Structure, stability, and electronic interactions of polyoxometalates on functionalized graphene sheets

Tuning Binding Strength of Multiple Intermediates towards Efficient pH-universal Electrocatalytic Hydrogen Evolution by Mo8 O26 -NbNx Oy Heterocatalysts

Developing efficient and robust hydrogen evolution reaction (HER) catalysts for scalable and sustainable hydrogen production through electrochemical water splitting is strategic and challenging. Herein, heterogeneous Mo8 O26 -NbNx Oy supported on N-doped graphene (defined as Mo8 O26 -NbNx Oy /NG) is synthesized by controllable hydrothermal reaction and nitridation process. The O-exposed Mo8 O26 clusters covalently confined on NbNx Oy nanodomains provide a distinctive interface configuration and appropriate electronic structure, where fully exposed multiple active sites give excellent HER performance beyond commercial Pt/C catalyst in pH-universal electrolytes. Theoretical studies reveal that the Mo8 O26 -NbNx Oy interface with electronic reconstruction affords near-optimal hydrogen adsorption energy and enhanced initial H2 O adsorption. Furthermore, the terminal O atoms in Mo8 O26 clusters cooperate with Nb atoms to promote the initial H2 O adsorption, and subsequently reduce the H2 O dissociation energy, accelerating the entire HER kinetics.

Development of polyoxometalate-based Ag-H2biim inorganic-organic hybrid compounds functionalized for the acid electrocatalytic hydrogen evolution reaction

The electrocatalytic hydrogen evolution reaction (HER) is an ideal method for hydrogen production. Transition metal complex electrocatalysts exhibit poor HER activity due to excessive or weak adsorption of H during the electrochemical reduction of water to molecular hydrogen in acidic environments. Developing specific functional complex materials as desired catalysts is challenging. Here, an electrochemical surface restructuring strategy of polyoxometalate (POM)-modified Ag materials toward the HER with a dramatically decreased overpotential under acidic aqueous conditions is established. We prepared two POM [SiW12O40]4- (SiW12)/[P2W18O62]6- (P2W18)-based Ag-2,2'-biimidazole (H2biim) inorganic-organic hybrid compounds (1 and 2) via the hydrothermal method and these two compounds undergo an electrochemical restructuring process in 0.5 M H2SO4 during the HER, in which Ag nanoparticles are in situ formed with the basic structures of SiW12 and P2W18 being maintained. The activated catalysts (1-AC-RDE and 2-AC-RDE) exhibit good electrocatalytic activity for the HER with good long-term stability, and the required overpotentials at a current density of 10 mA cm-2 are 112 mV (1-AC-RDE) and 91 mV (2-AC-RDE) with Tafel slopes of 77 mV dec-1 and 65 mV dec-1, respectively. The excellent electron-proton storage and transferability of SiW12 and P2W18 may provide a solution for the insufficient capture of H by Ag, leading to an effective self-optimizing behavior and superior acidic HER activity.

Reduced polyoxomolybdate immobilized on reduced graphene oxide for rapid catalytic decontamination of a sulfur mustard simulant

Keggin-type polyoxometalates (POMs) were immobilized on poly(diallyldimethylammonium chloride) (PDDA) functionalized reduced graphene oxide (rGO) by a facile and broad-spectrum hydrothermal method. The prepared POMs@PDDA-rGO composites (POM = H3PMo12O40, H3PW12O40, H5PMo10V2O40) have been thoroughly characterized using a series of techniques. The three composites can catalyze the oxidative decontamination of a sulfur mustard simulant, 2-chloroethyl ethyl sulfide (CEES) in the order of PMo12@PDDA-rGO > PMo10V2@PDDA-rGO > PW12@PDDA-rGO. Notably, under ambient conditions PMo12@PDDA-rGO can convert 99% of CEES within 30 min in the presence of nearly stoichiometric aqueous H2O2 (3 wt%) and its catalytic activity is significantly higher than that of homogeneous H3PMo12O40. XPS spectral analysis and control experiments indicate that the Mo center of POM is reduced from +6 to +5 during the hydrothermal process, and the excellent catalytic performance is related to the reduction of Mo. Moreover, the PMo12@PDDA-rGO composite is stable during the decontamination process and it can be used for at least five cycles without loss of activity.

