THE FORMATION OF ISOCYANIC ACID (HNCO) BY REACTION OF NO, CO, AND H-2OVER PT SIO2 AND ITS HYDROLYSIS ON ALUMINA/

The reaction of mixtures of NO, CO, and H-2 over Pt/SiO2 in combinatio n with Al2O3 has been investigated with on-line Fourier transform infr ared spectroscopy as the principal analytical method. With Pt/SiO2 alo ne isocyanic acid (HNCO) can be observed directly as a substantial pro duct. It comprises up to 45% of the CO converted in the temperature ra nge from 200 to 300 degrees C, The yield is maximized when the amounts of H-2 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 w ith a Pt/Al2O3 catalyst, Hydrolysis to NH3 and CO2 is then complete, F or this reason it is unlikely that HNCO will escape a catalytic conver ter 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 H NCO falls steeply above 300 degrees C when the CO coverage is falling, It is unclear if hydrolysis of HNCO occurs by direct reaction of mole cular HNCO on the Al2O3 surface or proceeds via the ubiquitious suppor t-bound isocyanate species which have been observed in many previous s tudies, 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 th rough the gas phase to form NCO groups. (C) 1996 Academic Press, Inc.