Effect of rare-earth elements in Pd ternary alloy catalysts on activity toward oxygen reduction reaction

To enhance the catalytic activity of Pd toward the oxygen reduction reaction (ORR), transition metals are alloyed with Pd. This changes the electronic structure of Pd because alloyed transition metal induces a lattice mismatch and ligand effects. Rare-earth elements have a much lower electronegativity, and can therefore donate electrons easily and fill the antibonding orbital of Pd. Therefore, rare-earth elements are effective in increasing the ORR activity by making Pd more metallic. Moreover, alloying Ir with noble metals enhances the ORR activity via the strain effect in an acid medium. In this study, rare-earth elements such as Y, Sc, and La were alloyed with Pd and Ir using a polyol process. As the rare-earth elements have larger atomic sizes and much lower reduction potentials than those of Pd, the amount that can be incorporated in Pd based alloy catalysts with rare-earth elements via the polyol process is limited. The addition of rare-earth elements increased the amount of metallic Pd; however, when added in excess, most of the rare-earth elements on the surface were located in the grain boundary in the form of oxides. When the composition ratio of Pd, Ir, and rare-earth elements was 4:1:0.1, Pd–Ir–Y, Pd–Ir–La and Pd–Ir–Sc alloy catalysts had higher metallic Pd portion, as determined by X-ray photoelectron spectroscopy, electrochemical surface area, and half-wave potential measurements, respectively. Among the Pd/C, Pd4Ir/C and Pd–Ir ternary alloy catalysts, Pd4IrY0.1/C exhibited the largest electrochemical surface area (26.46 m2/gPd) and the highest half-wave potential (0.626 V). In particular, the metallic Pd portion in the ternary alloy had a stronger relationship with the activity towards the ORR than the electrochemical surface area. Therefore, in order to enhance the ORR activity of Pd-based alloy catalysts, the alloying method, which reduces more rare-earth elements, should be developed to increase the metallic Pd portion.