Elucidating the role of H2O in promoting the formation of methacrylic acid during the oxidation of methacrolein over heteropolyacid compounds

The involvement of water in the selective oxidation of MAL to MAA over a pure Keggin-type H₃PMoO₁₂O₄₀ catalyst was investigated using an in situ DRIFTS reactor coupled with a mass spectrometer for the first time to elucidate the reaction pathway associated with water. Comparing the spectra and activity data using D₂O instead of H₂O during transient switching experiments has allowed us to evaluate the possible active sites where D₂O is activated. It has been found that, during the cycling switches of D₂O in and out of the MAL + O₂ gas feed at 320 °C, the formation of MAA-OD product is increased and decreased when D₂O is added and removed, respectively. This suggests that the deuterium from D₂O is involved in the production of gas phase MAA-OD. In addition, the in situ DRIFTS-MS results obtained from the isotopic switches between D₂O and H₂O reveal changes in the characteristic infrared bands of the Keggin unit between 1200 and 600 cm^-1. It is found that the isotopic exchange possibly occurs on the bridging oxygen of Mo-O-Mo unit, where water is activated for the formation of MAA. Based on the in situ DRIFTS-MS analysis from the transient switching experiments, the reaction mechanism associated with the effect of water on the selective oxidation of MAL to MAA over Keggin-type H₃PMoO₁₂O₄₀ catalyst is proposed.

Confinement of single polyoxometalate clusters in molecular-scale cages for improved flexible solid-state supercapacitors

Herein, we realized the supramolecular confinement of a single polyoxometalate (POM) cluster precisely in a polypyrrole (PPy) hydrogel-wrapped CNT framework with molecular-scale cages. This hybrid hydrogel framework demonstrated an ultra-high loading (67.5 wt%) and extremely uniform dispersion of individual of H₃[P(Mo₃O₁₀)₄] (PMo₁₂) molecules, as demonstrated by sub-ångström-resolution HAADF-STEM. Consequently, it exhibited a better supercapacitor performance than that of the conventional composite system. The flexible solid-state supercapacitor exhibited a high energy density of 67.5 μW h cm^-2 at a power density of 700 μW cm^-2 and delivered a high capacitance retention of 85.7% after 3000 cycles. Moreover, the flexible device exhibited excellent mechanical stability. Density functional theory calculations revealed that the wrapped "fishnet-like" hydrogel creates a cage structure with a size of 1.8 nm for the precise storage of the PMo₁₂ molecule (diameter = 1.05 nm), leading to the mono-dispersion of single PMo₁₂ molecules on the hybrid hydrogel. The "caging" effect also activates the PMo₁₂ molecule to enhance its charging/discharging performance by introducing new reactive sites for proton transfer. We believe that this design for suitable cage structures can be used for the construction of other POM-based hybrid hydrogels, thereby achieving mono-dispersity and performance enhancement.

Polyoxometalates on Functional Substrates: Concepts, Synergies, and Future Perspectives

Polyoxometalates (POMs) are molecular metal oxide clusters that feature a broad range of structures and functionalities, making them one of the most versatile classes of inorganic molecular materials. They have attracted widespread attention in homogeneous catalysis. Due to the challenges associated with their aggregation, precipitation, and degradation under operational conditions and to extend their scope of applications, various strategies of depositing POMs on heterogeneous substrates have been developed. Recent ground-breaking developments in the materials chemistry of supported POM composites are summarized and links between molecular-level understanding of POM-support interactions and macroscopic effects including new or optimized reactivities, improved stability, and novel function are established. Current limitations and future challenges in studying these complex composite materials are highlighted, and cutting-edge experimental and theoretical methods that will lead to an improved understanding of synergisms between POM and support material from the molecular through to the nano- and micrometer level are discussed. Future development in this fast-moving field is explored and emerging fields of research in POM heterogenization are identified.

A Comprehensive Study on the Dye Adsorption Behavior of Polyoxometalate-Complex Nano-Hybrids Containing Classic β-Octamolybdate and Biimidazole Units

Six new hybrids based on β-[Mo₈O26]4- polyoxometalates, [Ni(H₂biim)₃]₂[β-Mo₈O26]•8DMF(1); (DMA)₂[M(H₂biim)₂(H₂O)₂][β-Mo₈O26]•4DMF (M = Ni (2), Co (3)), DMA = dimethyl-ammonium, H₂biim=2,2'-biimidazole); [M(H₂biim)(DMF)₃]₂[β-Mo₈O26]•2DMF (M = Zn (4), Cu (5)); [(DMA)₂[Cu(DMF)₄][β-Mo₈O26]•2DMF]n (6), have been successfully synthesized and characterized. Compounds 2⁻5 show favorable capacity to adsorb methylene blue (MB) at room temperature, and they can selectively capture MB molecules from binary-mixture solutions of MB/MO (MO = Methyl Orange), or MB/RhB (RhB = Rhodamine B). Compound 3 can uptake up to 521.7 mg g-1 MB cationic dyes rapidly, which perform the maximum adsorption in an hour among the reported materials as far as we know. The compounds are stable and still work very efficiently after three cycles. For compound 3, the preliminary adsorption mechanism studies indicated that the adsorption is an ion exchange process and the adsorption behavior of polyoxometalate-complex can be benefited from additional exchangeable protons in the complex cations.

