Catalytic oxidation of a diesel soot formed in the presence of a cerium additive. II. Temperature-programmed experiments on the surface-oxygenated complexes and kinetic modeling

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 Gee-DS and nc-DS, respectively) are studied W via temperature-programmed experiments in the temperature range of 300-1300 K and (ii) adsorption of O-2, CO2 and M 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 CO2 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 CO2 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 CO2 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(SO4)(3), in the as-prepared soot), which provides oxygen species O-s to the soot; (ii) the oxidation of the SOCs into CO2 by the O-s species; and (iii) the de-sorption of the SOCs as CO. The kinetic model gives theoretical CO2 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 CO2 peak during the He-TPE. This kinetic model is W compared to literature data on the calcium-catalyzed gasification of carbon materials and (ii) used to suggest an orientation for the oxidation of diesel soots at lower temperatures.