Coming to the surface: The surface composition of carbon-supported Pt(3)Co catalyst particles changes upon a CO-annealing treatment. Platinum atoms segregate to the particle surface so that nanoparticles with a platinum shell surrounding an alloy core are formed. This modified catalyst has a superior activity in the oxygen reduction reaction compared to both a plain platinum catalyst and the untreated alloy particles.

Surface segregation of the non-noble component of a Pt bimetallic core-shell catalyst can occur even at room temperature under typical fuel cell cathode application conditions. While in an alkaline environment the nanoparticles remain stable, and the alteration in the surface composition can be tracked in situ; in an acidic electrolyte, any non-noble alloying material at the surface would immediately dissolve into the electrolyte. Therefore, such catalysts are expected to degrade steadily during operation in an acidic fuel cell until only Pt is left.

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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.