Developing Insoluble Polyoxometalate Clusters to Bridge Homogeneous and Heterogeneous Water Oxidation Photocatalysis

Abstract

Cluster catalysts are attractive for their atomically precise structures, defined compositions, tunable coordination environments, uniform active sites, and their ability to transfer multiple electrons, but they suffer from poor stability and recyclability. Here, we report a general approach to the direct insolubilization of a water soluble polyoxometalate (POM) [{(B‐α‐PW9O34)Co3(OH)(H2O)2(O3PC(O)‐(C3H6NH3)PO3)}2Co]14− (Co7) and formation of a series of POM‐based solid catalysts with the counter‐cations Ag+, Cs+, Sr2+, Ba2+, Pb2+, Y3+, and Ce3+. They exhibit improved catalytic activities for visible‐light‐driven water oxidation following the trend CsCo7>SrCo7>AgCo7>CeIIICo7>BaCo7>YCo7>PbCo7. While CsCo7 exhibits mainly homogeneous catalysis, the others are predominantly heterogeneous catalysts. An optimal oxygen yield of 41.3 % and a high apparent quantum yield (AQY) of 30.6 % for SrCo7 is obtained, which is comparable to that of the parent homogeneous POM. Band gap structures, UV/Vis spectra, and real‐time laser flash photolysis experiments collectively suggest that easier electron transfer from the solid POM catalyst to the photosensitizer promotes photocatalytic water oxidation performance. These solid POM catalysts exhibit good stability, which is directly confirmed by a combination of Fourier‐transform infrared spectroscopy, electron microscopy, X‐ray diffraction patterns, Raman spectroscopy, X‐ray photoelectron spectroscopy, five cycles of tests, and poisoning experiments.

Reactivity of Organoiridium Tungsten Oxide Clusters with Transition Metal Aquo Cations

“…Recently, water-insoluble POM compounds, particularly those containing cobalt, have gained interest as solid heterogeneous water oxidation catalysts. ,, All of our compounds are insoluble in water, and hence, we investigated their water-oxidation properties using a carbon paste electrode technique. Figure a shows linear sweep voltammograms of the carbon paste mixture of 1, 2-Co , or 3-Fe in comparison with the voltammograms of the pure carbon paste and carbon paste mixture of [(Cp*IrCl) 2 (μ-Cl) 2 ].…”

“…Recently, water-insoluble POM compounds, particularly those containing cobalt, have gained interest as solid heterogeneous water oxidation catalysts. 26,46,47 All of our compounds are insoluble in water, and hence, we investigated their water-oxidation properties using a carbon paste electrode technique. Figure 5a Increasing the concentration of 2-Co to 30% increases the current density to approximately 0.5 mA/cm 2 , which is similar to the current density generated by the 10 wt % (approximately 6.3 wt % total cobalt concentration) of cobalt(II) hydroxide/carbon paste mixture (Figure S10).…”

Fabrication of Polyoxometalate-Based Metal–Organic Frameworks Integrating Paddlewheel Rh2(OAc)4 for Visible-Light-Driven Oxidative Coupling of Amines

“…Polyoxometalates (POMs) are a class of discrete anionic metal–oxo clusters consisting of the early transition metals [Mo­(VI), W­(VI), V­(V), Nb­(V), and Ta­(V)] in the highest oxidation state. They have attracted much interest in the photocatalytic field because of their structural diversity, high negative charge, and remarkable redox ability. , Particularly, POMs are often used as “electron relays” in photocatalytic organic synthesis reactions because they can achieve structural modifications at the molecular level and enable fast, reversible, and multiple electron transfer. − However, POMs usually absorb only UV light and have high solubility in the reaction system, making them difficult to recycle and reuse, greatly restricting their usage. − To address this challenge, various supports, including zeolites, cationic carbon nanotubes, covalent organic frameworks, and metal–organic frameworks (MOFs), have been employed for the construction of heterogeneous POM-based photocatalysts. − Among these different supports, the porous structure and rich chemical tunability of MOFs make them very promising candidates for the fabrication of POM-based MOFs (POMOFs), which are reusable and possess enhanced photocatalytic properties. , The immobilization of POMs onto MOFs by the judicious selection of ligands or metal nodes not only enables one to tune the lowest unoccupied molecular orbital (LUMO) energy levels of POMs but also to preserve and enhance the functionality of individual components. , Especially, Rh 2 paddlewheel complexes are considered to be some of the most promising building blocks for the construction of MOFs because of their high symmetry and interesting functions in areas of photocatalytic reactions, solar energy conversion, and molecular sensing. − It is speculated that the combination of Rh 2 paddlewheel complexes and POMs to constitute novel POMOFs could not only overcome the deficiencies of POMs but also make full use of their synergistic effects, thus boosting the photocatalytic activity.…”

An MIL-88A(Fe) rod-like metal–organic framework decorated with tungstophosphate polyoxoanions and nickel ferrite nanoparticles for the removal of antibiotics from water

“…19–21 POMs are significant metal-oxide clusters with a highly negative charge and abundant topologies, which have been employed in many research fields such as optics, magnetism, catalysis, and biological medicine. 22,23 However, there are some disadvantages to the use of POMs as adsorbents: (1) their relatively small surface area seriously obstructs accessibility to the active sites and (2) due to their excellent solubility in aqueous solution they cannot be reused and recycled in the process of wastewater treatment. To solve these limitations, substrates including MOFs have been utilized to anchor POMs for the formation of heterogeneous composites.…”