In situ FTIR studies of the mechanism of NO x storage and reduction on Pt/Ba/Al 2O 3 catalysts

Abstract

The mechanism of NO x adsorption and reaction on a typical 1 wt.% Pt/20 wt.% BaO/Al 2O 3 NO x storage reduction (NSR) catalyst was studied by the use of in situ Fourier Transform InfraRed (FTIR) spectroscopy. Parallel studies were also conducted with the individual components of the NSR system (i.e., 1 wt.% Pt/Al 2O 3 and 20 wt.% BaO/Al 2O 3) in an effort to identify the role and importance of each component in the NSR process. Finally, the role of SO 2 was also investigated, since it has been reported that it represents a poison for the NSR process. The results indicate that at 350 °C NO x is stored on the NSR catalyst primarily in the form of barium nitrite and nitrate species. In the presence of Pt a conversion of the surface barium nitrite to bulk barium nitrate is taking place during the storage phase. This step is accelerated in the presence of gas phase oxygen and at elevated temperatures. Both barium nitrite and nitrate species are reduced in the presence of propylene, although at different rates. These reactions are catalyzed by Pt and do not proceed at any significant rate over the BaO/Al 2O 3 sample. Finally, the formation of surface and bulk barium sulfates was observed when SO 2 was introduced as a component of the NO-containing gas phase mixture. These species are stable on the catalyst surface under the reduction conditions studied and inhibit the NO x storage process.