CeO2-MxOy (M = Fe, Co, Ni, and Cu)-Based Oxides for Direct NO Decomposition

Direct decomposition of nitrogen oxides (NOx) into N-2 and O-2 offers the ideal route to catalytic removal of this air pollutant that contributes to photochemical smog and acid rain. Direct decomposition of NOx is simple and economic, because no reductants are required. Despite their extensive use in automotive exhaust catalytic applications, only a few ceria-based oxides are reported in the literature for direct NO decomposition. In the present study, transition-metal-doped (Fe, Co, Ni, and Cu) ceria-based catalysts were synthesized using a co-precipitation method and examined for direct NO decomposition in the temperature region 400-650 degrees C. Both CeO2-Co3O4 and CeO2-NiO catalysts exhibit higher activity and N2 selectivity compared to CeO2-Fe2O3 and CeO2-CuO catalysts. Remarkably, Ce-Ni catalysts exhibit 100% selectivity toward N-2 at 400 degrees C and more than 95% selectivity at the remaining temperatures (450, 550, and 650 degrees C). Structural characterization measurements (X-ray diffraction) indicate that the transition metal enters the ceria lattice, and the extent of doping into CeO2 depends on the transition-metal identity. Surface characterization by X-ray photoelectron spectroscopy reveals that both Co and Ni result in the presence of Ce3+/Ce4+ and M2+/M3+ (M = Co, Ni), and the synergy between these dual redox sites is the origin of their higher activity. The effect of this synergy on the direct NOx decomposition is confirmed via NO adsorption/desorption studies, identifying a change in the nature and stability of adsorbed surface NOx species. Conversely, Fe and Cu remain in single oxidation states (Fe3+ and Cu2+) in the Ce-Fe and Ce-Cu catalysts, respectively, thus their impact on the redox properties and NO adsorption species of ceria are less significant, yielding little effect on the direct NO decomposition activity.