Keggin-type Polyoxometalate Research Articles

Efficient Co-synthesis of glycolic acid and formic acid from 1,3-dihydroxyacetone by Keggin cesium phosphomolybdates

The increasing depletion of fossil fuel resources promote the study of conversion of renewable biomass resources to valuable chemicals. Formic acid (FA) is regarded as desirable liquid hydrogen carrier. Glycolic acid (GA) has been extensively used for the treatment of photoaging and wrinkles. It is meaningful to explore novel catalysts for the selective oxidation of bio-platform molecules into FA and GA. Keggin-type polyoxometalates (POMs) were firstly explored as catalysts for the oxidation of 1,3-dihydroxyacetone (DHA), which is an important biomass-based platform molecule, to obtain GA and FA in the presence H2O2.The effect of catalyst structure and composition, and the reaction conditions on DHA oxidation were investigated. Among the chosen compounds, Cs3PMo12O40 exhibited the best catalytic activity, with 86.4 % and 82.6 % yields of GA and FA, respectively. The five successive recycling experiments indicated that Cs3PMo12O40 had good stability and reusability. The catalytic activity of Cs3PMo12O40 was better than that of the other types of catalysts reported. This work give a clue for utilizing POMs as catalysts for the valorization of biomass derivatives to useful fine chemicals under mild conditions.

Trivacant Keggin-type polyoxometalate/TiO2 composite for enhanced sonocatalytic degradation of AR88 from aqueous solutions

Polyoxometalates (POMs) have been limited in their widespread use in catalysis in previous studies due to their strong carrier complexation and limited solubility. To overcome this limitation, in this study, Z-type heterojunction composites constructed with trivacant Keggin-type POMs K9SbW9O33·19.5H2O and TiO2 were synthesized by the sol–gel method and used for the sonocatalytic removal of the organic pollutant AR88. The results showed that, compared with pure K9SbW9O33·19.5H2O, the optimum K9SbW9O33·19.5H2O/TiO2 showed a significantly higher sonocatalytic degradation efficiency of 87.2 % for AR88, and the mineralization rate reached 51.8 %. Parameters such as dye pH and sonication power were optimized to determine the best conditions for sonocatalysis. In addition, the K9SbW9O33·19.5H2O/TiO2 Z-type heterojunction composites showed good stability and reusability. The excellent catalytic performance is attributed to the effective separation of charges and holes by the Z-type heterojunction, which enables them to produce active substances for AR88 decomposition. This study demonstrates the high potential of POMs for the application of sonocatalysis in the treatment of organic pollutants.

Electrochemical hydrogen storage using SrFe12O19 surface-immobilized polyoxometalate

A tri-nickel substituted Keggin-type polyoxometalate (PMo9Ni3O37) was synthesized and subsequently immobilized on the surface of the M-type strontium hexaferrite (SrFe12O19) via the sol-gel method. The investigation of hydrogen storage capacity for the synthesized PMo9Ni3O37@SrFe12O19 nanocomposite was conducted through electrochemical methodologies employing chronopotentiometry (CP). A conventional three-electrode configuration employing copper foam coated with PMo9Ni3O37@SrFe12O19 served as the working electrode and was examined across a current range of ±1.5 mA. The charge and discharge of the experimental procedure indicated the substantial potential of the PMo9Ni3O37@SrFe12O19 nanocomposite in the hydrogen storage process, which was determined 3125 mAh/g during the discharge phase. The successful synthesis was corroborated thorough FT-IR, UV-vis, XRD, SEM, EDX, and BET surface area analysis techniques by examining the characteristic absorption wavelengths or reflection patterns. The size of the nanoparticles was determined through SEM analysis yielding a diameter of 36.76 nm, and using the Scherrer equation, a diameter of 29.22 nm was obtained.

Assembled Structures and Electrical Conductivities of Salts Comprised of Cationic Sandwich Complexes and Polyoxometalates.

