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 (D2) on Pd particles deposited on a thin alumina film were studied by temperature-programmed desorption (TPD). The morphology and Pd particle size were controlled by the deposition parameters and monitored by scanning tunneling microscopy. 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 π- and more strongly bound di-σ-ethene, with the di-σ-ethene molecule preferentially adsorbed on the larger particles. Deuterium 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 strong and only weakly dependent on particle size. Hydrogen pre-adsorption shifts the distribution of ethene states toward the weakly bound π-state, while adsorbed ethene blocks the hydrogen adsorption. Upon co-adsorption, ethane is produced by the reaction of the π-bonded ethene with those hydrogen atoms residing in the most weakly bound sites. The ethene hydrogenation activity 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 observations.