Construction of a redox pathway through a polyoxometalate and covalent organic framework for H2O2 photosynthesis.

A novel type of electron donor-acceptor system was built from a nitrogen-rich covalent organic framework (PC) and a polyoxometalate (BW12), fabricating a composite material (BW12@PC-250), which shows significantly improved photocatalytic H2O2 yield (56.4 µM h-1) under full spectrum illumination in pure water, being about 30 times higher than that of PC. This is due to the opening of the electron and proton transport pathway between PC and BW12, which paves a new way for POMs to modulate the photocatalytic reactions of COFs.

Tunable covalent benzo-heterocyclic rings constructed using two-dimensional conjugated polymers for visible-light-driven water splitting.

Developing highly efficient, stable, and cost-effective two-dimensional (2D) conjugated polymers (CPs) for overall water splitting (OWS) is critical for producing clean and renewable hydrogen energy, yet it remains a great challenge. Here, we designed eight 2D CPs through the topological assembly of diacetylene and benzene-derived molecular linkers that can offer active sites for hydrogen and oxygen evolution reactions, and explored their structural, electronic, optical, and photocatalytic OWS properties by performing first-principles computations. It is shown that incorporating benzo-heterocyclic rings into CPs can significantly modulate the electronic structures of CPs and broaden the spectral absorption, suitable for visible-light-driven OWS. Remarkably, through a range of screening criteria, including stability, electronic band structures, band edge alignments, and photocatalytic activity, we found that CP-4 based on diacetylene and benzotrifuran can spontaneously trigger the OWS in a neutral environment under its own light-induced bias, eliminating the need for sacrificial agents or cocatalysts. Specifically, the HER active site is primarily located at diacetylene moieties, while the OER active site is mainly concentrated on the benzo-heterocyclic rings. Moreover, the ideal STH efficiency for OWS on CP-4 was estimated to be 13.87%, highlighting its potential as a prospective photocatalyst for large-scale industrial OWS. Our findings open a door to the rational design of novel polymer photocatalysts for OWS.

Preparation and isolation of mono-Nb substituted Keggin-type phosphomolybdic acid and its application as an oxidation catalyst for isobutylaldehyde and Wacker-type oxidation.

The potassium and proton mixed salt of mono-Nb substituted Keggin-type phosphomolybdate, KH3[PMo11NbO40], was isolated in a pure form by reacting Keggin-type phosphomolybdic acid (H3[PMo12O40]) and potassium hexaniobate (K8Nb6O19) in water, followed by freeze-drying. The all protonic form, H4[PMo11NbO40], was isolated via proton exchange with H-resin and subsequent freeze-drying. The most crucial factor to isolate KH3[PMo11NbO40] and H4[PMo11NbO40] in pure forms is the evaporation of water using the freeze-drying method. Using a similar procedure, the potassium salt of the di-Nb substituted compound K5[PMo10Nb2O40] was isolated. H4[PMo11NbO40] exhibited high catalytic activity for oxidizing isobutylaldehyde to methacrolein and moderate catalytic activity for the Wacker-type oxidation of allyl phenyl ether when combined with Pd(OAc)2.

Designed Tetranuclear Iron-oxo Clusters with Redox Activity for High-Performance Lithium Storage.

Polyoxometalates (POMs) are considered as promising catalysts with unique redox activity at the molecular level for energy storage. However, eco-friendly iron-oxo clusters with special metal coordination structures have rarely been reported for Li-ion storage. Herein, three novel redox-active tetranuclear iron-oxo clusters have been synthesized using the solvothermal method with different ratios of Fe3+ and SO4 2- . Further, they can serve as anode materials for Li-ion batteries. Among them, cluster H6 [Fe4 O2 (H2 O)2 (SO4 )7 ]⋅H2 O, the stable structure extended by SO4 2- with a unique 1D pore, displays a specific discharge capacity of 1784 mAh g-1 at 0.2 C and good cycle performance (at 0.2 C and 4 C). This is the first instance of inorganic iron-oxo clusters being used for Li-ion storage. Our findings present a new molecular model system with a well-defined structure and offer new design concepts for the practical application of studying the multi-electron redox activity of iron-oxo clusters.

