(399d) Solution Combustion Synthesis Boosts Ceria Activity Towards Diesel Soot Combustion

Catalysts suitable for promoting low temperature combustion of carbonaceous materials may be applied in the traps designed for the abatement of diesel particulate, which is one of the most harmful pollutants emitted by diesel engines. Nano-sized particles of ceria with high surface area were processed by solution combustion synthesis (SCS) and their catalytic activity towards combustion of carbonaceous materials was compared with that obtained by simple cerium nitrate thermal decomposition (ND, a more conventional method), exhibiting the same specific surface area. Both ceria obtained by combustion synthesis and thermal decomposition 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. Ceria processed by SCS was found to be more active than its counter part since it could decrease significantly both combustion activation energy (from 115 to 111 kJ/mol) and TPC-plot peak temperature (from 466 to 405 °C) of Printex U (154 kJ/mol and 560 °C), used as carbonaceous material. The activity of this material was found to be related to its microstructure, which favors intensification of catalyst-soot contact and to the nature, amount or spill-over aptitude of the oxygen species delivered by the SCS-CeO₂ towards the soot is higher than that of the nitrate-derived one. Moreover, to fully elucidate this point, the mechanism of the catalytic reaction of combustion was investigated by TPD/R experiments. Likewise other combustion catalysts acting as oxygen pumps, also ceria can exchange limited amounts of oxygen even below 300°C, the SCS-synthesized one showing a superior O₂ release at low temperatures (≤ 450°C). Finally, the effect of specific aging protocols (thermal treatment at 600°C for 96 hours) in the presence of potentially deactivating species present in diesel exhaust gases (SO₂ and water), was also studied showing that the here proposed materials suffers from less significant ageing than Ce(NO₃)₃ -derived ceria. Ceria produced by solution combustion synthesis shows good thermal stability and, after exposition to high temperature, it keeps a rather large surface area and good catalytic activity, as a likely consequence of the irregular shape of ceria particles and crystal aggregates, which hinder sintering phenomena.

microstructure, which favors intensification catalyst-soot contact points, as well as to exchange oxygen with the atmosphere and/or carbon through surface oxidation/reduction phenomena at temperatures below 600°C