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

Abstract

The mechanism of NO_x 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 NO_x 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 O₂, 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.