Catalytic performance of Ag/Co-Ce composite oxides during soot combustion in O2 and NOx: Insights into the effects of silver

Abstract

The composite oxides xAg/Co0.93Ce0.07 (x = Ag/(Co+Ce) molar ratio), intended for use as high performance catalytic materials, were successfully prepared via citric acid complexation. The effects of silver on the performance of these substances during soot combustion were subsequently investigated. Under O2, the 0.3Ag/Co0.93Ce0.07 catalyst resulted in the lowest ignition temperature, T10, of 197 °C, while the minimum light-off temperature was obtained from both 0.2Ag/Co0.93Ce0.07 and 0.3Ag/Co0.93Ce0.07 in the NOx atmosphere. These materials were also characterized by various techniques, including H2, soot and NOx temperature programmed reduction, X-ray diffraction, and electron paramagnetic resonance, Raman, X-ray photoelectron, and Fourier transform infrared spectroscopic analyses. The results demonstrated that silver significantly alters the catalytic behavior under both O2 and NOx, even though the lattice structure of the mixed oxide is not affected. Surface silver oxides generated under the O2 atmosphere favor soot combustion by participating in the redox cycles between soot and the silver oxide, whereas the AgNO3 that forms in a NOx-rich atmosphere facilitates soot abatement at a lower temperature. The inferior activity of AgNO3 relative to that of Ag2O results in the different catalytic performance in the presence of NOx or O2.