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

The oxidation mechanism of carbon by automotive exhaust gasses (O-2, H2O and NO2) was investigated in the range of temperature (300-400 degrees 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 NO2 below 450 degrees C. However, when O-2 (10 vol.%) and NO2 (100-600 ppmv) are both present in the gas mixture, two distinct carbon oxidation reactions take place. A direct reaction occurs between NO2 and the carbon surface along with a co-operative one involving simultaneously O-2 and NO2. In the latter case, NO2 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-NO2 rate. In contrast, water has no measurable effect on the co-operative C-NO2-O-2 reaction. A relationship between the water promoting effect on the direct oxidation rate and the amount of theoretical formation of HNO2 and HNO3 was established. (c) 2005 Elsevier Ltd. All rights reserved.