Two-Dimensional Covalent Heptazine-Based Framework Enables Highly Photocatalytic Performance for Overall Water Splitting.

Screening high-efficiency 2D conjugated polymers toward visible-light-driven overall water splitting (OWS) is one of the most promising but challenging research directions to realize solar-to-hydrogen (STH) energy conversion and storage. "Mystery molecule" heptazine is an intriguing hydrogen evolution reaction (HER) building block. By covalently linking with the electron-rich alkynyl and phenyl oxygen evolution reaction (OER) active units, 10 experimentally feasible 2D covalent heptazine-based frameworks (CHFs) are constructed and screened four promising visible-light-driven OWS photocatalysts, which are linked by p-phenyl (CHF-4), p-phenylenediynyl (CHF-7), m-phenylenediynyl (CHF-8), and phenyltriynyl (CHF-9), respectively. Their HER and OER active sites achieve completely spatially separated, where HER active sites focus on heptazine units and OER active sites located on alkynyl or phenyl units. Their lower overpotentials allow them to spontaneously trigger the surface OWS reaction under their own light-induced bias without using any sacrificial agents and cocatalysts. Among them, CHF-7 shows the best photocatalytic performance with an ideal STH energy conversion efficiency estimated at 12.04%, indicating that it is a promising photocatalyst for industrial OWS. This work not only provides an innovative idea for the exploration of novel polymer photocatalysts for OWS but also supplies a direction for the development of heptazine derivatives.

Copper-Bridged Tetrakis(4-ethynylphenyl)ethene Aggregates with Photo-Regulated 1 O2 and O2 .- Generation for Selective Photocatalytic Aerobic Oxidation.

Active species regulation is a key scientific issue that essentially determines the selectivity and activity of a photocatalyst. Herein, CuI -bridged tetrakis(4-ethynylphenyl)ethene aggregates (T4 EPE-Cu) with photo-regulated 1 O2 and O2 .- generation were demonstrated for selective photocatalytic aerobic oxidation. In this system, transient photovoltage combined with the density functional theory calculations confirmed that Cu-alkynyl was the main oxygen activation site. The adsorbed O2 tends to produce O2 .- because of the potential well effect of Cu-alkynyl under high-energy light excitation. But under low-energy light, O2 tends to produce 1 O2 via resonance energy transfer with Cu-alkynyl. For α-terpinene oxidation, the ratios of 1 O2 products to O2 .- products can be controlled from 1.3 (380 nm) to 10.7 (600 nm). Furthermore, T4 EPE-Cu exhibited ultrahigh photocatalytic performance for Glaser coupling and benzylamine oxidation, with a conversion and selectivity of over 99 %.

Molecular Iron Oxide Clusters Boost the Oxygen Reduction Reaction of Platinum Electrocatalysts at Near-Neutral pH.

The oxygen reduction reaction (ORR) is a key energy conversion process, which is critical for the efficient operation of fuel cells and metal-air batteries. Here, we report the significant enhancement of the ORR-performance of commercial platinum-on-carbon electrocatalysts when operated in aqueous electrolyte solutions (pH 5.6), containing the polyoxoanion [Fe28 (µ3 -O)8 (L-(-)-tart)16 (CH3 COO)24 ]20- . Mechanistic studies provide initial insights into the performance-improving role of the iron oxide cluster during ORR. Technological deployment of the system is demonstrated by incorporation into a direct formate microfluidic fuel cell (DFMFC), where major performance increases are observed when compared with reference electrolytes. The study provides the first examples of iron oxide clusters in electrochemical energy conversion and storage.

Enhanced Cuprophilic Interactions in Crystalline Catalysts Facilitate the Highly Selective Electroreduction of CO2 to CH4.

