Highly Efficient and Selective Two-Electron Reduction of Dioxygen Catalyzed By Cobalt Chlorin Complexes to Produce Hydrogen Peroxide

Hydrogen peroxide (H₂O₂) is an ideal energy carrier alternative to hydrogen, because H₂O₂ can be produced by photocatalytic oxidation of water by O₂ using solar energy,¹ and H₂O₂ can be used as a fuel in a one-compartment H₂O₂ fuel cell with low cost material.^2-4 H₂O₂ can also be produced by electrochemical two-electron reduction of O₂ using solar cells.⁵ Thus, it is highly desired to develop efficient catalysts for selective two-electron reduction of O₂ to produce H₂O₂.

We report herein that cobalt chlorin complexes in Chart 1 act as highly efficient and selective catalysts for two-electron reduction of O₂ to H₂O₂. The detailed study based on the kinetics and thermodynamics have provided valuable insight into the development of the more efficient electrocatalyst for two-electron re-duction of O₂ to H₂O₂, which is essential to realize H₂O₂-energy society. 

of O₂ by 1,1’-dimethylferrocene (Me₂Fc) efficiently and selectively to produce H₂O₂ in the presence of perchloric acid (HClO₄) in benzonitrile (PhCN) at 298 K (eq 1). The  rate of formation of Me₂Fc⁺was

2Me₂Fc + O₂ + 2H⁺ --> 2Me₂Fc⁺ + H₂O₂            (1)

                            Co^II(Ch_n)

proportinal to concentrations of Co^II(Ch_n) and O₂, whereas the rate was independent of concentraton of Me₂Fc. On the other hand, the catalytic rate increased with increasing concentraiton of HClO₄ to reach a constant value. Such kinetic results indicate that the rate-determining step in the catalytic cycle (Scheme 1) is the proton-coupled electron transfer reduction of O₂ by Co^II(Ch_n), followed by fast electron transfer from Me₂Fc to  [Co^II(Ch_n)]⁺.

The one-electron reduction potential of [Co^III(Ch₁)]⁺ was changed from 0.37 V (vs SCE) to 0.48 V by the addition of HClO₄ due to the protonation of [Co^III(Ch₁)]⁺.⁶ The introduction of electron-withdrawing groups on the chlorin ligand in Co^II(Ch_n) (n = 2, 3) resulted in the positive shift of the redox potential for Co(III/II) from 0.37 V to 0.45 and 0.40 V, respectively. In the presence of HClO₄, however, no positive shifts of the redox potential for [Co^III(Ch_n)]⁺/Co^II(Ch_n) (n = 2, 3) were observed. As a result, Co^II(Ch₃), which has the lowest one-electron oxidation potential in the presence of HClO₄ among the cobalt chlorin complexes in Chart 1, exhibited the highest catalytic reactivity for the two-electron recution of O₂ by Me₂Fc in the presence of HClO₄in PhCN. 

The electrocatalytic two-electron reduction of O₂ was also examined by using a Co^IICh₁-modified carbon paper electrode with multiwalled carbon nanotubes (MWCNTs) in an aqueous solution (pH = 1). A catalytic current of two-electron reduction of O₂ was observed at a small overpotential. The electrocatalytic two-electron reduction of O₂ was combined with the photocatalytic oxidation of water using semiconductor photocatalysts to produce H₂O₂ from H₂O and O₂using solar energy (eq 2).

                hν

2H₂O + O₂ --> 2H₂O₂              (2)

            catalysts

References

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