Surfactant encapsulated palladium-polyoxometalates: controlled assembly and their application as single-atom catalysts

Single Dispersion of Fe(H2O)2-Based Polyoxometalate on Polymeric Carbon Nitride for Biomimetic CH4 Photooxidation

Direct methane conversion to value-added oxygenates under mild conditions with in-depth mechanism investigation has attracted wide interest. Inspired by methane monooxygenase, the K9Na2Fe(H2O)2{[γ-SiW9O34Fe(H2O)]}2·25H2O polyoxometalate (Fe-POM) with well-defined Fe(H2O)2 sites is synthesized to clarify the key role of Fe species and their microenvironment toward CH4 photooxidation. The Fe-POM can efficiently drive the conversion of CH4 to HCOOH with a yield of 1570.0 µmol gPOM -1 and 95.8% selectivity at ambient conditions, much superior to that of [Fe(H2O)SiW11O39]5- with Fe(H2O) active site, [Fe2SiW10O38(OH)]2 14- and [P8W48O184Fe16(OH)28(H2O)4]20- with multinuclear Fe-OH-Fe active sites. Single-dispersion of Fe-POM on polymeric carbon nitride (PCN) is facilely achieved to provide single-cluster functionalized PCN with well-defined Fe(H2O)2 site, the HCOOH yield can be improved to 5981.3 µmol gPOM -1. Systemic investigations demonstrate that the (WO)4-Fe(H2O)2 can supply Fe═O active center for C-H activation via forming (WO)4-Fea-Ot···CH4 intermediate, similar to that for CH4 oxidation in the monooxygenase. This work highlights a promising and facile strategy for single dispersion of ≈1-2 Å metal center with precise coordination microenvironment by uniformly anchoring nanoscale molecular clusters, which provides a well-defined model for in-depth mechanism research.

Photoelectrochemical biosensor based on SiW12@CdS quantum dots for the highly sensitive detection of HPV 16 DNA

A highly sensitive biosensor for detecting HPV 16 DNA was prepared based on Keggin-type polyoxometalate (SiW₁₂)-grafted CdS quantum dots (SiW₁₂@CdS QDs) and colloidal gold nanoparticles (Au NPs), which exhibited remarkable selectivity and sensitivity upon target DNA detection because of its excellent photoelectrochemical (PEC) response. Here, an enhanced photoelectronic response ability was achieved with the strong association of SiW₁₂@CdS QDs by polyoxometalate modification, which was developed through a convenient hydrothermal process. Furthermore, on Au NP-modified indium tin oxide slides, a multiple-site tripodal DNA walker sensing platform coupled with T7 exonuclease was successfully fabricated with SiW₁₂@CdS QDs/NP DNA as a probe for detecting HPV 16 DNA. Due to the remarkable conductivity of Au NPs, the photosensitivity of the as-prepared biosensor was improved in an I3-/I- solution and avoided the use of other regents toxic to living organisms. Finally, under optimized conditions, the as-prepared biosensor protocol demonstrated wide linear ranges (15-130 nM), with a limit of detection of 0.8 nM and high selectivity, stability, and reproducibility. Moreover, the proposed PEC biosensor platform offers a reliable pathway for detecting other biological molecules with nano-functional materials.

Rapid detection of eugenol in perch utilizing electrochemical method by transition metal substituted polyoxometalates

Food safety concerns caused by the application of spice allergens to fish anaesthesia. In this paper, a chitosan-reduced graphene oxide/polyoxometalates/poly-l-lysine (CS-rGO/P₂Mo₁₇Cu/PLL) modified electrode was constructed by electrodeposition and successfully applied to the quantitative analysis of eugenol (EU). The detection limit was 0.4490 μM in the linear range of 2x10^-6 M to 1.4x10^-5 M. It was applied to the determination of EU residues in kidney, liver and meat tissues of perch with recoveries ranging from 85.43 to 93.60%. Besides, the electrodes demonstrate high stability (2.56% drop in current value after 70 days at room temperature), high reproducibility (RSD of 4.87% for 6 parallel electrodes) and extremely fast response time. This study provided a new material for the electrochemical detection of EU.

