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Mechanisms of pH-Dependent Activity for Water Oxidation to Molecular Oxygen by MnO2 Electrocatalysts
- Takashima, Toshihiro, Hashimoto, Kazuhito, Nakamura, Ryuhei
- Journal of the American Chemical Society 2012 v.134 no.3 pp. 1519-1527
- absorption, catalysts, electrochemistry, electrodes, manganese, manganese dioxide, models, nanoparticles, oxidation, oxygen, oxygen production, pH, superoxide anion
- Manganese oxides function as efficient electrocatalysts for water oxidation to molecular oxygen in strongly alkaline conditions, but are inefficient at neutral pH. To provide new insight into the mechanism underlying the pH-dependent activity of the electrooxidation reaction, we performed UV–vis spectroelectrochemical detection of the intermediate species for water oxidation by a manganese oxide electrode. Layered manganese oxide nanoparticles, δ-MnO₂ (K₀.₁₇[Mn⁴⁺₀.₉₀Mn³⁺₀.₀₇□₀.₀₃]O₂·0.53H₂O) deposited on fluorine-doped tin oxide electrodes were shown to catalyze water oxidation at pH from 4 to 13. At this pH range, a sharp rise in absorption at 510 nm was observed with a concomitant increase of anodic current for O₂ evolution. Using pyrophosphate as a probe molecule, the 510 nm absorption was attributable to Mn³⁺ on the surface of δ-MnO₂. The onset potential of the water oxidation current was constant at approximately 1.5 V vs SHE from pH 4 to pH 8, but sharply shifted to negative at pH > 8. Strikingly, this behavior was well reproduced by the pH dependence of the onset of 510 nm absorption, indicating that Mn³⁺ acts as the precursor of water oxidation. Mn³⁺ is unstable at pH < 9 due to the disproportionation reaction resulting in the formation of Mn²⁺ and Mn⁴⁺, whereas it is effectively stabilized by the comproportionation of Mn²⁺ and Mn⁴⁺ in alkaline conditions. Thus, the low activity of manganese oxides for water oxidation under neutral conditions is most likely due to the inherent instability of Mn³⁺, whose accumulation at the surface of catalysts requires the electrochemical oxidation of Mn²⁺ at a potential of approximately 1.4 V. This new model suggests that the control of the disproportionation and comproportionation efficiencies of Mn³⁺ is essential for the development of Mn catalysts that afford water oxidation with a small overpotential at neutral pH.