Characterization of the NO-soot combustion process over La0.8Ce0.2Mn0.7Bi0.3O3 catalyst

Abstract

The combustion of diesel soot in NO+O2/He atmospheres was investigated by temperature-programmed oxidation (TPO), X-ray photoelectron spectroscopy (XPS), and in-situ diffuse reflectance Fourier transform (DRIFT) technology, with the aim to demonstrate the participation of nitric oxide species and surface carbon-oxygenated complexes in the process. According to the TPO results, two main adsorption periods for nitric oxide species were observed: one was from 50 to about 180°C and the other occurred in the temperature range of 180–400°C. The DRIFT results indicated that the first adsorption period was mainly associated with weakly adsorbed NO, bidentate nitrite and unidentate nitrate, monodentate nitrate, chelating nitrate and bridging nitrate, while the second uptake was related to nitrite and free nitrate species. Based on the XPS and DRIFT results, possible reaction processes for the NOx-soot combustion are proposed based on the Langmuir–Hinshelwood mechanism. First, the carbon-oxygenated groups, including carboxyls, lactones, and anhydrides were formed at free carbon sites by the oxidation of dissociative nitrates. Such moieties were so stable that they could not be further oxidized, even at 400°C, without the presence of a catalyst. Second, the adsorption of NO+O2 resulted in the formation of nitrates on the MnOx surface of the catalyst. Then, the surface-activated nitrates, with activity decreasing in the order: chelating nitrates>bridging nitrates>monodentate nitrates>free ionic nitrates, further oxidized these carbon-oxygenated complexes with subsequent release of CO2, NO and N2.