Atomic Structure of Bimetallic Nanoparticles in PtAg/C Catalysts: Determination of Components Distribution in the Range from Disordered Alloys to “Core–Shell” Structures

Nanocatalysts PtAg/C with different distributions of components in bimetallic PtAg nanoparticles were synthesized by the methods of simultaneous and sequential reduction of Ag + and Pt(IV) in carbon suspension, prepared on the basis of ethylene glycol–water solvent. The characterization of atomic structure of as prepared PtAg nanoparticles and PtAg nanoparticles obtained after acid treatment at 90 °C was performed by Pt L 3 -edge extended X-ray absorption fine structure (EXAFS) using the technique for determining local structure parameters of the absorbing Pt atoms, which have the nearest surrounding consisting of Pt and Ag atoms. This technique enabled stable and unambiguous values of parameters for the local structure of Pt atoms in PtAg nanoparticles to be obtained under severe correlations between some of these parameters. The effects of averaging over the sizes and compositions of PtAg nanoparticles on the obtained values of structural parameters were studied by the method of cluster simulations. Analysis of changes in the values of parameters of Pt local structure determined from experimental EXAFS in differently prepared nanoparticles PtAg under acid treatment, supplemented with simulations of different particle architectures by the molecular dynamics (MD) method, enabled a set of evidence of the core–shell structure in these nanoparticles to be proposed. Application of the combined approach, based on the use of EXAFS data and MD simulations, enabled us to characterize the atomic structure of PtAg nanoparticles synthesized by the two methods used and to suggest the mechanism of formation of a platinum shell in nanoparticles obtained by sequential deposition of Pt and Ag atoms on the carbon support, followed by acid treatment.