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The kinetics of NO reduction by H 2 CO mixtures have been investigated over a Rh SiO 2 catalyst. Nearly 90% of the reduction can be ascribed to H 2. The presence of CO retards the rate of NO reduction, presumably due to competitive adsorption with H 2. The formation of urea is a direct result of NO reduction by H 2 and CO. The catalyst activity is 50% higher when the catalyst is preoxidized rather than prereduced. N 2 O, N 2, NH 3, and (NH 2) 2 CO are formed over the prereduced catalyst. NH 3 formation is largely suppressed when the catalyst is preoxidized and a product believed to be HNCO is formed instead of urea. In situ infrared spectra of the catalyst show that for NO conversions below 70%, the surface is dominated by adsorbed NO. At higher conversions, bands are also observed for adsorbed CO, but none for isocyanate species attached to Rh.