Metal Oxide Clusters Research Articles

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.

Transition metal oxide clusters: advanced electrocatalysts for a sustainable energy future.

The comprehensive utilization of sustainable green energy is essential to face the global energy and environmental crisis. The oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and electrocatalytic urea synthesis (EUS) are the pivotal electrocatalytic processes, necessitating the development of low-cost electrocatalysts with high efficiency. Small-sized transition metal oxide (TMO) clusters have attracted a lot of attention because of their exceptional qualities, such as exhibiting a dense array of low-coordinated metal active sites (e.g. abundant metal cation defects and oxygen vacancy), amorphous structures with high surface energy, high atom utilization efficiency, and cost-effectiveness. Furthermore, the synergistic actions between metal clusters and TM-Nx single atom active sites remarkably boost up the electrocatalytic performances, corroborated by density functional theory (DFT). More efforts in this comprehensive feature article are expected to achieve insights into the fundamental understanding of electrocatalytic reaction mechanisms in our lab and serve as a guide for creating cutting-edge electrocatalysts of transition metal oxide clusters.

Keplerate polyoxometalate compounds: a multifunctional nano-platform for advanced materials.

Polyoxometalates (POMs) are robust, discrete, and structurally well-defined metal-oxide cluster anions that have stimulated research in broad fields of science. Keplerates, as porous giant POMs, serve as a multifunctional nano-platform exhibiting fascinating chemical properties stemming from the porous molecular structure, substantial interior space, delocalization of d-electrons over the large molecular surface, etc. Consequently, Keplerates have attracted significant attention from scientists in the fields of chemistry, physics, biology, and materials sciences. This work reviews recent research progress on Keplerates as nanocontainers, catalysts, and battery materials. Furthermore, current challenges and potential future research directions are discussed, providing a reference for the development and effective application of Keplerates and Keplerate-based materials.

Advances in polyoxometalate-based electrochemical sensors in the last three years.

As a famous subclass of metal-oxide cluster materials, polyoxometalates (POMs) feature variable architectures, reversible multi-electron transport capability, catalytic activity, and redox capacity. These attributes endow POMs with great potential as promising electrode materials in electrochemical sensors (ECSs). Up to now, POM-based ECSs have been passionately studied, and diverse POM-based redox ECSs, aptasensors and immunosensors have emerged. And these POM-based ECSs generally demonstrate fast response, low detection limit, strong selectivity and high antijamming capability. This review mainly focuses on the remarkable advancement of POM-based ECSs in environmental monitoring, food safety and biomedicine from 2021, aiming to furnish theoretical insights that inform the design and development of innovative sensors.

Self-assembled molecular hybrids comprising lacunary polyoxometalates and multidentate imidazole ligands.

Self-assembly via coordination bonding facilitates the creation of diverse inorganic-organic molecular hybrids with distinct structures and properties. Recent advances in this field have been driven by the versatility of organic ligands and inorganic units. Lacunary polyoxometalates are a class of well-defined metal-oxide clusters with a customizable number of reactive sites and bond directions, which make them promising inorganic units for self-assembled molecular hybrids. Herein, we report a novel synthesis method for self-assembled molecular hybrids utilizing the reversible coordination of multidentate imidazole ligands to the vacant sites of lacunary polyoxometalates. We synthesized self-assembled molecular hybrids including monomer, dimers, and tetramer, demonstrating the potential of our method for constructing intricate hybrids with tailored properties and functionalities.

Hierarchically Structured Nanocomposites via Mixed-Graft Block Copolymer Templating: Achieving Controlled Nanostructure and Functionality.

Integrating inorganic and polymerized organic functionalities to create composite materials presents an efficient strategy for the discovery and fabrication of multifunctional materials. The characteristics of these composites go beyond a simple sum of individual component properties; they are profoundly influenced by the spatial arrangement of these components and the resulting homo-/hetero-interactions. In this work, we develop a facile and highly adaptable approach for crafting nanostructured polymer-inorganic composites, leveraging hierarchically assembling mixed-graft block copolymers (mGBCPs) as templates. These mGBCPs, composed of diverse polymeric side chains that are covalently tethered with a defined sequence to a linear backbone polymer, self-assemble into ordered hierarchical structures with independently tuned nano- and mesoscale lattice features. Through the coassembly of mGBCPs with diversely sized inorganic fillers such as metal ions (ca. 0.1 nm), metal oxide clusters (0.5-2 nm), and metallic nanoparticles (>2 nm), we create three-dimensional filler arrays with controlled interfiller separation and arrangement. Multiple types of inorganic fillers are simultaneously integrated into the mGBCP matrix by introducing orthogonal interactions between distinct fillers and mGBCP side chains. This results in nanocomposites where each type of filler is selectively segregated into specific nanodomains with matrix-defined orientations. The developed coassembly strategy offers a versatile and scalable pathway for hierarchically structured nanocomposites, unlocking new possibilities for advanced materials in the fields of optoelectronics, sensing, and catalysis.

