Direct observation of surface isocyanate (NCO) formation during the CO+NO reaction on Pt{100}

Abstract

The NO+CO reaction on Pt{100} has been studied using in-situ vibrational spectroscopy (IRAS) under low reaction rate, non-oscillatory conditions. Using a total pressure of 7.5×10^-7 mbar and a NO:CO ratio of 2:1, isocyanate (NCO) formation, characterised by an infrared absorption band at 2180 cm^-1, is observed over a narrow temperature range, 400–380 K. We propose that NCO formation becomes favourable once the concentration of N atoms on the surface is low and, more importantly, when the diffusion of these atoms becomes severely hindered by the high coverage of CO. The stability of the NCO, above its normal dissociation temperature, is due to the vacant site requirement for the dissociation reaction. The fact that recombination to NCO occurs, after dissociation at 453 K, and in the absence of gas phase NO, indicates that the diffusion of N atoms is hindered even at this higher temperature.