A Thermally Stable Cr–Cu Nanostructure Embedded in the CeO2 Surface as a Substitute for Platinum-Group Metal Catalysts

Abstract

High catalytic activities for CO–O2 and CO–NO reactions that were superior to or comparable with those of platinum-group metal catalysts were achieved by synchronous dual-mode arc plasma deposition of a very small amount of Cr and Cu (0.07 wt % each) onto CeO2, followed by subsequent thermal aging at 900 °C for 25 h. The turnover frequency for CO oxidation over Cr–Cu/CeO2 was 3-fold higher than that over Cu/CeO2 and exceeded values for the Rh, Pd, and Pt catalysts loaded on CeO2, despite a significant decrease in the surface area from 169 to 5 m2 g–1 caused by thermal aging. Experimental structure characterization and density functional theory calculations based on CeO2 (111) surface slab models revealed that Cu+ substitution for surface Ce atoms leads to the formation of asymmetric 3-fold oxygen coordination sites capable of efficient CO chemisorption and catalytic activity. In addition, Cr3+ was incorporated into the surface structure of CeO2; it plays an important role in enhancing the surface concentration of Cu+. A CO oxidation rate with nearly zero partial orders with respect to O2 and isotopic C16O–18O2 reactions yielding C16O2 as the primary product demonstrated that the reaction proceeds via the Mars–van Krevelen mechanism.