Mechanistic Insights into the Formation of Hydrogen Cyanide on Cu-SSZ-13 Zeolites During Ammonia-Assisted Selective Catalytic Reduction in the Presence of Formaldehyde: A Perspective from Ab Initio Energetic Span Modelling

The emission of hydrogen cyanide (HCN) from formaldehyde (CH2O) during ammonia-assisted selective catalytic reduction (NH3-SCR) remains a critical challenge for aftertreatment of bio-hybrid fuel combustion exhaust. The mechanistic details of HCN formation are still poorly understood, especially on widely deployed commercial catalysts like Cu-SSZ-13. In this work, we employed density functional theory calculations in combination with the Energetic Span Model to elucidate HCN formation pathways from CH2O in the presence of NO2 and H2O over Cu-SSZ-13. The results revealed the HCN formation pathway with intermediate methylene imine as the dominant one under typical reaction conditions. These findings resonate very well with reports of hexamethylenetetramine (HMT) formation during NH3-SCR with CH2O, for which methylene imine is a critical intermediate. Turnover frequency (TOF) estimations highlighted the strong influence of NO2 and H2O: higher NO2 concentrations promoted CO selectivity and suppressed HCN by oxidizing CH2O to HCOOH, while lower H2O enhanced HCN formation. These findings establish a detailed mechanistic framework for HCN emission on Cu-SSZ-13 and suggest that controlling NO2/NOx ratios and water content can mitigate HCN formation during NH3-SCR.