Metal-support spin orders: Crucial effect on electrocatalytic oxygen reduction.

Magnetic property (e.g. spin order) of support is of great importance in the rational design of heterogeneous catalysts. Herein, we have taken the Ni-supported ferromagnetic (FM) CrBr3 support (Nix/CrBr3) to thoroughly investigate the effect of spin-order on electrocatalytic oxygen reduction reaction (ORR) via spin-polarized density functional theory calculations. Specifically, Ni loading induces anti-FM coupling in Ni-Cr, leading to a transition from FM-to-ferrimagnetic (FIM) properties, while Ni-Ni metallic bonds create a robust FM direct exchange, benefiting the improvement of the phase transition temperature. Interestingly, with the increase in Ni loading, the easy magnetic axis changes from out-of-plane (2D-Heisenberg) to in-plane (2D-XY). The adsorption properties of Nix/CrBr3, involving O2 adsorption energy and configuration, are not governed by the d-band center but strongly correlate with magnetic anisotropy. It is noteworthy that the applied potential and electrolyte acidity triggers spin-order transition phenomena during the ORR and induces the catalytic pathway change from 4e- ORR to 2e- ORR with the excellent onset potential of 0.93 V/reversible hydrogen electrode, comparable to the existing most excellent noble-metal catalysts. Generally, these findings offer new avenues to understand and design heterogeneous catalysts with magnetic support.

Accurate Prediction of the Magnetic Ordering Temperature of Ultralow-Temperature Magnetic Refrigerants.

Adiabatic demagnetization refrigeration is known to be the only cryogenic refrigeration technology that can achieve ultralow temperatures (≪1 K) at gravity-free conditions. The key indexes to evaluate the performance of magnetic refrigerants are their magnetic entropy changes (-ΔSm) and magnetic ordering temperature (T0). Although, based on the factors affecting the -ΔSm of magnetic refrigerants, one has been able to judge if a magnetic refrigerant has a large -ΔSm, how to accurately predict their T0 remains a huge challenge due to the fact that the T0 of magnetic refrigerants is related to not only magnetic exchange but also single-ion anisotropy and magnetic dipole interaction. Here, we, taking GdCO3F (1), Gd(HCOO)F2, Gd2(SO4)3·8H2O, GdF3, Gd(HCOO)3 and Gd(OH)3 as examples, demonstrate that the T0 of magnetic refrigerants with very weak magnetic interactions and small anisotropy can be accurately predicted by integrating mean-field approximation with quantum Monte Carlo simulations, providing an effective method for predicting the T0 of ultralow-temperature magnetic refrigerants. Thus, the present work lays a solid foundation for the rational design and preparation of ultralow-temperature magnetic refrigerants in the future.

Magnetic Order Transition of a Two-Dimensional Square-Lattice Electrocatalyst Assembled by Fe-N4 Units: Crucial Role on Oxygen Reduction.

Herein, we theoretically investigate the effect of magnetic orders on electrocatalytic oxygen reduction reaction (ORR) properties on the Fe-N4 site-embedded two-dimensional (2D) covalent organic framework (Fe-N4@COF-C3N2) under realistic environments. The Fe-N4@COF-C3N2 shows a 2D square-lattice (sql) topology with three magnetic order states: one ferromagnetic state (FM) and two antiferromagnetic states (AFM1 and AFM2). Specially, the electrocatalyst in the AFM2 state shows a remarkable onset potential of 0.80 V/reversible hydrogen electrode (RHE) at pH 1, superior to the existing most excellent noble-metal catalysts. Thermodynamically, the onset potential for the 4e- ORR is 0.64 V/RHE at pH 1, with a magnetic state transition process of FM → AFM1 → FM → FM → FM, while at pH 13, the onset potential for the 4e- ORR is 0.54 V/RHE, with the magnetic transition process of FM → FM → AFM1 → FM → FM. Generally, this finding will provide new avenues to rationally design the Fe-N4 electrocatalyst.

Tuning Electrocatalytic Water Oxidation Activity: Insights from the Active-Site Distance in LnCu6 Clusters.

