Carbon-supported Pt x Co 1-x alloy nanoparticles are prepared via a modified Watanabe's process for oxygen reduction reaction (ORR). The relationship between the variations in alloying extent and Pt d-band vacancies in Pt-Co/C catalysts, which are tunable by changing the Pt and Co composition, are systematically studied. All of the catalysts are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammetry (CV), and XAS. TEM images indicate that the dispersion of the metal nanoparticles on the carbon support is uniform. The XAS technique containing both X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine spectroscopy (EXAFS) is utilized to extract the Pt d-band vacancies and alloying extent of Pt and Co in Pt x Co 1-x nanoparticles, respectively. Rotating disk electrode measurements of Pt x Co 1-x nanoparticle catalysts with various Pt:Co atomic compositions (3:1, 1:1, and 1:3) reveal that Pt 1 Co 1 /C nanocatalyst showed enhanced ORR activity. It is proposed that ORR activity enhancement of Pt 1 Co 1 /C nanocatalyst is attributed to the higher alloying extent of platinum and cobalt as well as the promising electronic structure offered by the lower unfilled Pt d-states when compared to the pure Pt.

The chemical dealloying mechanism of bimetallic Pt-Co nanoparticles (NPs) and enhancement of their electrocatalytic activity towards the oxygen reduction reaction (ORR) have been investigated on a fundamental level by the combination of X-ray absorption spectroscopy (XAS) and aberration-corrected scanning transmission electron microscopy (STEM). Structural parameters, such as coordination numbers, alloy extent, and the unfilled d states of Pt atoms, are derived from the XAS spectra, together with the compositional variation analyzed by line-scanning energy-dispersive X-ray spectroscopy (EDX) on an atomic scale, to gain new insights into the dealloying process of bimetallic Pt-Co NPs. The XAS results on acid-treated Pt-Co/C NPs reveal that the Co-Co bonding in the bimetallic NPs dissolves first and the remaining morphology gradually transforms to a Pt-skin structure. From cyclic voltammetry and mass activity measurements, Pt-Co alloy NPs with a Pt-skin structure significantly enhance the catalytic performance towards the ORR. Further, it is observed that such an imperfect Pt-skin surface feature will collapse due to the penetration of electrolyte into layers underneath and cause further dissolution of Co and the loss of Pt. The electrocatalytic activity decreases accordingly, if the dealloying process lasts for 4 h. The findings not only demonstrate the importance of appropriate treatment of bimetallic catalysts, but also can be referred to other Pt bimetallic alloys with transition metals.

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