Catalysts Based on Earth‐Abundant Metals for Visible Light‐Driven Water Oxidation Reaction

Tracking an FeV (O) Intermediate for Water Oxidation in Water

High-valent iron-oxo species are appealing for conducting O-O bond formation for water oxidation reactions. However, their high reactivity poses a great challenge to the dissection of their chemical transformations. Herein, we introduce an electron-rich and oxidation-resistant ligand, 2-[(2,2'-bipyridin)-6-yl]propan-2-ol to stabilize such fleeting intermediates. Advanced spectroscopies and electrochemical studies demonstrate a high-valent FeV (O) species formation in water. Combining kinetic and oxygen isotope labelling experiments and organic reactions indicates that the FeV (O) species is responsible for O-O bond formation via water nucleophilic attack under the real catalytic water oxidation conditions.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Electrochemical water oxidation catalyzed by a mononuclear cobalt complex of a pentadentate ligand: the critical effect of the borate anion

A cobalt complex is found as a homogeneous water oxidation electrocatalyst. Electrochemical examinations indicate that the implementation of proton-couple electron transfer process and formation of O–O bond are assisted by borate anion.

Theoretical insight of decatungstate photocatalyzed alkylation of N-tosylimine via hydrogen atom transfer and proton-coupled electron transfer

Decatungstate as a photocatalyst can activate various C(sp3)−H bond to successfully construct C(sp3)−C(sp2) bond with N-tosylimines. Herein density functional theory (DFT) calculations reveal the unique radical mechanism triggered by the...

Water-Stable Cobalt-Based MOF for Water Oxidation in Neutral Aqueous Solution: A Case of Mimicking the Photosystem II

Inspired by the highly efficient water oxidation of Mn₄CaO₅ in natural photosynthesis, development of novel artificial water oxidation catalysts (WOCs) with structure and function mimicked has inspired extensive interests. A novel 3D cobalt-based MOF (GXY-L8-Co) was synthesized for promising artificial water oxidation by employing the Co₄O₄ quasi-cubane motifs with a similar structure as the Mn₄CaO₅ as the core. The GXY-L8-Co not only shows good chemical stability in common organic solvents or water for up to 10 days but also exhibits oxygen evolution performance. It has been demonstrated that the uniform distribution of Co₄O₄ catalytic active sites confined in the MOF framework should be responsible for the good robustness and catalytic performance.

Photochemical Water Oxidation Using a Doubly N-Confused Hexaphyrin Dinuclear Cobalt Complex

A doubly N-confused hexaphyrin dinuclear cobalt complex (Co₂DNCH) is revealed as an efficient water oxidation catalyst, outperforming the mononuclear cobalt porphyrin with the same aryl group as those in Co₂DNCH. By photoirradiation of a water/acetone-d₆ (9:1) mixture containing Co₂DNCH, [Ru^II(bpy)₃]²⁺, and S₂O₈^2- as the water oxidation catalyst, photosensitizer, and sacrificial electron acceptor, respectively, with visible light, O₂ was obtained as the maximum with turnover number = 1200, turnover frequency = 3.9 s^-1, and quantum yield = 0.30.

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

The recent synthetic and mechanistic progress in molecular and heterogeneous water oxidation catalysts highlights the new, overarching strategies for knowledge transfer and unifying design concepts.

Polyoxometalates acting as a hole-transfer mediator and crystallization accelerant in a perovskite photoanode for the photoelectrocatalytic oxidation of benzene into phenol

Organic metal halide perovskite (OMHP) material shows promising applications in the photoelectrocatalytic field, but its efficiencies are unsatisfactory due to the bulk and surface carrier recombination. In this work, we used dual polyoxometalates (C₄H₉N)₃PW₁₂O₄₀ and [Ag₁₀[{Co(H₂O)₃}₂{CoBi₂W₁₉O₆₆(OH)₄}] to modify the OMHP photoanode; the former acted to improve the quality of the perovskite film and the latter could facilitate hole transfer. Such dual modifications effectively reduce carrier recombination and thus obviously boost photoelectrocatalytic efficiency. Hence, we explored the photoelectrocatalytic oxidation of benzene into phenol in aqueous solution by using the modified OMHP photoanode. The yield of phenol in the reaction using the modified OMHP photoanode reached about 31.8%, which was obviously superior to that using the pure OMHP photoanode. Furthermore, we carried out radical scavenger studies to investigate the active species involved in the photoelectrocatalytic benzene oxidation reaction, and thus proposed the plausible mechanism of the photoelectrocatalytic oxidation of benzene into phenol over the OMHP photoanode. These results provide new insights into the development of high performance OMHP photoanodes for photoelectrocatalytic organic transformation.

Homogeneous photochemical water oxidation with metal salophen complexes in neutral media

The development of water oxidation catalysts based on Earth-abundant metals that can function at neutral pH remains a basic chemical challenge.