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Iridium half-sandwich complexes of the types Cp*Ir(N-C)X, [Cp*Ir(N-N)X]X, and [CpIr(N-N)X]X are catalyst precursors for the homogeneous oxidation of water to dioxygen. Kinetic studies with cerium(IV) ammonium nitrate as primary oxidant show that oxygen evolution is rapid and continues over many hours. In addition, [Cp*Ir(H(2)O)(3)]SO(4) and [(Cp*Ir)(2)(μ-OH)(3)]OH can show even higher turnover frequencies (up to 20 min(-1) at pH 0.89). Aqueous electrochemical studies on the cationic complexes having chelate ligands show catalytic oxidation at pH > 7; conversely, at low pH, there are no oxidation waves up to 1.5 V vs NHE for the complexes. H(2)(18)O isotope incorporation studies demonstrate that water is the source of oxygen atoms during cerium(IV)-driven catalysis. DFT calculations and kinetic experiments, including kinetic-isotope-effect studies, suggest a mechanism for homogeneous iridium-catalyzed water oxidation and contribute to the determination of the rate-determining step. The kinetic experiments also help distinguish the active homogeneous catalyst from heterogeneous nanoparticulate iridium dioxide.

A series of Cp*Ir catalysts are the most active known by over an order of magnitude for water oxidation with Ce(IV). DFT calculations support a Cp*Ir=O complex as active species.

Surfactant-free mixed-metal hydroxide water oxidation nanocatalysts were synthesized by pulsed-laser ablation in liquids. In a series of [Ni-Fe]layered double hydroxides with intercalated nitrate and water, [Ni 1−x Fe x (OH) 2 ](NO 3 ) y (OH) x−y •nH 2 O, higher activity was observed as the amount of Fe decreased to 22%. Addition of Ti 4+ and La 3+ ions further enhanced electrocatalysis, with a lowest overpotential of 260 mV at 10 mA cm −2 . Electrocatalytic water oxidation activity increased with the relative proportion of a 405.1 eV N 1s (XPS binding energy) species in the nanosheets.

Molecular water-oxidation catalysts can deactivate by side reactions or decompose to secondary materials over time due to the harsh, oxidizing conditions required to drive oxygen evolution. Distinguishing electrode surface-bound heterogeneous catalysts (such as iridium oxide) from homogeneous molecular catalysts is often difficult. Using an electrochemical quartz crystal nanobalance (EQCN), we report a method for probing electrodeposition of metal oxide materials from molecular precursors. Using the previously reported [Cp*Ir(H(2)O)(3)](2+) complex, we monitor deposition of a heterogeneous water oxidation catalyst by measuring the electrode mass in real time with piezoelectric gravimetry. Conversely, we do not observe deposition for homogeneous catalysts, such as the water-soluble complex Cp*Ir(pyr-CMe(2)O)X reported in this work. Rotating ring-disk electrode electrochemistry and Clark-type electrode studies show that this complex is a catalyst for water oxidation with oxygen produced as the product. For the heterogeneous, surface-attached material generated from [Cp*Ir(H(2)O)(3)](2+), we can estimate the percentage of electroactive metal centers in the surface layer. We monitor electrode composition dynamically during catalytic turnover, providing new information on catalytic performance. Together, these data suggest that EQCN can directly probe the homogeneity of molecular water-oxidation catalysts over short times.

A high-potential porphyrin is codeposited on TiO 2 nanoparticles together with our Cp*-iridium water-oxidation catalyst to give a photoanode for a water-splitting cell. The photoanode optically resembles the porphyrin yet electrochemically responds like the Ir catalyst when it is immersed in aqueous solutions. Photoelectrochemical data show that illumination of the codeposited anode in water results in a marked enhancement and stability of the photocurrent, providing evidence for light-induced activation of the catalyst.Chart 1 Compounds 1 and 2.

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