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

The adsorption of nitric oxide (NO) on the Pd/MgO(100) model catalyst has b een studied ill the temperature range 160-430 degrees C, for various cluste r sizes (d = 2.8 to 45 nm), using a pulsed molecular beam. From angular dis tribution measurements. the physisorption probability of NO on MgO is deriv ed: alpha = 0.56 +/- 0.03. The physisorbed molecules call diffuse towards t he clusters and become chemisorbed. This phenomenon, which increases drasti cally the NO adsorption rate on the particles, has been quantified as a fun ction of the surface temperature and of the particle size. Once chemisorbed , NO molecules either dissociate to form nitrogen and oxygen adatoms, or de sorb. The Frequency factor and the activation energy for desorption have be en measured on large particles (d > 14 nm): v(des) =10(13) s(-1) and E-des = 32 kcal/mol. The equilibrium coverage of molecular NO is found much highe r 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 dissoci ation capacity of the surface is lowered due to oxygen poisoning, but remai ns high and stable. The dissociation efficiency is between 40 and 80% of th e molecules adsorbing on Pd, depending on the surface temperature and parti cle size. The dissociation of NO on palladium leads to the formation of N-2 (by association of two nitrogen adatoms), without production of N2O. From the transient kinetics of N-2 desorption, it is concluded that strongly bou nd nitrogen adatoms coexist, on the cluster surface, with loosely bound nit rogen species. (C) 2000 Elsevier Science B.V. All rights reserved.