Characterization, Structure, and Reactivity of Hydroxyl Groups on Metal-Oxide Cluster Nodes of Metal-Organic Frameworks: Structural Diversity and Keys to Reactivity and Catalysis.

Among the most stable metal-organic frameworks (MOFs) are those incorporating nodes that are metal oxide clusters with frames such as Zr6 O8 . This review is a summary of the structure, bonding, and reactivity of MOF node hydroxyl groups, emphasizing those bonded to nodes containing aluminum and zirconium ions. Hydroxyl groups are often present on these nodes, sometimes balancing the charges of the metal ions. They arise during MOF syntheses in aqueous media or in post-synthesis treatments. They are identified with infrared and 1 H nuclear magnetic resonance spectroscopies and characterized by their reactivities with polar compounds such as alcohols. Terminal OH, pairedµ2 -OH, and aqua groupson nodes are catalytic sites in numerous reactions. Relatively unreactive hydroxyl groups (such as isolatedµ2 -OH groups) may replace reactive groups and inhibit catalysis; some node hydroxyl groups (e.g.,µ3 -OH) are mere spectators in catalysis. There are similarities between MOF node hydroxyl groups and those on the surfaces of bulk metal oxides, zeolites, and enzymes, but the comparisons are mostly inexact, and much remains to be understood about MOF node hydroxyl group chemistry. It is posited that understanding and controlling this chemistry will lead to tailored MOFs and improved adsorbents and catalysts.

Electron-Sponge Nature of Polyoxometalates for Next-Generation Electrocatalytic Water Splitting and Nonvolatile Neuromorphic Devices.

Designing next-generation molecular devices typically necessitates plentiful oxygen-bearing sites to facilitate multiple-electron transfers. However, the theoretical limits of existing materials for energy conversion and information storage devices make it inevitable to hunt for new competitors. Polyoxometalates (POMs), a unique class of metal-oxide clusters, have been investigated exponentially due to their structural diversity and tunable redox properties. POMs behave as electron-sponges owing to their intrinsic ability of reversible uptake-release of multiple electrons. In this review, numerous POM-frameworks together with desired features of a contender material and inherited properties of POMs are systematically discussed to demonstrate how and why the electron-sponge-like nature of POMs is beneficial to design next-generation water oxidation/reduction electrocatalysts, and neuromorphic nonvolatile resistance-switching random-access memory devices. The aim is to converge the attention of scientists who are working separately on electrocatalysts and memory devices, on a point that, although the application types are different, they all hunt for a material that could exhibit electron-sponge-like feature to realize boosted performances and thus, encouraging the scientists of two completely different fields to explore POMs as imperious contenders to design next-generation nanodevices. Finally, challenges and promising prospects in this research field are also highlighted.

A Systematic Knowledge of Nanoparticles and Nanotechnology - A Review

The main aim is to describe the journey to beautiful and useful word Nano. Nano stands for one billionth. In science & technology, Nano stands for one billionth of meter, 1nm-10-9 m. Nanoparticles and nanotechnology words are relatively new but nanoparticles themselves has been around and studied long before the word were used. The beautiful colours of stained glass windows are a result of the presence of small metal oxide clusters in glass, having a size comparable to the wavelength of light. Particles of different sizes scatter different wavelength of light, imparting different colours to the glass. In photography the small colloidal particles of silver are a part of the processes of image formation and water at ambient temperature consists of clusters of hydrogen bonded water molecules. The description of variation of properties of nanoparticle with size, practical application of nanotechnology, uses in defence and biotechnology and as well as disadvantage of nanotechnology are given in this article.