Polyoxometalates (POMs) are recognized for their diverse structures and catalytic properties, yet their organometallic salts have not been thoroughly investigated. In this study, we synthesized salts of a Keggin-type POM with cationic sandwich complexes featuring various substituents, [X]2[SMo12O40] [X = Ru(C5H5)(C6H5R); R = H (1a), Me (1b), Bu (1c)], [Co(C5H5)2 (2), and Co(C5Me5)2 (3)]. These salts exhibited similar structural characteristics, with each anion surrounded by 10 cations, forming two-dimensional sheet arrangements through O···O anion-anion contacts, except for the salt of 1c, which exhibited one-dimensional contact owing to the larger cation substituent. Additionally, [2](THA)[Mo6O19], [3]2[Mo6O19], and [3](THA)[SMo12O40], containing the Lindqvist-type POM [Mo6O19] and/or tetrahexylammonium (THA) cation, were obtained, with the packing structure of [3]2[Mo6O19] closely resembling that of [X]2[SMo12O40]. Solvate crystals of [1a]3[PMo12O40] were also prepared. [1a]2[SMo12O40] exhibits an electrical conductivity σ of 1.4 × 10-7 S cm-1 at 373 K, which is 2 orders of magnitude higher than those of (THA)2[SMo12O40] and [1a]PF6.

Polyoxometalate-mediated film bearing near-infrared photothermal self-healing property for seawater desalination

The use of solar-powered seawater desalination films frequently results in damage, making repairable property a crucial concern. Here, a photothermal self-healing seawater desalination film is created using poly-thioctic aclhydrazine (P-TAH) as the bulk material and Keggin-type polyoxometalate (POM) doped with TAH as an additive. The film’s self-healing ability is supported by the dynamic covalent connections in TAH, while the addition of POM improves mechanical strength and adds photothermal properties. The Young’s modulus increases from 7 to 195 MPa when 1/100 M ratio of POM is added to bulk material. With an energy efficiency of 94.4 %, the produced film’s evaporation efficiency achieves a high value of 2.75 kg m−2h−1. Furthermore, the injured film can be repaired while maintaining nearly the same mechanical strength and evaporation efficiency. The outdoor experiments indicate that the prepared film has a high performance under the irradiation of natural sunlight, which makes its possibility for the further applications. This work demonstrates the unreported photothermal self-healing property of POM-containing materials, in addition to presenting a generic approach for building POM-based seawater desalination films.

Keggin-type polyoxometalates/Amberlite nanocomposites as efficient catalysts for the aqueous oxidation of sulfides: Crucial roles of mono-iron and mono-manganese substituents

The condensation of Keggin-type polyoxometalates including H3PW12O40, H3PMo12O40, and their Mn(III) and Fe(III) derivatives with nano-structured Amberlite, nanoAmbN(Me)3Cl led to the formation of electrostatically immobilized polyoxometalates on the anion exchange polymer with a size less than ca. 70 nm. The nanocomposites were characterized by FT-IR, XRD, solid state diffuse-reflectance and solution UV–vis spectroscopy, TGA, BET and FESEM-EDX-Mapping and used for the oxidation of sulfides with periodate under aqueous and non-aqueous conditions. In spite of the small difference between the catalytic activity of H3PMo12O40 and H3PW12O40, the corresponding mono-iron and manganese substituted derivatives showed significantly different catalytic activities. Furthermore, nanoAmbN(Me)3@PW11MnO39 showed a high selectivity towards sulfone (72 %), compared to the highly chemoselective formation of sulfoxide in the presence of nanoAmbN(Me)3@PW11FeO39. NanoAmbN(Me)3@PW11FeO39 as the best catalyst of the series was recovered and reused at least six times for the oxidation of methyl phenyl sulfide. The increased catalytic activity of the iron-containing polyoxometalates than that of the corresponding manganese compounds was attributed to the Jahn-Teller distortion around the manganese(III) center.

Two nickel-added poly(polyoxometalate)s built of Keggin-type {Ni6PW9} and Anderson-type NiW6O24via WO4/Sb2O bridges and Ni-O-W linkages with efficient hydrogen evolution activity.

