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.

Utilizing our recently developed method of identical location transmission electron microscopy (IL-TEM) in combination with electrochemical surface area determination, the degradation behavior of different carbon supported Pt catalysts for polymer electrolyte membrane fuel cells (PEMFCs) is investigated. Two different Pt based catalysts supported on a low surface area (LSA) carbon are compared to a Pt catalyst with standard high surface area (HSA) carbon support. One of the LSA carbon supports is of conventional type, while the other is modified by a transition metal. Relative to the standard, both catalysts with LSA carbon support exhibit improved degradation behavior in terms of loss in active surface area upon accelerated stress tests. The catalyst with transition metal modified carbon support thereby exhibits by far superior improvements. The characterization of the bare carbon supports indicates that the observed differences between both catalysts with LSA carbon support are not related to the resistance of the support to complete oxidation to carbon dioxide. Instead, the IL-TEM results reveal that the improved properties of the catalyst with transition metal modified support are due to a stabilization of the Pt particles attached to the support. Particle detachment thus can be drastically reduced and the degradation is limited to a migration and coalescence or sintering mechanism.

First results of investigations are presented, where size-selected metal clusters generated in ultra high vacuum (UHV) are transferred to ambient conditions and tested for suitable electrochemical applications. As demonstrated, the transfer allows for the characterization of clusters by transmission electron microscopy (TEM) as well as catalytic measurements, which is exemplified by the application of electrochemical measurements. It is demonstrated that well known electrochemical processes on the carbon supported Pt clusters are detected, and thus Pt clusters can be characterized with respect to their accessible surface area, an essential requirement for the study of catalytic processes. Furthermore, as an example for an important electrocatalytic process, it is shown that the oxygen reduction reaction can be probed on the cluster samples featuring a detrimental particle size effect, previously reported for industrial catalysts as well.