Characterization and CH4 Oxidation Activity of Cr/Al2O3 Catalysts

X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) have been used to characterize a series of Cr/Al2O3 catalysts (designated “Cry”). The information obtained from surface and bulk characterization has been correlated with the CH4 oxidation activity of the Cry catalysts. For catalysts with Cr/Al atomic ratio = 0.013, XPS results indicated that most of the Cr was present as a highly dispersed Cr6+ surface phase. Catalysts with intermediate Cr loadings (0.027 ≤ Cr/Al atomic ratio ≤ 0.080) showed no XRD peak characteristic of chromium oxide; however, XPS data indicated that the Cr dispersion decreased and the concentration of Cr3+ species increased with increasing Cr content. For catalysts with high Cr loadings (Cr/Al atomic ratio ≥ 0.107), large Cr2O3 crystallites were detected by XRD. The specific activity for CH4 oxidation increased with increasing Cr content up to Cr/Al = 0.107. This has been attributed to an increase in the amount of Cr(III)−Cr(VI) cluster present in the catalysts. A decrease in CH4 oxidation activity observed for Cr-rich catalysts (Cr/Al = 0.13) has been ascribed to the formation of poorly dispersed Cr2O3.