Influence of Ceria and Lanthana Promoters on the Kinetics of NO and N2O Reduction by CO over Alumina-Supported Palladium and Rhodium

The kinetic parameters of the NO+CO and N2O+CO reactions over alumina-, ceria/alumina-, and lanthana/alumina-supported Rh and Pd were determined from 425 to 625 K. For NO+CO, the presence of ceria and lanthana affected activity, apparent activation energy, dinitrogen selectivity, and reaction orders to varying degrees over both Pd and Rh. The turnover frequency (based on H2 chemisorption) for the ceria-promoted Pd catalyst was at least an order of magnitude greater than those for all other catalysts. Lanthana-promoted Pd was the next most active catalyst. Changes in kinetic parameters indicated that both ceria and lanthana facilitated NO dissociation on both Pd and Rh, but the effect of ceria on Pd was the most profound. For N2O+CO, ceria promoted the reaction rate on both Pd and Rh whereas lanthana did not promote the reaction on either metal. The ceria-promoted Pd catalyst was again the most active. However, the turnover frequencies for the N2O+CO reaction were about an order of magnitude lower than those for the NO+CO reaction under similar conditions. The rare earth components had very little effect on the activation energies and reaction orders for the N2O+CO reaction. In situ infrared spectroscopy showed the presence of isocyanate surface species on Rh/Al2O3 but not on Pd/Al2O3, which suggested two different reaction mechanisms for the N2O+CO reaction. No synergistic effect on NOx reduction was observed for a Pd/Rh bimetallic sample.

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