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 oxysulfidesCe 2 O 2 Ss 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.

Two diesel soots formed in a engine/exhaust line by using a fuel that contained 350 ppm (by weight) of sulfur with and without a cerium-based additive (diesel soots denoted Cec-DS and nc-DS, respectively) are studied (i) via temperature-programmed experiments in the temperature range of 300-1300 K and (ii) adsorption of O 2 , CO, and CO 2 at several adsorption temperatures (T a ). It is shown that, during the linear increase of temperature in a helium flow (a procedure denoted as the He-Temperature Programmed Experiment, He-TPE), the "as-prepared" Cec-DS soot leads to the detection of a high CO 2 peak, with a maximum at T m ) 905 K, which is not observed on nc-DS. After the treatment of Cec-DS at 1200 K in helium, the adsorption of O 2 at T a < 660 K leads, during the successive He-TPE, to observations similar to that on the freshly prepared soot, showing that the characteristic CO 2 peak is not linked to the formation of a particular surface-oxygenated complex (SOC), because of the experimental conditions of the engine/ exhaust line. This CO 2 peak is ascribed to the oxidation of SOCs by oxygen species coming from the cerium-containing particles. A kinetic modeling of the observations during He-TPE is presented as a first step of a microkinetic approach of the soot oxidation. Three main surface elementary steps are considered: (i) decomposition of the cerium-containing particles (identified as cerium sulfate, Ce 2 (SO 4 ) 3 , in the as-prepared soot), which provides oxygen species O s to the soot; (ii) the oxidation of the SOCs into CO 2 by the O s species; and (iii) the desorption of the SOCs as CO. The kinetic model gives theoretical CO 2 and CO productions that are consistent with the experimental observations for a set of activation energies that leads to the conclusion that it is the decomposition of the cerium-containing particles that controls the formation of the CO 2 peak during the He-TPE. This kinetic model is (i) compared to literature data on the calciumcatalyzed gasification of carbon materials and (ii) used to suggest an orientation for the oxidation of diesel soots at lower temperatures.

In previous studies, it has been shown, using a microkinetic approach of temperature-programmed experiments in a helium flow (denoted as He-TPE), that the surface-oxygenated complexes (SOCs) of a diesel soot, desorbing as CO, are preferentially oxidized in CO2 in the presence of cerium-containing particles that provide reactive oxygen species (Oa) to the soot. The limiting step of the CO2 production during He-TPE is the formation of the Oa species and not the oxidation of the SOCs. The objectives of the present study are (i) to verify this conclusion experimentally, (ii) to use this conclusion to improve the soot oxidation process (the decrease of the ignition temperature), and (iii) to reveal the process imposing the ignition temperature of a diesel soot (denoted as Cec-DS) formed with an engine test bench, using a sulfur-containing fuel with a cerium additive. It is shown that the impregnation of a model diesel soot (Printex U, from Degussa) with two cerium salts of different stabilitycerium nitrate (Ce(NO3)3, providing a solid denoted as CeN−PU), which is less stable that cerium sulfate (Ce2(SO4)3, providing a solid denoted as CeS−PU)significantly decreases the ignition temperature of the soot:  T i = 852, 768, and 587 K for pure Printex U, CeS−PU, and CeN−PU, respectively. This confirms clearly that cerium-containing particles favor the ignition of the soot and that the lower their stability, the lower the ignition temperature. For CeN−PU, it is observed that the cerium nitrate decomposes before the ignition with the formation of CeO2 particles. This allows us to study the impact of several processes on T i, such as sintering at high temperatures and the adsorption/reaction of SO2 at low (473 K) and high (1073 K) temperatures. These pretreatments moderately increase the value of T i (ΔT i ≤ +70 K), except for the reaction of SO2 at 1073 K in the absence of O2 that strongly increases T i (ΔT i = +183 K), leading to a T i value similar to that observed with Cec-DS. It is suggested that the high ignition temperature observed with a diesel soot formed on an engine test bench with a fuel that contains sulfur and a cerium additive is linked to the formation in the engine/exhaust line of well-dispersed Ce x O y S z particles with high stability.