Atomically precise metal clusters serve as a unique model for unraveling the intricate mechanism of the catalytic reaction and exploring the complex relationship between structure and activity. Herein, three series of water-soluble heterometallic clusters LnCu6, abbreviated as LnCu6-AC (Ln = La, Nd, Gd, Er, Yb; HAC = acetic acid), LnCu6-IM (Ln = La and Nd; IM = Imidazole), and LnCu6-IDA (Ln = Nd; H2IDA = Iminodiacetic acid) are presented, each featuring a uniform metallic core stabilized by distinct protected ligands. Crystal structure analysis reveals a triangular prism topology formed by six Cu2+ ions around one Ln3+ ion in LnCu6, with variations in Cu···Cu distances attributed to different ligands. Electrocatalytic oxygen evolution reaction (OER) shows that these different LnCu6 clusters exhibit different OER activities with remarkable turnover frequency of 135 s-1 for NdCu6-AC, 79 s-1 for NdCu6-IM and 32 s-1 for NdCu6-IDA. Structural analysis and Density Functional Theory (DFT) calculations underscore the correlation between shorter Cu···Cu distances and improves OER catalytic activity, emphasizing the pivotal role of active-site distance in regulating electrocatalytic OER activities. These results provide valuable insights into the OER mechanism and contribute to the design of efficient homogeneous OER electrocatalysts.

Visible-Light Photocatalytic Overall Water Splitting on a B4C3/CxNy Z-Scheme Heterojunction: Role of Ultrafast Carrier Recombination-Transfer Kinetics.

Herein, combining density functional theory (DFT) calculations with nonadiabatic molecular dynamics (NAMD), we built a computational framework to rationally screen from a series of 2D conjugated carbon nitrides (CNs) to match with B4C3, resulting in the excellent direct Z-scheme photocatalyst (B4C3/C6N6) for overall water splitting (OWS). Studies on interface engineering and ultrafast dynamics of carrier recombination-transfer show that in the B4C3/C6N6 system, compared with the slower interlayer migration process of carriers, strong nonadiabatic coupling and longer quantum decoherence time accelerates weak carrier interlayer recombination on a subpicosecond time scale, enabling simultaneous triggering of hydrogen evolution reaction (HER) with ΔG = -0.23 eV and spontaneous oxygen evolution reaction (OER) in the absence of sacrificial or cocatalysts. In general, our work will promote the design of efficient direct Z-scheme photocatalysts from an ultrafast dynamics perspective.

Conductive Lanthanide Metal-Organic Frameworks with Exceptionally High Stability.

Electrically conductive metal-organic frameworks (MOFs) have been extensively studied for their potential uses in energy-related technologies and sensors. However, achieving that goal requires MOFs to be highly stable and maintain their conductivity under practical operating conditions with varying solution environments and temperatures. Herein, we have designed and synthesized a new series of {[Ln4(µ4-O)(µ3-OH)3(INA)3(GA)3](CF3SO3)(H2O)6}n (denoted as Ln4-MOFs, Ln = Gd, Tm, and Lu, INA = isonicotinic acid, GA = glycolic acid) single crystals, where electrons are found to transport along the π-π stacked aromatic carbon rings in the crystals. The Ln4-MOFs show remarkable stability, with minimal changes in conductivity under varying solution pH (1-12), temperature (373 K), and electric field as high as 800 000 V/m. This stability is achieved through the formation of strong coordination bonds between high-valent Ln(III) ions and rigid carboxylic linkers as well as hydrogen bonds that enhance the robustness of the electron transport path. The demonstrated lanthanide MOFs pave the way for the design of stable and conductive MOFs.

Correlation of the spin state and catalytic property of M-N4 single-atom catalysts in oxygen reduction reactions.

