Presented herein is the experimental observation of the long-range ordered structure on Pt(111) in aqueous electrolytes by in situ surface X-ray scattering. The results confirmed the presence of two mirrored domains suggested earlier on the basis of scanning tunneling microscopy and infrared measurements. Based on the weak intensity of the second order adlattice reflections and earlier results obtained by other techniques, a further refinement of the structure with tilted CO molecules is proposed. The hystereses observed in transitions between and phases, as well as in CO adsorption and stripping, with change in electrode potential are discussed. © 2004 The Electrochemical Society. All rights reserved.

Bimetallic catalysts can undergo segregation or redistribution of the metals driven by oxidizing and reducing environments. Bragg coherent diffraction imaging (BCDI) was used to relate displacement fields to compositional distributions in crystalline Pt-Rh alloy nanoparticles. Three-dimensional images of internal composition showed that the radial distribution of compositions reverses partially between the surface shell and the core when gas flow changes between ${\mathrm{O}}_2$ and ${\mathrm{H}}_2$. Our observation suggests that the elemental segregation of nanoparticle catalysts should be highly active during heterogeneous catalysis and can be a controlling factor in synthesis of electrocatalysts. In addition, our study exemplifies applications of BCDI for in situ 3D imaging of internal equilibrium compositions in other bimetallic alloy nanoparticles.