ChemInform Abstract: Adsorbate‐Induced Surface Segregation for Core—Shell Nanocatalysts.

Abstract

Catalysts H 2000Adsorbate-Induced Surface Segregation for Core-Shell Nanocatalysts. -Nanoparticles consisting of a Pt shell around a Pt x Co y core are obtained from a carbon-supported Pt3Co alloy high-surface-area catalyst which is modified by either treatment with ambient pressure CO (200°C, 3 h) or electrochemically by potential cycling in a CO-saturated alkaline electrolyte (60 min), so-called CO annealing. The CO-annealed catalyst is inferior to the untreated Pt-Co alloy in oxidizing adsorbed CO. The core-shell nanoparticles also exhibit improved activity for oxygen reduction which is a key reaction in fuel cell technology. Catalysts prepared by the CO annealing procedure have very promising potential not only for oxygen reduction, but also for other reactions requiring noble-metal surfaces. -(MAYRHOFER*, K. J. J.; JUHART, V.; HARTL, K.; HANZLIK, M.; ARENZ, M.; Angew.

Potential-Dependent Structural Memory Effects in Au–Pd Nanoalloys

“…Agreement between the results presented here and theoretical 14,15 and experimental 8,16,17 studies suggests that these results can be extended to other binary systems and surface adsorbates, which demonstrates that surface adsorption indeed represents a crucial factor in nanoalloy segregation or realloying, even at room temperature. This is an important observation because it implies that segregation data obtained in vacuum may not reflect the real surface composition in a condensed phase, for instance, under (electro)catalytic operating conditions.…”

“…12,13 For example, the adsorption of hydrogen 14 or CO 8,15 can induce a displacement of Pd atoms in a Au−Pd alloy, from a subsurface position to the surface. As observed for Pt−Co nanoalloys, 16 Pt binds CO more strongly than Co, resulting in the formation of a Pt skin, but on CO removal, Co is able to diffuse back to the surface through the segregated Pt layer, as demonstrated from linear sweep voltammetry. 17 Au−Pd alloys have recently attracted a great deal of attention for possible applications in the direct 18−20 and the electrochemical synthesis of H 2 O 2 in fuel cells.…”

Self-Grown Ni(OH)2 Layer on Bimodal Nanoporous AuNi Alloys for Enhanced Electrocatalytic Activity and Stability

“…The composition at the surface may deviate from that in the interior, not only depending on the heat of segregation and the surface mixing energy of a bimetallic alloy but also additionally relying on the chemical potential in the gas or aqueous phase because of the energy gain of the whole system with the strong component/adsorbate bonding. 32,33 As a consequence, the component that binds a certain adsorbate more strongly may become rich at the surface of the bimetallic alloy. S1a; see the Supporting Information) that are produced by single-roller metal-spinning in vacuum from their ingot composed of a uniform α-Al metal and AuAl 2 Ni x alloy intermixture (Figure S1b).…”

Recent Advances in Earth-Abundant Core/Noble-Metal Shell Nanoparticles for Electrocatalysis

“…55−58 Surface segregation refers to the surface enrichment of PGM in a bimetallic alloy upon thermal treatment in selected gas atmosphere. 59,60 A predominant driving force for surface segregation is the reduction of surface energy. 61 For binary alloys, the element with lower surface energy usually tends to segregate onto the surface.…”

“…61 For binary alloys, the element with lower surface energy usually tends to segregate onto the surface. To induce PGM surface segregation, presynthesized alloy nanoparticles are usually annealed in reactive gas atmosphere such as H 2 and CO. 62,63 When the adsorption enthalpy of these adsorbates on PGM is higher than that on the nonprecious constituent, the diffusion and enrichment of PGM toward alloy surface is promoted. 64 Both approaches provide convenient access to core−shell particles, yet accurate structure-control over the product is often challenging.…”