Formation and reactivity of isocyanate (NCO) species on Ag/Al2O3

The formation of the reactivity of isocyanate species have been studied over Ag/Al2O3 by IR spectroscopy and mass spectrometry. Adsorbed CxHyNOz and NO3- species are produced by reaction among NO, O-2 and C3H6 at room temperature. Thermal decomposition of adsorbed CxHyNOz species leads to the formation of two types of NCO species (NCO on Ag and NCO on Al2O3) above 423 K. These NCO species are thermally stable in vacuum at 673 K, while adsorbed NO3- species decompose completely. The NCO species are highly reactive toward NO + O-2 at room temperature, being converted into N-2, CO2, CO and a small amount of N2O. The NCO species are less active in NO or O-2 alone than in the mixture of NO and O-2. Thus, excess oxygen added in the NO reduction by C3H6 plays an important role in the formation of adsorbed CxHyNOz species and in the reaction of adsorbed NCO with NO. It is suggested that the formation of adsorbed CxHyNOz and adsorbed NCO is essential for the progress of the NO reduction with C3H6 in the presence of O-2 under the present experimental conditions.