Size and Alloying Extent Dependent Physiochemical Properties of Pt−Ag/C Nanoparticles Synthesized by the Ethylene Glycol Method

Bimetallic alloy nanoparticles consisting of two noble metals Pt−Ag supported on carbon with a variable dimension were successfully prepared by ethylene glycol (EG) synthesis method. This work highlights the viability of EG synthesis methodology yielding a range of particle size from 1.2 to 3.1 nm with 1:1 atomic composition but with different alloying extents by a simple control over the solution pH of the preparation medium. The physical properties of resultant Pt−Ag/C nanoparticles such as size, structure, composition, coordination, and alloying extent parameters as well as d-band unfilled states of Pt atom were systematically studied by X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), transmission emission microscopy (TEM), and X-ray absorption spectroscopy (XAS) techniques. Both EDX and XAS analysis confirmed that the catalyst composition was nearly the same as that of the nominal value. It was realized that the lower preparation pH produces the Pt−Ag/C with larger dimension, wider particle size distribution (PSD), and worse alloying extent associated with lower d-band unfilled states. At higher preparation pH yields Pt−Ag/C particles of smaller size, narrower PSD, and better alloying extent along with higher d-band unfilled states. Increasing the d-band unfilled states of the bimetallic Pt−Ag/C nanoparticles leads to a negative shift in CO oxidation peak potential at identical experimental conditions. The observed d-band unfilled state of the Pt atom in the Pt−Ag/C nanoparticles may be due to the resultant of the two opposite effects, namely, the electron donation by Ag and the size effect of the Pt−Ag nanoparticles. The electron donation ability of Ag is believed to associate with the alloying extent of Ag and/or Pt atoms in the Pt−Ag nanoparticles, and a possible explanation was drawn on the basis of their charge transfer index scale values.