Dual Effects of C3H6 in Suppressing and Promoting NH3-SCR Performance over Cu-SSZ-16 Catalyst at Different Temperatures.

Unburned hydrocarbons (HCs), particularly light HCs such as propene (C3H6), have a significant effect on the performance of Cu-based small-pore zeolites in selective catalytic reduction with ammonia (NH3-SCR) for the control of nitrogen oxides (NOx) emitted from diesel vehicles. During this process, the formation of highly toxic hydrogen cyanide (HCN) occurs, posing significant risks to both the environment and human health. In this work, the temperature-dependent poisoning mechanism of C3H6 and the HCN formation pathway over the Cu-SSZ-16 catalyst were systematically investigated. It was found that C3H6 has dual effects on SCR activity. Firstly, competitive adsorption between C3H6 and NH3 as well as NOx on the active sites results in a decline in catalytic activity at low temperatures (<225 °C). Secondly, the ammoxidation of C3H6 takes place in the medium temperature range (225-400 °C), resulting in an absence of NH3 to participate in NOx reduction. During this process, HCN is produced by dehydration of formamide (HCONH2) formed from reaction between B-NH4+ and partial oxidation products of C3H6. Finally, the synergistic effects of the selective catalytic reduction of NOx by C3H6 (C3H6-SCR), supplementary NH3 released from -NCO hydrolysis and the inhibition of ammoxidation with C3H6, promote high-temperature (>400 °C) SCR activity.