Enzymatic versus Inorganic Oxygen Reduction Catalysts: Comparison of the Energy Levels in a Free-Energy Scheme

In this paper, we present a method to directly compare the energy levels of intermediates in enzymatic and inorganic oxygen reduction catalysts. We initially describe how the energy levels of a Pt(111) catalyst, operating at pH = 0, are obtained. By a simple procedure, we then convert the energy levels of cytochrome c oxidase (CcO) models obtained at physiological pH = 7 to the energy levels at pH = 0, which allows for comparison. Furthermore, we illustrate how different bias voltages will affect the free-energy landscapes of the catalysts. This allows us to determine the so-called theoretical overpotential of each system, which is shown to be significantly lower for the enzymatic catalysts compared to the inorganic Pt(111) catalyst. Finally, we construct theoretical polarization curves for the CcO models, in order to illustrate the effect of the low overpotentials on turnover rates per site.