Mechanism of propylene effect on the NH3-SCR performance of Cu-SSZ-13 catalyst

The ability of commercialized Cu-SSZ-13 to catalyze the Selective catalytic reduction of NOx by ammonia (NH3-SCR) is inhibited in the presence of hydrocarbons. The effect of C3H6 on the performance of Cu-SSZ-13 zeolite catalysts for NH3-SCR with various copper loadings and Si/Al ratios (SARs) was investigated in this work. At low temperatures, the competitive adsorption of C3H6 with NH3 and NOx is the main reason for the performance decline in the low-temperature range. When the temperature was above 250 °C, C3H6 began to be activated, resulting in carbon deposition and propylene ammoxidation, which are both major factors in the reduction of NOx conversion that accompanies C3H6 poisoning. The propylene ammoxidation reaction has a significant influence on the activity due to a shortage of the reducing agent NH3, and becomes progressively dominant with increasing Cu loading and decreasing SAR. C3H6 preferably participates in the direct oxidation of C3H6 and the selective catalytic reduction of NOx by C3H6 (C3H6-SCR) in the high-temperature range, contributing to the suppression of propylene ammoxidation and the restoration of NH3-SCR performance, and this phenomenon is facilitated by the increase in Cu loading and SAR. The accumulation of carbon deposits formed after high-temperature activation of C3H6 can affect the low-temperature NH3-SCR performance, but the carbon deposits can be burned off in the high-temperature range, thus restoring activity. The overall results can guide the control strategy of after-treatment systems in diesel engines.