Cluster Size Effects in Ethylene Hydrogenation over Palladium

Density functional theory calculations are used to investigate ethylene hydrogenation over palladium clusters in the range from Pd-13 to Pd-116. A comparison is made to single crystal surfaces, which exemplifies several novel aspects of clusters. We find that the adsorption energies are always higher for the clusters, even if the comparison is made to adsorption on stepped surfaces in the low-coverage regime. Clusters are found to accommodate higher adsorbate coverages than extended surfaces. The saturation coverage for Pd-13 is unity, whereas it is 0.33 on Pd(111). The activation energies for hydrogenation of C2H4 to C2H5 over Pd-38 are clearly different from Pd(111) and Pd(211), which stresses the limitation of extended surfaces as models for nanoparticles.