Molecular beam study of the adsorption and dissociation of NO on Pd clusters supported on MgO(100)

Abstract

The adsorption of nitric oxide (NO) on the Pd/MgO(100) model catalyst has been studied in the temperature range 160–430°C, for various cluster sizes ( d=2.8 to 45 nm), using a pulsed molecular beam. From angular distribution measurements, the physisorption probability of NO on MgO is derived: α=0.56±0.03. The physisorbed molecules can diffuse towards the clusters and become chemisorbed. This phenomenon, which increases drastically the NO adsorption rate on the particles, has been quantified as a function of the surface temperature and of the particle size. Once chemisorbed, NO molecules either dissociate to form nitrogen and oxygen adatoms, or desorb. The frequency factor and the activation energy for desorption have been measured on large particles ( d>14 nm): ν des=10 13 s −1 and E des=32 kcal/mol. The equilibrium coverage of molecular NO is found much higher on small particles (3.1 nm) than on large particles ( d>6 nm). After an initial stage of intense dissociation on the fresh Pd catalyst, the dissociation capacity of the surface is lowered due to oxygen poisoning, but remains high and stable. The dissociation efficiency is between 40 and 80% of the molecules adsorbing on Pd, depending on the surface temperature and particle size. The dissociation of NO on palladium leads to the formation of N 2 (by association of two nitrogen adatoms), without production of N 2O. From the transient kinetics of N 2 desorption, it is concluded that strongly bound nitrogen adatoms coexist, on the cluster surface, with loosely bound nitrogen species.