Tuning Proton Conduction by Staggered Arrays of Polar Preyssler-Type Oxoclusters.

Polyoxometalates (POMs), anionic nanosized oxoclusters that can be considered as fragments of metal oxides, have been extensively studied for their diverse composition and structure, showing promise in various fields such as catalysis and electronics. Proton conduction, relevant to catalysis and electronics, has attracted interest in materials chemistry, and POM anions are advantageous in terms of their proton carrier density and mobility. Recently, polar POMs have attracted attention for their unique ferroelectric behaviors, yet they have been little studied with regard to proton conduction, as their polarity has generally been believed to have a negative impact. Here, we propose that polar POMs can be used to align polar proton carriers, such as H2O and polymers, to construct efficient proton-conducting pathways. In this study, we present ionic crystals composed of polar Preyssler-type POMs ([Xn+(H2O)P5W30O110](15-n)-, Xn+ = Ca2+, Eu3+) and K+ exhibiting ultrahigh proton conductivity surpassing 10-2 S cm-1, which is required for practical applications. In contrast, ionic crystals with nonpolar Preyssler-type POMs show an order of magnitude lower proton conductivity. Structural and spectroscopic studies combined with theoretical calculations reveal that proton carriers align with the aid of staggered arrays of polar POMs, forming a hydrogen-bonding network favorable for proton conduction. This study integrates molecular chemistry by the design of POMs and solid-state chemistry by exploring long-range proton conduction mechanisms, offering novel insights for future materials design.

Insight into the Mechanism of CO2 Chemical Fixation into Epoxides by Windmill-Shaped Polyoxovanadate and n-Bu4NX (X = Br, I).

Carbon dioxide (CO2) coupled with epoxide to generate cyclic carbonate stands out in carbon neutrality due to its 100% atom utilization. In this work, the mechanism of CO2 cycloaddition with propylene oxide (PO) cocatalyzed by windmill-shaped polyoxovanadate, [(C2N2H8)4(CH3O)4VIV4VV4O16]·4CH3OH (V8-1), and n-Bu4NX (X = Br, I) was thoroughly investigated using density functional theory (DFT) calculations. The ring-opening, CO2-insertion, and ring-closing steps of the process were extensively studied. Our work emphasizes the synergistic effect between V8-1 and n-Bu4NX (X = Br, I). Through the analysis of an independent gradient model based on Hirshfeld partition (IGMH), it was found that the attack of n-Bu4NX (X = Br, I) on Cß of PO triggers a distinct attractive interaction between the active fragment and the surrounding framework, serving as the primary driving force for the ring opening of PO. Furthermore, the effect of different cocatalysts was explored, with n-Bu4NI being more favorable than n-Bu4NBr. Moreover, the role of V8-1 in the CO2 cycloaddition reaction was clarified as not only acting as Lewis acid active sites but also serving as "electron sponges". This work is expected to advance the development of novel catalysts for organic carbonate formation.

Construction of coherent interface between Cu2O and CeO2via electrochemical reconstruction for efficient carbon dioxide reduction to methane.

Developing an efficient electrocatalyst that enables the efficient electrochemical conversion from CO2 to CH4 across a wide potential range remains a formidable challenge. Herein, we introduce a precatalyst strategy that realizes the in situ electrochemical reconstruction of ultrafine Cu2O nanodomains, intricately coupled on the CeO2 surface (Cu2O/CeO2), originating from the heterointerface comprised of ultrafine CuO nanodomains on the CeO2 surface (CuO/CeO2). When served as the electrocatalyst for the electrochemical CO2 reduction reaction, Cu2O/CeO2 delivers a selectivity higher than 49 % towards CH4 over a broad potential range from -1.2 V to -1.7 V vs. RHE, maintaining negligible activity decay for 20 h. Notably, the highest selectivity for CH4 reaches an impressive 70 % at -1.5 V vs. RHE. Through the combination of comprehensive analysis including synchrotron X-ray absorption spectroscopy, spherical aberration-corrected high-angle annular dark field scanning transmission electron microscope as well as the density functional theoretical calculation, the efficient production of CH4 is attributed to the coherent interface between Cu2O and CeO2, which could converted from the original CuO and CeO2 interface, ensuring abundant active sites and enhanced intrinsic activity and selectivity towards CH4.

A single transition metal atom anchored on Nb2C as an electrocatalyst for the nitrogen reduction reaction.

