Three water-soluble cobalt porphyrins have been investigated as water oxidation catalysts via photo-initiation using Ru(II)(bpy)3(2+)/Na2S2O8. The pH dependence of the turnover frequency revealed maximum activity at pH 11. Based on the second order dependence on catalyst concentration for the rate of water oxidation, we suggest a bimolecular radical coupling process as the rate determining step.

The performance of a water-soluble cobalt porphyrin ([{meso-tetra­(4-sulfonatophenyl)­porphyrinato}­cobalt­(III)], CoTPPS) as a catalyst for the photoreduction of CO2 in fully aqueous media has been investigated under visible light irradiation using [Ru­(bpy)3]2+ as a photosensitizer and ascorbate as a sacrificial electron donor. CO is selectively produced (>82%) with high efficiency (926 TONCO; TONCO = turnover number for CO). Upon optimization, selectivities of at least 91% are achieved. Efficiencies up to 4000 TONCO and 2400 h–1 TOFCO (TOFCO = turnover frequency for CO) are reached at low catalyst loadings, albeit with loss in selectivity. This work successfully demonstrates the ability of CoTPPS to perform highly efficient photoreduction of CO2 in water while retaining its high selectivity for CO formation.

Enabling the production of solar fuels on a global scale through artificial photosynthesis requires the development of water oxidation catalysts with significantly improved stability. The stability of photosystems is often reduced owing to attack by singlet oxygen, which is produced during light harvesting. Here, we report photochemical water oxidation by CoFPS, a fluorinated Co-porphyrin designed to resist attack by singlet oxygen. CoFPS exhibits significantly improved stability relative to its non-fluorinated analogue, as shown by a large increase in turnover numbers. This increased stability results from resistance of CoFPS to attack by singlet oxygen, the formation of which was monitored in situ by using 9,10-diphenylanthracene as a chemical probe. Dynamic light scattering (DLS) confirms that CoFPS remains homogeneous, proving its stability during water oxidation catalysis.

A polypyridyl ruthenium sensitizer possessing pyridyl anchors (Ru-py) forms much stronger chemical linkages to TiO surfaces compared to the conventional carboxylate and phosphonate ones. A highly stable dye-sensitized photoelectrochemical cell for water reduction is successfully demonstrated using this technique.

Copper tetrasulfonatophthalocyanine (CuPcTS) is reported to serve as a catalyst for photochemical water oxidation via a radical coupling mechanism. Chloride greatly inhibits the water oxidation rate as a result of axial chloride binding to CuPcTS, preventing formation of the Cu oxyl or hydroxyl intermediate required for O−O bond formation.