CONICET | Buscador de Institutos y Recursos Humanos

The high catalytic activity of nanoscopic gold surfaces initially surprised the scientific community but is now well known and has given rise to a burgeoning new field of research. Most work in this area has been carried out on clean gold clusters in the 1 to 5 nm range deposited on oxide supports such as titania, while practically no attention has been paid to ligand-protected gold nanoparticles, so called monolayer protected clusters (MPCs), since these are generally assumed to be catalytically inactive due to the presence of the passivating ligand shell. Here we report that MPCs protected by mercaptoalkyl ethyleneglycol ligands exhibit unexpected catalytic activity for the electrochemical reduction of protons to adsorbed molecular hydrogen. There is no detectable equivalent reaction on macroscopic gold surfaces, where hydrogen evolution occurs at significantly more negative potentials and without a preceding adsorptive step. Similarly, under potential deposition (UPD) of certain metals including Cd can be observed on the same type of MPC. It is argued that the clusters exhibit highly reactive unsaturated surface sites (atoms) that are not covered by the thiol ligand and are therefore accessible to react with small molecules and ions capable of penetrating the ligand shell. The number of these active sites has been quantified for cluster sizes of 3 and 15 nm. Electrical contact is established by the spontaneous adsorption of the clusters to a hanging Hg drop electrode. The contact to the Hg electrode is very robust and does not lead to the formation of amalgams even after many hours of cycling the potential. We believe that, apart from its fundamental interest, this very unusual electrocatalytic behaviour of ligand-protected gold nanoparticles reported here, may lead to new strategies for the design of catalysts for the selective reduction or hydrogenation of small molecules.