Nitrogen (N2) reduction to produce ammonia (NH3) under milder conditions is attractive as NH3 has been widely used in various fields. The electrocatalytic nitrogen reduction reaction (NRR) is considered to be a more moderate and green method for ammonia synthesis. Herein, using density functional theory (DFT) computations, we investigated the potential application of single-atom catalysts (SACs) toward the NRR, in which transition metal (TM, TM = Ti, V, Mn, Fe, Co, Y, Zr, Mo) atoms are supported on Nb2C (TM-Nb2C). Through our screening, Fe-Nb2C is highlighted from 8 candidate systems as the superior SAC for the NRR with a low limiting potential of -0.47 V. Meanwhile, a volcano plot between UL (NRR) and the ICOHP values of the N-H bond in *NH2 is established to determine the optimal ICOHP values that can be used as a simple descriptor of the NRR performance of Fe-Nb2C.

N-H Bond Activation Catalyzed by an Anderson-Type Polyoxometalate-Based Compound: Key Role of Transition-Metal Heteroatom.

Polyoxometalates (POMs) have a broad array of applied platforms with well-characterized catalysis to achieve N-H bond activation. Herein, the mechanism of the Anderson-type POM-based catalyst [FeIIIMoVI6O18{(OCH2)3CNH2}2]3- ([TrisFeIIIMoVI6O18]3-, Tris = {(OCH2)3CNH2}2) for the N-H bond activation of hydrazine (PhHNNHPh) was investigated by density functional theory calculations. The results reveal that [TrisFeIIIMoVI6O18]3- as the active species is responsible for the continuous abstraction of two electrons and two protons of PhHNNHPh via a proton-coupled electron transfer pathway, resulting in the activation of two N-H bonds in PhHNNHPh and thus the product PhNNPh. H2O2 acts as an oxidant to regulate catalyst regeneration. Based on the proposed catalytic mechanism, the key role of the heteroatom FeIII in [TrisFeIIIMoVI6O18]3- was disclosed. The d-orbital of FeIII in [TrisFeIIIMoVI6O18]3- acts as an electron receptor to promote the electron transfer (ET) in the rate-determining step (RDS) of the catalytic cycle. The substitution of the heteroatom FeIII of [TrisFeIIIMoVI6O18]3- with CoIII, RuIII, or MnIII is expected to improve the catalytic activity for several reasons: (i) the unoccupied molecular orbitals of POM-based compounds containing CoIII or RuIII are low, which is beneficial for the ET of RDS; (ii) For N-H bond activation catalyzed by the MnIII-containing POM-based compound, the transition state of RDS is stable because the d-orbital of its active site is half-filled, which results in a low free-energy barrier.

Constructing Oxygen Vacancies via Engineering Heterostructured Fe3 C/Fe3 O4 Catalysts for Electrochemical Ammonia Synthesis.

Electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions provides an intriguing pathway to convert N2 into NH3 . However, significant kinetic barriers of the NRR at low temperatures in desirable aqueous electrolytes remain a grand challenge due to the inert N≡N bond of the N2 molecule. Herein, we propose a unique strategy for in situ oxygen vacancy construction to address the significant trade-off between N2 adsorption and NH3 desorption by building a hollow shell structured Fe3 C/Fe3 O4 heterojunction coated with carbon frameworks (Fe3 C/Fe3 O4 @C). In the heterostructure, the Fe3 C triggers the oxygen vacancies of the Fe3 O4 component, which are likely active sites for the NRR. The design could optimize the adsorption strength of the N2 and Nx Hy intermediates, thus boosting the catalytic activity for the NRR. This work highlights the significance of the interaction between defect and interface engineering for regulating electrocatalytic properties of heterostructured catalysts for the challenging NRR. It could motivate an in-depth exploration to advance N2 reduction to ammonia.

Regulating Efficient and Selective Single-atom Catalysts for Electrocatalytic CO2 Reduction.

