Infrared study of the interactions between NO and CO on Rh/Al2O3 catalysts

Abstract

Infrared spectra of NO and NO + CO mixtures on Rh/Al₂O₃ are reported. At ambient temperatures NO alone was adsorbed non-dissociatively and gave no detectable reaction products. At T 413 K NO underwent partial decomposition to N₂O(g) at surface sites which were rapidly poisoned as the reaction proceeded. The addition of NO to preadsorbed CO eliminated spectral bands characteristic of linear and bridged carbonyl species, but maxima due to gem-dicarbonyl species remained. The main products of the CO+NO reaction detectable by infrared spectroscopy were N₂O(g), adsorbed isocyanate and CO₂, either gaseous or chemisorbed. The extent of isocyanate formation depended on the relative amounts of NO and CO present, the gas-phase pressures of NO and CO and the order of addition of NO and CO to the Rh/Al₂O₃ catalyst. The order of addition did not affect the generation of N₂O. The results are discussed in terms of reaction mechanism and types of catalytically active adsorption site. Dissociative adsorption of CO₂ on Rh dispersed on alumina led to an adsorbed CO species giving an infrared band at 2015 cm^–1.