Structure-reactivity relationships on supported metal model catalysts: Adsorption and reaction of ethene and hydrogen on Pd/Al2O3/NiAl(110)

Adsorption and co-adsorption of ethene (C2H4, C2D4) and hydrogen (D-2) on P d particles deposited on a thin alumina film were studied by temperature-pr ogrammed desorption (TPD). The morphology and Pd particle size were control led by the deposition parameters and monitored by scanning tunneling micros copy. The TPD spectra change gradually as a function of particle size. The size effect for ethene adsorption is attributed to a redistribution between weakly bound pi- and more strongly bound di-sigma -ethene, with the di-sig ma -ethene molecule preferentially adsorbed on the larger particles. Deuter ium adsorption results in D atoms located on the surface and, in addition, presumably in subsurface sites. Hydrogen atoms adsorb more strongly on the surface of smaller particles, while binding in subsurface sites is less str ong and only weakly dependent on particle size. Hydrogen pre-adsorption shi fts the distribution of ethene states toward the weakly bound pi -state, wh ile adsorbed ethene blocks the hydrogen adsorption. Upon co-adsorption, eth ane is produced by the reaction of the pi -bonded ethene with those hydroge n atoms residing in the most weakly bound sites. The ethene hydrogenation a ctivity of Pd is independent of the particle size in the 1-3 nm range under the conditions studied. We rationalize this on the basis of the above obse rvations. (C) 2001 Academic Press.