Cu(I)-based catalysts have proven to play an important role in the formation of specific hydrocarbon products from electrochemical carbon dioxide reduction reaction (CO2RR). However, it is difficult to understand the effect of intrinsic cuprophilic interactions inside the Cu(I) catalysts on the electrocatalytic mechanism and performance. Herein, two stable copper(I)-based coordination polymer (NNU-32 and NNU-33(S)) catalysts are synthesized and integrated into a CO2 flow cell electrolyzer, which exhibited very high selectivity for electrocatalytic CO2-to-CH4 conversion due to clearly inherent intramolecular cuprophilic interactions. Substitution of hydroxyl radicals for sulfate radicals during the electrocatalytic process results in an in situ dynamic crystal structure transition from NNU-33(S) to NNU-33(H), which further strengthens the cuprophilic interactions inside the catalyst structure. Consequently, NNU-33(H) with enhanced cuprophilic interactions shows an outstanding product (CH4) selectivity of 82% at -0.9 V (vs reversible hydrogen electrode, j = 391 mA cm-2), which represents the best crystalline catalyst for electrocatalytic CO2-to-CH4 conversion to date. Moreover, the detailed DFT calculations also prove that the cuprophilic interactions can effectively facilitate the electroreduction of CO2 to CH4 by decreasing the Gibbs free energy change of potential determining step (*H2COOH → *OCH2). Significantly, this work first explored the effect of intrinsic cuprophilic interactions of Cu(I)-based catalysts on the electrocatalytic performance of CO2RR and provides an important case study for designing more stable and efficient crystalline catalysts to reduce CO2 to high-value carbon products.

Revival of Zeolite-Templated Nanocarbon Materials: Recent Advances in Energy Storage and Conversion.

Nanocarbon materials represent one of the hottest topics in physics, chemistry, and materials science. Preparation of nanocarbon materials by zeolite templates has been developing for more than 20 years. In recent years, novel structures and properties of zeolite-templated nanocarbons have been evolving and new applications are emerging in the realm of energy storage and conversion. Here, recent progress of zeolite-templated nanocarbons in advanced synthetic techniques, emerging properties, and novel applications is summarized: i) thanks to the diversity of zeolites, the structures of the corresponding nanocarbons are multitudinous; ii) by various synthetic techniques, novel properties of zeolite-templated nanocarbons can be achieved, such as hierarchical porosity, heteroatom doping, and nanoparticle loading capacity; iii) the applications of zeolite-templated nanocarbons are also evolving from traditional gas/vapor adsorption to advanced energy storage techniques including Li-ion batteries, Li-S batteries, fuel cells, metal-O2 batteries, etc. Finally, a perspective is provided to forecast the future development of zeolite-templated nanocarbon materials.

Semiconductor/Covalent-Organic-Framework Z-Scheme Heterojunctions for Artificial Photosynthesis.

A strategy to covalently connect crystalline covalent organic frameworks (COFs) with semiconductors to create stable organic-inorganic Z-scheme heterojunctions for artificial photosynthesis is presented. A series of COF-semiconductor Z-scheme photocatalysts combining water-oxidation semiconductors (TiO2 , Bi2 WO6 , and α-Fe2 O3 ) with CO2 reduction COFs (COF-316/318) was synthesized and exhibited high photocatalytic CO2 -to-CO conversion efficiencies (up to 69.67 µmol g-1 h-1 ), with H2 O as the electron donor in the gas-solid CO2 reduction, without additional photosensitizers and sacrificial agents. This is the first report of covalently bonded COF/inorganic-semiconductor systems utilizing the Z-scheme applied for artificial photosynthesis. Experiments and calculations confirmed efficient semiconductor-to-COF electron transfer by covalent coupling, resulting in electron accumulation in the cyano/pyridine moieties of the COF for CO2 reduction and holes in the semiconductor for H2 O oxidation, thus mimicking natural photosynthesis.

Pt-O bond as an active site superior to Pt0 in hydrogen evolution reaction.

The oxidized platinum (Pt) can exhibit better electrocatalytic activity than metallic Pt0 in the hydrogen evolution reaction (HER), which has aroused great interest in exploring the role of oxygen in Pt-based catalysts. Herein, we select two structurally well-defined polyoxometalates Na5[H3Pt(IV)W6O24] (PtW6O24) and Na3K5[Pt(II)2(W5O18)2] (Pt2(W5O18)2) as the platinum oxide model to investigate the HER performance. Electrocatalytic experiments show the mass activities of PtW6O24/C and Pt2(W5O18)2/C are 20.175 A mg-1 and 10.976 A mg-1 at 77 mV, respectively, which are better than that of commercial 20% Pt/C (0.398 A mg-1). The in situ synchrotron radiation experiments and DFT calculations suggest that the elongated Pt-O bond acts as the active site during the HER process, which can accelerate the coupling of proton and electron and the rapid release of H2. This work complements the knowledge boundary of Pt-based electrocatalytic HER, and suggests another way to update the state-of-the-art electrocatalyst.

