Study of the formation of isocyanate surface complexes on Pt/Al 2O 3 catalysts

Abstract

The interaction of NO, CO and NO + CO with 5% Pt/Al 2O 3 was investigated by adsorption, thermal decomposition and infrared spectroscopic measurements in the temperature range 25–400 °C. It was found that the extent of adsorption of NO + CO gas mixture was roughly 1.5–2 times larger above 200 °C than the sum of the separately adsorbed gases, and that the preadsorbed NO promoted the adsorption of CO. Infrared spectroscopic studies revealed the formation of surface isocyanate at 2267 cm −1 under these conditions. The formation of surface isocyanate was followed as functions of time, temperature and partial pressure of reacting gases. It was found that the isocyanate complex at 2267 cm −1 occurred at the remarkably low temperature of 150 °C, and it formed at maximum intensity at about 320 °C. The composition of the reacting gas greatly influenced the intensity of the band; it was also detected in high intensity in the presence of an excess of NO and even on oxidized surfaces. The change in intensity of the band due to isocyanate with time indicated that isocyanate reacted with NO at higher temperatures and in an NO excess. It is proposed that the dominant route for the formation of surface isocyanate is the reaction between gaseous CO and the adsorbed N formed in the dissociation of adsorbed NO at elevated temperature, but the possibility of molecular surface interaction between NO and CO at lower temperature, as the precursor of isocyanate formation, is also considered.