Polyoxometalate-Based Single-Atom Catalyst with Precise Structure and Extremely Exposed Active Site for Efficient H2 Evolution

Single-atom catalysts with precise structure and extremely high catalytic efficiency remain a fervent focus in the fields of materials chemistry and catalytic science. Herein, a nickel-substituted polyoxometalate (POM) {NiSb6O4(H2O)3[β-Ni(hmta)SbW8O31]3}15- (NiPOM) with one extremely exposed nickel site [NiO3(H2O)3] was synthesized using the conventional aqueous method. The uniform dispersion of single nickel center with well-defined structure was facilely achieved by anchoring nanosized NiPOM on graphene oxide (GO). The resulting NiPOM/GO can couple with CdS photoabsorber for the construction of low-cost and ultra-efficient hydrogen evolution system. The H2 yield can reach to 2753.27 mmol gPOM -1 h-1, which represents a record value among all the POM-based photocatalytic systems. Remarkablely, an extremely high hydrogen yield of 3647.28 mmol gPOM -1 h-1 was achieved with simultaneous photooxidation of commercial waste plastic, representing the first POM-based photocatalytic system for H2 evolution and waste plastic conversion. This work highlights a straightforward strategy for constructing extremely exposed single-metal site with precise microenvironment by facilely manipulating nanosized molecular cluster to control individual atom.

Polyoxometalate-Structured Materials: Molecular Fundamentals and Electrocatalytic Roles in Energy Conversion

Polyoxometalates (POMs), a kind of molecular metal oxide cluster with unique physical-chemical properties, have made essential contributions to creating efficient and robust electrocatalysts in renewable energy systems. Due to the fundamental advantages of POMs, such as the diversity of molecular structures and large numbers of redox active sites, numerous efforts have been devoted to extending their application areas. Up to now, various strategies of assembling POM molecules into superstructures, supporting POMs on heterogeneous substrates, and POMs-derived metal compounds have been developed for synthesizing electrocatalysts. From a multidisciplinary perspective, the latest advances in creating POM-structured materials with a unique focus on their molecular fundamentals, electrocatalytic roles, and the recent breakthroughs of POMs and POM-derived electrocatalysts, are systematically summarized. Notably, this paper focuses on exposing the current states, essences, and mechanisms of how POM-structured materials influence their electrocatalytic activities and discloses the critical requirements for future developments. The future challenges, objectives, comparisons, and perspectives for creating POM-structured materials are also systematically discussed. It is anticipated that this review will offer a substantial impact on stimulating interdisciplinary efforts for the prosperities and widespread utilizations of POM-structured materials in electrocatalysis.

Structurally-New Hexadecanuclear Ni-Containing Silicotungstate with Catalytic Hydrogen Generation Activity

A structurally-new, carbon-free hexadecanuclear Ni-containing silicotungstate, [Ni₁₆(H₂O)₁₅(OH)₉(PO₄)₄(SiW₉O₃₄)₃]^19-, has been facilely synthesized using a one-pot, solution-based synthetic method systematically characterized by single-crystal X-ray diffraction and several other techniques. The resulting complex works as a noble-metal-free catalyst for visible-light-driven catalytic generation of hydrogen, by coupling with a [Ir(coumarin)₂(dtbbpy)][PF₆] photosensitizer and a triethanolamine (TEOA) sacrificial electron donor. Under minimally optimized conditions, a turnover number (TON) of 842 was achieved for TBA-Ni₁₆P₄(SiW₉)₃-catalyzed hydrogen evolution system. The structural stability of TBA-Ni₁₆P₄(SiW₉)₃ catalyst under photocatalytic conditions was evaluated by the mercury-poisoning test, FT-IR, and DLS measurements. The photocatalytic mechanism was elucidated by both time-solved luminescence decay and static emission quenching measurements.

