Why Does Pt Shell Bearing Tensile Strain Still Have Superior Activity for the Oxygen Reduction Reaction?

Nanoparticles with a 55 nm Au core and a few monolayers of Pt shell (denoted as Au@Pt x ML, x = 1–4) are found to have superior activity toward the oxygen reduction reaction (ORR) compared to commercial Pt nanoparticles and polycrystalline Pt (pc-Pt) electrodes in 0.1 M HClO4. The half-wave potential for ORR at Au@Pt1 ML is ca. 20 mV positively shifted. With the decrease of the Pt shell thickness from 4 to 1 monolayer, both the extent of enhancement of the ORR activity and the adsorption strength of oxygen-containing species increase; the latter is confirmed by results from surface-enhanced Raman spectroscopy (SERS) and X-ray photon emission spectroscopy (XPS). Furthermore, SERS results also reveal that adsorbed CO on such catalysts displays a blue shift in Pt–CO and a red shift in C–O stretching frequencies compared to that at pc-Pt under otherwise identical conditions. Positive double layer effects induced by diminishing surface charge density, which leads to the less rigid first layer of water, are suggested to be the origin of the superior ORR performance of Au@Pt x ML to that of commercial Pt/C catalysts.