Low-temperature fuel cells are limited by the oxygen reduction reaction, and their widespread implementation in automotive vehicles is hindered by the cost of platinum, currently the best-known catalyst for reducing oxygen in terms of both activity and stability. One solution is to decrease the amount of platinum required, for example by alloying, but without detrimentally affecting its properties. The alloy PtxY is known to be active and stable, but its synthesis in nanoparticulate form has proved challenging, which limits its further study. Herein we demonstrate the synthesis, characterization and catalyst testing of model PtxY nanoparticles prepared through the gas-aggregation technique. The catalysts reported here are highly active, with a mass activity of up to 3.05 A mgPt(-1) at 0.9 V versus a reversible hydrogen electrode. Using a variety of characterization techniques, we show that the enhanced activity of PtxY over elemental platinum results exclusively from a compressive strain exerted on the platinum surface atoms by the alloy core.

A matter of size: The particle size effect on the activity of the oxygen reduction reaction of size‐selected platinum clusters was studied. The ORR activity decreased with decreasing Pt nanoparticle size, corresponding to a decrease in the fraction of terraces on the surfaces of the Pt nanoparticles (jk=kinetic current density, see picture).

Eine Frage der Größe: Der Einfluss der Partikelgröße auf die Aktivität größenselektierter Platin‐Cluster in der Sauerstoffreduktionsreaktion (ORR) wurde untersucht. Die Aktivität der Pt‐Nanopartikel sank mit kleiner werdender Größe der Nanopartikel, entsprechend einer Abnahme des Anteils an Terrassen auf der Oberfläche der Pt‐Nanopartikel (jk=kinetische Stromdichte, siehe Bild).

The oxidation and reduction of CuZn nanoparticles was studied using X-ray photoelectron spectroscopy (XPS) and in situ transmission electron microscopy (TEM). CuZn nanoparticles with a narrow size distribution were produced with a gas-aggregation cluster source in conjunction with mass-filtration. A direct comparison between the spatially averaged XPS information and the local TEM observations was thus made possible. Upon oxidation in O2, the as-deposited metal clusters transform into a polycrystalline cluster consisting of separate CuO and ZnO nanocrystals. Specifically, the CuO is observed to segregate to the cluster surface and partially cover the ZnO nanocrystals. Upon subsequent reduction in H2 the CuO converts into metallic Cu with ZnO nanocrystal covering their surface. In addition, a small amount of metallic Zn is detected suggesting that ZnO is reduced. It is likely that Zn species can migrate to the Cu surface forming a Cu–Zn surface alloy. The oxidation and reduction dynamics of the CuZn nanoparticles is of great importance to industrial methanol synthesis for which the direct interaction of Cu and ZnO nanocrystals synergistically boosts the catalytic activity. Thus, the present results demonstrate a new model approach that should be generally applicable to address metal–support interactions in coprecipitated catalysts and multicomponent nanomaterials.

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Using temperature-programmed desorption experiments, we have studied the coordination dependent adsorption of CO on a platinum (Pt) single crystal, and mass-selected Pt nanoparticles in the size range of 3 to 11 nm, for CO dosing pressures in 10–7 mbar and mbar ranges. From low pressure CO adsorption experiments on the Pt(111) crystal, we establish a clear link between the degree of presputtering of the surface prior to CO adsorption, and the amount of CO bound at high temperature. It was found that for rougher surfaces, i.e., with more undercoordinated surface atoms, a feature appears in the CO desorption spectra at high temperature. The result is consistent with literature results from stepped single crystals that have found high temperature CO desorption features due to the presence of undercoordinated step and kink sites on the crystal facets. For the nanoparticles, a study of the dependence of the CO desorption profile with particle size found more prominent high temperature CO desorption features as the nanoparticle size was decreased, consistent with the expectation for a higher proportion of undercoordinated sites at smaller particle sizes. Thus, for both systems there is a clear relation between surface atom coordination, and the desorption temperature of CO. Investigation of these structural features was then made for CO dosing pressures in the mbar range. Intriguingly, from the mbar pressure experiments it was observed that elevated CO pressures enhanced the annealing of the Pt(111) surface, but on the otherhand, caused an apparent roughening of the nanoparticles.

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High-quality mass spectrometry data of the oscillatory behavior of CO oxidation on SiO(2) supported Pt-nanoparticles at atmospheric pressure have been acquired as a function of pressure, coverage, gas composition and nanoparticle size. The oscillations are self-sustained for several days at constant temperature, pressure and CO/O(2) ratio. The frequency of the oscillations is very well defined and increases over time. The oscillation frequency is furthermore strongly temperature dependent with increasing temperature resulting in increasing frequency. A plausible mechanism for the oscillations is proposed based on an oxidation-reduction cycle of the nanoparticles which change the rate of CO oxidation on the particles.