Diesel soot combustion activity of ceria promoted with alkali metals

Catalytic oxidation technique for reducing particulate emissions from diesel engines is based on the use of a catalyst in order to decrease the combustion temperature of soot down to the same range of temperature of exhaust gases. The main objective of this work was to investigate the promotional effect of alkali metals in the soot combustion activity of ceria-based catalysts. Our study pointed out that the order of activity in soot oxidation depends on the nature (Cs > Rb ≈ K > Na) and the amount of the metal. Another key factor resulted to be the choice of the precursors (chlorides are almost inactive). The activity is correlated to the presence of carbonate species, carbon–oxygen complexes (C–O–M, with M alkali metals) being the active sites; these species react with carbon of soot producing CO2 and a reduced form “C–M” that is reoxidized by molecular oxygen to reform C–O–M. A major drawback resulted to be the loss of metal and consequent deactivation after prolonged aging at 1023 K, especially under water-containing atmosphere.