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/O2, NO2 or NO2/O2 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, appreciable amounts of NOx are stored on both systems. The storage occurs with the initial formation of ionic nitrites that completely evolve to ionic nitrates at high contact times (20 min). At 623 K on Pt-Ba/Al2O3 the storage also occurs through NO oxidation to NO2, rapidly adsorbed as ionic nitrates. When NO2 is used with or without O2, the storage occurs in very extensive amounts, both on Ba/Al2O3 and Pt-Ba/Al2O3 catalysts at all the temperatures studied, via ionic nitrates without the initial formation of nitrites. This suggests that NO2 can be stored through a fast mechanism that, on one hand, does not need the presence of oxygen and, on the other hand, is not promoted by the presence of Pt.