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

Abstract

The formation of the reactivity of isocyanate species have been studied over Ag/Al₂O₃ by IR spectroscopy and mass spectrometry. Adsorbed C_xH_yNO_z and NO₃⁻ species are produced by reaction among NO, O₂ and C₃H₆ at room temperature. Thermal decomposition of adsorbed C_xH_yNO_z 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+O₂ at room temperature, being converted into N₂, CO₂, CO and a small amount of N₂O. The NCO species are less active in NO or O₂ alone than in the mixture of NO and O₂. Thus, excess oxygen added in the NO reduction by C₃H₆ plays an important role in the formation of adsorbed C_xH_yNO_z species and in the reaction of adsorbed NCO with NO. It is suggested that the formation of adsorbed C_xH_yNO_z and adsorbed NCO is essential for the progress of the NO reduction with C₃H₆ in the presence of O₂ under the present experimental conditions.