Surface Reconstruction of Cobalt-Based Polyoxometalate and CNT Fiber Composite for Efficient Oxygen Evolution Reaction

Polyoxometalates (POMs), as carbon-free metal-oxo-clusters with unique structural properties, are emerging water-splitting electrocatalysts. Herein, we explore the development of cobalt-containing polyoxometalate immobilized over the carbon nanotube fiber (CNTF) (Co4POM@CNTF) towards efficient electrochemical oxygen evolution reaction (OER). CNTF serves as an excellent electron mediator and highly conductive support, while the self-activation of the part of Co4POM through restructuring in basic media generates cobalt oxides and/or hydroxides that serve as catalytic sites for OER. A modified electrode fabricated through the drop-casting method followed by thermal treatment showed higher OER activity and enhanced stability in alkaline media. Furthermore, advanced physical characterization and electrochemical results demonstrate efficient charge transfer kinetics and high OER performance in terms of low overpotential, small Tafel slope, and good stability over an extended reaction time. The significantly high activity and stability achieved can be ascribed to the efficient electron transfer and highly electrochemically active surface area (ECSA) of the self-activated electrocatalyst immobilized over the highly conductive CNTF. This research is expected to pave the way for developing POM-based electrocatalysts for oxygen electrocatalysis.

Structurally Precise Two-Transition-Metal Water Oxidation Catalysts: Quantifying Adjacent 3d Metals by Synchrotron X-Radiation Anomalous Dispersion Scattering

Mixed 3d metal oxides are some of the most promising water oxidation catalysts (WOCs), but it is very difficult to know the locations and percent occupancies of different 3d metals in these heterogeneous catalysts. Without such information, it is hard to quantify catalysis, stability, and other properties of the WOC as a function of the catalyst active site structure. This study combines the site selective synthesis of a homogeneous WOC with two adjacent 3d metals, [Co₂Ni₂(PW₉O₃₄)₂]^10- (Co₂Ni₂P₂) as a tractable molecular model for CoNi oxide, with the use of multiwavelength synchrotron X-radiation anomalous dispersion scattering (synchrotron XRAS) that quantifies both the location and percent occupancy of Co (∼97% outer-central-belt positions only) and Ni (∼97% inner-central-belt positions only) in Co₂Ni₂P₂. This mixed-3d-metal complex catalyzes water oxidation an order of magnitude faster than its isostructural analogue, [Co₄(PW₉O₃₄)₂]^10- (Co₄P₂). Four independent and complementary lines of evidence confirm that Co₂Ni₂P₂ and Co₄P₂ are the principal WOCs and that Co²⁺(aq) is not. Density functional theory (DFT) studies revealed that Co₄P₂ and Co₂Ni₂P₂ have similar frontier orbitals, while stopped-flow kinetic studies and DFT calculations indicate that water oxidation by both complexes follows analogous multistep mechanisms, including likely Co-OOH formation, with the energetics of most steps being lower for Co₂Ni₂P₂ than for Co₄P₂. Synchrotron XRAS should be generally applicable to active-site-structure-reactivity studies of multi-metal heterogeneous and homogeneous catalysts.

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.

Pt-Substituted polyoxometalate modification on the surface of low-cost TiO2 with highly efficient H2 evolution performance

In this study, Pt-substituted polyoxometalate was first modified on the surface of commercially available TiO₂, forming an efficient photocatalyst with high reactivity for hydrogen evolution. During the photocatalytic process, Pt-polyoxometalates not only increase the mobility rate of electrons but also improve the separation efficiency of photoinduced electrons and holes. After photoreduction, the in situ generated Pt⁰ species are anchored on the surface of polyoxometalate anion, which prevents further agglomeration. Then, the in situ formed Pt⁰ species and polyoxometalates synergistically promote the efficiency of photoinduced electron transfer from TiO₂ to the protons adsorbed on the Pt⁰ surface. Although the content of Pt⁰ in the nanocomposite is only 0.6%, the photocatalytic hydrogen production rate reaches 5.6 mmol g^-1 h^-1 and remains stable at 4.5 mmol g^-1 h^-1 after the continuous catalytic process. Due to the modification of TiO₂ by Pt-substituted polyoxometalate, this nanocomposite represents a practical model that possesses highly efficient photoelectric conversion performance. The presented work not only extends the family of new TiO₂-polyoxometalate-based materials but also takes a further step toward the practical application of commercial TiO₂ in photocatalytic hydrogen production.

