Effect of catalyst preparation for the simultaneous removal of soot and NOx

Abstract

La0.7Ag0.3MnO3 perovskite-like catalyst was prepared using three synthetic routes, microwaved-assisted (MW) microwaved-assisted hydrothermal (HMW) and solid state (SS) with the aim of studying catalyst preparation method effect on the simultaneous removal of soot and NOx under loose contact condition. The materials were characterized using various analytical techniques, such as XRD, XPS, H2-TPR and O2-TPD. Among the prepared solids, the MW catalyst showed the best catalytic performance for the simultaneous removal since it was the one with the lowest soot ignition temperature (160 °C), the lowest temperature for the maximum NOx reduction (328 °C), the highest NOx reduction efficiency (60%); and an intermediate T50 (371 °C) for soot oxidation. Results further showed that a rapid synthesis using short microwave irradiation time (6 min) for the precursor solution and short calcination times for the final solid (1 h) produced smaller particles (13 nm) with a high silver (Ag+) content incorporated into perovskite lattice structure thus facilitating the formation of oxygen vacancies as active sites. In fact, the increment in the oxygen vacancies correlated with the increment of surface oxygen content (α-oxygen) that along with the segregated metallic silver amount allowed inferring that both active sites are needed for the simultaneous NO and soot removal. Finally, based on our results it was suggested that both NO and O2 are adsorbed from the gaseous phase on the active sites (oxygen vacancies and Ago) of the catalyst surface to form NO2 as an intermediate that quickly reacts with soot to yield CO2 and N2.