Alloyed PtmAg nanostructures (m being the atomic Pt/Ag ratio, m = 0.03-1.0) supported on carbon black were prepared and comprehensively characterized by ultraviolet-visible spectroscopy, X-ray diffraction, and transmission electron microscopy techniques. Cyclic voltammetry (CV) and chronoamperometry studies of methanol electro-oxidation reaction (MOR) in alkaline electrolyte indicated that the catalytic activity and CO tolerance of Pt were improved remarkably by the co-presence of Ag. A volcano relationship between the activity and m was found for the PtmAg/C catalysts, and both the highest mass-specific activity and the highest intrinsic activity of Pt were reached at m = 0.5. Interestingly, we also found that the catalytic activity of the PtmAg/C catalysts strongly depended on the high-potential limit of CV. When the PtmAg/C catalysts were cycled up to 0.5 V (vs. saturated calomel electrode), which induced a significant redox process of Ag, they showed ca. 50 times higher catalytic activity than those not subjected to the Ag redox process (up to 0.1 V). Our study clearly demonstrated that the Ag oxide played a crucial role in promoting the catalytic role of Pt for MOR in alkaline electrolyte. (c) 2012 Elsevier Inc. All rights reserved.