Nature and thermal stability of adsorbed intermediates formed during the reaction of diesel soot with nitrogen dioxide

The reactivity of a diesel exhaust soot sample towards 1000 ppm NO2/balance Ar was investigated at moderate temperature using thermogravimetric analysis (TGA), temperature-programmed desorption (TPD-MS) and diffuse reflectance infrared spectroscopy (DRIFTS) with the aim to evidence the nature and thermal stability of adsorbed intermediates formed during the reaction. In the range 25–200 8C, the reaction pathways are influenced by the temperature dependency of the reaction regimes. While weak adsorption of NO2 prevails at temperatures under ca. 60 8C, it is found that associative and dissociative chemisorption occurs on the whole range of temperatures investigated, creating, respectively, nitrogenated (RNOx) and oxygenated (ROx) adsorbed intermediates on soot surface. DRIFTS and TPD experiments reveal that major surface species include on the one hand alkyl-nitrite and -nitrate esters adopting presumably a chelated or bidentate configuration on soot surface and on the other hand more stable acidic oxygenated surface complexes formed by reduction of NO2.