The formation of isocyanic acid (HNCO) by reaction of NO, CO, and H₂ over Pt/SiO₂ and its hydrolysis on alumina

The reaction of mixtures of NO, CO, and H₂ over Pt/SiO₂ in combination with Al₂O₃ has been investigated with on-line Fourier transform infrared spectroscopy as the principal analytical method. With Pt/SiO₂ alone isocyanic acid (HNCO) can be observed directly as a substantial product. It comprises up to 45% of the CO converted in the temperature range from 200 to 300°C. The yield is maximized when the amounts of H₂ and CO in the input stream are equal. No HNCO can be observed in experiments in which Al₂O₃ is placed downstream of the Pt/SiO₂ or with a Pt/Al₂O₃ catalyst. Hydrolysis to NH₃ and CO₂ is then complete. For this reason it is unlikely that HNCO will escape a catalytic converter in vehicle use but it may be a significant intermediate during the warm-up phase. The formation of HNCO can be rationalized as resulting from the hydrogenation of NCO groups present in small concentrations on the Pt surface under conditions of high CO coverage. The yield of HNCO falls steeply above 300°C when the CO coverage is falling. It is unclear if hydrolysis of HNCO occurs by direct reaction of molecular HNCO on the Al₂O₃ surface or proceeds via the ubiquitious support-bound isocyanate species which have been observed in many previous studies. Previous observations of isocyanate formation in hydrogen-free systems can be rationalized as arising from HNCO formation by reverse spillover of hydrogen from OH groups on the support to generate HNCO on the platinum metal. This is then transported back to the support through the gas phase to form NCO groups.