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

Noel W. Cant*, Dean C. Chambers, Andrew D. Cowan, Irene O Y Liu, Atsushi Satsuma

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    27 Citations (Scopus)


    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 NOx using cyanuric acid and when urea is used for the catalytic reduction of NOx in diesel exhaust. An investigation of reactions that can produce and consume HNCO over two kinds of catalysts active for reducing NOx 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.

    Original languageEnglish
    Pages (from-to)13-20
    Number of pages8
    JournalTopics in Catalysis
    Issue number1-2
    Publication statusPublished - Mar 2000


    • Deactivation
    • Hydrolysis
    • Isocyanic acid
    • Nitrogen oxides
    • Nitromethane
    • Pt/SiO


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