Systematic studies conducted on various catalyst types including metal oxides and elemental surfaces have shown the difference between the two constants to be K OOH -K OH ≈ 3.2 eV. In contrast, the ideal catalyst, where all elementary steps are isoenergetic should have K OH = 0 eV and K OOH = 2.46 eV. The main consequence of such scaling relations is the appearance of a minimum inaccessible free energy G ex . This term is the minimum potential difference from equilibrium at which all reactions are down-hill in energy and is frequently related to the overpotential η. By using the commonly accepted scaling relations, we expressed G ex as function of ∆ ḠO in Figure along with the change in ∆ ḠO for selected oxygen coverages. On the resulting volcano plot two regimes can be identified: for catalysts that bind oxygen too strongly, the Rate Determining Step (RDS) is the conversion of OH * to H 2 O, while for catalysts with weak oxygen binding, the formation of OOH * from O 2 becomes rate-limiting. At low coverages ∆ ḠO is lower than the optimal value of 1.73 eV, as expected for Pt.