Role of the Ni:Fe Ratio in Ethylene Hydrogenation Activity for Silica-Supported Ni-Fe Clusters Prepared by Dendrimer-Templating

Ni-Fe nanoparticles of similar to 1 nm in size were synthesized by the dendrimer-templating technique using hydroxyl-terminated fourth-generation poly aminoamide (PAMAM) dendrimers. Spectroscopy and electron microscopy results are consistent with 40 atom clusters, in which all atoms are at the surface, as dictated by the synthesis strategy. Following immobilization of the clusters onto silica, the role of Ni:Fe ratio in the bimetallic was addressed for ethylene hydrogenation. A distinctive enhancement (factor of 10) in the catalytic activity was observed for the Ni-rich (Ni:Fe = 3:1) nanoparticles as compared to pure Ni, which was more active than pure Fe and a physical mixture of the pure parent components. Examination of the rate dependencies upon temperature and ethylene partial pressure indicates ethylene and its intermediates' strong binding as a key kinetic limitation. Using an available semiempirical model, the d-band center of the Ni Fe systems correlated to turnover frequencies calculated for increasing strain (and not ligand effects). The estimation of the catalytic activity of a bimetallic system proposed here provides a novel strategy that facilitates catalytic advancements by its application to other bimetallic systems.