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.

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.

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.

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.

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.

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.

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).

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.

Theoretical Insight into the Performance of MnII/III-Monosubstituted Heteropolytungstates as Water Oxidation Catalysts.

The performance of MnII/III-monosubstituted heteropolytungstates [MnIII(H2O)GeW11O39]5- ([GT-MnIII-OH2]5-, where GT = GeW11O39) and [MnII(H2O)GeW11O39]6- ([GT-MnII-OH2]6-) as water oxidation catalysts at pH 9 was explored using density functional theory calculations. The counterion effect was fully considered, in which five and six Na+ ions were included in the calculations for water oxidation catalyzed by [GT-MnIII-OH2]5- and [GT-MnII-OH2]6-, respectively. The process of water oxidation catalysis was divided into three elemental stages: (i) oxidative activation, (ii) O-O bond formation, and (iii) O2 evolution. In the oxidative activation stage, two electrons and two protons are removed from [Na5-GT-MnIII-OH2] and three electrons and two protons are removed from [Na6-GT-MnII-OH2]. Therefore, the MnIV-O• species [Na5-GT-MnIV-O•] is obtained. Two mechanisms, (i) water nucleophilic attack and (ii) oxo-oxo coupling, were demonstrated to be competitive in O-O bond formation triggered from [Na5-GT-MnIV-O•]. In the last stage, the O2 molecule could be readily evolved from the peroxo or dinuclear species and the catalyst returns to the ground state after the coordination of a water molecule(s).

A Calix[4]resorcinarene-Based [Co12] Coordination Cage for Highly Efficient Cycloaddition of CO2 to Epoxides.

The design and synthesis of polynuclear metal cluster-based coordination cages is of considerable interest due to their appealing structural characteristics and potential applications. Herein, we report a calix[4]resorcinarene-based [Co12] coordination cage, [Co12(TPC4R-I)2(1,3-BDC)10(µ3-OH)4(H2O)10(DMF)2]·7DMF·23H2O (1), assembled with 2 bowl-shaped calix[4]resorcinarenes (TPC4R-I), 10 angular 1,3-benzenedicarboxylates (1,3-BDC), and 12 Co(II) cations. Remarkably, it is shown to be a highly efficient recyclable heterogeneous catalyst for CO2 conversion due to its exposed Co(II) Lewis acid sites.

Theoretical screening of promising donor and π-linker groups for POM-based Zn-porphyrin dyes in dye-sensitized solar cells.

A series of Zn-porphyrin dyes with different donor and π-linker groups based on the dye SM315 were systematically investigated to screen highly efficient candidates based on density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Compared with SM315, dye 1 not only has stronger and broader absorption spectra and larger intramolecular charge transition (ICT) parameters, but also has a larger maximum short circuit density (JmaxSC) and an open-circuit photovoltage (VOC) due to the insertion of polyoxometalate (POM). In addition, dye 3 with an electron donor 3-ethynyl-7-(4-methoxyphenyl)-10-methyl-10,10a-dihydro-4aH-phenothiazine (D3) exhibits better photovoltaic performance among dyes 1-3 due to its broadest absorption spectra and largest JmaxSC, hence, D3 can act as a promising donor candidate for the construction of efficient dyes. Furthermore, the performance can be further improved by replacing the π-linker 2,1,3-benzothiadiazole (dye 3) with benzo[1,2-b:4,5-b']difuran-2,6-dione (dye 4) and pyrrolo[2,3-f]indole-2,6(1H,5H)-dione (dye 5). Compared with dye 3, dyes 4 and 5 possess broader absorption spectra and light harvesting efficiency curves, larger ICT parameters, JmaxSC and conduction band energy shift. In particular, dye 4 might obtain a larger short-circuit photocurrent density, and dye 5 might obtain a larger VOC.

Mechanistic insight into electroreduction of carbon dioxide on FeNx (x = 0-4) embedded graphene.

Searching for non-precious, active and stable catalysts for CO2 electrochemical reduction (CO2ER) has attracted extensive attention, while the high overpotential and low efficiency hinder their widespread commercial applications on a large scale. In this work, density functional theory (DFT) calculations were conducted on the CO2ER process over FeNx embedded graphene (i.e., FeNx-gra, x = 0-4). The results reveal that the Fe atom strongly interacts with the unsaturated N atoms of the substrate and acts as the active site. Due to the small limiting potential of -0.78 V and the activation barrier (1.56 eV), FeN3-gra exhibits the highest catalytic activity towards CO2 reduction. The products of CO2ER catalyzed by FeN3-gra are CH4 and CH3OH, in which CO* → HCO* is the potential-determining step. It is expected that FeN3-gra would be a promising catalyst for CO2ER.

Theoretical insights into the catalytic mechanism for the oxygen reduction reaction on M3(hexaiminotriphenylene)2 (M = Ni, Cu).