Impact of PbO on structural, thermal, optical and gamma ray shielding behaviour of TeO2-CdO based glasses

A study was conducted to investigate the effect of an increase in the concentration of PbO on the structural, thermal, optical, and gamma ray shielding behavior of TeO2-CdO based glasses. The glasses were found to contain TeO4 and TeO3 units, as well as CdO metal oxide clusters, as indicated by the Raman spectrum. The glasses displayed minor weight loss (approximately 3%), suggesting high resistance to thermal degradation. With an increase in the concentration of PbO, the glass transition temperature (Tg), onset temperature (To), and crystallization temperature (Tc) all decreased. Additionally, the indirect band gap energy decreased from 3.497–3.337 eV as the lead oxide concentration increased. Moreover, the effects of PbO concentration on the linear attenuation coefficient (LAC) were investigated. The LAC values were found to be highest at 39.5 keV and decreased with increasing energy. As the PbO concentration increased, LAC values also increase. The Zeff displayed an inverse relationship with energy, except for an abrupt increase at 121.8 keV. The half value layer was found to be positively correlated with energy and varied with changes in PbO and CdO concentrations. The glasses demonstrated effective gamma radiation shielding properties, with the HVL remaining small even at higher energies, suggesting potential applications in industries such as medical imaging where thin shielding materials are preferred.

Reticular Chemistry for Optical Sensing of Anions.

In the last few decades, reticular chemistry has grown significantly as a field of porous crystalline molecular materials. Scientists have attempted to create the ideal platform for analyzing distinct anions based on optical sensing techniques (chromogenic and fluorogenic) by assembling different metal-containing units with suitable organic linking molecules and different organic molecules to produce crystalline porous materials. This study presents novel platforms for anion recognition based on reticular chemistry with high selectivity, sensitivity, electronic tunability, structural recognition, strong emission, and thermal and chemical stability. The key materials for reticular chemistry, Metal-Organic Frameworks (MOFs), Zeolitic Imidazolate Frameworks (ZIFs), and Covalent-Organic Frameworks (COFs), and the pre- and post-synthetic modification of the linkers and the metal oxide clusters for the selective detection of the anions, have been discussed. The mechanisms involved in sensing are also discussed.

Divalent Heterometal Doped Titanium-Oxide Cluster Polymers: Structures, Photoresponse, and Photocatalysis.

Five cluster polymers based on heterometal-doped titanium-oxide cluster (TOC) monomers are reported. The monomers feature Ti10-oxide cluster cores and are connected to the divalent closed-shell heterometal anchors by salicylate ligands. The Sr2+, Ba2+, and Pb2+ dopants cause the monomers to bind head-to-head and generate linear chains, while the Ca2+ and Cd2+ lead to head-to-tail connections and zigzag chains. The cluster polymers are responsive to visible-light up to 565 nm and photo-catalytically active in both H2 evolution and CO2/epoxide cycloaddition reactions. The photo-absorption, photo-charge separation, and photocatalytic properties of the cluster polymers are dependent on the heterometal dopants in order Cd > Pb > Ba > Sr > Ca. Heterometals serve as the catalytic sites in the cluster polymers, which depending on the contribution of the pCB bottom, facilitate photo-charge separation and interfacial charge transfer, further enhancing catalytic activity. The tunable compositions and topologies of the cluster polymers shown herein may inspire the design and synthesis of more multidimensional functional metal-oxide cluster materials for a variety of applications in the future.

H8[PV5Mo7O40] (HPA-5) - a unique polyoxometalate for acid and RedOx catalysis: synthesis, characterization, and modern applications in green chemical processes.

Polyoxometalates (POMs) are a fascinating group of anionic metal-oxide clusters with a broad variety of structural properties and several catalytic applications, especially in the conversion of bio-derived platform chemicals. H8[PV5Mo7O40] (HPA-5) is a unique POM catalyst that ideally links numerous fascinating research fields for the following reasons: a) HPA-5 can be synthesized by rational design approaches; b) HPA-5 can be well characterised using multiple analytical tools explaining its catalytic properties; and c) HPA-5 is suitable for multiple important catalytic transformations of bio-based feedstocks. This review combines the fields of synthesis, spectroscopic, electrochemical and crystallographic characterization of HPA-5 with those of sustainable catalysis and green chemistry. Selected catalytic applications include esterification, dehydration and delignification of biomass as well as selective oxidation and fractionation of bio-based feedstock. We are fascinated from this unique POM and hope that the reader can get deeper insights into the chemistry of HPA-5 and its broad scope for biomass valorisation.