The rational design of high-performance catalysts for oxygen reduction reactions (ORRs) is of great importance for large-scale applications in the field of proton-exchange membrane fuel cells and the green synthesis of H2O2. The effect of spin states of paramagnetic metal ions on the selectivity of ORRs is significant for single-atom catalysts (SACs). In this work, via spin-polarization density functional theory (DFT) calculations, we systematically investigated the popular paramagnetic metal-nitrogen graphene (M-N4-C, M = Mn, Fe, and Co) SACs to mainly focus on the correlation of spin states and catalytic performance (e.g. activity and selectivity). Both thermodynamically and kinetically, it was found that Co-N4-C (S = 1/2) has excellent 2e- oxygen reduction performance (hydrogen peroxide production) with an ultralow overpotential of 0.03 V, and the hydrogenation of OOH* is the rate-determining step (RDS) with an energy barrier of 1.20 eV. The 4e- ORR tends to occur along the OOH dissociation pathway (O* + OH*) on Co-N4-C (S = 3/2), in which OOH* decomposition is the RDS with an energy barrier of 1.01 eV. It is proved that the spin magnetic moment is the key factor to regulate the ORR property via multi-angle electronic analysis. The spin states of catalysts play a crucial role in the activity and selectivity of ORRs mainly by manipulating the bond strength between OOH and catalysts. This will provide new insights for the rational design of ORR catalysts with magnetic metals.

Rational design of bimetallic MXene solid solution with High-Performance electrocatalytic N2 reduction.

Electrocatalytic N2 reduction reaction (eNRR) was an effective alternative method for green synthesis of NH3. By combining the first-principal Density functional theory (DFT) calculations and Monte Carlo (MC) simulation, we systematacially investigated 24 types equal-ratio bimetallic MXene solid solution, involving 88 different catalysts. Our focus was on the catalytic performance of these materials in eNRR. The computational result indicate that MoW(3Mo) has high stability, selectivity (93.8 % against the hydrogen evolution reaction (HER)) and activity (UL = -0.26 V), which is significantly better than that of monometal Mo2CO2 and W2CO2. This improvement in catalytic properties is attributed to the unique electronic structure (e.g. d-band center, charge) of bimetallic MXene solid solution. In explicit solvent conditions, the microenvironment of hydrogen bond in aqueous liquid thermodynamically promotes the catalytic property for eNRR and reduce the catalytic property of HER side reaction, but the kinetic barrier is also increased due to the effect of the hydrogen-bond microenvironment on proton migration. Overall, the obtained bimetallic MXene solid solution MoW(3Mo) exhibits excellent catalytic performance in eNRR.

Co-Dissolved Isostructural Polyoxovanadates to Construct Single-Atom-Site Catalysts for Efficient CO2 Photoreduction.

We explored a co-dissolved strategy to embed mono-dispersed Pt center into V2 O5 support via dissolving [PtV9 O28 ]7- into [V10 O28 ]6- aqueous solution. The uniform dispersion of [PtV9 O28 ]7- in [V10 O28 ]6- solution allows [PtV9 O28 ]7- to be surrounded by [V10 O28 ]6- clusters via a freeze-drying process. The V centers in both [PtV9 O28 ]7- and [V10 O28 ]6- were converted into V2 O5 via a calcination process to stabilize Pt center. These double separations can effectively prevent the Pt center agglomeration during the high-temperature conversion process, and achieve 100 % utilization of Pt in [PtV9 O28 ]7- . The resulting Pt-V2 O5 single-atom-site catalysts exhibit a CH4 yield of 247.6 µmol g-1 h-1 , 25 times higher than that of Pt nanoparticle on the V2 O5 support, which was accompanied by the lactic acid photooxidation to form pyruvic acid. Systematical investigations on this unambiguous structure demonstrate an important role of Pt-O atomic pair synergy for highly efficient CO2 photoreduction.

Magnetic and Dielectric Switchings Actuated by the Rotation of the Picoline Ligand in an Iron-Based Dinuclear Phase-Transition Complex.

