Promotive Effect of Bicarbonate Ion on Two-Electron Water Oxidation to Form H₂ O₂ Catalyzed by Aluminum Porphyrins

Two-electron water oxidation initiated by one-electron oxidation of aluminum porphyrins (AlTMPyP) is an alternative water oxidation to the conventional four-electron pathway and could help to avoid the bottleneck subject of photon-flux density in artificial photosynthesis. Here, a dramatic enhancement of the reactivity by bicarbonate ion in the two-electron water oxidation to form H₂ O₂ is reported. An addition of sodium carbonate (Na₂ CO₃ ) controlled both catalytic current and product selectivity of the two-electron water oxidation to enhance the activity of AlTMPyP at pH≈10-11. Controlled potential electrolysis experiments at different concentrations of Na₂ CO₃ (10-100 mm) showed that peroxide selectivity was improved up to approximately 73 % by the increase of [Na₂ CO₃ ] added to the system. The promotion of the reaction cycle was induced by an enhanced dynamic capturing of H₂ O₂ from the hydroperoxy complex of AlTMPyP through an attack of a bicarbonate ion. The detailed electrochemical studies and product selectivity indicated that the bicarbonate ion served as a good cofactor for producing H₂ O₂ from water. At stronger alkaline conditions (pH 12.5), however, a retardative effect of the addition of Na₂ CO₃ on the catalytic reactivity was observed.