Giant iron polyoxometalate that works as a catalyst for water oxidation

A polyoxometalate (POM) cluster [Mo₇₂Fe₃₀O₂₅₂(CH₃COO)₁₂{Mo₂O₇(H₂O)}₂ {H₂Mo₂O₈(H₂O)} (H₂O)₉₁]. ca. 150 H₂O (catalyst I) has been explored as a light-driven water oxidation catalyst. The catalyst is stable and could be reused/recycled several times.

Thin Films of Fully Noble Metal-Free POM@MOF for Photocatalytic Water Oxidation

P₂W₁₈Co₄@MOF-545, which contains the sandwich-type polyoxometalate (POM) [(PW₉O₃₄)₂Co₄(H₂O)₂]^10- (P₂W₁₈Co₄) immobilized in the porphyrinic metal-organic framework (MOF), MOF-545, is a "three-in-one" (porosity + light capture + catalysis) heterogeneous photosystem for the oxygen-evolution reaction (OER). Thin films of this composite were synthesized on transparent and conductive indium tin oxide (ITO) supports using electrophoretic (EP) or drop-casting (DC) methods, thus providing easy-to-use devices. Their electro- and photocatalytic activities for OER were investigated. Remarkably, both types of films exhibit higher turnover numbers (TONs) than the original bulk material previously studied as a suspension for the photocatalytic OER, with TONs after 2 h equal to 1600 and 403 for DC and EP films, respectively, compared to 70 for the suspension. This difference of catalytic activities is related to the proportion of efficiently illuminated crystallites, whereby a DC thin film offers the largest proportion of POM@MOF crystallites exposed to light due to its lower thickness when compared to an EP film or crystals in suspension. Such devices can be easily recycled by simply removing them from the reaction medium and washing them before reuse. The films were fully characterized with extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies, Raman, scanning electron microscopy, and electrochemistry before and after catalysis. The combination of all of these techniques shows the stability of both the POM and the MOF within the composite upon water-oxidation reaction.

9-Cobalt(II)-Containing 27-Tungsto-3-germanate(IV): Synthesis, Structure, Computational Modeling, and Heterogeneous Water Oxidation Catalysis

The 9-cobalt(II)-containing trimeric, cyclic polyanion [Co₉(OH)₃(H₂O)₆(PO₄)₂(B-α-GeW₉O₃₄)₃]^21- (1) was synthesized in an aqueous phosphate solution at pH 8 and isolated as a hydrated mixed sodium-cesium salt. Polyanion 1 was structurally and compositionally characterized in the solid state by single-crystal X-ray diffraction, Fourier transform infrared spectroscopy, as well as thermogravimetric and elemental analyses. The magnetic and electrochemical properties of 1 were also studied and compared with those of its phosphorus analogue, [Co₉(OH)₃(H₂O)₆(HPO₄)₂(B-α-PW₉O₃₄)₃]^16- (Co₉-P). The electrochemical water oxidation activity of the cesium salt of 1 under heterogeneous conditions was also studied and shown to be superior to that of Co₉-P. The experimental results were supported by computational studies.

