Oxidation mechanism of carbon black by NO2 : Effect of water vapour

Abstract The oxidation mechanism of carbon by automotive exhaust gasses (O 2 , H 2 O and NO 2 ) was investigated in the range of temperature (300–400 °C) corresponding to the operation conditions of the continuous regeneration regime of the soot trap. No carbon gasification occurs when O 2 is injected in absence of NO 2 below 450 °C. However, when O 2 (10 vol.%) and NO 2 (100–600 ppmv) are both present in the gas mixture, two distinct carbon oxidation reactions take place. A direct reaction occurs between NO 2 and the carbon surface along with a co-operative one involving simultaneously O 2 and NO 2 . In the latter case, NO 2 promotes the decomposition of the intermediate C(O) complex. Injection of water vapour increases the overall oxidation rate but does not modify the global mechanism. The promoting effect of water is attributed to the intermediate formation of traces of nitric and nitrous acids which enhance the direct reaction C–NO 2 rate. In contrast, water has no measurable effect on the co-operative C–NO 2 –O 2 reaction. A relationship between the water promoting effect on the direct oxidation rate and the amount of theoretical formation of HNO 2 and HNO 3 was established.