NOx storage in barium-containing catalysts

Artikel i vetenskaplig tidskrift, 1999

The effect of key parameters on the characteristics of barium oxide-based NOx storage catalysts was systematically investigated. Model Pt/BaO/Al2O3, BaO/Al2O3, Pt–Rh/Al2O3, and Pt–Rh/BaO/Al2O3 catalysts were prepared and evaluated with respect to NOx storage capacity using transient flow reactor studies, temperature-programmed desorption studies (TPD), and in situ Fourier transform infrared (FTIR) absorption spectroscopy. The influence of temperature, storage and regeneration times, NOx source (NO or NO2), oxygen concentration, reducing agent (C3H6, C3H8, CO, or H2), and carbon dioxide concentration onNOx storage capacity was studied. Significant amounts of NOx were found to be stored in the catalysts containing both barium oxide and noble metals. For these catalysts the following observations were made: (1) maximum NOx storage was observed at about 380C; (2) around this temperature no significant differences between NO and NO2 on NOx storage capacity could be observed; (3) a slow increase in stored NOx could be observed with increasing oxygen concentration during the lean phase; (4) significant NOx desorption peaks, mainly of NO, were observed immediately after the switch from lean to rich conditions; and (5) at about 380±C the in situ FTIR spectra show characteristic nitrate peaks in the region 1300–1400 cm¡1 when NOx was stored under lean conditions and isocyanate peaks around 2230 cm-1 when the catalysts were regenerated under rich conditions in the presence of hydrocarbons. The step leading to stored NOx is believed to involve NO2 and the presence of atomic oxygen. During the rich period, the noble metal surfaces are probably reduced, leading to breakthrough peaks when NO desorbs.