Reduction of Oxygen on Dispersed Nanocrystalline CoS₂

The electrocatalytic properties of nanocrystalline CoS₂ have been investigated for the oxygen reduction reaction (ORR) in 0.1 M HClO₄. CoS₂ with pyrite structure was prepared by hydrothermal synthesis and attached to a glassy carbon electrode from solution with a mixture of carbon and Nafion. The prepared CoS₂ electrode layers showed high activity toward the ORR and very good stability under oxygen reducing conditions. Selectivity of the ORR toward H₂O₂ was determined by rotating (ring) disk electrode measurements, and relatively high selectivity was obtained with up to 80% H₂O₂ formation around 0.4 V (vs Ag/AgCl), but this dropped to zero for potentials below 0.0 V. The amount of H₂O₂ produced between 0.6 and 0.0 V was dependent on the quality of the CoS₂ dispersion within the electrode layer, and decreasing CoS₂ particle size resulted in significant improvement in the ORR electrocatalytic activity, both by increasing the turnover frequency and through decreasing the selectivity toward H₂O₂ production

Potential-Dependent Structural Memory Effects in Au–Pd Nanoalloys

Alloying of metals offers great opportunities for directing reactivity of catalytic reactions. For nanoalloys, this is critically dependent on near-surface composition, which is determined by the segregation energies of alloy components. Here Au–Pd surface composition and distribution of Pd within a Au_0.7Pd_0.3 nanoalloy were investigated by monitoring the electrocatalytic behavior for the oxygen reduction reaction used as a sensitive surface ensemble probe. A time-dependent selectivity toward the formation of H₂O₂ as the main oxygen reduction product has been observed, demonstrating that the applied potential history determines surface composition. DFT modeling suggests that these changes can result both from Pd surface diffusion and from exchange of Pd between the shell and the core. Importantly, it is shown that these reorganizations are controlled by surface adsorbate population, which results in a potential-dependent Au–Pd surface composition and in remarkable structural memory effects