Dynamic Behavior of Adsorbed NO and CO under Transient Conditions on Pd/Al2O3

Abstract

The dynamic behavior of adsorbed NO and CO under transient NO–CO reaction conditions on Pd/Al2O3has been studied byin situinfrared (IR) spectroscopy coupled with TPR and pulse reaction techniques in the 303–673 K range. Below the light-off temperature (i.e., 561 K), Pd0–NO and Pd0–CO are the dominant adsorbates on the Pd surface. Pd0–NO competes favorably over Pd0–CO for the same reduced Pd0site when the temperature is increased. Pulse reaction studies at 473 K suggest that Pd0–NO dissociates to form adsorbed nitrogen and adsorbed oxygen. Adsorbed oxygen further reacts with Pd0–CO to produce CO2. Concentration profiles of CO2and Pd0–CO during the pulse reaction studies indicate that removal of adsorbed oxygen from the Pd surface to produce CO2is the rate-limiting step. Prolonged exposure of the catalyst to the NO flow at 473 K results in oxidation of Pd0to Pd+and produces Pd–NO+; the presence of gaseous CO reduces Pd+to Pd0and increases the surface coverage of Pd0–NO. Above the light-off temperature, Pd–NO+, Al–NCO, nitrate, and carbonate species are the dominant adsorbates. The presence of Pd–NO+indicates that the process for Pd0oxidation to Pd+by NO is faster than that of Pd+reduction to Pd0by CO. This study demonstrates that careful selection of transient IR techniques allows (i) determination of the modes of adsorbed NO and CO participating in the reaction and (ii) development of a comprehensive mechanism for the NO–CO reaction on Pd/Al2O3catalyst.