Across chemical disciplines, an interest in developing artificial water splitting to O(2) and H(2), driven by sunlight, has been motivated by the need for practical and environmentally friendly power generation without the consumption of fossil fuels. The central issue in light-driven water splitting is the efficiency of the water oxidation, which in the best-known catalysts falls short of the desired level by approximately two orders of magnitude. Here, we show that it is possible to close that 'two orders of magnitude' gap with a rationally designed molecular catalyst [Ru(bda)(isoq)(2)] (H(2)bda = 2,2'-bipyridine-6,6'-dicarboxylic acid; isoq = isoquinoline). This speeds up the water oxidation to an unprecedentedly high reaction rate with a turnover frequency of >300 s(-1). This value is, for the first time, moderately comparable with the reaction rate of 100-400 s(-1) of the oxygen-evolving complex of photosystem II in vivo.

With the inspiration from an oxygen evolving complex (OEC) in Photosystem II (PSII), a mononuclear Ru(II) complex with a tetradentate ligand containing two carboxylate groups has been synthesized and structurally characterized. This Ru(II) complex showed efficient catalytic properties toward water oxidation by the chemical oxidant cerium(IV) ammonium nitrate. During the process of catalytic water oxidation, Ru(III) and Ru(IV) species have been successfully isolated as intermediates. To our surprise, X-ray crystallography together with HR-MS revealed that the Ru(IV) species is a seven-coordinate Ru(IV) dimer complex containing a [HOHOH](-) bridging ligand. This bridging ligand has a short O...O distance and is hydrogen bonded to two water molecules. The discovery of this very uncommon seven-coordinate Ru(IV) dimer together with a hydrogen bonding network may contribute to a deeper understanding of the mechanism for catalytic water oxidation. It will also provide new possibilities for the design of more efficient catalysts for water oxidation, which is the key step for solar energy conversion into hydrogen by light-driven water splitting, the ultimate challenge in artificial photosynthesis.

A molecular device with a photocathode for hydrogen generation has been successfully demonstrated, based on an earth abundant and inexpensive p-type semiconductor NiO, an organic dye P1 and a cobalt catalyst Co1.

Artificial nitrogen fixation through the nitrogen reduction reaction (NRR) under ambient conditions is a potentially promising alternative to the traditional energy-intensive Haber–Bosch process. For this purpose, efficient catalysts are urgently required to activate and reduce nitrogen into ammonia. Herein, by the combination of experiments and first-principles calculations, we demonstrate that copper single atoms, attached in a porous nitrogen-doped carbon network, provide highly efficient NRR electrocatalysis, which compares favorably with those previously reported. Benefiting from the high density of exposed active sites and the high level of porosity, the Cu SAC exhibits high NH3 yield rate and Faradaic efficiency (FE), specifically ∼53.3 μgNH3 h–1 mgcat –1 and 13.8% under 0.1 M KOH, ∼49.3 μgNH3 h–1 mgcat –1 and 11.7% under 0.1 M HCl, making them truly pH-universal. They also show good stability with little current attenuation over 12 h of continuous operation. Cu–N2 coordination is identified as the efficient active sites for the NRR catalysis.

