Catalytic oxidation of a diesel soot formed in the presence of a cerium additive. III. Microkinetic-assisted method for the improvement of the ignition temperature

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 (O-a) to the soot. The limiting step of the CO2 production during He-TPE is the formation of the O-a 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 (Ce-2(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 Ti, 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(Delta T-i <= +70 K), except for the reaction of SO2 at 1073 K in the absence of O-2 that strongly increases T-i (Delta T-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 CexOySz particles with high stability.