Multifunctional materials with switchable magnetic and dielectric properties are crucial for the development of memory and sensor devices. Herein, we report a methoxy-bridged dinuclear iron-pyridyl complex [Fe2(4-picoline)4(NCS)4(µ-OCH3)2] (1), which shows simultaneous thermal-induced magnetic and dielectric switchings. Within the phase-transition temperature range, both magnetic switching and the dielectric anomaly were detected, in which the thermal hysteresis loops were 23 and 21 K, respectively. Detailed structural analyses revealed that these simultaneous switchings were rooted in the flexible rotatable ligands, which were actuated by readjusting the π-π intermolecular interactions between the pyridine ligands in the trans positions of the metal centers. These results were comprehensively investigated both experimentally and theoretically. This study presents a new guideline to control both the magnetic and dielectric properties of molecular complexes by external stimuli.

Construction of a High Nuclear Gadolinium Cluster with Enhanced Magnetocaloric Effect through Structural Transition.

Starting from a dinuclear complex {Gd2(L)2(NO3)4(H2O)2}·2(CH3CN) (1) based on 2,6-dimethoxyphenol (HL), a nonanuclear cluster {Gd9(L)4(µ4-OH)2(µ3-OH)8(µ2-OCH3)4(NO3)8 (H2O)8}(OH)·2H2O (2) was obtained via modulating the amount of the ligand and base. Both of them have been structurally and magnetically characterized. Complex 1 decorates the Gd2 core bridged by double µ2-phenoxyl oxygen atoms and coordinated neutral CH3CN molecules, while 2 features the Gd9 core with a sandglass-like topology. Magnetic investigations reveal that the weaker antiferromagnetic interactions between adjacent metal ions exist in complex 2 than in 1, which is in agreement with the theoretical results. Meanwhile, the magnetocaloric effect with a maximum -ΔS m value changes from 27.32 to 40.60 J kg-1 K-1 at 2 K and 7 T.

Family of Nanoclusters, Ln33 (Ln = Sm/Eu) and Gd32, Exhibiting Magnetocaloric Effects and Fluorescence Sensing for MnO4.

A family of nanoclusters, [Ln33(EDTA)12(OAc)2(CO3)4(µ3-OH)36(µ5-OH)4(H2O)38]·OAc·xH2O (x ≈ 50, Ln = Sm for 1; x ≈ 70, Ln = Eu for 2) and [Gd32(EDTA)12(OAc)2(C2O4)(CO3)2(µ3-OH)36(µ5-OH)4(H2O)36]·x(H2O) (x ≈ 70 for 3; H4EDTA = ethylene diamine tetraacetic acid), was prepared through the assembly of repeating subunits under the action of an anion template. The analysis of the structures showed that compounds 1 and 2 containing 33 Ln3+ ions were isostructural, which were constructed by three kinds of subunits in the presence of CO32- as an anion template, while compound 3 had a slightly different structure. Compound 3 containing 32 Gd3+ ions was formed by three types of subunits in the presence of CO32- and C2O42- as a mixed anion template. The CO32- anions came from the slow fixation of CO2 in the air. Meanwhile, one kind of high-nuclearity lanthanide clusters showed high chemical stability. The quantum Monte Carlo (QMC) calculation suggested that weak antiferromagnetic interactions were dominant between Gd3+ ions in 3. Magnetocaloric studies showed that compound 3 had a large entropy change of 43.0 J kg-1 K-1 at 2 K and 7 T. Surprisingly, compound 2 showed excellent recognition and detection effects for permanganate in aqueous solvents based on the fluorescence quenching phenomenon.

W Single-Atom Catalyst for CH4 Photooxidation in Water Vapor.

Solar-driven high-efficiency and direct conversion of methane into high-value-added liquid oxygenates against overoxidation remains a great challenge. Herein, facile and mass fabrication of low-cost tungsten single-atom photocatalysts is achieved by directly calcining urea and sodium tungstate under atmosphere (W-SA-PCN-m, urea amount m = 7.5, 15, 30, and 150 g). The single-atom photocatalysts can manage H2 O2 in situ generation and decomposition into ·OH, thus achieving highly efficient CH4 photooxidation in water vapor under mild conditions. Systematic investigations demonstrate that integration of multifunctions of methane activation, H2 O2 generation, and decomposition into one photocatalyst can dramatically promote methane conversion to C1 oxygenates with a yield as high as 4956 µmol gcat -1 , superior to that of the most reported non-precious photocatalysts. Liquid-solid phase transition can induce the products to facilely switch in from HCOOH to CH3 OH by pulling the catalyst above water with CH3 OH/HCOOH ratio from 10% (in H2 O) to 80% (above H2 O).

