One Electron‐Initiated Two‐Electron Oxidation of Water by Aluminum Porphyrins with Earth's Most Abundant Metal

Abstract

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.

Promotive Effect of Bicarbonate Ion on Two‐Electron Water Oxidation to Form H2O2 Catalyzed by Aluminum Porphyrins

“…As already reported by us, the most earth‐abundant metal ion, aluminum(III), incorporated in porphyrin (AlTMPyP) was synthesized in one pot at ambient temperature, and an electrochemical one‐electron oxidation of AlTMPyP initiates the two‐electron oxidation of water to form H 2 O 2 . The reaction was most reactive under rather strong alkaline conditions (pH≈12.5), in which the two axially ligated water molecules on the central aluminum existed in the fully deprotonated form, [AlTMPyP(O − ) 2 ](I).…”

“…H 2 O 2 was found to be the primary product in the oxidation of water but underwent further oxidation on the electrode to form oxygen as the major product in the early stage of the controlled potential electrolysis. The source of electron and oxygen atom in both the four‐electron‐oxidized product (oxygen) and the two‐electron one (H 2 O 2 ) was confirmed to be water itself by isotope‐labelling studies . As shown in Scheme , the key point of the two‐electron oxidation initiated by one‐electron oxidation of the catalyst involves an attack of the hydroxide ion on the one‐electron‐oxidized AlTMPyP ([AlTMPyP(O − )(O⋅)] 2+ , II ), which has an oxyl radical character, to form an adduct of the radical anion on the porphyrin ring ([AlTMPyP(O − )(OO − )]; III ).…”

“… Structure of AlTMPyP exhibiting a protolytic equilibrium of axial ligands; mechanism of water oxidation catalyzed by AlTMPyP under NaOAc/NaOH conditions at pH 12.5 …”

“…Although ordinary electrochemical water oxidation through simultaneous multiple‐electron transfer from molecular catalysts is not affected by the photon‐flux density issue, a stepwise photochemical four‐electron oxidation is affected. In this context, the catalytic reactions initiated by a one‐electron process on an electrode can provide a way for the primary process in photochemical water oxidation to avoid the photon‐flux‐density problem. In the present study, we have performed a detailed study on the effect of bicarbonate and carbonate ions in the two‐electron water oxidation initiated by one‐electron electrochemical oxidation of a molecular catalyst.…”

“…To avoid this bottleneck, one alternative approach is to develop a one‐electron‐induced pathway of water oxidation that completes within the one‐electron oxidation step and does not need to wait for the next photon . In this context we have investigated a two‐electron water oxidation initiated through one‐electron oxidation of catalysts to form H 2 O 2 catalyzed by metalloporphyrins centered with earth‐abundant elements such as Al III , Si IV , and Sn IV[21] . In addition to the novel approach of water oxidation initiated through a one‐electron process, which enables a one‐photon‐induced process to avoid the bottleneck, a more favorable energy accumulation by the H 2 O 2 /H 2 couple (+341 kJ mol −1 ) compared with the conventional O 2 /H 2 couple (243 kJ mol −1 ) as well as the utilization of abundant elements such as Al III[19] and Si IV[20] ions as the reaction center strongly motivated us to further develop the two‐electron oxidation of water to form H 2 O 2 .…”