NOx removal catalysis under lean conditions

The storage-reduction of NOx over Pt–Ba/Al2O3 samples is analyzed in this work. It is pointed out that the storage of NO in the presence of excess oxygen proceeds through two parallel routes: (i) the “nitrate route” where NO is oxidized to NO2 on Pt sites followed by NO2 disproportionation on Ba sites to form nitrates with evolution of gaseous NO; (ii) the “nitrite route”, where NO is oxidized on Pt sites and directly stored on Ba neighbouring sites in the form of nitrites that are then oxidized to nitrates. The latter route possibly dominates at high Ba loadings. Concerning the reduction of the stored NOx, the bulk of data indicates that the process under isothermal conditions is not initiated by the release of NOx in the gas phase but occurs through a Pt-catalysed reduction of adsorbed NOx species. This route requires the co-presence of Pt and Ba on the same support. It is speculated that H2 is activated on Pt sites and spills on the alumina support towards nitrate ad-species where it operates the reduction, although alternative explanations are possible.