Characterization and CO Oxidation Activity of Cu/Cr/Al2O3 Catalysts

X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) have been used to characterize a series of Cu/Cr/Al2O3 catalysts prepared by stepwise incipient wetness impregnation of first chromium followed by copper (designated “CuCry”). The copper loading was held constant at 8 wt % CuO, and chromium loadings were varied from 0 to 20 wt % Cr2O3. The information obtained from surface and bulk characterization has been correlated with the CO oxidation activity of the catalysts. XPS and XRD results of analogous Cry indicated that the Cr dispersion decreased and the concentration of Cr3+ species increased with increasing Cr content. The decrease in Cu dispersion of CuCry with increasing Cr content has been attributed to the formation of large crystalline CuO and CuCr2O4. Copper addition decreased the Cr dispersion by reacting selectively with a dispersed Cr3+ species to form CuCr2O4 species. However, the Cu addition did not affect the Cr oxidation state distribution compared to that of Cry. For low Cr loading CuCry catalysts (Cr/Al ≤ 0.027), the CO oxidation activity increased with increasing Cr content due to the formation of crystalline CuO on the Cr-modified alumina. This has been attributed to the inhibition of Cu ion diffusion into alumina lattice vacancies by highly dispersed chromium species. The CuCry catalyst of Cr/Al = 0.054 showed the highest CO oxidation activity due to the formation of CuCr2O4 which was more active than the CuO phase. For Cr-rich catalysts (Cr/Al ≥ 0.080), the decrease in CO oxidation activity has been ascribed to the encapsulation of the active site with Cr2O3 species.