Dual Effects of C 3 H 6 in Suppressing and Promoting NH 3 –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 (NO_x) 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 NO_x 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 NO_x 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 NO_x by C₃H₆ (C₃H₆-SCR), supplementary NH₃ released from -NCO hydrolysis and the inhibition of ammoxidation with C₃H₆, promote high-temperature (>400 °C) SCR activity.