Electrocatalysis by Mass-Selected Pt n Clusters

• Published in: Accounts of chemical research 2016-11, Vol.49 (11), p.2632-2639
• Main Authors: von Weber, Alexander, Anderson, Scott L
• Format: Article
• Language: English
• Subjects: Chemistry
• ISSN: 0001-4842; 1520-4898
• DOI: 10.1021/acs.accounts.6b00387

Mass-selected Pt n + ion deposition in ultrahigh vacuum (UHV) was used to prepare a series of size-selected electrodes with Pt n (n ≤ 14) clusters supported on either glassy carbon (GC) or indium tin oxide (ITO). After characterization of the physical properties of the electrodes in UHV, an in situ method was used to study electrocatalytic activity for the oxygen reduction and ethanol oxidation reactions, without significant air exposure. For each reaction studied, there are similarities between the catalytic properties of Pt n -containing electrodes and those of nanoparticulate or bulk Pt electrodes, but there are also important differences that provide mechanistic insights. For all systems, strong cluster size effects were observed. For comparison, select experiments were done under identical conditions but with the Pt n electrodes exposed to air prior to electrochemical studies, resulting in strong modification/suppression of catalytic activity due to adventitious contaminants. For ethanol oxidation at Pt n /ITO, activity varies with size nonmonotonically, by more than an order of magnitude. The sharp size dependence persists during at least 30 to 40 cycles through the Pt redox potential, indicating that processes that would tend to broaden the size distribution are not efficient. All but the least active sizes are substantially more active per mass of Pt, than Pt nanoparticles under the same conditions. The oscillatory dependence of activity on size is anticorrelated with the binding energy of the Pt 4d core level, demonstrating that activity is controlled by the electronic structure of the supported clusters. For oxygen reduction at Pt n /ITO, the branching between water and hydrogen peroxide production is strongly dependent on cluster size, with small clusters selectively producing peroxide with high activity. The selectivity appears to be related to the size of the active site, with no obvious correlation to Pt electronic properties. The most unusual effect seen was for Pt n /GC, studied under acid conditions appropriate to oxygen reduction. Pt7 and a few other cluster sizes show “normal” oxygen reduction activity, similar to what is measured for Pt nanoparticles on GC under the same conditions. Many of the small clusters, however, are found to catalyze highly efficient oxidation, by water, of the glassy carbon support, with essentially no overpotential. The high activity for carbon oxidation for many Pt n /GC electrodes and the absence of significa