Polyoxometalate-based electron transfer modulation for efficient electrocatalytic carbon dioxide reduction.

The electrocatalytic carbon dioxide (CO2) reduction reaction (CO2RR) involves a variety of electron transfer pathways, resulting in poor reaction selectivity, limiting its use to meet future energy requirements. Polyoxometalates (POMs) can both store and release multiple electrons in the electrochemical process, and this is expected to be an ideal "electron switch" to match with catalytically active species, realize electron transfer modulation and promote the activity and selectivity of the electrocatalytic CO2RR. Herein, we report a series of new POM-based manganese-carbonyl (MnL) composite CO2 reduction electrocatalysts, whereby SiW12-MnL exhibits the most remarkable activity and selectivity for CO2RR to CO, resulting in an increase in the faradaic efficiency (FE) from 65% (MnL) to a record-value of 95% in aqueous electrolyte. A series of control electrochemical experiments, photoluminescence spectroscopy (PL), transient photovoltage (TPV) experiments, and density functional theory (DFT) calculations revealed that POMs act as electronic regulators to control the electron transfer process from POM to MnL units during the electrochemical reaction, enhancing the selectivity of the CO2RR to CO and depressing the competitive hydrogen evolution reaction (HER). This work demonstrates the significance of electron transfer modulation in the CO2RR and suggests a new idea for the design of efficient electrocatalysts towards CO2RR.

A Hydrolytically Stable Vanadium(IV) Metal-Organic Framework with Photocatalytic Bacteriostatic Activity for Autonomous Indoor Humidity Control.

Metal-organic frameworks (MOFs) with long-term stability and reversible high water uptake properties can be ideal candidates for water harvesting and indoor humidity control. Now, a mesoporous and highly stable MOF, BIT-66 is presented that has indoor humidity control capability and a photocatalytic bacteriostatic effect. BIT-66 (V3 (O)3 (H2 O)(BTB)2 ), possesses prominent moisture tunability in the range of 45-60 % RH and a water uptake and working capacity of 71 and 55 wt %, respectively, showing good recyclability and excellent performance in water adsorption-desorption cycles. Importantly, this MOF demonstrates a unique photocatalytic bacteriostatic behavior under visible light, which can effectively ameliorate the bacteria and/or mold breeding problem in water adsorbing materials.

Highly Efficient Photoreduction of Low-Concentration CO2 to Syngas by Using a Polyoxometalates/RuII Composite.

At present, the fixation of CO2 always requires it to be extracted from the atmosphere first, which leads to more energy consumption. Thus, direct photoreduction of low-concentration CO2 to useful chemicals (e.g., syngas) under sunlight is significant from an energy-saving and environmentally friendly perspective. Here, the design and fabrication of a [Ru(bpy)3 ]/[Co20 Mo16 P24 ] composite is demonstrated for visible-light-driven syngas production from diluted CO2 (3-20 %) gas with a high yield of approximately 1000 TONs (turnover number of syngas). This activity is an order of magnitude higher than the reported system with [Ru(bpy)3 ]2+ participation. With evidence from ultrafast transient absorption, GC-MS, 1 H NMR spectroscopy and in situ transient photovoltage tests, a clear and fundamental understanding of the highly efficient photoreduction of CO2 by the [Ru(bpy)3 ]/[Co20 Mo16 P24 ] composite is achieved. Making use of the structure and property designable polyoxometalates towards the photo-fixation of CO2 is a conceptually distinct and commercially interesting strategy for making useful chemicals and environmental protection.

Superfine CoNi alloy embedded in Al2O3 nanosheets for efficient tandem catalytic reduction of nitroaromatic compounds by ammonia borane.