Atomically Dispersed Pd Sites on ZrO2 Hybridized N-Doped Carbon for Efficient Suzuki–Miyaura Reaction

Researchers studying heterogeneous catalysis are intrigued by single-atom catalysts (SACs) due to their ultrahigh atomic utilization. However, only a few reports on SAC-catalyzed classical organic transformations are available. In this work, atomically dispersed Pd sites are confined to a ZrO2 hybridized N-doped carbon skeleton with a smart design. UiO-66-NH2 is used to anchor Pd atoms by the coordination of the donor atoms including lone pairs of electrons and metal atoms. Subsequently, the in situ introduction of ZrO2 doping is achieved using pyrolysis, which helps improve the catalytic performance by modulating the electronic state. The Pd@ZrO2/N–C catalyst obtained from the unique design exhibits a high yield (99%) in eco-friendly media with an extremely low noble metal dosage (0.03 mol% Pd) for the Suzuki reaction. Moreover, Pd@ZrO2/N–C remains highly active after being reused several times and possesses versatility in a variety of substrates. This strategy offers a feasible alternative to designing SACs with atomically dispersed noble metals for heterogeneous reactions.

Tunable Ordered Nanostructured Phases by Co-assembly of Amphiphilic Polyoxometalates and Pluronic Block Copolymers

The assembly of polyoxometalate (POM) metal-oxygen clusters into ordered nanostructures is attracting a growing interest for catalytic and sensing applications. However, assembly of ordered nanostructured POMs from solution can be impaired by aggregation, and the structural diversity is poorly understood. Here, we present a time-resolved small-angle X-ray scattering (SAXS) study of the co-assembly in aqueous solutions of amphiphilic organo-functionalized Wells-Dawson-type POMs with a Pluronic block copolymer over a wide concentration range in levitating droplets. SAXS analysis revealed the formation and subsequent transformation with increasing concentration of large vesicles, a lamellar phase, a mixture of two cubic phases that evolved into one dominating cubic phase, and eventually a hexagonal phase formed at concentrations above 110 mM. The structural versatility of co-assembled amphiphilic POMs and Pluronic block copolymers was supported by dissipative particle dynamics simulations and cryo-TEM.

Recent progress of sub-1 nm nanomaterials: synthesis, polymer-analogue properties and applications in redox catalysis

Sub-1 nm nanomaterials (SNMs) have attracted attention for their novel structures and size-related properties. In the past decade, various SNMs were synthesized, such as nanowires, nanorings, nanosheets, nanobelts, nanotubes, and other superstructures. We discussed their synthetic strategies systematically, including the ligand pathway and the cluster-nuclei co-assembly pathway. In addition, SNMs exhibit unique size-related properties. Firstly, SNWs show polymer-analogue properties due to their sub-nanometric size and ultrahigh aspect ratio. We illustrate the polymer-analogue properties on both microscopic and macroscopic scales. The macroscopic assemblies of SNWs can be widely applied for organic liquid storage, energy, and optical applications. Finally, we summarized the applications of SNMs in redox catalysis. Their extraordinary catalytic activity is attributed to their large specific surface area and electronic delocalization at the sub-nanometric scale. We hope this feature article can provide new viewpoints on the design and applications of SNMs.

Guided Synthesis of a Mo/Zn Dual Single-Atom Nanozyme with Synergistic Effect and Peroxidase-like Activity

We present a facile route towards a dual single-atom nanozyme composed of Zn and Mo, which utilizes the non-covalent nano-assembly of polyoxometalates, supramolecular coordination complexes as the metal-atom precursor, and a macroscopic amphiphilic aerogel as the supporting substrate. The dual single-atoms of Zn and Mo have a high content (1.5 and 7.3 wt%, respectively) and exhibit a synergistic effect and a peroxidase-like activity. The Zn/Mo site was identified as the main active center by X-ray absorption fine structure spectroscopy and density functional theory calculation. The detection of versatile analytes, including intracellular H₂ O₂ , glucose in serum, cholesterol, and ascorbic acid in commercial beverages was achieved. The nanozyme has an outstanding stability and maintained its performance after one year's storage. This study develops a new peroxidase-like nanozyme and provides a robust synthetic strategy for single-atom catalysts by utilizing an aerogel as a facile substrate that is capable of stabilizing various metal atoms.

