Catalytic Oxidation of a Diesel Soot Formed in the Presence of a Cerium Additive. I. Characterization of the Cerium Fraction Using Magnetic Susceptibility and Temperature-Programmed Desorption

The present study concerns the characterization (by magnetic susceptibility measurements and temperature-programmed desorption (TPD) experiments) of the cerium fraction of a diesel soot (denoted Cec-DS) collected in the exhaust line of an engine using a fuel containing 50 ppm of cerium additive and 350 ppm of sulfur. The impact of this cerium fraction on the reactivity/stability of the surface oxygenated complexes (denoted SOCs) is studied by comparison with (i) a diesel soot obtained with a fuel without additive and (ii) a commercial soot that is considered as a model of diesel soot. Magnetic susceptibility measurements indicate that 49% of the additive in Cec-DS is present as Ce 3+, corresponding to cerium(III) sulfate (Ce 2 (SO 4 ) 3 ), and the remainder is present as Ce 4+, which is associated with CeO 2 particles (average diameter of ∼23 nm, using X-ray diffractometry). The Ce 3+ ions of the as-prepared soot are very stable in air. During TPD of Cec-DS at a temperature of T > 1000 K, the production of SO 2 (decomposition of the Ce 2 (SO 4 ) 3 ), as well as large amounts of CO 2 and CO, is observed. The number of Ce 3+ ions in the solid is slightly decreased during the TPD. However, the Ce 3+ fraction is now able to activate O 2 at room temperature. In particular, all the Ce 3+ ions are oxidized to Ce 4+ ions by O 2 adsorption at 300 K while, in addition, a significant amount of SOC is formed on the soot (contrary to the observations of the two soots without cerium). Moreover, a sulfur mass balance during TPD indicates that a significant amount of sulfur remains associated to CeO x -containing particles. According to the literature data, this is tentatively ascribed to the formation of a very stable oxysulfide Ce 2 O 2 S during the course of the TPD. It is shown that, after the sulfate decomposition, the oxygen species from the cerium-containing particles are involved in the formation and removal of the SOC species. The decomposition of the Ce 2 (SO 4 ) 3 seems to be an important step for the catalytic oxidation of a Cec-DS soot.