Two Ni-added poly(polyoxometalate)s built of Keggin-type {Ni6PW9} and Anderson-type NiW6O24 units via WO4/Sb2O bridges and Ni-O-W linkages, Na4H8[Ni(enMe)2][(Sb2O)2(NiW6O24)-{Ni12O2(OH)4(enMe)4(H2O)3(WO4)2(B-α-PW9O34)2}2]·39H2O (1) and H9[Ni(en)2(H2O)][Ni0.5(en)2(H2O)][Ni-(enMe)2(H2O)][(Sb2O)2(NiW6O24){Ni12O2(OH)4(en)2(enMe)2(H2O)3(WO4)2}-{Ni12O2(OH)4(en)4(H2O)3(WO4)2}(B-α-PW9O34)4]·45H2O (2), have been hydrothermally synthesized and characterized, in which the {Ni12(WO4)2(PW9)2} subunit was obtained by the synergistic directing effect of 2 lacunary PW9O34 (PW9) fragments and further linked by a central Anderson-type (Sb2O)2(NiW6O24) bridge. Both compounds represent the first example of Ni-added polyoxometalates (POMs) simultaneously based on Keggin-type and Anderson-type POM components. Photocatalytic studies revealed that 2 can work as an efficient heterogeneous catalyst towards a light-driven H2 evolution reaction, achieving a hydrogen evolution rate of as high as 19 214 μmol g-1 h-1 (TON = 1500), which is superior to most of the reported POM-based heterogeneous catalysts.

Enhanced Electrical Conductivity of Polyoxometalates by Bridging with Mixed-Valent Multinuclear Platinum Complexes.

Polyoxometalates (POMs) are nanosized molecular metal oxide anion clusters with tuneable structures and functionalities, and they exhibit a redox chemistry and catalytic activity in multielectron redox processes. These are typically poor electrical conductors (<10-10 Scm-1), which is attributed to negligible electronic interactions among anions in the solid state. Since the reduced electrons on the d0 metals in POMs are delocalized, electrical conductivity was improved when judicious pathways for the electrons were created by bridging the POMs. Utilized with the electronic interactions between bridging oxygen atoms with the highest occupied molecular orbital in the POMs and the metal dz2 orbitals in the multinuclear platinum complexes, and three mixed-valent assemblies were synthesized and characterized. Simply mixing Keggin-type or Dawson-type POMs with tetranuclear or trinuclear platinum complexes in solution afforded three single crystals, and all three compounds were paramagnetic with mixed oxidation states and better conductivities at room temperature than the parent compounds.

Electrochemical hydrogen storage by a new nanocomposite based on tri zinc substituted keggin-type polyoxometalate (PMo9Zn3O37) and strontium hexaferrite (SrFe12O19)

Blending the capabilities of the strontium hexaferrite SrFe12O19 (SRF) with tri zinc substituted Keggin-type polyoxometalate PMo9Zn3O37 (PMZ), to yield PMo9Zn3O37@SrFe12O19 (PMZ@SRF) will have notable ability of hydrogen storage. The use of hydrogen gas as an alternative gas fuel for solving the air pollution has increased its demand by far and need for investigating its storage ways can be clearly observed. Herein, the hypothesis of the ability of novel PMZ@SRF on hydrogen storage was confirmed by chronopotentiometry analysis under a constant current of ±1.5 mA in an electrochemical cell containing of the 6 M of KOH as an electrolyte and the modified cupper foam plate coated with PMZ@SRF nanocomposite as a working electrode. The novel nanocomposite was synthesized by the sol-gel method and characterized by FT-IR, XRD, UV–vis, SEM, EDX, and BET analysis. The characteristic spectra showed the proper synthesis and SEM analysis evaluated the mean size of the nanoparticles 39.44 nm. However, the size of the nanoparticles was calculated by the Debye-Scherrer formula as 29.75 nm with an XRD pattern of the as-prepared PMZ@SRF nanocomposite. Also, BET analysis proved that when PMZ is immobilized on the SRF host the whole new composite will have 0.08 (cm3/g) more pore volume than the raw compound of PMZ. The hydrogen storage capacity will be in priority for thinking new ways of how to have less air pollution, and introducing the significant capabilities of the novel PMZ@SRF on hydrogen storage technology will make a full impact on research areas.

