The reaction between NH3 and NO in excess CO over silica-supported platinum, palladium, and rhodium has been investigated for temperatures from 100 to 450 degrees C. As found previously for the corresponding reactions of H-2/NO/CO mixtures, isocyanic acid (HNCO) is produced with each catalyst. With Pd/SiO2, the peak yield when NH3 is used is 46% based on the total nitrogen converted and 55% based on the hydrogen taken from NH3, with the remaining hydrogen converted to water. The reaction over Pt/SiO2 requires a somewhat higher temperature and is more efficient with a maximum HNCO yield of 70% based on nitrogen and 95% on hydrogen. The near-absence of water as a product correlates with the high activity of platinum for the production of HNCO from H2O/NO/CO mixtures, which in turn is driven by activity for the water-gas shift reaction. The peak yield of HNCO from NH3/NO/CO mixtures is much lower with Rh/SiO2 (10% based on nitrogen), and, unlike the yield of the platinum and the palladium catalysts, it is less than that observed during the H-2 + NO + CO reaction (30%). In experiments using (NO)-N-15 and (NH3)-N-14, the N-15 content of N-2, HNCO, and the end nitrogen of N2O are similar, consistent with dissociation of both nitric oxide and ammonia to form a single surface pool of nitrogen atoms. However, neither dissociation is rapidly reversible, since there is little exchange of N-14 from ammonia into unreacted (NO)-N-15, and significant formation of (NH3)-N-15 is confined to Pt/SiO2 at temperatures where it could be formed by hydrolysis of product H15NCO. It is concluded that HNCO is formed by the rapid pick-up of surface hydrogen atoms by metal-bound NCO groups existing in equilibrium with N atoms and surrounding CO molecules. The trends in product distribution and in activity between the three metals can be rationalised in terms of competition between NO and CO for surface sites with CO favoured on Pt, NO favoured on Rh, and Pd exhibiting intermediate characteristics. (c) 2005 Elsevier Inc. All rights reserved.