Keggin-type POM Research Articles

Polyoxometalate-based crown ether supramolecular hybrid electrocatalyst for the hydrogen evolution reaction and robust nitrite reduction

The novel crown ether–polyoxometalate assembly, [Na(H24C12O6)(CH3CN)(H2O)]2[Na(H24C12O6)(CH3CN)2][PW12O40] (PW12@Crown), based on Keggin-type phosphotungstic acid, was successfully synthesized in acetonitrile solution, and characterized by single-crystal/powder X-ray diffraction and a wide range of analytical methods, including FT-IR, Raman spectroscopies, thermogravimetric analysis. This POM-based supramolecular structure is the first example of a Keggin-type POM linked to three [18]-crown-6 ether components forming unique supramolecular host-guest structures by hydrogen bonding and other intermolecular forces. In addition, the hydrogen evolution reaction (HER) and the reduction of sodium nitrite (NaNO2) activity were studied by cyclic voltammetry to verify the catalytic performance of the compound.

Constructing of crystalline CoP/amorphous CoMoP coupling composites for efficient water splitting

Bifunctional electrocatalysts with the joint merits of being highly efficient, stable, and low-cost are essential for overall water splitting applications. Herein, we reported a unique amorphous/crystalline coupling structure consisting of crystalline CoP nanocrystals with amorphous CoMoP in-situ growth on nickel foam (termed as CoP/a-CoMoP/NF). This was achieved through oxidation-phosphating in two successive steps by Keggin-type POM (H3PMo12O40) embedded into the zeolitic imidazolate framework-67 (ZIF-67) as the precursor. The Mo doping and crystalline/amorphous coupling structures of CoP and CoMoP modulate the electronic structure, increase the electrocatalytic active sites, and promote the interfacial electron transfer, thus accelerating the catalytic kinetics of both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The amorphous CoMoP serves a protective role in preventing corrosion during the electrochemical process. The catalyst exhibits low overpotentials of 301 mV and 250 mV to drive a current of 100 mA cm−2 for OER and HER, respectively, and demonstrates excellent stability for both reactions. Furthermore, the alkaline electrolytic cell assembled with CoP/a-CoMoP/NF catalysts as both the anode and cathode requires only 1.66 V and 1.73 V to drive current densities of 50 and 100 mA cm−2, respectively. This study offers a rational strategy for the development of efficient bifunctional electrocatalysts for hydrogen generation.

In-situ growth of polyoxometalate-based metal-organic frameworks on wood as a promising dual-function filter for effective hazardous dye and iodine capture

Environmental contaminants have become a major global issue in recent years. Adsorption has been considered as one of the most effective methods of removing contaminants from the environment. However, there are still some challenges to their practical applications, such as complex preparation, expensive cost, and low efficiency. In this study, we used solvothermal conditions to introduce polyoxometalate-based metal-organic frameworks (POMOFs) into a natural wood (POMOF/wood) for the effective removal of organic dye and iodine capture. The Keggin-type POM anions (PMo12) with a highly negative charge were encapsulated to adjust the charge of the UiO-66 MOF, where the charge overcompensation in the POMOFs allows them to efficiently adsorb cationic dyes. Benefiting from the unique microstructure of wood, the POMOF/wood exhibits up to 94.07% and 95.23% removal efficiency for cationic dyes methylene blue (MB) and gentian violet (GV) with a permeance of 1.0×104 L·m−2·h−1·bar−1. Furthermore, the POMOF/wood has high adsorption capacity for capturing iodine, with a maximum adsorption capacity of 1232.8 mg·g−1 in vapor. We believe that a low-cost and scalable filter is advantageous for environmental remediation and this work provides a promising strategy for developing more multifunctional POMOF/wood materials.

An unprecedented polyoxometalate-encapsulated organo–metallophosphate framework as a highly efficient cocatalyst for CO2 photoreduction

Both compounds have host organo–metallophosphate (OMPO) frameworks, whereas the Keggin-type POM as a guest encapsulated in this size-matched framework. More interestingly, compound 2 displays outstanding performance in CO2 photoreduction.

