Enhanced Electrocatalytic Activity of Carbon-Supported Ordered Intermetallic Palladium–Lead (Pd3Pb) Nanoparticles toward Electrooxidation of Formic Acid

Nanosized ordered intermetallic Pd3Pb nanoparticles (NPs)/carbon black (CB) (1–8 nm), Pd3Pb NPs/CB, in which Pd3Pb has a Cu3Au-type structure and its NPs are supported on CB, were prepared by the polyol method under an air atmosphere and characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and X-ray photoelectron spectroscopy (XPS). The XRD and XPS measurements confirmed the formation of ordered intermetallic Pd3Pb NPs with a super lattice phase, and the TEM and STEM images indicated a relatively uniform dispersion of Pd3Pb NPs on the CB surface with an average size of 4.3 nm and an atomic ratio (Pd:Pb) of 75.9:24.1. The surface of the as-prepared Pd3Pb NPs/CB was found to be covered with the Pb (and its oxide) layer and to possess actually no electrocatalysis for the electrooxidation of formic acid (FA). However, this "inactive" as-prepared Pd3Pb NPs/CB could be changed drastically to the "active" one with a high level of electrocatalysis by the electrochemical treatment using cyclic voltammetry, i.e., the pertinent electrooxidation of the Pb surface coating in a 0.1 M HClO4 aqueous solution. The atomic-resolution STEM measurements confirmed that the surface state of the "inactive" as-prepared Pd3Pb NPs/CB can be controlled by changing the number of potential scans employed in the electrochemical treatment. That is, when the potential scan number is suitably chosen, the surface covered with the Pb coating dissolves and becomes an active, ideal structure of Pd3Pb, and further scanning leads to a surface close to that of Pd NPs. The thus electrochemically treated ideal Pd3Pb NPs/CB possessed a largely higher level of electrocatalysis for the FA oxidation than Pd NPs/CB, which could be explained reasonably on the basis of the experimentally measured and/or theoretically calculated d-band center values of both catalysts and CO binding energies on them.