Precipitation in gold-platinum alloys—I thermodynamics

Surface Composition Tuning of Au–Pt Bimetallic Nanoparticles for Enhanced Carbon Monoxide and Methanol Electro-oxidation

“…We hypothesize the enhancement in the CO electro-oxidation kinetics of Au 0.5 Pt 0.5 /C to stem from the modified electronic structure of surface Pt that is surrounded by Au neighbors, which results in a weakened binding strength of CO ad . This hypothesis is supported by the recent single-crystal work, whereby isolated surface Pt clusters surrounded by Au neighbors (see Supporting Information, Scheme S1) were shown to have weaker CO binding than Pt due to lateral interactions and the absence of binding contribution from the next nearest Pt atoms. , We believe that this configuration, where the Pt or Pt-rich clusters are surrounded by Au or Au-rich atoms, well represents the surface structure of our Au 0.5 Pt 0.5 nanoparticles with surface Au enrichment as Pt prefers to have Pt as its nearest neighbor due to favorable enthalpy of the Pt–Pt bond . Further evidence on the importance of weakened CO ad binding came from a positive reaction order of CO concentration on the COR activity observed for Au 0.5 Pt 0.5 nanoparticles with 30% surface Pt and a zero reaction order (invariant activity with CO concentration) for ∼70 atom % Pt (Supporting Information, Figure S7).…”

“…25,26 We believe that this configuration, where the Pt or Pt-rich clusters are surrounded by Au or Aurich atoms, well represents the surface structure of our Au 0.5 Pt 0.5 nanoparticles with surface Au enrichment as Pt prefers to have Pt as its nearest neighbor due to favorable enthalpy of the Pt−Pt bond. 27 Further evidence on the importance of weakened CO ad binding came from a positive reaction order of CO concentration on the COR activity observed for Au 0.5 Pt 0.5 nanoparticles with 30% surface Pt and a zero reaction order (invariant activity with CO concentration) for ∼70 atom % Pt (Supporting Information, Figure S7). This finding is in contrast to the case of Pt/C, which has a negative reaction order, indicative of the poisonous nature of CO for COR.…”

Formation and Growth of Modulated Structure in Ni–Cu–Si Alloys

“…It may be incorrect to apply the mechanism of Ostwald-ripening to the zone-coarsening of the present alloy, because this mechanism must be applied to the coarsening behavior of pre-cipitates without compositional change of solute atoms in the matrix and the precipitate. A new growth mechanism for the early stage of aging should be proposed, which can explain simultaneously the following phenomena: (1) orderly repeated extinction of the nearest neighbor zones, (2) different growth rates of wavelength in different compositions of one alloy system, and (3) increment in amplitude during the aging period in which the spacing and size of zones increase.…”

Alliages à structure « modulée »

“…A haute température les deux rrétaux sont miscibles en toutes proportions, la phase unique est c. f. c. A basse température deux solutions solides terminables c. f. c. coexistent. Tiedema, Bouman et Burgers [3], ont montré dans les alliages Au-Pt, l'existence du stade de pré-précipitation et en ont expliqué la forination par des considérations thermodynamiques [4], [5]. Fukano [6] a trouvé récemment que le vieillissement vers 150 OC des alliages Au-Ni faisait apparaître de la diffusion en forme de satellites sur les diagrammes de diffraction d'électrons.…”

“…Tiedema and al. [4], [5], pour les alliages Au-Pt, ont trouvé des satellites sur les diagrammes pour des points situés au-dessous de la courbe spinodale et n'en ont pas trouvé au-dessus. Leur raisonnement est le suivant : au-dessous de la courbe spinodale, l'alliage peut se décomposer sans énergie d'activation, car toute fluctuation de composition s'accentue par diffusion « ascendante » et peut se propager en créant une modulation.…”