Immobilization of Polyoxometalates on Tailored Polymeric Surfaces

Herein we describe the preparation of hybrid polymer-inorganic interfaces by the immobilization of polyoxometalate nanoclusters on functionalized polymer surfaces. The polymeric surfaces were made of polystyrene-b-poly(acrylic acid)/polystyrene (PS-b-PAA/PS) blends by spin coating on a silicon wafer. The functionalization of the polymer film was obtained by interfacial migration of the amphiphilic block copolymer toward the interface upon water vapor annealing. The carboxylic acid functional groups contained in the PAA block were then employed to anchor the [LnIII(α-SiW11O39)]5- polyoxometalates (Ln: Ce, Er). This purpose was achieved by immersing the films in aqueous solutions of the in situ-formed inorganic nanoclusters. X-ray photoelectron and confocal Raman spectroscopies, together with atomic force microscopy, confirmed the immobilization of the inorganic species at the interface.

Heterooctamolybdate-Based Clusters H3[(Cp*Rh)4PMo8O32] and H5[Na2(Cp*Ir)4PMo8O34] and Derived Hybrid Nanomaterials with Efficient Electrocatalytic Hydrogen Evolution Reaction Activity

Polyoxometalates (POMs), emerging as a new class of porous molecular materials, play a promising role in homo- and heterogeneous catalysis. Among them, noble-metal-decorated POMs have a profound impact as catalytic materials. Thus, it is imperative to design and structurally explore new catalysts including noble metals. Herein, two new clusters, H₃[(Cp*Rh)₄PMo₈O₃₂]·14H₂O (1) and H₅[Na₂(Cp*Ir)₄PMo₈O₃₄]·13H₂O (2) (Cp* = pentamethylcyclopentadienyl), based on a heterooctamolybdate anionic core were successfully obtained via a one-pot reaction using [Cp*MCl₂]₂ [M = Rh (1) and Ir (2)] and Na₂MoO₄ in acidic conditions. Compounds 1 and 2 were well characterized in the solid state by single-crystal X-ray diffraction, IR, and thermogravimetric analysis and in solution by UV-vis, electrospray ionization mass spectrometry, and electrochemistry. Compounds 1 and 2 represent an important class of structurally isolated organometallic POM-based clusters that were successfully nanostructured onto Ni foam and electrochemically reduced after 48 h of electrolysis to M/MoO₂, where M = Rh (3) and Ir (4), nanocomposite hybrid materials on a Ni foam surface in a 0.1 M KOH solution. The modified electrocatalysts (3 and 4) show efficient hydrogen evolution reaction activities almost comparable to those of high-grade Pt/C at 0.1 M KOH. The nanostructured POMs [1- and 2@NF (Ni foam)] and their corresponding reduced products (3 and 4) were observed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and X-ray photoelectron spectroscopy and further proven by transmission electron microscopy (TEM) and high-resolution TEM.

Tungsten addenda mixed heteropolymolybdates supported on functionalized graphene for high-performance aqueous supercapacitors

A novel honeycomb-like porous composite based on H₃PMo_12−xW_xO₄₀ and functionalized graphene was fabricated for high-performance aqueous supercapacitors.

Organic–inorganic hybrid polyoxometalate and its graphene oxide–Fe3O4nanocomposite, synthesis, characterization and their applications as nanocatalysts for the Knoevenagel condensation and the synthesis of 2,3-dihydroquinazolin-4(1H)-ones

Organic–inorganic hybrid of H₆Cu₂[PDA]₆[SiW₉Cu₃O₃₇]·12H₂O (HybPOM), was synthesized and grafted on graphene oxide decorated with Fe₃O₄. The structures of HybPOM and GO@Fe₃O₄@HybPOM nanocomposites were characterized and their catalytic activities investigated.