Nuclearity enlargement from [PW9O34@Ag51] to [(PW9O34)2@Ag72] and 2D and 3D network formation driven by bipyridines.

The structural transformations of metal nanoclusters are typically quite complex processes involving the formation and breakage of several bonds, and thus are challenging to study. Herein, we report a case where two lacunary Keggin polyoxometallate templated silver single-pods [PW9O34@Ag51] (SD/Ag51b) fuse to a double-pod [(PW9O34)2@Ag72] by reacting with 4,4'-bipyridine (bipy) or 1,4-bis(4-pyridinylmethyl)piperazine (pi-bipy). Their crystal structures reveal the formation of a 2D 44-sql layer (SD/Ag72a) with bipy and a 3D pcu framework (SD/Ag72c) with pi-bipy. The PW9O349- retains its structure during the cluster fusion and cluster-based network formation. Although the two processes, stripping of an Ag-ligands interface followed by fusion, and polymerization, are difficult to envisage, electrospray ionization mass spectrometry provides enough evidences for such a proposal to be made. Through this example, we expect the structural transformation to become a powerful method for synthesizing silver nanoclusters and their infinite networks, and to evolve from trial-and-error to rational.

Cooperatively interface role of surface atoms and aqueous media on single atom catalytic property for H2O2 synthesis.

Direct electrosynthesis of hydrogen peroxide (H2O2) from H2 and O2 is a promising alternative to currently industrial Riedl-Pfleiderer route. Utilizing a combination of density functional theory (DFT) and ab-initio-molecular dynamic simulation (AIMD), we presented an effective computational framework to identify the cooperative role of surface atoms(e.g. O, N and S) and aqueous media on catalytic performance of single-atom catalysts (SACs) supported Nb2C MXenes. Computational results shown that both Ni/Nb2CN2 and Co/Nb2CS2 have low overpotentials of 0.17 V and 0.20 V, and the barrier of 0.89 eV and 0.67 eV for 2e- ORR under gas phase, respectively, while in aqueous phase, hydrogen bond framework on the surface promotes the transfer of proton, resulting in the lower 2e- ORR overpotential (0.05 V) in Co/Nb2CS2 and lower barrier (almost 0.01 eV) for rate-determining step (RDS) in Ni/Nb2CN2. Electronically, we found that the less-electronegativity N and S relative to O more benefit to mediate the activation degree of O2 on SACs and thereby improve catalytic selectivity. Thus, it is concluded that both surface atom and aqueous medium synergistically promote catalytic property for H2O2 synthesis.

Tuning the (Chir)Optical Properties and Squeezing out the Inherent Chirality in Polyphenylene-Locked Helical Carbon Nanorings.

Distorting linear polyaromatic hydrocarbons (PAHs) out of planarity affects their physical properties and breaks their symmetry to induce inherent chirality. However, the chirality cannot be achieved in large distorted PAHs-based macrocycles due to a low racemization barrier for isomerization. Herein, we report the precise synthesis and tuning size-dependent (chir)optical properties of a new class of chiral PAHs-containing conjugated macrocycles (cyclo[n]paraphenylene-2,6-anthrylene, [n]CPPAn2,6 ; n=6-8). Their inherent chiralities were squeezed out in small anthrylene-based macrocycles. Efficient resolutions for chiral enantiomers with (P)/(M)-helicity of small [6-7]CPPAns were achieved by HPLC. Interestingly, these macrocycles showed enriched size-dependent physical, chiral, and (chir)optical properties. Theoretical calculations indicate that these macrocycles have high strain energy (Estrain =60.8 to 73.4 kcal/mol) and very small Egap (∼3.0 eV). Notably, these enantiomers showed strong chiroptical properties and dissymmetry factors (|gabs | and |glum |∼0.01 for an enantiomer of [6]CPPAn2,6 ), which can give them potential applications in optically active materials.