Anchoring transition metal (TM) atoms on suitable substrates to form single-atom catalysts (SACs) is a novel approach to constructing electrocatalysts. Graphdiyne with sp-sp2 hybridized carbon atoms and uniformly distributed pores have been considered as a potential carbon material for supporting metal atoms in a variety of catalytic processes. Herein, density functional theory (DFT) calculations were performed to study the single TM atom anchoring on graphdiyne (TM1 -GDY, TM=Sc, Ti, V, Cr, Mn, Co and Cu) as the catalysts for CO2 reduction. After anchoring metal atoms on GDY, the catalytic activity of TM1 -GDY (TM=Mn, Co and Cu) for CO2 reduction reaction (CO2 RR) are significantly improved comparing with the pristine GDY. Among the studied TM1 -GDY, Cu1 -GDY shows excellent electrocatalytic activity for CO2 reduction for which the product is HCOOH and the limiting potential (UL ) is -0.16 V. Mn1 -GDY and Co1 -GDY exhibit superior catalytic selectivity for CO2 reduction to CH4 with UL of -0.62 and -0.34 V, respectively. The hydrogen evolution reaction (HER) by TM1 -GDY (TM=Mn, Co and Cu) occurs on carbon atoms, while the active sites of CO2 RR are the transition metal atoms . The present work is expected to provide a solid theoretical basis for CO2 conversion into valuable hydrocarbons.

Defective Mo2C as a promising electrocatalyst for the nitrogen reduction reaction.

The electrocatalytic nitrogen reduction reaction under ambient conditions is considered as a promising alternative to the Haber-Bosch process for NH3 production. However, developing low-cost and high-efficiency electrocatalysts for N2 reduction remains a challenge. Herein, we propose VC-Mo2C with C vacancies as a novel nitrogen reduction reaction (NRR) electrocatalyst based on density functional theory (DFT) calculations. The computational results show that N2 in the gas phase can be fully activated on the surface of VC-Mo2C and can be efficiently reduced to ammonia via a dissociative-associative path with a low limiting potential (-0.43 V). The presence of vacancies enhances the catalytic performance and the collaboration between Mo3 around the vacancies and the remaining substrate d-Mo2C facilitates the overall catalytic reaction. VC-Mo2C also well suppresses the hydrogen evolution reaction (HER) with high selectivity. The present work opens up a new way to promote the sustainable production of NH3.

Vanadium-substituted polycationic Al-oxo cluster in a porous ionic crystal exhibiting Lewis acidity.

Herein we present the first example of a vanadium-substituted δ-Keggin Al13-based (Al28V4) porous Ionic crystal (PIC) (Al28V4-PW9V3). Structural and theoretical studies suggested that Al28V4 may exhibit Lewis acidity, which was confirmed by acetalization reaction with Al28V4-PW9V3 as a solid catalyst and investigations with pyridine or 2,6-lutidine as basic probe molecules. A comparative study with the δ-Keggin Al13 monomer based PIC indicated that the vanadium-substitution gave rise to the Lewis acidity.

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.

Incorporating highly basic polyoxometalate anions comprising Nb or Ta into nanoscale reaction fields of porous ionic crystals.

Polyoxometalates (POMs) are oxide cluster anions composed of high-valence early transition metals and are widely used as catalysts. Yet base catalysis of POMs remains an ongoing challenge; group V (V, Nb, and Ta) elements form more negatively charged POMs than group VI (Mo and W) elements, and in particular, polyoxoniobates and polyoxotantalates are known to show strong basicity in solution due to the highly negative surface oxygen atoms. Herein, we report for the first time porous ionic crystals (PICs) comprising Nb or Ta. The PICs are composed of Dawson-type Nb/W or Ta/W mixed-addenda POMs with oxo-centered trinuclear CrIII carboxylates and potassium ions as counter cations to control the crystal structure. Among the PICs, those with Nb or Ta tri-substituted POMs exhibit the highest yield (78-82%) and selectivity (99%) towards the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate (353 K, 6 h), which is a typical base-catalyzed reaction, as reusable solid catalysts, and they can also catalyze the reaction of other active methylene compounds. A detailed investigation into the crystal structures together with DFT calculations and in situ IR spectroscopy with methanol as a basic probe molecule shows that the exposure of [Nb3O13] or [Ta3O13] units with highly negative surface oxygen atoms to the pore surface of PICs is crucial to the catalytic performance. These findings based on the composition-structure-function relationships show that Nb- and Ta-containing PICs can serve as platforms for rational designing of heterogeneous base catalysts.

An efficient chromium(III)-catalyzed aerobic oxidation of methylarenes in water for the green preparation of corresponding acids.

