(292b) Increased Accessibility in Gold Clusters Using Bis-Diphenyphosphine Ligands

The synthesis of small ( < 1 nm), yet catalytically accessible, metallic clusters using bound organic ligands is a challenge. Organic ligands, such as thiols and phosphines, provide stability to the clusters, but also block potential catalytic binding sites on the cluster surface. When the size of the organic ligand approaches the size of cluster, a packing problem occurs. Pockets of accessible surface arise from the inefficient ligand packing on the cluster surface.

We demonstrate the synthesis of a series of phosphine-bound Au clusters using different bis-diphenylphosphine ligands. The number of binding sites is quantified with steady-state fluorescence using 2-napthalenethiol titration experiments. The number of binding sites depends on the size of the bis-diphenylphosphine ligand used. An unprecedented 60% of the total Au atoms in the cluster are accessible in the most accessible cluster that was synthesized. This improves upon pioneering work by de Silva, et al.(Nature Chem. 2010, 2, 1062) where they demonstrate 25% of the total Au atoms are accessible to 2-napthalenethiol molecules. These clusters are further characterized by UV-Vis, HAADF-STEM, XPS, and CO binding experiments. Catalytic activity is measured using resazurin reduction and CO oxidation, and the activity scales with the number of open sites on the Au clusters. The synthesis methods that are utilized in this work provide a route to synthesize ligand-bound catalytic active metal nanoparticles while maintaining a high number of accessible sites using a minimal quantity of precious metal.