Authors: T. P. Johansson, E. T. Ulrikkeholm, P. Hernandez-Fernandez, P. Malacrida, H. A. Hansen, A. S. Bandarenka, J. K. Nørskov, J. Rossmeisl, I. E. L. Stephens, I. Chorkendorff Year of publication: 2013 Journal: Topics in Catalysis DOI: 10.1007/s11244-013-0179-y In order for low temperature polymer electro- lyte membrane fuel cells to become economically viable Pt catalyst loading must be significantly reduced. The cathode of the polymer electrolyte membrane fuel cell, where oxygen reduction takes place, is responsible for the main activity loss. The development of new materials for this reaction is essential in order to increase the overall effec- iency of the fuel cell. Herein, we study the effect of ultra high vacuum annealing on the structure and activity of polycrystalline Pt3Sc. Upon annealing in ultra high vacuum a Pt overlayer is formed on the polycrystalline Pt3Sc. The reactivity of the overlayer is probed by temperature pro- grammed desorption of CO. The onset of CO desorption is around 130 K lower on UHV annealed Pt3Sc than on Pt(111) and the temperature of the desorption peak maxi- mum at saturation was *50 K lower on UHV annealed Pt3Sc, relative to Pt(111), consistent with the CO adsorption energies calculated using density functional theory calculations. Exposing the annealed Pt3Sc sample to 200 mbar O2 at room temperature results in *14 % Sc oxide as measured by X-ray photoelectron spectroscopy. Electrochemical testing of the oxygen reduction reaction shows the same activity as sputter cleaned Pt3Sc.