Supramolecular assembled Keggin based porous material: Synthesis, crystal structure, electrochemical and spectroscopic properties

In this paper, a new supramolecular assembled Keggin polyoxometalate (POM) based organic-inorganic porous material (A; [HSiMo12O40].3[C10H11N4O2S].48H2O) containing the compound sulfadiazine (SZD) has been prepared and its molecular structure was determined. To characterize the hybrid compound (A), X-ray crystallography, MALDI-TOF, UV–Vis, photoluminescence and FTIR methods were used. The electrochemical properties of the Keggin POM, SZD and porous hybrid (A) compounds were investigated in the −1.8 – 1.8 V range and the redox processes at some potentials of the hybrid compound (A) are irreversible at 250 and 500 mV/s scan rates. Thermal stability properties of the hybrid compound (A) were investigated by the TGA/DTA instruments. The loss of the adsorbed water molecules on the hybrid compound (A) starts at 28 °C temperature. Crystal structure of the hybrid compound contains a Keggin type polyoxometalate [HSiMo12O40]3− and three monoprotonated sulfadiazinium (HSZD)+ cations. In the structure of the hybrid material, there are solvent accessible voids along a axis filled with water molecule.

How Polyoxometalate Promote the Aggregation of a Cyanine Dye in the Aqueous Solution of Non-Ionic Copolymers of Varying Hydrophilic-Lipophilic Balance?

The nonradiative pathway leading to the photoisomerization of a cyanine dye is well-established information. However, the modulations induced in the photoisomerization pathway by a Keggin-type polyoxometalate in a confined media is new. Our study reveals that, in the presence of pluronic block copolymers F-108 and P-123, phosphomolybdic acid hydrate (PMA) promotes the aggregation of 3,3'-diethylthiadicarbocyanine iodide (DTDCI). The absorption spectra show a clear indication of a red-shifted trimer band in F-108 and P-123, whereas it is absent in F-127 and P-84. Fluorescence emission studies suggest that, in the presence of PMA, the rate of photoisomerization is accelerated in F-108, P-123, and P-84 micelles, whereas it is retarded in F-127 micelles. Time-resolved studies in the presence of PMA indicate the preference of F-108, P-84, and P-123 toward the trapped conformer of DTDCI, whereas F-127 favors the formation of photoisomer of DTDCI. Our findings imply the importance of the interplay between the hydrophobic and electrostatic interactions between the DTDCI cations and the PMA anions in nonionic micelles of varying hydrophilic-lipophilic balance (HLB). Dynamic light scattering (DLS) data suggest a modulation by PMA in the intermicellar electrostatic repulsions of a hydrophilic copolymer micelle, whereas its unaffected in a hydrophobic copolymeric micelle.

Polyoxometalate Supported Single Transition Metal Atom as a Redox Mediator for Li-O2 Batteries.

Keggin-type polyoxometalate (POM) supported single transition metal (TM) atom (TM1/POM) as an efficient soluble redox mediator for Li-O2 batteries is comprehensively investigated by first-principles calculations. Among the pristine POM and four kinds of TM1/POM (TM = Fe, Co, Ni, and Pt), Co1/POM not only maintains good structural and thermodynamic stability in oxidized and reduced states but also exhibits promising electro(chemical) catalytic performance for both oxygen reduction reaction and oxygen evolution reaction (OER) in Li-O2 batteries with the lowest Gibbs free energy barriers. Further investigations demonstrate that the moderate binding strength of Li2-xO2 (x = 0, 1, and 2) intermediates on Co1/POM guarantees favorable Li2O2 formation and decomposition. Electronic structure analyses indicate that the introduced Co single atom as an electron transfer bridge can not only efficiently improve the electronic conductivity of POM but also regulate the bonding/antibonding states around the Fermi level of [Co1/POM-Li2O2]ox. The solvent effect on the OER catalytic performance and the electronic properties of [Co1/POM-Li2O2]ox with and without dimethyl sulfoxide solvent are also investigated.

