(Keynote) Effects of Catalytic Site Size on Activity of Model Electrodes Prepared By Mass-Selected Cluster Deposition

Model size-selected electrodes were prepared by deposition of mass-selected metal cluster ions on electrode substrates, cleaned and characterized in ultra-high vacuum (UHV).  After analysis by X-ray photoelectron spectroscopy (XPS), the electrodes were transferred and mated to a small electrochemical cell without air exposure, and then studied by cyclic voltammetry.  Electrodes with small Pt_n (n = 1 - 16) on glassy carbon and indium tin oxide substrates were studied.  The ethanol oxidation reaction (EOR), the oxygen reduction reaction (ORR), and carbon oxidation by water were all found to depend strongly on cluster size, with peak currents varying by factors of up to 1000.  For oxidation reactions, the activity was found to be anticorrelated with Pt core level binding energies measured by XPS, i.e., the dominant factor appears to be the electronic properties of the metal centers.  For ORR, the dominant factor appears to be the physical size of the active site for O₂ binding.