A highly efficient method to oxidize methylarenes to their corresponding acids with a reusable Cr catalyst was developed. The reaction can be carried out in water with 1 atm oxygen and K2S2O8 as cooxidants, proceeds under green and mild conditions, and is suitable for the oxidation of both electron-deficient and electron-rich methylarenes, including heteroaryl methylarenes, even at the gram level. The excellent result, together with its simplicity of operation and the ability to continuously reuse the catalyst, makes this new methodology environmentally benign and cost-effective. The generality of this methodology gives it the potential for use on an industrial scale. Differing from the accepted oxidation mechanism of toluene, GC-MS studies and DFT calculations have revealed that the key benzyl alcohol intermediate is formed under the synergetic effect of the chromium and molybdenum in the Cr catalyst, which can be further oxidized to afford benzaldehyde and finally benzoic acid.

Isomeric effects on the acidity of Al13 Keggin clusters in porous ionic crystals.

We demonstrate a facile synthesis method of a porous ionic crystal (PIC) composed of the little-known δ-Keggin-type cationic polyoxoaluminum cluster ([δ-Al13O4(OH)24(H2O)12]7+, δ-Al13) with an oppositely-charged polyoxometalate, which enabled us to investigate the activity as a solid acid. The δ-Al13 based PIC exhibited much higher activity in pinacol rearrangement, a typical acid-catalyzed reaction, than the PIC based on the well-known and thermodynamically stable rotational isomer (ε-Al13). This work is a rare example of rotational isomers of polyoxoaluminum clusters exhibiting remarkably different catalytic activities.

Transition-Metal-Modified Vanadoborate Clusters as Stable and Efficient Photocatalysts for CO2 Reduction.

Photocatalytic carbon dioxide reduction (CO2RR) is considered to be a promising sustainable and clean approach to solve environmental issues. Polyoxometalates (POMs), with advantages in fast, reversible, and stepwise multiple-electron transfer without changing their structures, have been promising catalysts in various redox reactions. However, their performance is often restricted by poor thermal or chemical stability. In this work, two transition-metal-modified vanadoborate clusters, [Co(en)2]6[V12B18O54(OH)6]·17H2O (V12B18-Co) and [Ni(en)2]6[V12B18O54(OH)6]·17H2O (V12B18-Ni), are reported for photocatalytic CO2 reduction. V12B18-Co and V12B18-Ni can preserve their structures to 200 and 250 °C, respectively, and remain stable in polar organic solvents and a wide range of pH solutions. Under visible-light irradiation, CO2 can be converted into syngas and HCOO- with V12B18-Co or V12B18-Ni as catalysts. The total amount of gaseous products and liquid products for V12B18-Co is up to 9.5 and 0.168 mmol g-1 h-1. Comparing with V12B18-Co, the yield of CO for V12B18-Ni declines by 1.8-fold, while that of HCOO- increases by 35%. The AQY of V12B18-Co and V12B18-Ni is 1.1% and 0.93%, respectively. These values are higher than most of the reported POM materials under similar conditions. The density functional theory (DFT) calculations illuminate the active site of CO2RR and the reduction mechanism. This work provides new insights into the design of stable, high-performance, and low-cost photocatalysts for CO2 reduction.

Carbon nitride derivatives as photocatalysts for the CO2 reduction reaction: computational study.

Photocatalytic reduction of CO2 to hydrocarbons is considered to be a promising strategy to solve the energy crisis and environmental problems. Herein, the electronic and optical properties, and catalytic performance of g-C3N4 derivatives [C6N7(C6H4)1.5]n (systems 1 and 2), and [C6N7(C12H8)1.5]n (system 3) were studied by density functional theory (DFT) computations. Compared to g-C3N4 the band gaps of systems 1-3 are smaller, and the absorption intensities of the three derivatives in the visible light region increase, indicating that these derivatives can produce more electrons under visible light irradiation and enhance the photocatalytic performance. The computational results show that the main products of CO2 reduction catalyzed by system 1 are HCOOH and CH3OH. The rate-determining step is CO2→ COOH* with a ΔG of 1.22 eV. Therefore, system 1 is predicted to be a promising catalyst for the CO2 reduction reaction.

The interesting luminescence behavior and rare nonlinear optical properties of the {Ag55Mo6} nanocluster.

The remarkably reversible thermochromic luminescence behavior and the rare nonlinear optical (NLO) properties of the [Ag55(MoO4)6(C[triple bond, length as m-dash]C t Bu)24(CH3COO)18(CH3COO)]·2H2O ({Ag55Mo6} for short) nanocluster reported were investigated experimentally. The important contributions of Ag+, C[triple bond, length as m-dash]C- ions and MoO4 2- groups to the NLO properties were proved by further density functional theory (DFT) calculations.

