Dynamic behavior of adsorbed NO and CO under transient conditions on Pd/Al2O3

The dynamic behavior of adsorbed NO and CO under transient NO-CO reaction conditions on Pd/Al2O3 has been studied by in situ infrared (IR) spectroscopy coupled with TPR and pulse reaction techniques in the 303-673 K range. Below the light-off temperature (i.e., 561 K), Pd-0-NO and Pd-0-CO are the dominant adsorbates on the Pd surface. Pd-0-NO competes favorably over Pd-0-CO for the same reduced Pd-0 site when the temperature is increased. Pulse reaction studies at 473 K suggest that Pd-0-NO dissociates to form adsorbed nitrogen and adsorbed oxygen. Adsorbed oxygen further reacts with Pd-0-CO to produce CO2. Concentration profiles of CO2 and Pd-0-CO during the pulse reaction studies indicate that removal of adsorbed oxygen from the Pd surface to produce CO2 is the rate-limiting step. Prolonged exposure of the catalyst to the NO flow at 473 K results in oxidation of Pd-0 to Pd+ and produces Pd-NO+; the presence of gaseous CO reduces Pd+ to Pd-0 and increases the surface coverage of Pd-0-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 Pd-0 oxidation to Pd+ by NO is faster than that of Pd+ reduction to Pd-0 by 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/Al2O3 catalyst. (C) 1999 Academic Press.