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.
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.
Artificial
photosynthesis, which splits water into H2/O2 or reduces CO2 and N2 by visible
light, catalyzed by molecular catalysts (MCs) with/without being coupled
with a solar cell should serve as one of the most promising renewable
energy systems. There still, however, exist bottleneck subjects to
be resolved in the MCs’ approach, despite the recent progress.
The key subject is the “photon-flux-density problem”
of the rarefied sunlight radiation, which leads to a difficulty for
the stepwise four-photon excitation of MCs to induce oxygen evolution
from water. On the basis of our recent challenges on the two-electron
oxidation of water by one-photon visible light excitation of an MC,
where the MC does not need to wait for the next photon’s arrival,
here we report an artificial photosynthesis system that produces H2 and H2O2 simultaneously on highly earth-abundant
element-based aluminum-porphyrins by only one-photon excitation of
visible light as the first exemplum to overcome the bottleneck.
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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