Enhanced Durability of a Cu/Zeolite Based SCR Catalyst 2008-01-1025
Passenger and light duty diesel vehicles will require up to 90% NOx conversion over the Federal Test Procedure (FTP) to meet future Tier 2 Bin 5 standards. This accomplishment is especially challenging for low exhaust temperature applications that mostly operate in the 200 - 350°C temperature regime. Selective catalytic reduction (SCR) catalysts formulated with Cu/zeolites have shown the potential to deliver this level of performance fresh, but their performance can easily deteriorate over time as a result of high temperature thermal deactivation. These high temperature SCR deactivation modes are unavoidable due to the requirements necessary to actively regenerate diesel particulate filters and purge SCRs from sulfur and hydrocarbon contamination. Careful vehicle temperature control of these events is necessary to prevent unintentional thermal damage but not always possible. As a result, there is a need to develop thermally robust SCR catalysts. Fe/zeolite formulations are known to exhibit superior hydrothermal stability over Cu/zeolite formulations. However, current Fe/zeolite formulations are not very active for NOx conversion in the desired 200 - 350°C temperature regime under conditions having low NO2/NOx ratios. From previous studies, Cu/zeolite formulations have demonstrated never-to-exceed temperatures up to 775°C. In this work, a laboratory flow reactor was utilized to hydrothermally age and evaluate the latest state-of-the-art Cu/zeolite formulations. Results confirm remarkable high temperature hydrothermal stability up to 950°C while maintaining stable low temperature NOx activity. A broad range of time-at-temperature hydrothermal aging was carried out to clearly define the full durability range. The aging time was varied from 1 hour to 256 hours while the aging temperature was varied from 670°C to 1100°C. The catalyst performance was evaluated under a synthetic exhaust gas mixture commonly known as the “Standard” SCR reaction.