Albert K. Engstfeld

Oxygen Reduction on Structurally Well Defined, Bimetallic PtRu Surfaces: Monolayer PtxRu1−x/Ru(0001) Surface Alloys Versus Pt Film Covered Ru(0001)

The electrocatalytic activity of different, structurally well defined bimetallic PtRu surfaces in the oxygen reduction reaction was investigated by a combination of scanning tunnelling microscopy and electrochemical measurements performed under controlled mass transport conditions in a flow cell. We compare the effect of pseudomorphic Pt cover layers, mimicking the situation in a core–shell Pt/Ru nanoparticle, and of mixed PtxRu1−x monolayer surface alloys, reflecting the situation in an alloyed nanoparticle. The results unambiguously demonstrate that these bimetallic surfaces can reach activities well in excess of that of Pt(111), both for the film surfaces and the surface alloys, by optimizing the Pt surface content (surface alloys) or the Pt film thickness (film surfaces). The results are compared with simulated kinetic current–potential profiles based on existent density functional theory calculations (Greeley and Nørskov, J Phys Chem C 113:4932, 2009; Lischka et al., Electrochim Acta 52:2219, 2007) revealing very good agreement in trends. Potential and limits of this approach are discussed.

Potential-Induced Surface Restructuring—The Need for Structural Characterization in Electrocatalysis Research†

The necessity of the careful structural characterization of model electrodes before and after the electrochemical measurements for a proper mechanistic interpretation is demonstrated for a well-known electrocatalytic system, bulk CO oxidation on PtRu model electrodes. Bimetallic, Pt-monolayer-island-modified Ru(0001) electrodes, which were prepared and characterized by scanning tunneling microscopy under ultrahigh-vacuum conditions, were found to undergo a distinct restructuring when they were potential cycled to 1.05 VRHE , while up to 0.90 VRHE they are stable. The restructuring, which is not evident in base voltammograms, is accompanied by a significant increase in the CO oxidation activity at low potentials (0.5-0.8 V), indicating that it is caused by new active sites created by the restructuring, and not by the PtRu sites that existed in the original surface and that were previously held responsible for the high activity of these bimetallic surfaces in terms of a bifunctional mechanism.

Formation, atomic distribution and mixing energy in two-dimensional PdxAg1-x surface alloys on Pd(111)

The formation and atom distribution in two-dimensional Pd(x)Ag(1-x)/Pd(111) monolayer surface alloys were studied by high resolution scanning tunnelling microscopy (STM) with chemical contrast. From short-range order (SRO) parameters, we calculate preferences for like or unlike nearest neighbours to elucidate the mixing behaviour of the two components for various sub monolayer Ag surface contents. In the regime of low Ag surface contents (<40% Ag), the system shows a weak tendency towards phase separation, high Ag coverages (>60% Ag) result in a disperse distribution of the atoms in the surface. Effective pair interactions (EPIs) were derived by comparing the measured distribution with distributions obtained using Monte Carlo (MC) simulations. From the EPIs, we derived a function for the mixing energy, which can describe the change from clustering to a disperse distribution. The effects of the resulting surface atom distributions and of the Ag coverage dependent surface mixing/demixing on catalytic reactions are discussed.

Growth of PtRu Clusters on Ru(0001)-Supported Monolayer Graphene Films

We report on results of a detailed scanning tunnelling microscopy study on the formation, size and size distribution, and internal structure of small bimetallic PtRu clusters on a graphene monolayer film supported on a Ru(0001) substrate. These clusters, with sizes around ∼15 (Ru) or ∼40 (Pt) atoms per cluster at the lowest coverage, are interesting model systems for the catalytic behaviour of small metal PtRu particles, for example for application in electrocatalytic oxidation reactions. The clusters were generated by sequential deposition of the two metals at room temperature. The data reveal a distinct influence of the deposition sequence on the cluster formation process, with Ru pre-deposition followed by Pt deposition leading to predominantly bimetallic clusters, possibly with a core-shell-type structure, while the reverse sequence results in co-existent mono- and bimetallic clusters, where the latter are likely to intermix at the interface. The observations are related to the nucleation process of the respective metals on the templated surface, and the 2D growth behaviour of the two metals.