Abstract
The interaction of NO + CO with 5% Rh/Al 2O 3 was investigated by infrared spectroscopic measurements in the temperature range 25–400°C. Absorption bands attributed to the surface isocyanate (M–NCO) were observed at 2195 and 2272 cm -1. The formation of isocyanate was followed as a function of time, temperature and partial pressure of reacting gases. It was detected at as low as 100°C; maximum intensity was obtained at 200°C and at an NO/CO ratio of 1:2. The formation of isocyanate was detected on oxidized surfaces, but measurable intensities occured only at higher temperatures. The study of the reactions of adsorbed NO with CO revealed a molecular interaction between the adsorbed NO species and CO. It is assumed that this surface interaction promotes the dissociation of NO and then the formation of isocyanate at lower temperatures. Some results are presented concerning the effect of supports which basically determine the location, formation and stability of the isocyanate band at higher frequencies (the absorption band varied with the support from 2210 to 2317 cm -1). It is proposed that the isocyanate complex is formed on rhodium (the absorption band is at 2195 cm -1), but it rapidly migrates to the supports.
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