Polyoxometalate‐Incorporated CuI‐Resorcin[4]arene Metal‐Organic Complexes as Heterogeneous Catalysts for Catalytic Oxidation of Mustard Gas Simulant

AbstractExploring efficient heterogeneous catalysts for catalytic oxidation of chemical warfare agents (CWAs) is highly desired. As a class of discrete anionic metal oxide clusters, polyoxometalates (POMs) provide abundant catalytic active sites, thus resulting their rich redox properties. Here, a family of known POM‐incorporated CuI‐resorcin[4]arene metal‐organic complexes, namely, [Cu4(TPTR4A)2][PW12O40](OH) ⋅ 0.5DMA ⋅ 5H2O (Cu‐PW), [Cu4(TPTR4 A)2][PMo12O40](OH) ⋅ 2DMA ⋅ H2O (Cu‐PMo) and [Cu4(TPTR4A)2][SiW12O40] ⋅ 2.5DMA (Cu‐SiW) were utilized as catalysts to promote the oxidation of 2‐chloroethyl ethyl sulfide (CEES). Strikingly, compared to the novel compound [Cu3Cl6(TPTR4A)(DMA)] ⋅ CH3CH2OH (defined as Cu‐T), the three complexes exhibited excellent stability, indicating that the integration of POMs and metal–organic units could improve the stability of the compounds. Moreover, Cu‐PMo and Cu‐PW showed higher activities for the catalytic oxidation of CEES to CEESO with selectivities both of 99 %.

Molecular Complexation of Polymers and Metal Oxide Clusters for Semicrystalline, Thermoplastic Anhydrous Proton Exchange Membranes in Fuel Cells.

With the manipulation of surface charges and loadings, 1 nm super-acidic metal oxide clusters can co-crystallize with poly(ethylene glycol) (PEG) at molecular scale for thermoplastic anhydrous proton exchange membranes (PEMs). The coexistence of crystalline and amorphous regions endows the PEMs with a high Young's modulus and high flexibility, while the noncovalent complex interactions enable facile preparation and (re)processing. Furthermore, the diffusive dynamics of PEG chains is slowed by the confinement effect, while the local segmental dynamics is accelerated due to the transition of the chain conformation from helix to zigzag when confined in the crystalline framework. This greatly facilitates proton transportation in the crystalline region for an excellent anhydrous proton conductivity of 4.5 × 10-3 S cm-1 at 90 °C. The balanced proton conductivity, mechanical strength, and processability of the PEMs contribute to the promising power density of H2/O2 fuel cells assembled with co-crystalline PEMs at high temperatures under dry conditions.

Investigating the adsorption behavior of NOx molecules on γ-Al2O3(1 0 0)-supported proximal Pt-Ba clusters using density functional theory

The advancement of low to zero carbon fuels for internal combustion engines has made Nitrogen oxides (NOx) emissions a focal point of engine aftertreatment. Lean NOx Trap (LNT) is one of the most promising technologies for reducing NOx emissions in light-duty vehicles. The proximity of barium (Ba) to platinum (Pt) sites plays a significant role in the adsorption, reduction, and desorption of NOx inside LNT. A simultaneous loading of Pt clusters (Pt3) and barium oxide clusters ((BaO)2) on γ-Al2O3(100) was innovatively established in this study, and the effect of oxide carrier - triatomic catalyst clusters - bimolecular alkaline earth metal oxide clusters was considered comprehensively to examine the effect of Pt-Ba proximity on the NOx adsorption behavior. Interesting trends in the adsorption energy and activation of NOx molecules (including NO/NO2 and the strong greenhouse gas N2O) due to differences in the adsorption sites (from Pt sites to Pt-Ba proximity sites and finally to Ba sites) were found, and the essential reasons for such variations were elucidated by an exhaustive electronic structure analysis. The results of this study are an interesting complement to the current mechanism for storage and reduction of NOx.

Reusable and Highly Active Copper-Based Lacunary-type Polyoxometalate (LPMo-Cu) as an Effective Catalyst for Nitroarene Reduction in Aqueous Solution.

Lacunary polyoxometalates (LPOMs), a class of anionic metal oxide clusters, are promising materials as catalysts, photocatalysts, and electrocatalysts. Designing and functionalizing this type of compound are key factors for the discovery and development of novel materials. Herein, a new lacunary polyoxometalate-based compound was designed as a heterogeneous catalyst, prepared through functionalization of a lacunary Keggin-type polyoxometalate, i.e., [PMo11O39]7-, with 3-aminopropyltrimethoxysilane (APTS) and 2-pyridine carboxaldehyde. Subsequent reactions of this compound with Cu2+ ions led to the desired catalyst, LPMo-Cu. The catalytic activity of the obtained LPMo-Cu was examined in nitroarene reduction using sodium borohydride as a reducing agent in aqueous solution. It was specified that the synthesized LPMo-Cu exhibits high catalytic efficiency in the reduction of a wide variety of nitroarenes in a short time (5 min). Furthermore, the stability and recoverability of the prepared material were proved as a result of four consecutive reduction cycles without a significant decrease in its efficiency.

