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 less than or equal to Cr/Al atomic ratio less than or equal to 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 greater than or equal to 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.