Enabling High Loading in Single-Atom Catalysts on Bare Substrate with Chemical Scissors by Saturating the Anchoring Sites

Atomically dispersed metal catalysts often exhibit high catalytic performances, but the metal loading density must be kept low to avoid the formation of metal nanoparticles, making it difficult to improve the overall activity. Diverse strategies based on creating more anchoring sites (ASs) have been adopted to elevate the loading density. One problem of such traditional methods is that the single atoms always gather together before the saturation of all ASs. Here, a chemical scissors strategy is developed by selectively removing unwanted metallic materials after excessive loading. Different from traditional ways, the chemical scissors strategy places more emphasis on the accurate matching between the strength of etching agent and the bond energies of metal-metal/metal-substrate, thus enabling a higher loading up to 2.02 wt% even on bare substrate without any pre-treatment (the bare substrate without any pre-treatment generally only has a few ASs for single atom loading). It can be inferred that by combining with other traditional methods which can create more ASs, the loading could be further increased by saturating ASs. When used for CH₃ OH generation via photocatalytic CO₂ reduction, the as-made single-atom catalyst exhibits impressive catalytic activity of 597.8 ± 144.6 µmol h^-1 g^-1 and selectivity of 81.3 ± 3.8%.

Pd(ii)-Based polyoxometalate polymers as highly efficient heterogeneous catalysts for Suzuki–Miyaura reactions

Five Pd(ii)/polyoxometalate catalysts with a unique atomic arrangement and high Pd loading were synthesized under solvothermal conditions, and their structures were determined by single crystal X-ray diffraction.

Theoretical Understandings of Graphene-based Metal Single-Atom Catalysts: Stability and Catalytic Performance

Research on heterogeneous single-atom catalysts (SACs) has become an emerging frontier in catalysis science because of their advantages in high utilization of noble metals, precisely identified active sites, high selectivity, and tunable activity. Graphene, as a one-atom-thick two-dimensional carbon material with unique structural and electronic properties, has been reported to be a superb support for SACs. Herein, we provide an overview of recent progress in investigations of graphene-based SACs. Among the large number of publications, we will selectively focus on the stability of metal single-atoms (SAs) anchored on different sites of graphene support and the catalytic performances of graphene-based SACs for different chemical reactions, including thermocatalysis and electrocatalysis. We will summarize the fundamental understandings on the electronic structures and their intrinsic connection with catalytic properties of graphene-based SACs, and also provide a brief perspective on the future design of efficient SACs with graphene and graphene-like materials.

Single-Atom Catalysts across the Periodic Table

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

Recent Progress in Adipic Acid Synthesis Over Heterogeneous Catalysts

Adipic acid is one of the most important feedstocks for producing resins, nylons, lubricants, plasticizers. Current industrial petrochemical process, producing adipic acid from KA oil, catalyzed by nitric acid, has a serious pollution to the environment, due to the formation of waste nitrous oxide. Hence, developing cleaner methods to produce adipic acid has attracted much attention of both industry and academia. This mini-review article discussed advances on adipic acid synthesis from bio-renewable feedstocks, as well as most recent progress on cleaner technology from fossil fuels over novel catalytic materials. This work on recent advances in green adipic acid production will provide insights and guidance to further study of various other industrial processes for producing nylon precursors.

Polyoxometalate-based composite materials in electrochemistry: state-of-the-art progress and future outlook

Polyoxometalates (POMs) have been developed as a class of promising smart material candidates not only due to their multitudinous architectures but also their good redox activities and outstanding electron and proton transport capacities. Recently, abundant studies on POMs composited with metal nanoparticles (NPs), carbon materials (e.g., carbon nanotubes (CNTs), carbon quantum dots (CQDs), graphene), and conducting polymers or highly-porous framework materials (e.g., MOFs, ZIFs) have been performed and POM-based composite materials (PCMs) undoubtedly show enhanced stability and improved electrochemical performances. Therefore, POMs and PCMs are of increasing interest in electrocatalysis, electrochemical detection and energy-related fields (such as fuel cells, redox flow batteries and so on), thus, developing novel PCMs has long been the key research topic in POM chemistry. This review mainly summarizes some representative advances in PCMs with electrochemical applications in the past ten years, expecting to provide some useful guidance for future research.

Beyond Charge Balance: Counter-Cations in Polyoxometalate Chemistry

Polyoxometalates (POMs) are molecular metal-oxide anions applied in energy conversion and storage, manipulation of biomolecules, catalysis, as well as materials design and assembly. Although often overlooked, the interplay of intrinsically anionic POMs with organic and inorganic cations is crucial to control POM self-assembly, stabilization, solubility, and function. Beyond simple alkali metals and ammonium, chemically diverse cations including dendrimers, polyvalent metals, metal complexes, amphiphiles, and alkaloids allow tailoring properties for known applications, and those yet to be discovered. This review provides an overview of fundamental POM-cation interactions in solution, the resulting solid-state compounds, and behavior and properties that emerge from these POM-cation interactions. We will explore how application-inspired research has exploited cation-controlled design to discover new POM materials, which in turn has led to the quest for fundamental understanding of POM-cation interactions.

