Abstract
Fossil fuels (coal, oil, natural gas) are becoming increasingly disfavored as long-term energy options due to concerns of scarcity and environmental consequences (e.g., release of anthropogenic CO2). Hydrogen gas, on the other hand, has gained popularity as a clean-burning fuel because the only byproduct from its reaction with O2 is H2O. In recent decades, hydrogen derived from water splitting has been a topic of extensive research. The bottleneck of the water splitting reaction is the difficult water oxidation step (2H2O → O2 + 4H+ + 4e−), which requires an effective and robust catalyst to overcome its high kinetic barrier. Research in water oxidation by molecular ruthenium catalysts enjoys a rich history spanning nearly 40 years. As the diversity of novel ligands continues to widen, the relationship between ligand geometry or electronics, and catalyst activity is undoubtedly becoming clearer. The present review highlights, in the authors’ opinion, some of the most impactful discoveries in the field and explores the evolution of ligand design that has led to the current state of the art.
Highlights
Technologies for harnessing wind, geothermal, hydropower, and solar energy are continually advancing and are beginning to play a formidable role in world energy production
The important implications of this discovery are the following: (1) proton-coupled electron transfer (PCET) prevents excessive charge build-up that would normally disfavor the formation of RuIV species (as exemplified by the high RuIII/IV oxidation potential of 1.78 V for Ru(bpy)2 Cl2 ), and (2) PCET allows for the facile formation of stable Ru-oxo species that can be exploited for further oxidative chemistry such as oxidation of water or organic substrates
In 2010, Meyer’s group made an important discovery in the field showing that electrocatalytic water oxidation, the O-O bond formation step in water nucleophilic attack (WNA), is significantly enhanced by the addition of external bases [97]
Summary
Technologies for harnessing wind, geothermal, hydropower, and solar energy are continually advancing and are beginning to play a formidable role in world energy production. Raising the pH will vary the standard potentials according to the Nernst equation (∆E = −59 mV/pH unit); the overall potential difference will remain ∆E = −1.23 V, which translates to an extremely endothermic reaction with a ∆G value of ~ + 475 kJ per mol of O2 formed It is already apparent from a thermodynamic standpoint that splitting water is an uphill battle. Two prevalent mechanistic scenarios discussed in the literature are: (1) water nucleophilic attack (WNA) and (2) bimolecular radical oxo-coupling (I2M) (Figure 1) Requisite to both scenarios is the generation of a high-valent metal oxo intermediate formed from an aquo ligand through successive proton-coupled electron transfers.
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