Study of the reaction of NOx and soot on Fe2O3 catalyst in excess of O2

Abstract

This study addresses the catalytic reaction of NOx and soot into N2 and CO2 under O2-rich conditions. To elucidate the mechanism of the soot/NOx/O2 reaction and particularly the role of the catalyst α-Fe2O3 is used as model sample. Furthermore, a series of examinations is also made with pure soot for reference purposes. Temperature programmed oxidation and transient experiments in which the soot/O2 and soot/NO reaction are temporally separated show that the NO reduction occurs on the soot surface without direct participation of the Fe2O3 catalyst. The first reaction step is the formation of CC(O) groups that is mainly associated with the attack of oxygen on the soot surface. The decomposition of these complexes leads to active carbon sites on which NO is adsorbed. Furthermore, the oxidation of soot by oxygen provides a specific configuration of active carbon sites with suitable atomic orbital orientation that enables the chemisorption and dissociation of NO as well as the recombination of two adjacent N atoms to evolve N2. Moreover, carbothermal reaction, high resolution transmission electron microscopy and isotopic studies result in a mechanistic model that describes the role of the Fe2O3 catalyst. This model includes the dissociative adsorption of O2 on the iron oxide, surface migration of the oxygen to the contact points of soot and catalyst and then final transfer of O to the soot. Moreover, our experimental data suggest that the contact between both solids is maintained up to high conversion levels thus resulting in continuous oxygen transfer from catalyst to soot. As no coordinative interaction of soot and Fe2O3 catalyst is evidenced by diffuse reflectance infrared Fourier transform spectroscopy a van der Waals type interaction is supposed.