Aromatic amino compounds are important and universally used chemical intermediates in a wide range of industrial fields. Thus, their production with high efficiency and selectivity under ambient conditions is expected and demanded in modern industry. Herein, a series of superfine CoNi alloy nanoparticles embedded in Al2O3 nanosheet (CoxNi1-x/Al2O3, where x represents the content of Co in the precursor) catalysts was fabricated from CoNiAl-LDH and used to catalyze the tandem dehydrogenation of ammonia borane (AB) and hydrogenation of nitroaromatics to the corresponding amines. Systematic experiments indicate that the composition, size, morphology and catalytic performance of the CoxNi1-x/Al2O3 catalysts can be easily controlled by changing the content of Ni in the CoNiAl-LDH precursor. Particularly, Co0.67Ni0.33/Al2O3 exhibited the best tandem catalytic performance among the six samples. This as-prepared catalyst not only showed a moderate turn-over-frequency value (TOF: 34.5 molH2 molCo0.67Ni0.33-1 min-1 at 298 K without base or additives) and relatively low activation energy (32.4 kJ mol-1) for the dehydrogenation of AB, but also superior catalytic activity (conversion yield reaching up to 100%) and selectivity (>99%) for the tandem reductive transformation of in excess of sixteen types of nitroaromatics to aromatic amines. Density functional theory (DFT) calculations suggest that the construction of the CoNi alloy optimized the electronic structure with respect to the pure component, promoting its activity for AB hydrolysis and nitroaromatics hydrogenation. Finally, the catalyst could be easily recycled using a magnet due to the magnetic properties of the Co0.67Ni0.33 alloy.

Tetravalent Metal Ion Guests in Polyoxopalladate Chemistry: Synthesis and Anticancer Activity of [MO8Pd12(PO4)8]12- (M = SnIV, PbIV).

The first two examples of polyoxopalladates(II) (POPs) containing tetravalent metal ion guests, [MO8Pd12(PO4)8]12- (M = SnIV, PbIV), have been prepared and structurally characterized in the solid state, solution, and gas phase. The interactions of the metal ion guests and the palladium-oxo shell were studied by theoretical calculations. The POPs were shown to possess anticancer activity by causing oxidative stress inducing caspase activation and consecutive apoptosis of leukemic cells.

Controlling the Activity and Stability of Electrochemical Interfaces Using Atom-by-Atom Metal Substitution of Redox Species.

Understanding the molecular-level properties of electrochemically active ions at operating electrode-electrolyte interfaces (EEI) is key to the rational development of high-performance nanostructured surfaces for applications in energy technology. Herein, an electrochemical cell coupled with ion soft landing is employed to examine the effect of "atom-by-atom" metal substitution on the activity and stability of well-defined redox-active anions, PMo xW12- xO403- ( x = 0, 1, 2, 3, 6, 9, or 12) at nanostructured ionic liquid EEI. A striking observation made by in situ electrochemical measurements and further supported by theoretical calculations is that the substitution of only one to three tungsten atoms by molybdenum atoms in the PW12O403- anions results in a substantial spike in their first reduction potential. Specifically, PMo3W9O403- showed the highest redox activity in both in situ electrochemical measurements and as part of a functional redox supercapacitor device, making it a "super-active redox anion" compared with all other PMo xW12- xO403- species. Electronic structure calculations showed that metal substitution in PMo xW12- xO403- causes the lowest unoccupied molecular orbital (LUMO) to protrude locally, making it the "active site" for reduction of the anion. Several critical factors contribute to the observed trend in redox activity including (i) multiple isomeric structures populated at room temperature, which affect the experimentally determined reduction potential; (ii) substantial decrease of the LUMO energy upon replacement of W atoms with more-electronegative Mo atoms; and (iii) structural relaxation of the reduced species produced after the first reduction step. Our results illustrate a path to achieving superior performance of technologically relevant EEIs in functional nanoscale devices through understanding of the molecular-level electronic properties of specific electroactive species with "atom-by-atom" precision.

Oxygen-Doped Nickel Iron Phosphide Nanocube Arrays Grown on Ni Foam for Oxygen Evolution Electrocatalysis.

