Investigations of NOx storage catalysts

Abstract

NO_x storage catalysts are used to reduce nitrogen oxides from lean-bum vehicles. The nitrogen oxides are stored in the catalyst during lean conditions and subsequently released and reduced during short periods of rich conditions. In the present study, we systematically investigate the sequence of elementary steps in the NO_x reduction cycle, and the extent to which these steps influence the maximum NO_x reduction potential of the catalyst. As a model system, we use barium oxide as the NO_x storing compound in a Pt/Rh/Al₂O₃ system. Kinetics of NO oxidation, NO and NO₂ adsorption, NO and NO2 release and reduction are studied under controlled conditions with systematic variations of temperature, gas composition, and storing/release times. The transient experiments comprise a storing phase using a lean NO/C₃H₆/O₂/N₂ gas mixture, and a regenerating phase where the O₂ flow is turned off. Experimentally, a significant amount of NO_x is found to be stored in the Ba-containing material. A maximum in NO_x storage is observed around 380 °C. For most of the experiments, there are clear NO and NO₂ desorption peaks upon switching from the storing to the regeneration phase. TPD studies of NO and NO₂ reveal a significant difference between prereduced and pre-oxidised samples where the former produce predominantly N₂ and N₂O at around 200 °C while NO and O₂ desorb from the latter around 500 °C. In situ FTIR spectra show nitrate peaks in the region 1300 - 1400 cm^− 1when NO_x is stored under lean conditions.