Highly Efficient and Selective Two-Electron Reduction of Dioxygen Catalyzed By Cobalt Chlorin Complexes to Produce Hydrogen Peroxide
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)
proportinal to concentrations of CoII(Chn) 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 CoII(Chn), followed by fast electron transfer from Me2Fc to [CoII(Chn)]+.
The one-electron reduction potential of [CoIII(Ch1)]+ was changed from 0.37 V (vs SCE) to 0.48 V by the addition of HClO4 due to the protonation of [CoIII(Ch1)]+.6 The introduction of electron-withdrawing groups on the chlorin ligand in CoII(Chn) (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 [CoIII(Chn)]+/CoII(Chn) (n = 2, 3) were observed. As a result, CoII(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 CoIICh1-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).
2H2O + O2 --> 2H2O2 (2)
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