Role of nitrogen dioxide in the oxidation of diesel soot on promoted mixed catalysts with fluorite and perovskite structures

Abstract

The oxidation of soot on catalysts with the perovskite and fluorite structures (including platinum-promoted catalysts) in the presence and in the absence of NO2 was studied using in situ IR spectroscopy and temperature-programmed techniques (TPR, TPD, and TPO). It was found that, as a rule, the temperature of the onset of soot oxidation considerably decreased upon the addition of NO2 to a flow of O2/N2, whereas the amount of oxygen consumed in soot oxidation considerably increased. To explain these facts, we hypothesized that the initiation of soot combustion in the presence of NO2 was related to the activation of the NO2 molecule through the formation (at a low temperature) and decomposition (at a high temperature) of nitrate structures on the catalyst. Superequilibrium amounts of NO2 resulted from the decomposition of nitrate complexes immediately on the catalyst for soot combustion. Based on a comparison between catalyst activities and data obtained by TPR and the TPD of oxygen, a conclusion was drawn that the presence of labile oxygen in the catalyst is a necessary but insufficient condition for the efficient occurrence of a soot oxidation reaction in the presence of NO2. The introduction of platinum as a constituent of the catalyst increased the amount of labile oxygen and, as a consequence, increased the amount of highly reactive nitrate complexes. As a result, this caused a decrease in the temperature of the onset of soot combustion.