Reduction of nitric oxide by carbon monoxide over a silica supported platinum catalyst infrared and kinetic studies

Abstract

The reduction of nitric oxide by carbon monoxide over a 4.5 weight precent platinum catalyst supported on silica was studied at 300 C. Reaction rate data was obtained together with in situ infrared spectra of species on the catalyst surface. The kinetics of the system were found to exhibit two distinct trends, depending on the molar ratio of CO/NO in the reactor. For net reducing conditions (CO/NO> 1) the catalyst underwent a transient deactivation, the extent of which was dependent on the specific CO/NO ratio during reaction. Reactivation of the catalyst was obtained with both oxidizing and reducing pretreatments. For molar feed ratios of CO/NO less than one, carbon monoxide conversion was typically 95 to 100%, resulting in strongly oxidizing conditions over the catalyst. Under these conditions no deactivation was apparent. Infrared spectra recorded under reaction conditions revealed intense bands at 2075 and 2300 cm{sup -1} , which were identified as carbon monoxide adsorbed on Pt and Si-NCO, respectively. Isocyanate bands formed under reducing conditions were more intense and exhibited greater stability than those formed under oxidizing conditions. A reaction mechanism based on the dissociation of nitric oxide as the rate-limiting step was used to correlate nitric oxide reaction rates and nitrous oxide selectivities observed under reducing conditions. As part of this mechanism it is assumed that nitrous bxide is formed via a Langmuir-Hinshelwood process in which an adsorbed nitrogen atom reacts with an adsorbed nitric oxide molecule. The nitric oxide reaction rate was found to be first order in nitric oxide partial pressure, and inverse second order in carbon monoxide partial pressure. A mechanism is proposed to qualitatively explain the deactivation process observed under reducing conditions. The essential part of this mechanism is the formation of an isocyanate species on the Pt crystallites of the catalyst and the subsequent transient diffusion of these species to the silica support. The deactivation is believed to result from the build-up of NCO on vacant sites necessary for the dissociation of nitric oxide.