Catalytic effect of platinum on the kinetics of carbon oxidation by NO2 and O2

The effect of a commercial Pt/Al2O3 catalyst on the oxidation by NO2 and O2 of a model soot (carbon black) in conditions close to automotive exhaust gas aftertreatment is investigated. Isothermal oxidations of a physical mixture of carbon black and catalyst in a fixed bed reactor were performed in the temperature range 300–450 °C. The experimental results indicate that no significant effect of the Pt catalyst on the direct oxidation of carbon by O2 and NO2 is observed. However, in presence of NO2–O2 mixture, it is found that besides the well established catalytic reoxidation of NO into NO2, Pt also exerts a catalytic effect on the cooperative carbon–NO2–O2 oxidation reaction. An overall mechanism involving the formation of atomic oxygen over Pt sites followed by its transfer to the carbon surface is established. Thus, the presence of Pt catalyst increases the surface concentration of –C(O) complexes which then react with NO2 leading to an enhanced carbon consumption. The resulting kinetic equation allows to model more precisely the catalytic regeneration of soot traps for automotive applications.