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

Thermodynamically, isocyanic acid (HNCO) is the most stable molecule with one atom each of hydrogen, nitrogen, carbon, and oxygen. It is a primary reactant species in the RAPRENOx scheme for the homogeneous reduction of NO_x using cyanuric acid and when urea is used for the catalytic reduction of NO_x in diesel exhaust. An investigation of reactions that can produce and consume HNCO over two kinds of catalysts active for reducing NO_x showed that > 1000 ppm HNCO can be produced by reducing 3000 ppm NO with H₂/CO mixtures over a Pt/SiO₂ catalyst. Complete hydrolysis of HNCO to ammonia (NH₃) and CO₂ occurred when even weakly catalytic materials, e.g., CeO₂/SiO₂ and BaO/SiO₂, were placed downstream. HNCO was involved as an intermediate in the reaction of nitromethane over Co-ZSM5 and Cu-ZSM5 under hydrocarbon SCR conditions. In the early stages of reaction there was a complete conversion through to N₂ with Cu-ZSM5, but the process stopped at NH₃ with Co-ZSM5 < 350°C. In both cases, but particularly with Co-ZSM5, isocyanic acid became observable as the catalyst deactivated during continuous exposure below ~ 290°C. In situ FTIR measurements showed that deactivation was due to a reaction between HNCO and NH₃ that generated cyclic s-triazine compounds.