Effect of Formaldehyde in Selective Catalytic Reduction of NO by Ammonia (NH-SCR) on a Commercial VO-WO/TiO Catalyst under Model Conditions.

The impact of formaldehyde (HCHO, formed in vehicle exhaust gases by incomplete combustion of fuel) on the performance of a commercial VO-WO/TiO catalyst in NH-SCR of NO under dry conditions has been analyzed in detail by catalytic tests, in situ FTIR and transient studies using temporal analysis of products (TAP). HCHO reacts preferentially with NH to a formamide (HCONH) surface intermediate. This deprives NH partly from its desired role as a reducing agent in the SCR and diminishes NO conversion and N selectivity. Between 250 and 400 °C, HCONH decomposes by dehydration (major pathway) and decarbonylation (minor pathway) to liberate toxic HCN and CO, respectively. HCN was proven to be oxidized by lattice oxygen of the catalyst to CO and NO, which enters the NH-SCR reaction.