Functionalization of Hollow Nanomaterials for Catalytic Applications: Nanoreactor Construction

Hollow nanomaterials have attracted a broad interest in multidisciplinary research due to their unique structure and preeminent properties. Owing to the high specific surface area, well-defined active site, delimited void space, and tunable mass transfer rate, hollow nanostructures can serve as excellent catalysts, supports, and reactors for a variety of catalytic applications, including photocatalysis, electrocatalysis, heterogeneous catalysis, homogeneous catalysis, etc. Based on state-of-the-art synthetic methods and characterization techniques, researchers focus on the purposeful functionalization of hollow nanomaterials for catalytic mechanism studies and intricate catalytic reactions. Herein, an overview of current reports with respect to the catalysis of functionalized hollow nanomaterials is given, and they are classified into five types of versatile strategies with a top-down perspective, including textual and composition modification, encapsulation, multishelled construction, anchored single atomic site, and surface molecular engineering. In the detailed case studies, the design and construction of hierarchical hollow catalysts are discussed. Moreover, since hollow structure offers more than two types of spatial-delimited sites, complicated catalytic reactions are elaborated. In summary, functionalized hollow nanomaterials provide an ideal model for the rational design and development of efficient catalysts.

Self-Assembly of Ultrathin Nanocrystals to Multidimensional Superstructures

The self-assembly of ultrathin nanocrystals (UTNCs) into well-organized multidimensional superstructures is one of the key topics in material chemistry and physics. Highly ordered nanocrystal assemblies also known as superstructures or synthetic structures have remained a focus for researchers over the past few years due to synergy in their properties as compared to their components. Here, we aim to present the recent progress being made in this field with highlights of our research group endeavors in the engineering of self-assembled complex multidimensional superstructures of various inorganic materials, including polyoxometalates. The driving forces for the assembly process and its kinetics along with the potential applications associated with these unique ordered and spatially complex superstructures are also discussed.

Atomically Dispersed Pt1-Polyoxometalate Catalysts: How Does Metal-Support Interaction Affect Stability and Hydrogenation Activity?

Unlike nanostructured metal catalysts, supported single-atom catalysts (SACs) contain only atomically dispersed metal atoms, hinting at much more pronounced metal-support effects. Herein, we take a series of polyoxometalate-supported Pt catalysts as examples to quantitatively investigate the stability of Pt atoms on oxide supports and how the Pt-support interaction influences the catalytic performance. For this entire series, we show that the Pt atoms prefer to stay at a 4-fold hollow site of one polyoxometalate molecule and that the least adsorption energy to obtain sintering-resistant Pt SACs is 5.50 eV, which exactly matches the cohesive energy of bulk Pt metal. Further, we compared their catalytic performance in several hydrogenation reactions and simulated the reaction pathways of propene hydrogenation by density functional theory (DFT) calculations. Both experimental and theoretical approaches suggest that despite the Pt₁-support interactions being different, the reaction pathways of various Pt₁-polyoxometalate catalysts are very similar and their effective reaction barriers are close to each other and as low as 24 kJ/mol, indicating the possibility of obtaining SACs with improved stability without compromising activity. DFT calculations show that all reaction elementary steps take place only on the Pt atom without involving neighboring O atoms and that hydrogenation proceeds from the molecularly adsorbed H₂ species. Pt SACs give a weaker H₂ adsorption energy than Pt clusters or surfaces, resulting in small adsorption equilibrium constants and small apparent activation barriers, which agree between experiment and theory.

Magnetically Hollow Pt Nanocages with Ultrathin Walls as a Highly Integrated Nanoreactor for Catalytic Transfer Hydrogenation Reaction

Fabricating efficient and stable nanocatalysts for chemoselective hydrogenation of nitroaromatics is highly desirable because the amines hold tremendous promise for the synthesis of nitrogen containing chemicals. Here, a highly reactive and stable porous carbon nitride encapsulated magnetically hollow platinum nanocage is developed with subnanometer thick walls (Fe3O4@snPt@PCN) for this transformation. This well-controlled nanoreactor is prepared via the following procedures: the preparation of core template, the deposition of platinum nanocage with subnanometer thick walls, oxidative etching, and calcination. This highly integrated catalyst demonstrates excellent performance for the catalytic transfer hydrogenation of various nitroaromatics and the reaction can reach >99% conversion and >99% selectivity. With the ultrathin wall structure, the atom utilization of platinum atoms is highly efficient. The X-ray photoelectron spectroscopy results indicate that partial electrons transfer from the iron oxides to Pt nanowalls, and this increases the electron density of snPt nanoparticles, thus promoting the catalytic activity for the transfer hydrogenation of nitroaromatics. For the reduction of 4-nitrophenol, the reaction rate constant K app is 0.23 min-1 and the turnover frequency (TOF) is up to 3062 h-1. Additional reaction results illustrate that this magnetic nanoreactor can be reused more than eight times and it is a promising catalytic nanoplatform in heterogeneous catalysis.

