We report herein a new molecular catalyst for efficient water splitting, aluminum porphyrins (tetra-methylpyridiniumylporphyrinatealuminum: AlTMPyP), containing earth's most abundant metal as the central ion. One-electron oxidation of the aluminum porphyrin initiates the two-electron oxidation of water to form hydrogen peroxide as the primary reaction product with the lowest known overpotential (97 mV). The aluminum-peroxo complex was detected by a cold-spray ionization mass-spectrometry in high-resolution MS (HRMS) mode and the structure of the intermediate species was further confirmed using laser Raman spectroscopy, indicating the hydroperoxy complex of AlTMPyP to be the key intermediate in the reaction. The two-electron oxidation of water to form hydrogen peroxide was essentially quantitative, with a Faradaic efficiency of 99 %. The catalytic reaction was found to be highly efficient, with a turnover frequency up to ∼2×10 s . A reaction mechanism is proposed involving oxygen-oxygen bond formation by the attack of a hydroxide ion on the oxyl-radical-like axial ligand oxygen atom in the one-electron-oxidized form of AlTMPyP(O ) , followed by a second electron transfer to the electrode.

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The reaction mechanism of photochemical water oxidation catalyzed by AlIII porphyrins (AlTIIIMPyP(OH)2) covalently adsorbed through the coordination of one axial OH group on the surface of TiO2 particles was investigated by time‐resolved nanosecond laser flash photolysis (NLFP). Three types of transient species with completely different absorption and decay characteristics were detected by means of NFLP, demonstrating that the photochemical production of H2O2 from water is initiated only by one‐photon excitation of the catalyst followed by stepwise two‐electron conversion processes, providing a promising model for photochemical water oxidation. Laser flash excitation of AlIIITMPyP adsorbed on a transparent TiO2 film leads to the formation of the AlTMPyP(OH) radical cation through one‐electron injection from the excited singlet state of the catalyst to the conduction band of TiO2. The radical cation is subsequently deprotonated into AlTMPyP(O.), which further reacts with OH−/H2O to form a key intermediate, the radical anion of AlTMPyP(O‐OH), having a characteristic O−O bond at the axial position within a delay time of 2–3 μs. Thereafter the radical anion efficiently transfers a second electron to TiO2 leading to the formation of AlTIIIMPyP(O‐OH) (≈67 μs), which reacts further with OH−/H2O to liberate free hydrogen peroxide as an end‐product with regeneration of the starting catalyst, AlTIIIMPyP(OH), (≈500 μs).

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