HCN production from formaldehyde during the selective catalytic reduction of NOx with NH3 over V2O5/WO3-TiO2

[Display omitted] •Formaldehyde present in exhaust gases can influence the SCR of NOx with NH3.•HCN production is observed when formaldehyde is added to the SCR feed on V2O5/WO3-TiO2.•The more NH3 is dosed to compensate for the parasitic reaction, the more HCN is produced.•HCN production from formic acid and NH3 occurs only below 300 °C under dry conditions.•HCN production above 300 °C is not associated with the formate route. Raw exhaust gases may contain notable levels of formaldehyde that can negatively impact the efficiency of after-treatment systems. In the selective catalytic reduction (SCR) of NOx over V2O5/WO3-TiO2, formaldehyde was found to react with NH3 to produce HCN at concentrations above the threshold limit value set by environmental/safety organizations. Due to this side reaction, NH3 is consumed parasitically and the NOx conversion decreases by up to 15 %, even after compensating for the fraction of lost NH3. Under similar conditions, the non-reducible TiO2 support also produced HCN moderately, thereby showing that redox sites promote the reaction but are not a necessary condition. To understand the chemistry responsible for HCN formation, the roles of reaction temperature, water, and oxygen were investigated. Our results suggest a new pathway for HCN production through the direct reaction of formaldehyde and NH3, which is active at high temperature and does not proceed through the formate route previously proposed.