Can Polyoxometalates Be Considered as Superchaotropic Ions?

In this work, we have studied the influence of different anionic species, including typical Hofmeister ions (citrate3– and SCN–), hydrophobic ions (tetraphenyl borate, Ph4B–), and polyoxometallic clusters (Keggin-type POM, PW3–), on the electrophoretic mobility of colloidal particles of a different nature, charge, and wettability. We focused our attention on the adsorption properties of PW3– in comparison with Ph4B– because both species have previously been termed as superchaotropes. However, their different structural properties make it difficult to reconcile the same classification for both anions. We found that the influence of Ph4B– is similar to and surpasses the characteristic effect of chaotropic anions (SCN–), which particularly interacts more strongly with hydrophobic interfaces. On the contrary, we observed clear POM adsorption on the different particles investigated, regardless of their charge or wettability. These observations are complemented by a direct adsorption measurement that combines a...

A CsCl-type inorganic cluster-based high-symmetry crystal built from {Mo4.55V7.45PO40}10.45- with a high ratio of vanadium to molybdenum and {(H2O)0.3@K6(H2O)12}6+ clusters exhibiting proton conduction below the freezing point of water.

As a class of anionic oxoclusters of early transition metals in their highest oxidation states, polyoxometalates (POMs) show considerable structural versatility and unique chemical and physical properties, making them promising multifunctional materials. In this study, a Keggin-type POM has been achieved, with a formula of [(H2O)0.3@K6(H2O)12]H4.45[PV7.45Mo4.55O40]·11H2O (1), and its microcrystals and nanocrystals have been obtained, respectively. This POM was characterized by elemental analysis for C, H and N, ICP-MS, TG, PXRD, SEM, X-band EPR and XPS techniques. Single crystal X-ray diffraction analysis demonstrated that 1 shows a rare extended structure with a high-connected three-dimensional (3D) all inorganic network of a Keggin-type POM, built from {Mo4.55V7.45PO40}10.45- polyoxoanions and {(H2O)0.3@K6(H2O)12}6+ clusters with CsCl-type crystal structure. In addition, to the best of our knowledge, 1 shows the highest ratio of vanadium to molybdenum among Keggin-type POMs reported thus far. Most interestingly, 1 exhibits intrinsic proton conduction below the freezing point of water, with a proton conductivity of 6.90 × 10-7 S cm-1 at 249 K and further reaching 3.36 × 10-6 S cm-1 at 272 K and Ea = 0.44 eV at 249-272 K.

A novel WO2@FeWO4 composite derived from polyoxometalates@Fe–metal–organic frameworks and its electrochemical properties

In this paper, we synthesize a needle-like polyoxometalates@Fe-based metal–organic framework (POM@Fe–MOFs) composites, in which the Keggin-type W-based POM with negative charges exist as secondary building units of Fe-based MOFs. Based on the synthesized POM@Fe–MOFs, WO2@FeWO4 is easily prepared by appropriate thermal treatment. On the one hand, the derivative retains the original POM@Fe–MOFs morphology. Owing to the face that the Fe element contains variable valence state and the W element in tungstophosphoric acid exists in the form of high valence, both of which have great potential in the field of electrochemistry. According to the systematical characterization, the results show that WO2@FeWO4 composite can be used as positive electrodes in supercapacitors which is reported for the first time. In addition, the WO2@FeWO4 is confirmed to exhibit an excellent capacity of 124 F g−1 at a current density of 0.5 A g−1 in 6 M KOH when used in supercapacitors, which can be attributed to the redox reaction of W element.

Computational Study on M1/POM Single-Atom Catalysts (M = Cu, Zn, Ag, and Au; POM = [PW12O40]3-): Metal-Support Interactions and Catalytic Cycle for Alkene Epoxidation.

