Aging Effects of Catalytic Converters in Diesel Exhaust Gas Systems and Their Influence on Real Driving NOx Emissions for Urban Buses

The selective catalytic reduction (SCR) of nitrogen oxides seems to be the most promising technique to meet prospective emission regulations of diesel-driven commercial vehicles. In the case of developing cost-effective catalytic converters with comparably high activity, selectivity, and resistance against aging, ion-exchanged zeolites play a major role. This study presents, firstly, a brief literature review and subsequently a discussion of an extensive conversion analysis of exemplary Cu/and Fe/zeolites, as well as a homogeneous admixture of both. The aging stages of SCR catalysts deserve particular attention in this study. In addition, the aging condition of the diesel oxidation catalyst (DOC) was analyzed, which influences the nitrogen dioxide (NO2) formation, because the NO2/nitrogen oxides (NOx) ratio upstream from the SCR converter could be identified as a key factor for low temperature NOx conversion. Furthermore, it could be proved that a surplus of NO2 has the potential to suppress the overall efficiency in NOx conversion. In the final step of this investigation, the data gained during the conversion analysis were used as input parameters for a numerical model. This previously published simulation model was applied to predictively determine NOx emissions for one representative public bus route, for all investigated configurations of SCR catalysts and DOCs in different aging stages. It could be shown that an exhaust gas system in a moderate aging stage has the potential to deliver lower NOx emissions than its fresh equivalent. The reason for this could be a more favorable NO2/NOx ratio caused by an aged DOC compared to a fresh oxidation catalyst, which overcompensates for the losses in activity of aged SCR converters, particularly at low temperatures.