Recently two dimensional (2D) metal-organic frameworks have been successfully used as electrocatalysts, which exhibited a high catalytic activity. Herein, we investigated the catalytic mechanism of the oxygen reduction reaction (ORR) on M3(hexaiminotriphenylene)2 (M3(HITP)2, M = Ni, Cu) in an acidic medium using the density functional theory (DFT) method. The results indicate that the first electron transfer (ET) to nonadsorbed O2 is a process of long-range ET on the outer Helmholtz plane (i.e. the ET-OHP mechanism). On the surface of M3(HITP)2 (M = Ni, Cu), both the 2e reduction pathway and the 4e reduction pathway are feasible, while the 2e pathway to form H2O2 is more favorable. In the several competing reactions for the 4e reduction pathway on M3(HITP)2, the favorable path is OOH* → O* + H2O → OH* → H2O. Our study provides theoretical guidance for gaining deeper insights into the reaction mechanism of the ORR on M3(HITP)2 (M = Ni, Cu) catalysts.

The Effect of Dyes with Different π-Linkers on the Overall Performance of P-DSSCs: Lessons from Theory.

A series of Keggin-type polyoxometalate (POM)-based organic-inorganic complexes (systems 2-8) were designed with different π-linkers on the basis of the molybdate-pyrene hybrid (system 1). UV-vis spectra and charge transfer (CT) parameters of designed systems were systematically analyzed by density functional theory (DFT) and time-dependent DFT (TD-DFT). The results indicate that the absorption spectra are red-shifted and the absorption intensities are enhanced with increasing number of phenylacetylene π-oligomers and introducing benzodifuranone between benzene and ethyne. However, the π-linkers near POMs are "dissolved" in the total system and the excitation occurs in a local region with increasing π-linkers. Systems 3 and 6 possess the maximum CT distance and CT charge among systems 1-5 and systems 6-8, respectively, resulting from the balance point between effectiveness of structures and delocalization. The absorption spectra of systems 6-8 obviously red-shift in comparison to systems 1-5. The present study is a further step toward the optimal absorption and CT properties.

Thermally Activated Delayed Fluorescence in CuI Complexes Originating from Restricted Molecular Vibrations.

The mechanism of thermally activated delayed fluorescence (TADF) in molecules in aggregated or condensed solid states has been rarely studied and is not well understood. Nevertheless, many applications of TADF emitters are strongly affected by their luminescence properties in the aggregated state. In this study, two new isomeric tetradentate CuI complexes which simultaneously show aggregation induced emission (AIE) and TADF characteristics are reported for the first time. We provide direct evidence that effectively restricting the vibrations of individual molecules is a key requisite for TADF in these two CuI complexes through in-depth photophysical measurements combined with kinetic methods, single crystal analysis and theoretical calculations. These findings should stimulate new molecular engineering endeavours in the design of AIE-TADF active materials with highly emissive aggregated states.

Two-State Reactivity Mechanism of Benzene C-C Activation by Trinuclear Titanium Hydride.

The cleavage of inert C-C bonds is a central challenge in modern chemistry. Multinuclear transition metal complexes would be a desirable alternative because of the synergetic effect of multiple metal centers. In this work, carbon-carbon bond cleavage and rearrangement of benzene by a trinuclear titanium hydride were investigated using density functional theory. The reaction occurs via a novel "two-state reactivity" mechanism. The important elementary steps consist of hydride transfer, benzene coordination, dehydrogenation, oxidative addition, hydride-proton exchange, and reductive elimination. Most importantly, the ground-state potential energy surface switches from nearly degenerate triplet and antiferromagnetic singlet states to a closed-shell singlet state in the dearomatization of benzene, which effectively decreases the activation barrier. Furthermore, the roles of the transition metal centers and hydrides were clarified.

The transport properties of silicon and carbon nanotubes at the atomic scale: a first-principles study.

Nanotubes are one of the most promising functional materials in nanotechnology. Silicon nanotubes (SiNTs) have been experimentally validated; they are unique puckered nanotubular structures unlike carbon nanotubes (CNTs). Although the electronic and optical properties of SiNTs have been previously studied, their structure-related capability for electron transport has not been investigated. Here we report a comparative study of the intrinsic electronic and transport properties of four pairs of SiNTs and CNTs (one armchair nanotubes (3,3) and three zigzag nanotubes (5,0), (6,0) and (7,0)) using density functional theory (DFT) combined with the nonequilibrium Green's function (NEGF) method. All our investigated systems of SiNTs and CNTs are conductors. Both the armchair SiNTs and CNTs possess superior electron transport performance to their zigzag counterparts. Compared with CNTs, SiNTs have more advantages in the high bias voltage region. Especially, Si(3,3) possesses around double the potential charge capacity of C(3,3) under the bias voltage of 2.0 V. In particular, the CNT(6,0) exhibits distinct negative differential resistance (NDR) behavior and the peak-valley ratio (PVR) for C(6,0) is about 1.2.