The synergetic effect of an aqua ligand and metal site on the performance of single-atom catalysts in H2O2 synthesis: a density functional theory study.

Studying the effect of the coordination field on the catalytic property is critical for the rational design of outstanding electrocatalysts for H2O2 synthesis. Herein, via density functional theory (DFT) calculations and ab initio molecular dynamic (AIMD) simulations, we built an effective computational framework to identify the synergetic effect of an aqua ligand and metal ion on the 2e- ORR catalytic performance under gas condition and aqua solvent. Specifically, the screening results of 29 single-atom catalysts (SACs), TM@C6N6 (TM = transition metal), indicated that Cu@C6N6 features excellent catalytic property with thermal stability, lowest 2e- ORR overpotential (0.02 V) and high selectivity of 99.99%. Once an aqua ligand binds with the Cu site, the activity is reduced to the overpotential of 0.42 V and the selectivity decreased slightly (99.98%) due to the reduction of the adsorption strength for the reaction intermediates. A combination of geometric structures and electronic properties revealed that such changes are correlated with the charge of the Cu site. Furthermore, based on molecular orbital theory, the essence of the high catalytic property deeply lies in the effect of the moderate electron back donation bond (dyz & dxz→) between Cu and O2. This work will provide a route to better design high-performance SACs for H2O2 synthesis effectively.

Integration of bio-inspired lanthanide-transition metal cluster and P-doped carbon nitride for efficient photocatalytic overall water splitting.

Photosynthesis in nature uses the Mn4CaO5 cluster as the oxygen-evolving center to catalyze the water oxidation efficiently in photosystem II. Herein, we demonstrate bio-inspired heterometallic LnCo3 (Ln = Nd, Eu and Ce) clusters, which can be viewed as synthetic analogs of the CaMn4O5 cluster. Anchoring LnCo3 on phosphorus-doped graphitic carbon nitrides (PCN) shows efficient overall water splitting without any sacrificial reagents. The NdCo3/PCN-c photocatalyst exhibits excellent water splitting activity and a quantum efficiency of 2.0% at 350 nm. Ultrafast transient absorption spectroscopy revealed the transfer of a photoexcited electron and hole into the PCN and LnCo3 for hydrogen and oxygen evolution reactions, respectively. A density functional theory (DFT) calculation showed the cooperative water activation on lanthanide and O-O bond formation on transition metal for water oxidation. This work not only prepares a synthetic model of a bio-inspired oxygen-evolving center but also provides an effective strategy to realize light-driven overall water splitting.

Enantioselective Recognition and Separation of C2 Symmetric Substances via Chiral Metal-Organic Frameworks.

A promising route toward the enantioselective recognition and separation of racemic molecules is the design of chiral metal-organic frameworks (CMOFs) with high enantioselectivity and stability. Herein, we report porous CMOFs Δ- and Λ-RuEu-MOFs constructed from the D3-symmetry helical chiral Ru(phen)3-derived tricarboxylate ligand and Eu2 units, which can be utilized as adsorbents for the enantioselective recognition and separation of 1,1'-bi-2-naphthol (BINOL) derivatives. Investigation of the circular dichroism enantiodifferentiation between the host and guest suggested that Δ- and Λ-RuEu-MOFs can be employed as chiral sensors to discriminate axial enantiomers due to their diastereomeric host-guest relationship. Density functional theory calculations reveal that chiral recognition is attributed to the distinguishing binding affinities stemming from N···H-O hydrogen bonds and π-π stacking between the host and guest. Moreover, the reticulate structure of Δ- and Λ-RuEu-MOFs can be readily recycled and reused for the successive enantioselective separation of BINOL up to 80% ee.

Thermally induced transformation of a Cu4I4-based cluster to a Cu2I2-based cluster under mild conditions.

A reaction of 6,6'-bis((benzylthio)methyl)-2,2'-bipyridine (L) with CuI at room temperature led to one Cu4I4-based cluster, which could be thermally transformed to a Cu2I2-based one under mild conditions due to the formation of a Cu-S bond. Along with the structural transformation, remarkable changes in the color and luminescence have been triggered.