Formation and reactions of isocyanic acid during the catalytic reduction of nitrogen oxides

Reactions which can produce and consume isocyanic acid (HNCO) over two types of catalysts active for the reduction of nitrogen oxides have been investigated. More than 1000 ppm HNCO can be produced by the reduction of 3000 ppm NO with H2/CO mixtures over a Pt/SiO2 catalyst. Complete hydrolysis of HNCO to ammonia and carbon dioxide occurs if even weakly catalytic materials, such as CeO2/SiO2 and BaO/SiO2, are placed downstream. Isocyanic acid is also involved as an intermediate in the reaction of nitromethane over Co‐ZSM5 and Cu‐ZSM5 under the conditions of hydrocarbon SCR. In the initial stages of reaction there is complete conversion through to N2 with Cu‐ZSM5 but the process stops at ammonia with Co‐ZSM5 at temperatures below 350°C. In both cases, but especially with Co‐ZSM5, isocyanic acid becomes observable as the catalyst deactivates during continuous exposure at temperatures below about 290°C. In situ FTIR measurements indicate that deactivation is due to a reaction between isocyanic acid and ammonia which generates cyclic s‐triazine compounds.