Highly Efficient and Selective Photocatalytic CO₂ Reduction to CO in Water by a Cobalt Porphyrin Molecular Catalyst

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 TON_CO; TON_CO = turnover number for CO). Upon optimization, selectivities of at least 91% are achieved. Efficiencies up to 4000 TON_CO and 2400 h^-1 TOF_CO (TOF_CO = 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.