Structures and Catalytic Properties of Cr-Cu Embedded CeO2 Surfaces with Different Cr/Cu Ratios

The effects of the Cr/Cu ratios of Cr-Cu-embedded CeO2 surfaces (0.065 wt % Cu loading) on their local structures and catalytic activities were studied using experimental and theoretical approaches. The sample with a weight ratio of Cr/Cu = 1, which was prepared by wet impregnation followed by subsequent thermal aging at 900 degrees C for 25 h, showed catalytic activity higher than that of the Cu/CeO2 sample in both CO-O-2 and CO-NO reactions. The activity of the catalyst was enhanced by increasing the Cr/Cu ratio. The highest activity occurred for a Cr/Cu ratio of around 3, and after it had been thermally aged, its activity was superior to that of Rh/CeO2. Having more Cr than Cu increases the surface concentration of the Cu+ sites, which promotes CO adsorption and its reaction with surface O atoms. As-formed surface O vacancies are filled by the dissociative adsorption of O-2 and/or NO. At the optimum composition, almost all of the Ce sites on the outermost layer are replaced by Cr and Cu, and oxidative chemisorption of CO and NO as carbonate and nitrate/nitrite, respectively, on the CeO2 surface becomes difficult. This situation enables more efficient dissociation of adsorbed NO and faster desorption of CO2, thereby leading to higher catalytic activity. Isocyanate species (NCO) that form sites are a possible reaction intermediate for the CO-NO reaction.