CO oxidation over the polyoxometalate-supported single-atom catalysts M1/POM (Fe, Co, Mn, Ru, Rh, Os, Ir, and Pt; POM = [PW12O40]3–): a computational study on the activation of surface oxygen species

Density functional theory calculations have been carried out to explore the catalytic performance of a series of the M₁/POM (M = Fe, Co, Mn, Ru, Rh, Os, Ir, and Pt; POM = [PW₁₂O₄₀]³⁻) single-atom catalysts for CO oxidation.

Calculations of NO reduction with CO over a Cu1/PMA single-atom catalyst: a study of surface oxygen species, active sites, and the reaction mechanism

Density functional theory calculations have been employed to probe the reaction mechanism of NO reduction with CO over a Cu₁/PMA (PMA is the phosphomolybdate, Cs₃PMo₁₂O₄₀) single-atom catalyst.

Microfluidic synthesis and on-chip enrichment application of two-dimensional hollow sandwich-like mesoporous silica nanosheet with water ripple-like surface

The merits of microfluidics bring new opportunities for engineering of nanomaterials with well controlled chemical, physical and biological properties for a variety of applications. Herein, using a two-run spiral-shaped microfluidic device, we first develop a facile and straightforward flow synthesis strategy to create two-dimensional mesoporous silica nanosheet (MSN). Such MSN exhibits typical hollow sandwich-like bilayer and unique water ripple-like wrinkle surface, which greatly increase the particulate accessibility for mass transfer. The enhanced on-chip enrichment performance of MSN is further confirmed toward different substrates (dye, protein, drug). These findings not only shed new light on microfluidics-enabled chemicals engineering, but also open up numerous new possibilities by microfluidics in adsorption, separation, and biomedical fields.

Synthesis of Single Atom Based Heterogeneous Platinum Catalysts: High Selectivity and Activity for Hydrosilylation Reactions

Catalytic hydrosilylation represents a straightforward and atom-efficient methodology for the creation of C-Si bonds. In general, the application of homogeneous platinum complexes prevails in industry and academia. Herein, we describe the first heterogeneous single atom catalysts (SACs), which are conveniently prepared by decorating alumina nanorods with platinum atoms. The resulting stable material efficiently catalyzes hydrosilylation of industrially relevant olefins with high TON (≈10⁵). A variety of substrates is selectively hydrosilylated including compounds with sensitive reducible and other functional groups (N, B, F, Cl). The single atom based catalyst shows significantly higher activity compared to related Pt nanoparticles.

Remarkable effect of alkalis on the chemoselective hydrogenation of functionalized nitroarenes over high-loading Pt/FeO x catalysts

Alkali metals transform non-selective high-loading Pt/FeO_x to a highly chemoselective catalyst by helping to form isolated and electropositive Pt centers.

The chemoselective hydrogenation of substituted nitroarenes to form the corresponding functionalized anilines is an important type of reaction in fine chemistry, and the chemoselectivity is critically dependent on the rational design of the catalysts. This reaction has rarely been accomplished over high-loading Pt catalysts due to the formation of Pt crystals. Here, for the first time, we report that alkali metals (Li⁺, Na⁺, K⁺, etc.) can transform the non-selective high loading Pt/FeO_x catalyst to a highly chemoselective one. The best result was obtained over a 5% Na–2.16% Pt/FeO_x catalyst, which enhanced the chemoselectivity from 66.4% to 97.4% while the activity remained almost unchanged for the probe reaction of 3-nitrostyrene hydrogenation to 3-aminostyrene. Using aberration-corrected HAADF-STEM, in situ XAS, ⁵⁷ and Fe Mössbauer and DRIFT spectroscopy, the active site of a Pt–O–Na–O–Fe-like species was proposed, which ensures that the Pt centers are isolated and positively charged for the preferential adsorption of the –NO₂ group.