Abstract
The effect of potassium on the formation of isocyanate species in the NO + CO reaction on supported Rh catalysts was investigated by means of infrared spectroscopy. Addition of potassium to 5% Rh/Al 2O 3 significantly decreased the uptake of H2, CO, and NO at 300 K, as indicated by adsorption and infrared spectroscopic measurements. All absorption bands of adsorbed CO and NO registered for K-free Rh/Al 2O 3 have been identified for K-containing samples. However, the relative intensity of bands of Rh x CO and Rh(CO) 2 species decreased with the increase in potassium content. At the same time, the band at 1650 cm −1, attributed to negatively charged NO −, intensified. Preadsorbed potassium exerted a significant increase on the formation of the NCO complex, characterized by a broad absorption band at 2240–2265 cm −1 (Al-NCO): the maximum intensity was obtained for a sample containing 2.5% K. The same effect was observed on Rh/SiO 2, where two NCO species were identified (2180 cm −1 for Rh-NCO and 2310 cm −1 for Si-NCO). The effect of potassium is attributed to the promotion of the dissociation of NO and to the stabilization of adsorbed nitrogen atoms. It is concluded that both the migration of NCO from Rh onto the supports and the stability of NCO located on the support are influenced only slightly by potassium.
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