Geometrical structures, metal-support interactions, and infrared (IR) spectroscopy of a series of M1/POM (M = Cu, Zn, Ag, and Au; POM = [PW12O40]3-) single-atom catalysts (SACs), and catalytic cycle for alkene epoxidation catalyzed by M1/POM SACs were studied using density functional theory (DFT) calculations. The calculations demonstrate that the most probable anchoring sties for the isolated single atoms studied here in the M1/POM SACs are the fourfold hollow sites on the surface of POM support. The bonding interaction between single metal atom and surface of POM support comes from the molecular orbitals with a mixture of d atomic orbital of metal and 2p group orbital of surface oxygen atoms of POM cage. The calculated adsorption energy of isolated metal atoms in these M1/POM SACs indicates that the early transition metals (Cu and Zn) have high thermal stability. The DFT-derived IR spectra show that the four characteristic peaks of free Keggin-type POM structure split into six because of introduction of isolated metal atom. Compared with other metal atoms, the Zn1/POM SAC has the high reactivity for activity of dioxygen molecule, because the dioxygen moiety in Zn1/POM SAC displays O2-· radical feature with [POM4-·Zn2+O2-·]3- configuration. Finally, a catalytic cycle for ethylene epoxidation by O2 catalyzed by Zn1/POM SAC was proposed based on our DFT calculations. Supported noble-metal SACs are among the most important catalysts currently. However, noble metals are expensive and of limited supply. Development of non-noble-metal SACs is of essential importance. Therefore, the reported Zn1/POM SAC would be very useful to guide the search for SACs into non-noble metals.

Sensitive determination of chlorogenic acid in pharmaceutical products based on the decoration of 3D macroporous carbon with Au nanoparticles via polyoxometalates.

A simple and sensitive electrochemical sensor is constructed for the detection of chlorogenic acid (CGA) based on Au nanoparticles (NPs)/polyoxometalates/3D macroporous carbon (Au-POMs-MPC). Serving as both a reducing and stabilizing agent, the Keggin-type POM, H3PW12O40, is used for the synthesis of stable colloidal Au NPs and then used to link them to MPC at a mild temperature. Because of the unique structural properties and synergetic catalytic effect, Au-POMs-MPC can be developed as an effective sensing platform for the detection of CGA, which showed high activity and excellent analytical performance towards CGA, such as a wide linear range of 2.28 nM-3.24 μM, a high sensitivity of 30 554.71 μA mM-1, and a low limit of detection of 2.15 nM. Importantly, the successfully fabricated Au-POMs-MPC device accurately measured the amount of CGA in pharmaceutical samples.

A polyoxometalate-encapsulating cationic metal-organic framework as a heterogeneous catalyst for desulfurization.

A new cationic triazole-based metal-organic framework encapsulating Keggin-type polyoxometalates, with the molecular formula [Co(BBPTZ)3][HPMo12O40]⋅24 H2O [compound 1; BBPTZ = 4,4'-bis(1,2,4-triazol-1-ylmethyl)biphenyl] is hydrothermally synthesized and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and single-crystal X-ray diffraction. The structure of compound 1 contains a non-interpenetrated 3D CdSO4 (cds)-type framework with two types of channels that are interconnected with each other; straight channels that are occupied by the Keggin-type POM anions, and wavelike channels that contain lattice water molecules. The catalytic activity of compound 1 in the oxidative desulfurization reaction indicates that it is not only an effective and size-selective heterogeneous catalyst, but it also exhibits distinct structural stability in the catalytic reaction system.

Control of the Grafting of Hybrid Polyoxometalates on Metal and Carbon Surfaces: Toward Submonolayers

A Keggin-type POM is attached to gold or glassy carbon surfaces by electro(chemical) or peptidic coupling. In addition to demonstrating the robust attachment of the POMs (by electrochemistry, XPS, and IRRAS), the surface concentration, layer thickness, and rate constant for electron transfer from the surface to the POMs have been measured. The use of such complementary techniques is mandatory to characterize the modified electrodes properly. Whatever the grafting method, experimental conditions are found to allow monolayer or submonolayer coverage. Besides covalently grafted species, additional electrostatically bonded POMs are present in the film. Cathodic polarization allows removing them to get a grafted film that is stable with time and potential, which is a requirement in the design of molecular memories.

