Researchers in this field have investigated photochemical and electrochemical CO 2 reduction to CO or formate, even to methanol, using transition metal electrodes, metal complexes, semiconductors, and also organic molecules, and the details of these achievements can be found in many reviews recently published. 14−31 While recent progress in this field is quite remarkable in photochemical CO 2 reduction using semiconductors or heterogeneous systems, most experiments have not been confirmed using labeled CO 2 (i.e., 13 CO 2 ) and H 2 O (i.e., H 2 18Figure 1. Z-Scheme for photocatalytic CO 2 reduction coupled to methanol oxidation. Reproduced with permission from ref 46.

Green plants convert CO(2) to sugar for energy storage via photosynthesis. We report a novel catalyst that uses CO(2) and hydrogen to store energy in formic acid. Using a homogeneous iridium catalyst with a proton-responsive ligand, we show the first reversible and recyclable hydrogen storage system that operates under mild conditions using CO(2), formate and formic acid. This system is energy-efficient and green because it operates near ambient conditions, uses water as a solvent, produces high-pressure CO-free hydrogen, and uses pH to control hydrogen production or consumption. The extraordinary and switchable catalytic activity is attributed to the multifunctional ligand, which acts as a proton-relay and strong π-donor, and is rationalized by theoretical and experimental studies.

New strategy for the simultaneous tuning of catalytic activity and water solubility of complex catalysts is described on the basis of an acid−base equilibrium between pyridinol and pyridinolate as the catalyst ligands. Herein, half-sandwich complexes with 4,4‘-dihydroxy-2,2‘-bipyridine (DHBP) or 4,7-dihydroxy-1,10-phenanthroline (DHPT) served as highly efficient and recyclable catalysts for the hydrogenation of bicarbonate in water. The oxyanion generated from the phenolic hydroxy group shows strong electronic donation and polarity, which play significant roles in the catalytic activity and water solubility, respectively. As a result, turnover frequencies (TOF) up to 42 000 h-1 and turnover numbers (TON) up to 222 000 have been obtained by using iridium catalysts under 6 MPa at 120 °C. Furthermore, an iridium DHPT catalyst was spontaneously precipitated at the end of the reaction. Iridium leaching was found to be 0.11 ppm (1.2% of the loaded catalyst), and the added base was completely consumed. The recovered catalyst could be recycled for four cycles with high catalytic activity. Consequently, the catalyst was homogeneous and highly activated at the beginning of the reaction, whereas it was heterogeneous and deactivated at the end. The catalytic system offers an environmentally benign process with high efficiency, easy separation, catalyst recycling, waste-free process, and aqueous catalysis.

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