Kagóme Cobalt(II)‐Organic Layers as Robust Scaffolds for Highly Efficient Photocatalytic Oxygen Evolution

Abstract

Two Kagóme cobalt(II)‐organic layers of [Co₃(μ₃‐OH)₂(bdc)₂]_n (1) and [Co₃(μ₃‐OH)₂(chdc)₂]_n (2) (bdc=o‐benzenedicarboxylate and chdc=1,2‐cyclohexanedicarboxylate) that bear bridging OH⁻ ligands were explored as water oxidation catalysts (WOCs) for photocatalytic O₂ production. The activities of 1 and 2 towards H₂O oxidation were assessed by monitoring the in situ O₂ concentration versus time in the reaction medium by utilizing a Clark‐type oxygen electrode under photochemical conditions. The oxygen evolution rate (R ) was 24.3 μmol s⁻¹ g⁻¹ for 1 and 48.8 μmol s⁻¹ g⁻¹ for 2 at pH 8.0. Photocatalytic reaction studies show that 1 and 2 exhibit enhanced activities toward the oxidation of water compared to commercial nanosized Co₃O₄. In scaled‐up photoreactions, the pH value of the reaction medium decreased from 8.0 to around 7.0 after 20 min and the O₂ production ceased. Based on the amounts of the sacrificial oxidant (K₂S₂O₈) used, the yield of O₂ produced is 49.6 % for 2 and 29.8 % for 1. However, the catalyst can be recycled without a significant loss of catalytic activity. Spectroscopic studies suggest that the structure and composition of recycled 1 and 2 are maintained. In isotope‐labeling H₂¹⁸O (97 % enriched) experiments, the distribution of ¹⁶O¹⁶O/¹⁶O¹⁸O/¹⁸O¹⁸O detected was 0:7.55:92.45, which is comparable to the theoretical values of 0.09:5.82:94.09. This work not only provides new catalysts that resemble ligand‐protected cobalt oxide materials but also establishes the significance of the existence of OH⁻ (or H₂O) binding sites at the metal center in WOCs.