Promotive Effect of Bicarbonate Ion on Two‐Electron Water Oxidation to Form H2O2 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 H2O2 is reported. An addition of sodium carbonate (Na2CO3) 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 Na2CO3 (10–100 mm) showed that peroxide selectivity was improved up to approximately 73 % by the increase of [Na2CO3] added to the system. The promotion of the reaction cycle was induced by an enhanced dynamic capturing of H2O2 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 H2O2 from water. At stronger alkaline conditions (pH 12.5), however, a retardative effect of the addition of Na2CO3 on the catalytic reactivity was observed. Two is better than four: A dramatic enhancement of the reactivity and peroxide selectivity by bicarbonate ion in the one‐electron‐initiated two‐electron oxidation of water catalyzed by aluminum porphyrins is studied. The incorporation of the carbonate cofactor results in approximately 73 % product selectivity with a lowest overpotential of 55–100 mV for two‐electron water oxidation.