(426d) Nox and Diesel Soot Abatement over Catalytic Traps Based on Mixed Trasition Metal Oxides

The automotive industry is currently facing serious challenges to meet the specific requirements of future regulations concerning both NO_x and particulate, the two prevalent pollutants in Diesel exhaust gases. These limits cannot be accomplished by simple engine modifications, fuel pre-treatments or just a better tuning of the combustion process: a convenient way of treating diesel off-gases has thus to be worked out. The present investigation concerns the development of Nanostructured perovskite-type catalysts (ABO₃, where A = La, K and B = Ni, Cr, Co, Cu) for the simultaneous removal of soot and NO_x. A series of catalysts were prepared via a highly exothermic and self-sustaining reaction, the so-called ?solution combustion synthesis? method. They have been firstly submitted to physical and chemical characterization. The catalysts were analysed by X-ray diffraction in order to asses their purity, crystalline structure and approximate crystal grain size. Field Emission Scanning Electron Microscope was employed to analyze the microstructure of the crystal aggregates of the catalysts as prepared and after aging. The BET specific surface area of the catalysts has been finally evaluated from the linear parts of the BET plot of the N₂ isotherms. A detailed investigation was carried out on these oxides by temperature programmed oxidation of soot and reduction of NO (TPO/R) tests in fixed bed microreactors, by assessing the effect of the prevalent operating parameters. The encouraging results obtained with the catalytic activity tests, proved to be effective in the simultaneous removal of soot and NO_x, the two prevalent pollutants in diesel exhaust gases in the temperature range 350?450 °C. The best compromise between soot and nitrogen oxide abatement was shown by La_0.8K_0.2Ni_0.95Cu_0.05O₃ catalyst, which could promote soot combustion and appreciable NO_x reduction below 380°C, the maximum temperature reached in the exhaust line of a diesel engine. On the grounds of transient thermal analysis studies (TPD), the prevalent activity of the this catalysts could be explained by their higher concentration of suprafacial, weakly chemisorbed oxygen, which contributes actively to soot combustion by spillover in the temperature range 300?500°C. The role of the various elements present in the catalysts was also defined through a variety of techniques and will be described only in the full paper. Along with further catalysts development undertaken to possibly lower the operating temperatures at which the simultaneous removal of NO_x and particulate is promoted, the process is currently being scaled-up to a catalytic trap prototype, based on the best catalyst developed so far (La_0.8K_0.2Ni_0.95Cu_0.05O₃), to be tested on real exhaust gases.