NO decomposition and NO reduction by CO over Pd/α-Al 2O 3

Abstract

The stoichiometric reduction of NO by CO has been studied over Pd/α-Al 2O 3 catalysts of varying metallic dispersion. The samples prepared by impregnation of the alumina support with benzene solutions of palladium acetylacetonate, have been characterized by oxygen uptake measurements at 300°C before and after reaction. The decomposition of NO was also used as a test reaction. NO adsorption and dissociation takes place over metallic palladium but it deactivates rapidly due to oxygen retention. On the other hand, the activity for NO–CO reaction is not modified by the pretreatment conditions. Both the turnover number and the apparent activation energy for the reaction change with palladium particle size. Highly dispersed catalysts enhanced the NO reduction activity while decreasing the apparent activation energy for the reaction. This modification of the kinetics may be associated with the presence of metal–support interactions induced by the preparation method, which are also responsible for the much higher amount of oxygen that small particles are able to uptake. Under reaction conditions, the palladium surface is oxidized. This behaviour suggests a redox mechanism based on the adsorption and/or the oxidation of CO as the rate-limiting step.