Water oxidation catalysts are essential components of light-driven water splitting systems, which could convert water to H 2 driven by solar radiation (H 2 O þ hν → 1∕2O 2 þ H 2 ). The oxidation of water (H 2 O → 1∕2O 2 þ 2H þ þ 2e − ) provides protons and electrons for the production of dihydrogen (2H þ þ 2e − → H 2 ), a clean-burning and high-capacity energy carrier. One of the obstacles now is the lack of effective and robust water oxidation catalysts. Aiming at developing robust molecular Ru-bda (H 2 bda ¼ 2,2 0 -bipyridine-6,6′-dicarboxylic acid) water oxidation catalysts, we carried out density functional theory studies, correlated the robustness of catalysts against hydration with the highest occupied molecular orbital levels of a set of ligands, and successfully directed the synthesis of robust Ru-bda water oxidation catalysts. A series of mononuclear ruthenium complexes ½RuðbdaÞL 2 (L ¼ pyridazine, pyrimidine, and phthalazine) were subsequently synthesized and shown to effectively catalyze Ce IV -driven ½Ce IV ¼ CeðNH 4 Þ 2 ðNO 3 Þ 6 water oxidation with high oxygen production rates up to 286 s −1 and high turnover numbers up to 55,400.catalysis | density function theory | seven coordination | photosystem II | solar fuels I n pursuit of sustainable energy systems such as solar fuels, much effort has been spent on water splitting to hydrogen and oxygen since hydrogen is a potential clean energy carrier and water is an abundant and environmentally benign resource (1-5). Water splitting consists of two half reactions: (i) water oxidationIn practice, an overpotential is always present, leading to an even higher applied potential. The former half reaction requires strongly oxidizing conditions and is generally considered as the bottleneck of the whole water-splitting process due to the multiple protonelectron transfers and the formation of the O─O bond. Over the last few years, there has been an increasing development of water oxidation catalysts (WOCs) and many transition metal-based catalysts, including Ru (4, 5), Ir (6-8), Co (9-13), Fe (14,15), and Mn (16-18), have been reported with oxygen production rates (OPRs: mole oxygen produced per mole catalyst per second) ≤5 s −1 . Very recently, we reported a family of highly active Ru-based WOCs ½RuðbdaÞL 2 (H 2 L ¼ 2;2 -bipyridine-6,6′-dicarboxylic acid; L ¼ 4-picoline, A; L ¼ isoquinoline, B) ( Fig. 1) with OPRs up to 300 s −1 (19, 20); a seven-coordinate dimeric Ru IV intermediate (D7Ru IV ) (Fig. 1) is involved in the O─O bond formation step (20,21).A general problem encountered in molecular WOCs is the decomposition of catalysts. Ligand dissociation and oxidative decomposition have been considered as the major deactivation pathways. The groups of Llobet, Meyer, and Sun have demonstrated an improved durability of their catalysts by immobilizing catalysts on the electrode/material surface and thereby dramatically suppressing the intermolecular oxidative decomposition pathway (22-25).For our Ru-bda catalysts, the main deactivation pathway has been found to b...

The oxygen evolving complex (OEC) of the natural photosynthesis system II (PSII) oxidizes water to produce oxygen and reducing equivalents (protons and electrons). The oxygen released from PSII provides the oxygen source of our atmosphere; the reducing equivalents are used to reduce carbon dioxide to organic products, which support almost all organisms on the Earth planet. The first photosynthetic organisms able to split water were proposed to be cyanobacteria-like ones appearing ca. 2.5 billion years ago. Since then, nature has chosen a sustainable way by using solar energy to develop itself. Inspired by nature, human beings started to mimic the functions of the natural photosynthesis system and proposed the concept of artificial photosynthesis (AP) with the view to creating energy-sustainable societies and reducing the impact on the Earth environments. Water oxidation is a highly energy demanding reaction and essential to produce reducing equivalents for fuel production, and thereby effective water oxidation catalysts (WOCs) are required to catalyze water oxidation and reduce the energy loss. X-ray crystallographic studies on PSII have revealed that the OEC consists of a Mn4CaO5 cluster surrounded by oxygen rich ligands, such as oxyl, oxo, and carboxylate ligands. These negatively charged, oxygen rich ligands strongly stabilize the high valent states of the Mn cluster and play vital roles in effective water oxidation catalysis with low overpotential. This Account describes our endeavors to design effective Ru WOCs with low overpotential, large turnover number, and high turnover frequency by introducing negatively charged ligands, such as carboxylate. Negatively charged ligands stabilized the high valent states of Ru catalysts, as evidenced by the low oxidation potentials. Meanwhile, the oxygen production rates of our Ru catalysts were improved dramatically as well. Thanks to the strong electron donation ability of carboxylate containing ligands, a seven-coordinate Ru(IV) species was isolated as a reaction intermediate, shedding light on the reaction mechanisms of Ru-catalyzed water oxidation chemistry. Auxiliary ligands have dramatic effects on the water oxidation catalysis in terms of the reactivity and the reaction mechanism. For instance, Ru-bda (H2bda = 2,2'-bipyridine-6,6'-dicarboxylic acid) water oxidation catalysts catalyze Ce(IV)-driven water oxidation extremely fast via the radical coupling of two Ru(V)═O species, while Ru-pda (H2pda = 1,10-phenanthroline-2,9-dicarboxylic acid) water oxidation catalysts catalyze the same reaction slowly via water nucleophilic attack on a Ru(V)═O species. With a number of active Ru catalysts in hands, light driven water oxidation was accomplished using catalysts with low catalytic onset potentials. The structures of molecular catalysts could be readily tailored to introduce additional functional groups, which favors the fabrication of state-of-the-art Ru-based water oxidation devices, such as electrochemical water oxidation anodes and photo-electrochemical anodes. The d...

