Influence of supports, cluster structure, and cluster composition on hydrogenation reactions catalyzed by oxide-supported metal clusters

Metals are among the most important catalysts, usually applied as nano-clusters or -particles dispersed on supports, exposing many metal atoms at surfaces, where they are catalytically engaged. This work is an investigation of some of the smallest metal catalysts on supports, Ir₄, Ir ₆, and Rh₆ with the goals of understanding the influence of supports, cluster size, and cluster composition on catalytic hydrogenation activity. Nearly uniform supported metal clusters were synthesized from metal carbonyl precursors and characterized during catalysis with extended X-ray absorption fine structure and infrared spectroscopies. The data identify the structures of the active catalysts (the clusters) and the reactant-derived ligands bonded to the clusters during catalysis, including reactive intermediates (e.g., alkyl).; Propylidyne on Ir₄/γ-Al₂O₃ is virtually inert, whereas propylidyne on extended metal surfaces readily hydrogenates and thermally decomposes. The striking reactivity difference of propylidyne on clusters and propylidyne on extended metal surfaces implies the requirement of ensembles of metal atoms in the surface reactions. The results highlight the unique reactivity of small site-isolated metal clusters. The rate of catalytic hydrogenation of ethene depends strongly on cluster size, being four times faster on Ir₄ than on Ir₆ (both supported on γ-Al ₂O₃). This difference is attributed to the increased adsorbate-adsorbate interactions on the smaller Ir₄ clusters (because the adsorbates are forced closer together on smaller Ir₄ clusters than on larger Ir₆ clusters), which become more pronounced at elevated alkene partial pressures.; Rates of alkene hydrogenation catalysis on clusters are affected by supports. Hydrogenation of ethene (and of propene) is slower on Ir₄ supported on basic MgO than on Ir₄ supported on less-strongly basic γ-Al ₂O₃. Electron density, transferred from the MgO support to Ir₄, hinders the alkene's adsorption and the rate of the catalytic reaction. Likewise, the cluster composition affects the cluster-reactant bond strength and thus the catalytic activity. Ethene, which is adsorbed more strongly on Ir₆ than on Rh₆ (both supported on MgO), is hydrogenated at a lower rate on Ir₆ than on Rh₆.; This work suggests that the catalytic properties of supported metal clusters can be tuned by balancing the cluster size, cluster composition, and support influence.