A rationally designed oxygen evolution reaction (OER) catalyst with advanced structural and compositional superiority is highly desirable to optimize electrocatalytic performance. Prussian blue analogues (PBAs) with adjustable element compositions and accessible porous structures represent a promising precursor for the preparation of OER catalysts. Herein, oxygen-doped nickel iron phosphide nanocube arrays (Ni2 P/(NiFe)2 P(O) NAs) grown on Ni foam is rationally designed and fabricated from PBAs. The porous structure and the synergistic effect of Ni and Fe enable superior electrocatalytic performance and stability toward the OER in alkaline electrolytes. Density functional theory calculations reveal that Fe-incorporated Ni2 P can generate new active sites on the Fe atoms, and the energy barriers of the intermediates and products are decreased efficiently in the presence of surface doped oxygen, both processes are crucial factors for enhanced catalytic performances. In 1 m KOH, the Ni2 P/(NiFe)2 P(O) NAs afford current densities of 10 and 800 mA cm-2 at overpotentials of 150 and 530 mV, respectively, which outperform the commercial noble metal IrO2 . Ni2 P/(NiFe)2 P(O) NAs also have long-term stability over 100 h at a high current density. The present approach may provide a new avenue for the controlled assembly of nanoarrays for energy storage and conversion applications.

Electrocatalytic performance of ultrasmall Mo2C affected by different transition metal dopants in hydrogen evolution reaction.

Molybdenum carbides are considered as one type of privileged noble-metal-free electrocatalysts for hydrogen evolution reactions (HER) due to their d-band electron structure, which is similar to Pt. Especially, the electronic structure of such materials can be further adjusted by elemental doping to improve their electrocatalytic activity. Herein, we selected the Anderson-type polyoxometalates (POMs) (NH4)n[TMMo6O24H6]·5H2O (TM = Ni2+, Co2+, n = 4; TM = Fe3+, Cr3+, n = 3) as precursors to prepare new transition-metal-doped Mo2C materials. When these POMs were mixed with dicyandiamide (DCA) by solid grinding, and carbonized at a high temperature, a series of Ni-, Co-, Fe-, and Cr-doped Mo2C composite nanoparticles covered by few-layer graphitic carbon shells (abbr. TM-Mo2C@C) were obtained. All these nanoparticles possess a similar size, morphology, and TM/Mo component ratio, and thus it is feasible to systematically investigate the influence of different TM dopants on the electrocatalytic activity of Mo2C for HER. Both electrocatalytic experiments and DFT calculations reveal that TM dopants have a significant effect on the hydrogen binding energy (ΔGH*) and the catalytic activity of Mo2C. The sequence of HER electrocatalytic activity is as follows: Ni-Mo2C > Co-Mo2C > Fe-Mo2C > Cr-Mo2C. As a result, Ni-Mo2C@C possesses the best HER performance, which required an overpotential of 72 mV at a current density of 10 mA cm-2 and the Tafel slope is 65.8 mV dec-1. This work suggests a shortcut to reasonably investigate the effects of elemental doping on molybdenum carbides and explore new high-efficient and low-cost electrocatalysts for HER.

Self-Sorting of Heteroanions in the Assembly of Cross-Shaped Polyoxometalate Clusters.

Heteroanion (HA) moieties have a key role in templating of heteropolyoxometalate (HPA) architectures, but clusters templated by two different templates are rarely reported. Herein, we show how a cross-shaped HPA-based architecture can self-sort the HA templates by pairing two different guests into a divacant {XYW15O54} building block, with four of these building block units being linked together to complete the cross-shaped architecture. We exploited this observation to incorporate HA templates into well-defined positions within the clusters, leading to the isolation of a collection of mixed-HA templated cross-shaped polyanions [(XYW15O54)4(WO2)4]32-/36- (X = H-P, Y = Se, Te, As). The template positions have been unambiguously determined by single crystal X-ray diffraction, NMR spectroscopy, and high-resolution electrospray ionization mass spectrometry; these studies demonstrated that the mixed template containing HPA clusters are the preferred products which crystallize from the solution. Theoretical studies using DFT calculations suggest that the selective self-sorting originates from the coordination of the template in solution. The cross-shaped polyoxometalate clusters are redox-active, and the ability of molecules to accept electrons is slightly modulated by the HA incorporated as shown by differential pulse voltammetry experiments. These results indicate that the cross-shaped HPAs can be used to select templates from solution, and themselves have interesting geometries, which will be useful in developing functional molecular architectures based upon HPAs with well-defined structures and electronic properties.