Mechanistic investigation of NOx storage reduction (NSR) over Pt- and Rh-based BaO/Al2O3 catalysts

The removal of nitrogen oxides from diesel engines based on the NOx storage Reduction concept (also referred as Lean NOx Trap, NSR) is an appealing solution to successfully control exhaust emissions under high air/fuel ratio. In this technique, NOx are stored on the catalyst surface during the normal engine operation; once the catalyst becomes saturated with NOx, the engine is made running rich for a short period of time and reduction of the stored NOx is attained. Typical catalytic materials comprise noble metals like Pt, Pd, Rh and storage elements (Ba, K) dispersed over an high surface area carrier. This thesis work addresses mechanistic and reactivity aspects of the NSR catalysis. In particular catalysts with different noble metals (Pt, Rh) have been considered to better enlighten the role of the noble metal on the catalyst reactivity (adsorption of NOx and reduction). Particular emphasis has been given to the analysis of the reduction phase; for this purpose different reductants have been used (H2, NH3, n-heptane) and aspects related to the formation of N2O, an highly undesired byproduct, have been deepened. It is found that n-heptane is an effective reductant of both gaseous NO and stored NOx. The reduction leads to the selective formation of N2 above 300 °C, whereas significant amounts of N2O are also formed at lower temperatures. The reactivity is not markedly affected by the presence of water, despite the fact that in the presence of water the hydrocarbon molecules are in involved in the SR reaction and WGS reactions, resulting in H2 formation with a higher reduction capacity of stored NOx. However H2 formation is seen at high temperatures, above the onset temperature of the reaction of heptane with the stored NOx. Notably, under the investigated temperature range, no adspecies like NCO have been observed on the surface of the catalyst. These results suggest that n-heptane directly participates in the reduction of the stored NOx. It is expected that the Pt sites are kept in reduced state by the hydrocarbon, which invokes the migration of stored NOx toward active metal sites followed by dissociation into N- and O- adatoms. Then O- adatoms are scavenged by the hydrocarbon, while N adatoms and NO ad-species recombine to generate the main products of reaction, i.e. N2 and N2O. Concerning the influence of nature of the noble metal (Pt vs. Rh) on the NOx storagereduction, it was found that the storage of NOx is affected by the oxidizing properties of the noble metal (with Pt being more active than Rh) and by the metal dispersion. Also the reduction of the stored NOx is affected by the nature of the noble metal, with Pt being more active than the Rh counterpart. The isotopic labelling study shows that the reduction implies the release of NO followed by its reduction; the NO release is a redox reaction favoured by the presence of reductant species. In this respect, Pt is more easily reducible than Rh and this leads to its superior reactivity. Finally, concerning N2O formation, mechanistic aspects concerning the formation of N2O over Pt and Rh- based BaO/Al2O3 catalyst clearly shows that N2O formation involves the presence of gas phase NO. Notably N2O formation originates from the coupling of undissociated NO molecules with N-adspecies formed upon NO dissociation on the redox centers provided by PGM sites. The formation of N2O is favoured at low temperatures, when PGM centers are being reduced, and is prevented at higher temperatures when redox sites are in reduced state and complete dissociation occurs. Furthermore reactivity study in the presence of reducing has shown that N2O is observed at lower temperature in the reaction with H2, if compared to NH3. These results are in line with higher reduction capacity of H2 in the scavenging of O ad atoms from the PGM sites; in particular of Pt containing catalyst due better redox properties with respect to Rh based catalytic samples.