The nature of O2 adsorption, and of the various intermediates, such as OOH, OH, O, H2O2, and H2O in the presence of hydrated protons (acid medium) on Pt catalysts studied with density functional theory on small clusters and extended surfaces, and Car-Parrinello molecular dynamics simulations, permit us to elucidate aspects of the four electron reduction steps. The effect of the electrode potential on the complete the four-electron reduction mechanism of O2 on a Pt(111) surface is incorporated via simulated cyclic voltammetry obtained from Dynamic Monte Carlo simulations. The simulations qualitatively agree with the experiments in the detection of H2O2 on the surface, as well as in the magnitude of the cathode overpotential. We discuss the time evolution of surface species and turnover frequencies as functions of electrode potential.