In situ FT-IR and reactivity study of NOx storage over Pt-Ba/Al2O3 catalysts

The mechanism of NOx storage on Pt - Ba/Al2O3 and, for comparison, on Ba/ Al2O3 catalysts was investigated. In situ FT-IR spectroscopy was used to follow temporal changes in the types and relative amounts of surface species formed at three different temperatures (423, 523, 623 K) during the reaction with NO, NO/O-2, NO2 or NO2/O-2 atmospheres. At 623 K the NOx storage capability was also investigated by a mass-spectrometry transient response method (TRM) as a complementary technique. The results show that in absence of oxygen appreciable amounts of NO are adsorbed only on the Pt-Ba/Al2O3 system mainly as ionic nitrites. When oxygen is present, NO is stored according to two different mechanisms, both promoted by Pt. The first occurs through the formation of surface nitrites that evolve to nitrates; the second occurs through the NO oxidation to NO2 over Pt, followed by NO2 migration to the Ba phase and nitrate formation. When NO2 is used with or without O-2, an extensive storage in the form of ionic nitrates occurs both on Ba/ Al2O3 and Pt - Ba/ Al2O3 catalysts at all the temperatures studied, without the formation of nitrites. NO2 storage occurs initially through a very fast mechanism, giving nitrates without NO release. In parallel, it proceeds via a disproportionation reaction of NO2 to nitrate and NO. Both mechanisms are not promoted by Pt.