-doped catalysts, oxygen deficiency and a strong surface enrichment in lithium and chromium were detected. Zirconium substitution enhanced the reducibility of zirconium-doped catalysts and decreased the strength constant of both the Co-O band and the Cr-O band, which may contribute to the catalytic activity toward methane combustion. In addition, the prevalent catalytic combustion activity of the zirconium-substituted catalysts could be explained by their higher concentration of suprafacial, weakly chemisorbed oxygen.