Enhancement of Oxidation of Formic Acid in Acid Medium on Zirconia-Supported Phosphotungstate-Decorated Noble Metal (Pd, Pt) Nanoparticles

Palladium and platinum nanoparticles have been modified at sub-monolayer level with Keggin-type phosphododecatungstate (by spontaneous adsorption of H3PW12O40) and considered as catalysts for the electrooxidation of formic acid in sulfuric acid electrolyte. The presence of phosphotungstate adsorbates has been confirmed using Fourier transform infrared spectroscopy (by reflectance). The enhancement effect (described in terms of the oxidation current increases) is even more pronounced when the catalytic centres have been supported on nanostructured ZrO2. In the case of Pd catalysts, a large population of hydroxyl groups and a high mobility of interfacial protons (on zirconia) seem to favour the direct oxidation path to CO2, whereas in the case of Pt-based systems, the enhancement effect is related to the oxidative removal of otherwise passivating CO adsorbates (indirect oxidation path). The role of polytungstate adsorbates on Pd or Pt nanoparticles relates to their ability to interact competitively with the CO adsorbates at noble metal catalytic sites (‘third-body’ effect).

Synthesis and high proton conductive performance of a quaternary vanadomolybdotungstosilicic heteropoly acid

A novel quaternary vanadomolybdotungstosilicic heteropoly acid with excellent proton conductivity.

PW9V3/rGO/SPEEK hybrid material: an excellent proton conductor

Reduced graphene oxide (rGO) and sulfonated polyether ether ketone (SPEEK) were introduced into a tungstovanadophosphoric acid (H₆PW₉V₃O₄₀, abbreviated as PW₉V₃) to prepare a novel PW₉V₃/rGO/SPEEK hybrid proton conduction material.

Noncovalent Functionalization of Graphene Nanosheets with Cluster-Cored Star Polymers and Their Reinforced Polymer Coating

A noncovalent and phase-transfer-assisted method is developed for the fabrication of polymer-functionalized graphene, in which a series of cluster-cored star polymers (CSPs) containing a polyoxometalate core and polystyrene (PS) arms are used as modifiers. Through the electron transfer interaction between polyoxometalate and graphene, the CSPs can strongly adsorb on graphene nanosheets and transfer them from aqueous media to organic solvents like chloroform, forming individually dispersed graphene. Moreover, the CSP-functionalized graphene is well compatible with additional polymer matrices and can serve as a reinforcing nanofiller for polymer composites. A 0.2 wt% loading of them in PS coating achieves a 98.9% high enhancement in Young's modulus.

A Novel pseudo-Four-Component Domino Reaction for the Synthesis of Naphtho[2,1-b]furan-2(1H)-ones Using a Nanocatalyst

In this article, an original one-pot method is utilized to synthesize a variety of derivatives of naphtho[2,1-b]furan-2(1H)-one via a pseudo-four-component domino reaction of aryl aldehydes, acetic anhydride, hippuric acids, and 2-naphthols catalyzed by HSW@SPIONs. This reaction illustrates an array of attractive features including, with particular interest in this report, a convenient and unique process of creating and utilizing a powerful recyclable nanocatalyst.

Heterogeneous catalysis

A heterogeneous catalyst is a functional material that continually creates active sites with its reactants under reaction conditions. These sites change the rates of chemical reactions of the reactants localized on them without changing the thermodynamic equilibrium between the materials.

Electron transfer to covalently immobilized Keggin polyoxotungstates on gold

Spontaneously adsorbed monolayers have been formed on gold electrodes using a Keggin polyoxotungstate with covalently attached alkanethiol linkers of two different lengths. Films of both polyoxotungstates show two well-defined reduction processes associated with the polyoxotungstate centers where the ionic liquid, [BMIM][BF4], acts as supporting electrolyte. The surface coverages are both less than that expected for a close-packed monolayer. For the short and long linkers, the voltammetric response can be described in terms of the Butler-Volmer response involving a surface confined species using standard heterogeneous electron transfer rate constants of 170 and 140 s(-1) for the first reduction and 150 and 100 s(-1) for the second reduction processes, respectively. The rate of electron transfer to a solution phase redox probe, ferrocyanide, is significantly more sensitive to the length of the linker than the rate of electron transfer to the tungstate centers. This behavior probably arises due to potential-induced changes in the film structure.

Phase transfer and dispersion of reduced graphene oxide nanosheets using cluster suprasurfactants

Surfactant-encapsulated polyoxometalates (SEP) can act as cluster suprasurfactants to transfer reduced graphene oxide (RGO) from water to low polar organic solvents with single-layer RGO dispersion.