Self-Assembled Supramolecular Hybrid of Carbon Nanodots and Polyoxometalates for Visible-Light-Driven Water Oxidation

Water splitting is considered the most attractive pursuit in the field of solar energy conversion. In this study, we report the synthesis and application of a supramolecular hybrid of carbon nanodot (CD) and cobalt polyoxometalate (Co-POM) to solar water oxidation. The self-assembly of the alginate-based CD and Co-POM led to the formation of a spherical hybrid of CD/Co-POM. Owing to the facile transfer of photogenerated holes from CD under visible light irradiation, the hybrid donor-acceptor type of CD/Co-POM enabled the rapid scavenging of holes and accumulation of a long-lived oxidation state of Co-POM for efficient solar water oxidation, outperforming conventional [Ru(bpy)₃]²⁺-based systems. We believe that this study offers new insights into the development of CD-based nanocomposites with various photocatalytic and optoelectronic applications.

In situ X-ray absorption spectroscopy of transition metal based water oxidation catalysts

X-ray absorption studies of the geometric and electronic structure of primarily heterogeneous Co, Ni, and Mn based water oxidation catalysts are reviewed. The X-ray absorption near edge and extended X-ray absorption fine structure studies of the metal K-edge, characterize the metal oxidation state, metal-oxygen bond distance, metal-metal distance, and degree of disorder of the catalysts. These properties guide the coordination environment of the transition metal oxide radical that localizes surface holes and is required to oxidize water. The catalysts are investigated both as-prepared, in their native state, and under reaction conditions, while transition metal oxide radicals are generated. The findings of many experiments are summarized in tables. The advantages of future X-ray experiments on water oxidation catalysts, which include the limited data available of the oxygen K-edge, metal L-edge, and resonant inelastic X-ray scattering, are discussed.

A Fully Noble Metal-Free Photosystem Based on Cobalt-Polyoxometalates Immobilized in a Porphyrinic Metal-Organic Framework for Water Oxidation

The sandwich-type polyoxometalate (POM) [(PW₉O₃₄)₂Co₄(H₂O)₂]^10- was immobilized in the hexagonal channels of the Zr(IV) porphyrinic MOF-545 hybrid framework. The resulting composite was fully characterized by a panel of physicochemical techniques. Calculations allowed identifying the localization of the POM in the vicinity of the Zr₆ clusters and porphyrin linkers constituting the MOF. The material exhibits a high photocatalytic activity and good stability for visible-light-driven water oxidation. It thus represents a rare example of an all-in-one fully noble metal-free supramolecular heterogeneous photocatalytic system, with the catalyst and the photosensitizer within the same porous solid material.

Honeycomb-patterned hybrid films of surfactant-encapsulated polyoxometalates by a breath figure method and its electrocatalysis for BrO3−

Surfactant-encapsulated POMs can self-assemble into ordered porous honeycomb films under a moist atmosphere. We successfully fabricated (DODA)₁₀{Cu₄(PW₉)₂} honeycomb films by using a one-step method.

Cobalt-to-vanadium charge transfer in polyoxometalate water oxidation catalysts revealed by 2p3d resonant inelastic X-ray scattering

2p3d RIXS spectra reveal electronic structures ofCo4V2WOC, which offers insights into its enhanced catalytic activity thanCo4P2WOC.

Layer-by-Layer Assembly of Polyoxometalates for Photoelectrochemical (PEC) Water Splitting: Toward Modular PEC Devices

Artificial photosynthesis is considered one of the most promising solutions to modern energy and environmental crises. Considering that it is enabled by multiple components through a series of photoelectrochemical processes, the key to successful development of a photosynthetic device depends not only on the development of novel individual components but also on the rational design of an integrated photosynthetic device assembled from them. However, most studies have been dedicated to the development of individual components due to the lack of a general and simple method for the construction of the integrated device. In the present study, we report a versatile and simple method to prepare an efficient and stable photoelectrochemical device via controlled assembly and integration of functional components on the nanoscale using the layer-by-layer (LbL) assembly technique. As a proof of concept, we could successfully build a photoanode for solar water oxidation by depositing a thin film of diverse cationic polyelectrolytes and anionic polyoxometalate (molecular metal oxide) water oxidation catalysts on the surface of various photoelectrode materials (e.g., Fe₂O₃, BiVO₄, and TiO₂). It was found that the performance of photoanodes was significantly improved after the deposition in terms of stability as well as photocatalytic properties, regardless of types of photoelectrodes and polyelectrolytes employed. Considering the simplicity and versatile nature of LbL assembly techniques, our approach can contribute to the realization of artificial photosynthesis by enabling the design of novel photosynthetic devices.

