Biodiesel soot combustion: Analysis of the soot-catalyst contact in different experimental conditions

To simulate the oxidation of Biodiesel soot in a catalyzed Diesel Particulate Filter, oxidation experiments of undoped or Na-doped model soot were performed in the presence of Pt-Pd/Al2O3 under Temperature Programmed Oxidation and different carbon-catalyst configurations. Loose or tight contact was simulated by mixing soot and catalyst either with a spatula or in a mortar. Samples were characterized by Flame Atomic Adsorption Spectroscopy, Inductively Coupled Plasma-Optical Emission Spectroscopy, N-2-physisorption, X-ray Photoelectron Spectroscopy, Raman Spectroscopy and Transmission Electron Microscopy coupled with Energy Dispersive X-Ray. The results showed that reducing the carbon granulometry led to an enhancement of the reactivity under NO2, due to the external combustion mechanism of carbon crystallite. In the absence of catalysts, no impact of the granulometry was observed under a flue gas containing NO and O-2. A better catalyst efficiency was obtained when the carbon-catalyst contact was increased. In tight contact, a mechanism where NO2 can react adsorbed on a metallic site with a carbon site was proposed. In the presence of water, a simple addition of the catalytic activity of both water and Pt-Pd/Al2O3 catalyst impact on carbon oxidation was observed. The use of Biodiesel, simulated by Na-doped model soot, led to a significant increase in the catalyst efficiency in loose contact. Thus, a second mechanism was proposed, assuming that the doped sites play the role of NO2-reservoir and carrier between catalyst and carbon sites.