Scaling Relation between the Reduction Potential of Copper Catalysts and the Turnover Frequency for the Oxygen and Hydrogen Peroxide Reduction Reactions

Changes in the electronic structure of copper complexes can have a remarkable impact on the catalytic rates, selectivity, and overpotential of electrocatalytic reactions. We have investigated the effect of the half-wave potential (E-1/2) of the Cu-II/Cu-I redox couples of four copper complexes with different pyridylalkylamine ligands. A linear relationship was found between E-1/2 of the catalysts and the logarithm of the maximum rate constant of the reduction of O-2 and H2O2. Computed binding constants of the binding of O-2 to Cu-I, which is the rate-determining step of the oxygen reduction reaction, also correlate with E-1/2. Higher catalytic rates were found for catalysts with more negative E-1/2 values, while catalytic reactions with lower overpotentials were found for complexes with more positive E-1/2 values. The reduction of O-2 is more strongly affected by the E-1/2 than the H2O2 rates, resulting in that the faster catalysts are prone to accumulate peroxide, while the catalysts operating with a low overpotential are set up to accommodate the 4-electron reduction to water. This work shows that the E-1/2 is an important descriptor in copper-mediated O-2 reduction and that producing hydrogen peroxide selectively close to its equilibrium potential at 0.68 V vs reversible hydrogen electrode (RHE) may not be easy.