Single-cluster Functionalized TiO2 Nanotube Array for Boosting Water Oxidation and CO2 Photoreduction to CH3OH.

Solar-driven CO2 reduction and water oxidation to liquid fuels represents a promising solution to alleviate energy crisis and climate issue, but it remains a great challenge for generating CH3OH and CH3CH2OH dominated by multi-electron transfer. Single-cluster catalysts with super electron acceptance, accurate molecular structure, customizable electronic structure and multiple adsorption sites, have led to greater potential in catalyzing various challenging reactions. However, accurately controlling the number and arrangement of clusters on functional supports still faces great challenge. Herein, we develop a facile electrosynthesis method to uniformly disperse Wells-Dawson- and Keggin-type polyoxometalates on TiO2 nanotube arrays, resulting in a series of single-cluster functionalized catalysts P2M18O62@TiO2 and PM12O40@TiO2 (M=Mo or W). The single polyoxometalate cluster can be distinctly identified and serves as electronic sponge to accept electrons from excited TiO2 for enhancing surface-hole concentration and promote water oxidation. Among these samples, P2Mo18O62@TiO2-1 exhibits the highest electron consumption rate of 1260 μmol g-1 for CO2-to-CH3OH conversion with H2O as the electron source, which is 11 times higher than that of isolated TiO2 nanotube arrays. This work supplied a simple synthesis method to realize the single-dispersion of molecular cluster to enrich surface-reaching holes on TiO2, thereby facilitating water oxidation and CO2 reduction.

Detailed Study on the Interfacial Interaction between Different Polyoxometalates and Tetronic Block Copolymers Exploring the Langmuir-Blodgett Technique.

Polyoxometalates (POMs) interact with various biologically relevant entities. A basic understanding of this interaction is very important for various applications in the biological field. In this work, the focus is on the study of the interaction between tetronics and Keggin POMs. T701 and T90R4 are the two tetronics considered here; they have different solubilities in water due to different PPO/PEO ratios. The arrangement of PPO and PEO is also different with respect to the central ethylenediamine groups. Three different Keggin-type POMs, phosphomolybdic acid (PMA), phosphotungstic acid (PTA), and silicotungstic acid (STA), with different charge densities are chosen for an elaborate investigation using Langmuir-Blodgett technique. The observation is analyzed thoroughly, which shows both electrostatic interaction and adsorption of POMs on the PPO blocks of the tetronics due to the chaotropic effect, which is responsible for the binding of POMs (in subphase) with the tetronic monolayer. This interaction results in an expanded yet rigid monolayer for POM-tetronic association on the surface. Surface pressure vs mean molecular area isotherm is the key characterization to reach the conclusion. UV-vis spectroscopy, NMR, ITC, ellipsometric studies, FTIR, and SEM also serve as supportive characterization techniques.

Polyoxometalate exerts broad-spectrum activity against human respiratory viruses hampering viral entry