Frontiers of water oxidation: the quest for true catalysts

Development of advanced analytical techniques is essential for the identification of water oxidation catalysts together with mechanistic studies.

An Effective Strategy To Construct Novel Polyoxometalate-Based Hybrids by Deliberately Controlling Organic Ligand Transformation In Situ

Deliberately controlling organic ligand transformation in situ has remained a challenge for the construction of polyoxometalate (POM)-based inorganic-organic hybrids. In this work, four POM-based hybrids assembled from an in situ bifurcating organic ligand-[Cu2(DIBA)4](H3PMo12O40)·6H2O (1), [Cu2(DIBA)4](H4SiW12O40)·6H2O (2), [Ag(HDIBA)2](H2PMo12O40)·2H2O (3), [Ag3(HDIBA)2(H2O)][(P2W18O62)1/2]·4H2O (4) (DIBAH = 3,5-di(1H-imidazol-1-yl) benzoic acid)-have been designed and obtained under hydrothermal conditions. Compounds 1 and 2 are isostructural, displaying a three-dimensional (3D) 2-fold interpenetrating framework with two types of channels, and the bigger channels are occupied by Keggin polyoxoanions and crystallization water molecules, but only crystallization water molecules in the smaller ones. Compound 3 displays a 3D supramolecular structure constructed from {Ag(HDIBA)2} segments and PMo12O40(3-) polyoxoanions through hydrogen bonding interactions. Compound 4 shows a 3D 2-fold interpenetrating framework based on (3, 3, 4)-connected network, which is constructed from {Ag3(HDIBA)2}n chains and P2W18O62(6-) polyoxoanions as linkers. The DIBAH ligand was generated in situ from 3,5-di(1H-imidazol-1-yl)benzonitrile by deliberate design, which illustrates that the strategy to construct novel POM-based hybrids by controlling ligand transformation in situ is rational and feasible. In addition, the effects of the central metal and POMs on the structures of the target compounds were discussed. Finally, the electrochemical and photocatalytic properties of compounds 1-4 have been investigated in this paper.

Nanoscale cobalt metal–organic framework as a catalyst for visible light-driven and electrocatalytic water oxidation

Co-ZIF-67 is proposed as an efficient water oxidation catalyst under visible light.

Assembly of cyanometalate-functionalized phosphotungstates with magnetic properties and bifunctional electrocatalytic activities

Two cyanometalate-functionalized Dawson type phosphotungstates with a “Netherlands windmills” shape have been prepared under conventional reaction conditions. Their magnetic properties and electrocatalytic activity were investigated.

An organic–inorganic hybrid photocatalyst based on sandwich-type tetra-Co-substituted phosphotungstates with high visible light photocatalytic activity

A novel hybrid photocatalyst constructed from tetra-Co^II-substituted sandwich-type phosphotungstates exhibits highly efficient visible light photocatalysis activity.

Sailing into uncharted waters: recent advances in the in situ monitoring of catalytic processes in aqueous environments

This review presents recent advances inin situstudies of catalytic processes in the aqueous environment with an outlook of mesoscale imaging.

Progress in base-metal water oxidation catalysis

This minireview provides a brief overview of the progress that has been made in developing homogeneous water oxidation catalysts based on base metals (manganese, iron, cobalt, nickel, and copper) from the 1990s to mid-2014. The impact of each contribution is analyzed, and opportunities for further improvement are noted. In addition, the relative stabilities of the base-metal catalysts that have been reported are compared to illustrate the importance of developing more robust catalytic systems by using these metals. This manuscript is intended to provide a firm foundation for researchers entering the field of water oxidation based on base metals and a useful reference for those currently involved in the field.