Electrocatalysis on Shape-Controlled Palladium Nanocrystals: Oxygen Reduction Reaction and Formic Acid Oxidation

A systematic study was conducted on small Pd nanocrystals (5–6 nm) to understand the effects of catalyst structure and electrolyte on the oxygen reduction reaction (ORR) and formic acid oxidation (FAO). The ORR activities of Pd catalysts strongly depended on their structure and the electrolyte used. It was found that Pd cubes were 10 times more active than Pd octahedra for ORR in an aqueous HClO<sub>4</sub> solution due to higher onset potential of OH<sub>ad</sub> formation on the cubic surface. In the case of a H<sub>2</sub>SO<sub>4</sub> solution, the ORR activity of Pd cubes was 17 times higher than that of Pd octahedra due to the stronger adsorption of (bi)­sulfate on the surface of octahedral nanocrystals in addition to OH<sub>ad</sub>. In alkaline solutions, however, no structure dependence was observed for ORR due to the outer-sphere electron-transfer mechanism in the potential region for Pd oxide formation. For FAO, no advantage was observed on shape-controlled Pd nanocrystals in comparison to conventional Pd catalysts. The FAO current densities, both at peak current and at 0.4 V, followed the order of conventional Pd > octahedral Pd > cubic Pd. It was hypothesized that steps and defects were more active for FAO than terraces, which could be used to explain why the shape-selective materials were less active than conventional Pd because they contained fewer defects and edge sites.