ChemInform Abstract: Hydrogen Peroxide Activation over TiIV: What Have We Learned from Studies on Ti‐Containing Polyoxometalates?

AbstractReview: [<60refs.]

Keggin heteropolyacid H3PW12O40 supported on different oxides for catalytic and catalytic photo-assisted propene hydration

Catalytic and catalytic photo-assisted hydration of propene to form 2-propanol in gas-solid regime at atmospheric pressure and 85 °C were carried out by using a heteropolyacid (POM) supported on different oxides. Binary materials were prepared by impregnation of H3PW12O40 on different commercial and home prepared supports (TiO2, SiO2, WO3, ZrO2, ZnO, Al2O3). Some of the composites were active both for catalytic and catalytic photo-assisted reactions. The Keggin type POM was completely and partially degraded, when supported on ZnO and Al2O3, respectively, and these binary solids always resulted as inactive for both catalytic and catalytic photo-assisted reactions. The supported Keggin POM species played a key role both for the catalytic and the photo-assisted catalytic reactions, due to their strong acidity and ability to form strong oxidant species under UV irradiation, respectively. The contemporary presence of heat and UV light improved the activity of almost all POM supported materials. All materials were characterized by X-ray diffraction (XRD), scanning electron microscopy observations (SEM), diffuse reflectance spectroscopy (DRS), determination of the conduction and valence band energy by photovoltage measurements, Fourier transform infrared spectroscopy (FTIR), NH3-TPD experiments and time resolved microwave conductivity (TRMC).

PH-Dependent syntheses of copper–quinoxaline–polyoxotungatate hybrids: variable role of Keggin-type polyanion in different pH conditions

Four new hybrids based on the Keggin-type polyoxometalate, formulated as (Hbm)3(PW12O40)·4H2O (1) (bm = benzimidazole, which is synthesized in situ by quinoxaline), [Cu6(qx)9(PW12O40)2] (2), [Cu3(qx)5(PW12O40)(H2O)] (3) and [Cu4(qx)4(HPCuIIW11O39)]·1.5H2O (4) (qx = quinoxaline), were hydrothermally synthesized and structurally characterized by routine techniques and single-crystal X-ray diffraction. In 1, the [PW12O40]3− (PW12) clusters only act as counteranions and combine with the protonated organic ligand by electrostatic interactions. In 2, a 63 topological 2D layer is formed with the PW12 as the template. In 3, the PW12 anions link with four metal–organic chains via Cu–O weak interactions to construct a 3D framework. In 4, the Cu(II)-substituted Keggin unit as a hexadentate inorganic ligand coordinates with four Cu(I) ions from metal–organic chains and two O atoms from neighboring POMs to form a sandwich-like 2D layer. The structure difference of compounds 1–4 reveals that the self-assembly process is pH-dependent and the Keggin-type POM plays different role in different pH conditions.

Prediction of second-order optical nonlinearity of porphyrin–metal–polyoxometalate sandwich compounds

The second-order nonlinear optical (NLO) properties of porphyrin-metal-polyoxometalate (por-metal-POM) sandwich structures [(por)M(PW(11)O(39))](5-) (por = TPP, TPyP, TPPF(20), M = Hf; por = TPP, M = Zr) and [(TPP)Hf(XW(11)O(36))](6-) (X = Si, Ge) are investigated by time-dependent density functional theory (TDDFT). The character of charge-transfer transition indicates that the porphyrin ligand acts as an electron acceptor and the lacunary Keggin-type POM acts as an electron donor. Our results show that this kind of organic-inorganic hybrid compound possesses remarkably large molecular second-order NLO polarizability, approximately 100 x 10(-30) esu, and might be an excellent second-order NLO material. Furthermore, the NLO response can be tuned by the element substituents. The computed beta(0) values increase with the auxiliary electron-accepting group on the porphyrin ring (TPPF(20) > TPyP > TPP) and a heavy central heteroatom (Ge > Si > P). The present investigation provides important insight into the NLO properties of this class of por-metal-POM sandwich compound.