FT-IR investigation of NOx storage properties of Pt–Mg(Al)O and Pt/Cu–Mg(Al)O catalysts obtained from hydrotalcite compounds

NOx storage-reduction catalysts based on Pt and Pt/Cu supported on Mg(Al)O mixed oxides obtained from hydrotalcite-type compounds were studied. In particular, NOx storage capability upon admission of NO and NO2 in the absence or in the presence of excess oxygen at temperatures up to 623 K was investigated by in situ FT-IR spectroscopy. For comparative purposes, the bare mixed oxide support was also investigated. Pure NO2 is adsorbed according to a dismutation reaction with the simultaneous formation of nitrates and nitrites that progressively evolve to nitrates (“dismutation route”), the nitrite evolution to nitrates is promoted by the metal phase. When metal phase is present, the nitrites oxidation is further accelerated by the oxygen presence. Pure NO is not stored. When O2 is present, NO is stored according to two different pathways, both promoted by the metal phases: (i) the NO oxidation to nitrites followed by their oxidation to nitrates (“nitrite route”); (ii) the NO oxidation to NO2, followed by the “dismutation route”.