Calculation of Self, Corrected, and Transport Diffusivities of Isopropyl Alcohol in UiO-66.

The UiO-6x family of metal-organic frameworks has been extensively studied for applications in chemical warfare agent (CWA) capture and destruction. An understanding of intrinsic transport phenomena, such as diffusion, is key to understanding experimental results and designing effective materials for CWA capture. However, the relatively large size of CWAs and their simulants makes diffusion in the small-pored pristine UiO-66 very slow and hence impractical to study directly with direct molecular simulations because of the time scales required. We used isopropanol (IPA) as a surrogate for CWAs to investigate the fundamental diffusion mechanisms of a polar molecule within pristine UiO-66. IPA can form hydrogen bonds with the μ3-OH groups bound to the metal oxide clusters in UiO-66, similar to some CWAs, and can be studied by direct molecular dynamics simulations. We report self, corrected, and transport diffusivities of IPA in pristine UiO-66 as a function of loading. Our calculations highlight the importance of the accurate modeling of the hydrogen bonding interactions on diffusivities, with about an order of magnitude decrease in diffusion coefficients when the hydrogen bonding between IPA and the μ3-OH groups is included. We found that a fraction of the IPA molecules have very low mobility during the course of a simulation, while a small fraction are highly mobile, exhibiting mean square displacements far greater than the ensemble average.

Limits of Detection for EXAFS Characterization of Heterogeneous Single-Atom Catalysts

Single-atom catalysts (SACs), consisting of individual metal atoms dispersed on a support, attract attention due to their unique reactivity, efficient use of precious metals, and precise chemical tunability. Characterization of the metal species is crucial to substantiate structure–function relationships. Authors often use─and referees often require─X-ray absorption spectroscopy (XAS) data to prove the absence of clustered metal (or metal oxide) structures after pre-treatment and under in situ or operando conditions. However, there has been no critical assessment of the limitations of XAS in substantiating such conclusive statements, which is particularly important given the potential outsized influence of minority catalyst structures in dictating catalytic activity. In this article, we quantitatively assess the detection limits of XAS to identify metal (or metal oxide) clusters in samples containing predominantly single atoms by modeling the extended X-ray absorption fine structure (EXAFS) of mixtures of structures. We identified that a significant fraction of clusters can coexist with SAC active sites (e.g., ∼10% metallic Pt or ∼40% oxidized Pt clusters in Pt/CeO2 SACs), while eluding detection via EXAFS with any statistical significance. To generalize these conclusions, a descriptor-based screening of bulk metal oxides using a continuous Cauchy wavelet transform was proposed that suggests certain materials for which differentiating atomically dispersed metal species and metal oxide clusters would be infeasible by EXAFS (e.g., ReOx). Based on this analysis, we suggest best practices for the study of SACs using EXAFS and provide recommendations to ensure that conclusions do not outpace the evidence used to support them. In this rapidly expanding research area, rigorous characterization will lead to greater understanding of the behavior of SACs and ultimately improved catalytic materials.

Size-Dependent Geometrical Structures of Platinum Oxide Cluster Cations Studied by Ion Mobility–Mass Spectrometry

Structures of platinum oxide cluster cations (PtnOm+) were studied by ion mobility-mass spectrometry in combination with theoretical calculations. Structures of oxygen-equivalent PtnOn+ (n = 3-7) clusters were discussed from the comparison between their collision cross sections (CCSs) obtained by mobility measurement and simulated CCSs of their structural candidates from structural optimization calculations. Assigned structures of PtnOn+ were found to be composed of Pt frameworks and bridging O atoms, which follows the previous theoretical prediction on the neutral clusters. The structures change from planar (n = 3 and 4) to three-dimensional (n = 5-7) with increasing cluster size by deforming platinum frameworks. Comparison with other group-10 metal oxide cluster cations (MnOn+; M = Ni and Pd) showed that the PtnOn+ structures have a similar tendency to PdnOn+ rather than NinOn+.