Research PaperCarbon-supported Pd100-XAuX alloy nanoparticles for the electrocatalytic oxidation of formic acid: Influence of metal particles composition on activity enhancement

- Preparation of PdAu/C catalysts by the “water-in-oil” microemulsion method.
- Pd surface segregation was determined by concentration of precursor solutions.
- Pd segregation was much pronounced at high concentration of PdCl2-HAuCl4 mixture.
- Activity for formic acid electrooxidation correlated with surface Pd/Au composition.
- Surface atomic ratio Pd/Au = 9/1 resulted in the highest activity.

Two series of carbon-supported Pd100-XAuX catalysts of different Au contents have been synthesized by the “water-in-oil” microemulsion method (Triton X 114, cyclohexane) using precursor solutions of low (series L) and high concentration (series H). The metal nanoparticles are characterized by XRD, electron microscopy (SEM, HRTEM) and XPS to determine their crystal structure, particles size, composition and morphology. The electrochemical measurements provide data concerning surface properties of the PdAu particles, including surface fraction of Pd and Au and the reactivity towards hydrogen sorption/desorption processes. The metal particles of low bulk Au-content (Au/Pd <1) are of high monodispersity and of smaller size than their monometallic Pd and Au counterparts, synthesized by the same method. At higher bulk Au-content (Au/Pd >1), the particle size increases approaching almost the size of Au particles. An enrichment of Pd toward the surface of all the Pd100-XAuX particles occurs, much pronounced in the samples H series. The activity of catalysts (based on the current densities, and TOF values) in the electrooxidation of formic acid (FA) is correlated with the bulk and surface composition of the Pd100-XAuX particles. The high monodispersity of the metal particle size in the studied catalysts made it possible to observe relation between FA electrooxidation activity and surface composition of the PdAu particles. The catalytically active sites formed at the Au surface fraction ca. 0.1-0.12 (Pd 0.9-0.88) display 1.6-times higher activity (TOF) as compared to pure Pd, while higher surface Au fraction strongly reduces activity. The occurrence of electronic modification of Pd by Au in the Pd100-XAuX particles also results in a reduced hydrogen solubility, accompanied by weaker PdH bonds, which might influence the poisoning ability by CO formed through electroreduction of CO2, the product of FA oxidation.

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