MnOx-CeO2 mixed oxides for the low-temperature oxidation of diesel soot

TG-FTIR and flow reactor experiments were performed to study soot oxidation on MnOx-CeO2 mixed oxide catalysts in model diesel exhaust gas (10% O2, 5% H2O, 1000ppm NO, balance N2). The ignition temperature in TG-FTIR experiments for a 1:20 mixture of soot and catalyst was found to be ∼280°C, which is significantly lower than for the individual oxides. It was found that NO is oxidized over the catalyst to NO2, which is stored on the catalyst as nitrate at low temperatures. At higher temperatures the nitrates decompose, releasing NO2 to the gas phase which acts as the oxidizing agent for soot. The nitrate storage capacity of MnOx-CeO2 is three to five times higher than that of the individual oxides resulting in a major contribution of the released NO2 to the soot oxidation process. This explains the strong synergistic effect of manganese and cerium in the mixed oxide on the soot oxidation. Adding SO2 to the model gas resulted in a deactivation of the catalyst, which is traced to the loss of the NO oxidation activity. The sulphates could be decomposed by heating the catalyst under reducing as well as oxidizing conditions. However, the initial activity of the catalyst could not be restored.