n-Heptane As a Reducing Agent in the NOxRemoval over a Pt–Ba/Al2O3NSR Catalyst

The reduction with n-heptane of gaseous NO and NOx stored over a model PtBa/Al2O3 catalyst is investigated in this paper by transient microreactor flow experiments coupled with in situ FT-IR spectroscopy for the analysis of the surface species. It is found that n-heptane is an effective reductant of both gaseous NO and stored NOx at temperatures above 200 and 250 °C, respectively. The reduction leads to the selective formation of N2 above 300 °C, whereas significant amounts of other species (N2O) are also formed at lower temperatures. A reaction pathway for the reduction of stored NOx is suggested where n-heptane initially reduces the oxygen-covered Pt sites. This leads to the activation/destabilization of the stored NOx, leading to the NOx release. Released NOx dissociate over Pt sites into N- and O-adatoms: the O-adatoms are scavenged by the hydrocarbon leading to COx and H2O, whereas N-adatoms may recombine with undissociated NO molecules, with other N-adatoms, or with H-ad-species to give N2O, N2, and NH3, respectively. The Pt oxidation state (or the oxygen coverage) drives the selectivity of the process. Under wet conditions the hydrocarbon molecule is involved in the SR reaction and WGS reactions as well, leading to the formation of H2 that participates in the reduction of stored NOx. This route is however of minor importance over the selected catalyst. Finally isocyanate species are also detected under conditions favoring the formation of CO, that may participate in the reduction of the stored NOx.