Abstract
A comparative study of the infrared spectra of reaction intermediates and chemisorbed species during the reaction of NO + CO over Pt, Pd, Rh, Ir, and Ru-on-alumina catalysts at elevated temperatures has been made. A strong band which is assigned to a surface isocyanate species has been found in the 2260–2270 cm −1 region on all the noble metals except Ru where the band is either quite weak or entirely absent. The weakness of the band on Ru Al 2O 3 seems to correlate with its unique low ammonia-forming properties during NO x removal from automobile exhaust. Hydrolysis of the surface isocyanate species provides a pathway to NH 3 which does not involve reduction of NO directly with hydrogen already present in the exhaust stream or generated via a water-gas shift mechanism. No assessment of the relative importance of this mechanism with respect to other mechanisms for ammonia formation in actual automobile exhaust can be made from the present results. Sulfur poisoning of the Rh and Ru catalysts is observed to inhibit the formation of the isocyanate species. However, this poisoning is temporary and the -NCO sites can be regenerated by oxidation treatment.
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