Strong Effects of Cluster Size and Air Exposure on Oxygen Reduction and Carbon Oxidation Electrocatalysis by Size-​Selected Ptn (n ≤ 11) on Glassy Carbon Electrodes

Abstract

Model Ptn/glassy C electrodes (Ptn/GCE) were prepd. by deposition of mass-​selected Ptn+ (n ≤ 11) on GCE substrates in ultrahigh vacuum. Electrocatalysis under conditions appropriate for the O redn. reaction (ORR) was studied, for samples both in situ with no exposure to lab. air and with air exposure prior to electrochem. measurements. Of the small clusters, only a few cluster sizes show the expected ORR activity, and in those cases, the activity per Pt atom is similar to that seen under identical conditions with a conventionally prepd. electrode with Pt nanoparticles grown on a GCE. For other small Ptn on GCE, any ORR signal is overwhelmed by large oxidative currents attributed to catalysis of C oxidn. by H2O. If the samples are exposed to air prior to electrochem., both ORR and C oxidn. signals are absent, and instead only small capacitive currents or currents attributed to redox chem. of adventitious org. adsorbates are obsd., indicating that air exposure results in passivation of the small Pt clusters.