Human respiratory viruses have an enormous impact on national health systems, societies, and economy due to the rapid airborne transmission and epidemic spread of such pathogens, while effective specific antiviral drugs to counteract infections are still lacking. Here, we identified two Keggin-type polyoxometalates (POMs), [TiW11CoO40]8- (TiW11Co) and [Ti2PW10O40]7- (Ti2PW10), endowed with broad-spectrum activity against enveloped and non-enveloped human respiratory viruses, i.e., coronavirus (HCoV-OC43), rhinovirus (HRV-A1), respiratory syncytial virus (RSV-A2), and adenovirus (AdV-5). Ti2PW10 showed highly favorable selectivity indexes against all tested viruses (SIs >700), and its antiviral potential was further investigated against human coronaviruses and rhinoviruses. This POM was found to inhibit replication of multiple HCoV and HRV strains, in different cell systems. Ti2PW10 did not affect virus binding or intracellular viral replication, but selectively inhibited the viral entry. Serial passaging of virus in presence of the POM revealed a high barrier to development of Ti2PW10-resistant variants of HRV-A1 or HCoV-OC43. Moreover, Ti2PW10 was able to inhibit HRV-A1 production in a 3D model of the human nasal epithelium and, importantly, the antiviral treatment did not determine cytotoxicity or tissue damage. A mucoadhesive thermosensitive in situ hydrogel formulation for nasal delivery was also developed for Ti2PW10. Overall, good biocompatibility on cell lines and human nasal epithelia, broad-spectrum activity, and absence of antiviral resistance development reveal the potential of Ti2PW10 as an antiviral candidate for the development of a treatment of acute respiratory viral diseases, warranting further studies to identify the specific target/s of the polyanion and assess its clinical potential.

The synergistic effect of POM-π interaction and solid phase amidation on the visible light photocatalytic of g–C3N4–based photocatalyst

Due to the unique ability of Keggin-type polyoxometalate (POM) to remain stable when accepting electrons to form mixed valence POM, organic-POM hybrid materials can complete multi-electron transfer while maintaining structural stability. The organic-POM hybrids photocatalysts prepared using organic materials and POM as raw materials have been developed to form new energy bands and crystal structures. This not only enhances their quantum efficiency and but also expands their visible light response range. Nevertheless, organic-POM hybrids, featuring organic amines as ligands, are prone to photoetching, impeding g-C3N4's progress in this area. In this paper, the amino group of g-C3N4 was converted into imide ring by solid phase amidation method, which overcame this obstacle and converted g-C3N4 into g-C3N4 based polyimide (CNPI) with enhanced photoelectric properties. XRD, IR spectrum, and XPS data indicate that CNPI is more tightly integrated with potassium phosphotungstate (KPW) than g-C3N4, demonstrating superior electron transport capabilities and potentiated POM-π interactions. In terms of photocatalytic efficacy, CNPI/KPW-30 % exhibits an impressive degradation rate constant of 0.5458 h−1 for imidacloprid, attributed to its compact microstructure and superior charge transfer capability. While maintaining good photocatalytic stability, the mineralization rate of CNPI/KPW-30 % to imidacloprid was 55.25 %. In addition, h+ and ·O2− were identified as active species through trapping agent experiments, and the degradation pathway was advanced by ESR technology analysis.

Synthesis and characterization of new nanocomposite PW11Ba@BaMnO2 as an efficient nanocatalyst for water oxidation process

In this study, a new nanocomposite was synthesized using the sol-gel method by immobilizing of mono-substituted Keggin-type polyoxometalate containing barium (PW11Ba) on the surface of barium manganese oxide (BaMnO2) nanoceramics. The corresponding nanocomposite, denoted as PW11Ba@BaMnO2, was utilized as an effective heterogeneous catalyst in the water splitting for O2 evolution reaction (OER). The characterization of the PW11Ba@BaMnO2 nanocatalyst was performed using various techniques, such as FT-IR, UV–vis, XRD, SEM, and EDX methods. After the successful preparation of the PW11Ba@BaMnO2, its catalytic performance in the OER process was evaluated at different pH values (pH = 7 and 9) using ceric ammonium nitrate (CAN) as an efficient oxidant. The as-prepared nanocomposite exhibited remarkable turnover frequency (TOF), turnover number (TON), and O2 evolution values of 0.702 mmolO2molCo.s, 971.68 mmolO2molCo, and 0.0014 mgO2L.S at pH = 9, respectively. Moreover, the influence of CAN concentration, pH value, and the amounts of KOH and BaCl2 on the OER process was assessed, and a plausible reaction mechanism was suggested. In this research, the PW11Ba@BaMnO2 nanocatalyst has provided valuable insights on the design of other catalysts for water oxidation reactions and beyond.