Discovery of an efficient catalyst bearing low overpotential toward water oxidation is a key step for light-driven water splitting into dioxygen and dihydrogen. A mononuclear ruthenium complex, Ru(II)L(pic)(2) (1) (H(2)L = 2,2'-bipyridine-6,6'-dicarboxylic acid; pic = 4-picoline), was found capable of oxidizing water eletrochemically at a relatively low potential and promoting light-driven water oxidation using a three-component system composed of a photosensitizer, sacrificial electron acceptor, and complex 1. The detailed electrochemical properties of 1 were studied, and the onset potentials of the electrochemically catalytic curves in pH 7.0 and pH 1.0 solutions are 1.0 and 1.5 V, respectively. The low catalytic potential of 1 under neutral conditions allows the use of [Ru(bpy)(3)](2+) and even [Ru(dmbpy)(3)](2+) as a photosensitizer for photochemical water oxidation. Two different sacrificial electron acceptors, [Co(NH(3))(5)Cl]Cl(2) and Na(2)S(2)O(8), were used to generate the oxidized state of ruthenium tris(2,2'-bipyridyl) photosensitizers. In addition, a two-hour photolysis of 1 in a pH 7.0 phosphate buffer did not lead to obvious degradation, indicating the good photostability of our catalyst. However, under conditions of light-driven water oxidation, the catalyst deactivates quickly. In both solution and the solid state under aerobic conditions, complex 1 gradually decomposed via oxidative degradation of its ligands, and two of the decomposed products, sp(3) C-H bond oxidized Ru complexes, were identified. The capability of oxidizing the sp(3) C-H bond implies the presence of a highly oxidizing Ru species, which might also cause the final degradation of the catalyst.

Practical-efficiency catalysis of the water oxidation process (2 H 2 O!O 2 + 4 e À + 4 H + ) is the highly sought element of emerging artificial photosynthetic energy-conversion technology. [1,2] While oxygen evolution in naturally occurring photosynthesis, which supports nearly all existing life forms, relies on a sophisticated Mn-based complex, [1a, 3] the majority of artificial molecular water-oxidation catalysts (WOCs) are Ru-based complexes with relatively simple polypyridyl ligands. [4, 5] Recently emerged evidence in favor of alternative O 2 -evolving mechanisms for Ru-catalyzed water oxidation, [5] such as solvent water nucleophilic attack (WNA) and direct O À O coupling via interaction of two M-O units (I2M), [5a, 6-8] demonstrated dramatic mechanistic consequences of different ligand designs, which are yet to be fully rationalized. Understanding of intricate ligand-dependent preferences for one mechanism over the other is necessary for further progress to be made. [5, 7] Unfortunately, vast structural differences between Ru-bound ligands of WOCs which operate by the WNA or I2M mechanism hampers determinations of ligand influence on catalytic pathways.