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

Reactions which can produce and consume isocyanic acid (HNCO) over two type s of catalysts active for the reduction of nitrogen oxides have been invest igated. More than 1000 ppm HNCO can be produced by the reduction of 3000 pp m NO with H-2/CO mixtures over a Pt/SiO2 catalyst. Complete hydrolysis of H NCO 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-ZS M5 and Cu-ZSM5 under the conditions of hydrocarbon SCR. In the initial stag es of reaction there is complete conversion through to N-2 with Cu-ZSM5 but the process stops at ammonia with Co-ZSM5 at temperatures below 350 degree s C. In both cases, but especially with Co-ZSM5, isocyanic acid becomes obs ervable as the catalyst deactivates during continuous exposure at temperatu res below about 290 degrees C. In situ FTIR measurements indicate that deac tivation is due to a reaction between isocyanic acid and ammonia which gene rates cyclic s-triazine compounds.