Development of a chromatographic method for optimizing the thiol-maleimide coupling of polyoxometalate-polymer hybrids

The covalent attachment of polyoxometalates (POMs) to polymers has been developed as a strategic approach for the advancement of POM-based hybrid materials with versatile applications. In this study, we utilized thiol-maleimide Michael addition to investigate the kinetics and efficacy of the “one-to-one” conjugation between Keggin type POM and polystyrene. We explored the effects of solvent polarity, catalyst, molecular weight of PS and synthetic strategies on the reaction kinetics and efficiency, by means of reverse-phase high-performance liquid chromatography (RP-HPLC). A series of comparative analysis affirmed the superior efficiency of the one-pot method, particularly when facilitated by the addition of a high-polarity solvent and an excess of maleimide. These findings offer valuable insights into the intricate interplay between reaction conditions, kinetics, and selectivity in thiol-maleimide reactions of POMs and polymers. They hold profound implications for advancing the study of POM-based multifunctional materials and the synthesis of complex hybrid molecules.

Tuning the bulk behavior and 2D interfacial self-assembly of microgels by keggin-type polyoxometalate ionic specificity

Finding new ways to tune the behavior of thermoresponsive microgels in bulk and confined at 2D liquid interfaces is key to achieve a deeper understanding and control of these smart materials. We studied the interaction of positively and negatively charged pNIPAM microgels with the Keggin-type polyoxometalates Na3PW12O40 (POM3−) and H4SiW12O40 (POM4−). In bulk, we observed charge inversions of the positively charged microgels below and above the volume phase transition temperature (VPTT) at significantly low POM concentrations as 5⋅10−5 M. In the presence of POM, both microgels exhibited a deswelling-swelling-deswelling behavior below the VPTT, and a two-step further deswelling above the VPTT for the positively charged microgels. When the later were confined at 2D water/air interfaces, adding 10−5 M of POM3− below the VPTT was equivalent to heating above the VPTT and compressing the monolayer from 5 to 20mN m−1. Above the VPTT, the diameter at the interface did not change while the portion immersed in the subphase further deswelled, in agreement with the behavior in bulk. Adding more POM3− did not change the diameter at the interface nor the height of the microgels, showing a saturation effect in 2D. The restructuring of the pNIPAM polymeric network by the POM3− was characterized by EDS mapping and XPS. The microgel monolayers with POM3− improved their resistance to plasma etching, which could be useful for soft colloidal lithography.

Constructing polyoxometalates-based electrocatalytic nanofiltration membranes for nitrite removal

Nitrite-containing wastewater poses an imminent threat to both human health and ecosystems. Conventional methods like nitrification and denitrification rely heavily on chemical reagents, while emerging membrane separation has limitations in permselectivity. Here, a conductive membrane with both electroresponsive and electrocatalytic capabilities was developed by integrating Keggin-type polyoxometalates (POMs) through combining layer-by-layer self-assembly and interfacial polymerization. The pore size of the membranes remained largely unaffected by an electric field, but its Donnan effect was significantly enhanced, leading to a substantial increase in nitrite removal from 23.1% to an impressive 81%. Nitrite underwent a reductive degradation at the cathode, producing nitrogen-containing gases (8.3%), while oxidation at the anode yielded hypotoxic NO3− (11.3%), respectively exhibiting 79.9% rejection of NO2− and 83.1% rejection of NO3− under the electric field-enhanced Donnan effect. Therefore, the high-level total nitrogen removal was composed of three key components: 64.2% from direct NO2− rejection, 9.4% from oxidation-generated NO3− rejection, and 7.4% attributed to gas generation. Significantly, our work underscored the consistently robust structural stability of the POMs-based membrane under an electric field, maintaining an outstanding total nitrogen removal exceeding 75%. Consequently, this research introduced a promising electrochemical-membrane fusion approach for efficiently removing high-activity nitrite-containing wastewater, emphasizing the potential for environmental applications.