Investigations of NOx storage catalysts

Abstract

NOx 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 NOx reduction cycle, and the extent to which these steps influence the maximum NOx reduction potential of the catalyst. As a model system, we use barium oxide as the NOx 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 NOx is found to be stored in the Ba-containing material. A maximum in NOx 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 NOx is stored under lean conditions.