Density functional theory study on a nitrogen-rich carbon nitride material C3N5 as photocatalyst for CO2 reduction to C1 and C2 products.

A new-type nitrogen-rich carbon nitride material C3N5 has been synthesized recently, in which the C:N ratio increases from 3:4 in g-C3N4 to 3:5 due to the introduction of azo linkage (NN) connecting segments in two C6N7 units. Herein, C3N5 as a photocatalyst for CO2 reduction was investigated by density functional theory methods. The electronic and optical properties indicate that C3N5 has a longer visible-light region with 2.0 eV of band gap in comparison with g-C3N4. The spatial distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) show that the π network of C3N5 is extended by introducing -NN- linkage, which results in much higher photocatalytic efficiency than g-C3N4. The Gibbs free energies for possible CO2 reaction paths on C3N5 were computed. The results show that CO2 can be reduced to CH4 with a low limiting potential of -0.54 V and to CH3CH2OH with a low limiting potential of -0.61 V, which all driven by solar energy. The present work is expected to provide useful guide for new-type nitrogen-rich C3N5 as promising photocatalyst for CO2 reduction reaction (CO2RR).

Configuration effect in polyoxometalate-based dyes on the performance of DSSCs: an insight from a theoretical perspective.

The electronic properties of dyes can be readily tuned by modifying the structure. Herein, the polyoxometalate (POM)-based dyes derived from dye XW11 with new patterns, donor-acceptor-π linker-acceptor (D-A-π-A) structure (dye 1), and D-π-A-π-A structure (dye 2) were designed by inserting a POM moiety besides the extensively exploited D-π-A structure (dye 3). Based on density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, the configuration effect on the designed dyes was investigated. The results indicate that dye 3 possesses the largest short-circuit photocurrent density JSC due to the red-shifted absorption spectra, superior intramolecular charge transfer (ICT) parameters and the largest electron injection efficiency. At the same time, dye 1 with a D-A-π-A structure not only benefits the conduction band energy shift, but also retards the charge recombination and dye aggregation effect, which is beneficial for open-circuit photovoltage VOC. Moreover, the dynamics analysis of interfacial electron transfer shows that the electrons in dye 1 are almost completely injected after 14 fs, while it takes a long time for dyes 2 and 3. The present work is expected to establish a structure-property relationship for future dye design.

Hollow Porous MnFe2 O4 Sphere Grown on Elm-Money-Derived Biochar towards Energy-Saving Full Water Electrolysis.

The development of inexpensive and efficient bifunctional electrocatalysts is significant for widespread practical applications of overall water splitting technology. Herein, a one-pot solvothermal method is used to prepare hollow porous MnFe2 O4 spheres, which are grown on natural-abundant elm-money-derived biochar material to construct MnFe2 O4 /BC composite. When the overpotential is 156 mV for both the oxygen evolution reaction and the hydrogen evolution reaction, the current density reaches up to 10 mA cm-2 , and its duration is 10 h. At 1.51 V, the overall water decomposition current density of 10 mA cm-2 can be obtained in 1 m KOH. This work proves that elm-money-derived biochar is a valid substrate for growing hollow porous spheres. MnFe2 O4 /BC give a promising general strategy for preparing the effective and stable bifunctional catalysis that can be expand to multiple transition metal oxide.

Ruthenium-based catalysts for water oxidation: the key role of carboxyl groups as proton acceptors.

In this work, the mechanism of water oxidation catalyzed by an Ru-based complex [Ru(L)]+ (L = 5,5-chelated 2-carboxyl-phen, 2,2';6',2''-terpyridine) was studied by density functional theory (DFT) calculations. In [Ru(L)]+, a carboxyl group is included in the second coordination sphere and plays an important role in the catalytic process. In the oxidation activation stage of water oxidation catalysis, the carboxyl group is proposed as a promising proton acceptor to promote proton transfer, which results in active RuV[double bond, length as m-dash]O species. Then, O-O bond formation can proceed via water nucleophilic attack (WNA) or oxo-oxo coupling mechanisms. In the O2 release stage, similar to the oxidation activation process, the carboxyl group promotes proton transfer as a promising proton acceptor. In the present work, the favorable mechanism is WNA that involves proton transfer to the carboxyl group. It is expected that this work will provide meaningful information for synthesizing excellent water oxidation catalysts (WOCs).