(600a) Nanoengineering and Application of Protected but Accessible Metal Cluster Catalysts

The synthesis of metal clusters that are protected against aggregation while possessing exposed metal-surface sites for catalysis is crucial in both conventional catalysis as well as electrocatalysis, where the trigger for aggregation is high temperature of reducing environment and high reducing potential, respectively. We have relied on calixarene-bound metal clusters as highly accessible yet protected metal colloids in solution and when supported. Here, we demonstrate that they also serve as such under high reducing potentials, as required for CO2 reduction. For example, 0.9 nm gold clusters stabilized with five calixarene diphosphine ligands are stable at -1.4 V, conditions under which intrinsically more stable (larger) 4 nm gold nanoparticles aggregate readily. We demonstrate that for electrocatalysis, the ability to control the organic ligand sphere as well as the size and composition of the metal cluster are all key in the molecular engineering of the active site. In addition, relying on calixarene-bound iridium clusters as precursors to supported active sites, we demonstrate the synthesis of smaller nanoparticles, compared with conventional approaches. These have direct catalytic benefit as a result of their higher surface to mass ratio, when dealing with noble metals. Altogether, our results demonstrate the benefit of creating high dispersions of aggregation-resistant supported metal-cluster sites, with calixarene-ligated clusters.