Catalytic removal of NOx and soot from mobile sources

Commercial vehicles and diesel passenger cars will be subjected in a near future to very stringent emission regulations regarding nitrogen oxides (NOx) and particulate. To handle these limits the use of exhaust after-treatment technologies is required. Different strategies have been proposed for lean-burn engines; one such strategy is the Diesel Particulate NOx Reduction (DPNR) system that accomplishes the simultaneous removal of NOx and particulate. In this thesis work the behavior of a model PtBa/Al2O3 and PtK/Al2O3 NSR catalyst in both NOx storage/reduction and soot oxidation is investigated. It is found that the presence of soot reduces the NOx storage capacity of the catalyst, evaluated in presence of water and CO2 in the feed stream in the range 200–350 °C and with different values of the NO inlet concentration. Besides the presence of soot favors the decomposition and the reduction of the stored nitrates, while soot is being oxidized. A direct reaction between the stored nitrates and soot is suggested, that has been explained on the basis of the surface mobility of the adsorbed nitrates. This soot oxidation pathway involves surface species and parallels the NO2-soot oxidation that occurs in the presence of gas-phase NO2. This has also been confirmed by dedicated TPD and TPO experiments. However the presence of soot does not appreciably affect the behavior of the catalysts in the reduction by H2 or CO of the stored nitrates, being in all cases N2 the major reaction product along with minor amounts of ammonia. Mechanistic aspects involved in the formation of N2 and of N2O during the reduction of gas-phase NO and of NOx stored over PtBa/Al2O3 catalyst are also investigated using unlabelled ammonia and labelled NO. It appears that N2 formation occurs primarily through the statistical coupling of N-atoms formed by dissociation of NOx- and NH3-related surface intermediates, although an SCR-pathway (involving the coupling of NH3- and NO-derived ad-species) is also likely to occur. It appears as well that the formation of nitrous oxide involves either the coupling of two adsorbed NO molecules or the recombination of an adsorbed NO molecule with an adsorbed NHx fragment. Mecanicistic aspects of NO oxidation reaction are also investigated with other catalytic systems of interest like Rh and Co-based catalysts. It is shown that NO oxidation on RhO2 and Co3O4 is limited by O2 activation at vacancies on oxygen-saturated surfaces, as also found on Pt and PdO. Oxygen binding energies set vacancy densities and turnover rates. One electron reductions accessible to RhO2 and Co3O4 facilitate O2 activation and allow faster 16O2-18O2 exchange and NO oxidation than expected from their oxygen binding strengths.