Investigation on the Component Evolution of a Tetranuclear Nickel-Cluster-Based Metal–Organic Framework in an Electrochemical Oxidation Reaction

A highly robust cluster-based indium(III)-organic framework with efficient catalytic activity in cycloaddition of CO2 and Knoevenagel condensation

The efficient catalytic performance displayed by MOFs is decided by an appropriate charge/radius ratio of defect metal sites, large enough solvent-accessible channels and Lewis base sites capable of polarizing substrate molecules. Herein, the solvothermal self-assembly led to a highly robust nanochannel-based framework of {[In₄(CPDD)₂(μ₃-OH)₂(DMF)(H₂O)₂]·2DMF·5H₂O}_n (NUC-66) with a 56.8% void volume, which is a combination of a tetranuclear cluster [In₄(μ₃-OH)₂(COO)₁₀(DMF)(H₂O)₂] (abbreviated as {In₄}) and a conjugated tetracyclic pentacarboxylic acid ligand of 4,4'-(4-(4-carboxyphenyl)pyridine-2,6-diyl)diisophthalic acid (H₅CPDD). To the best of our knowledge, NUC-66 is a rarely reported {In₄}-based 3D framework with embedded hierarchical triangular-microporous (2.9 Å) and hexagonal-nanoporous (12.0 Å) channels, which are shaped by six rows of {In₄} clusters. After solvent exchange and vacuum drying, the surface of nanochannels in desolvated NUC-66a is modified by unsaturated In³⁺ ions, N_pyridine atoms and μ₃-OH groups, all of which display polarization ability towards polar molecules due to their Lewis acidity or basicity. The catalytic experiments performed showed that NUC-66a had high catalytic activity in the cycloaddition reactions of epoxides with CO₂ under mild conditions, which should be ascribed to its structural advantages including nanoscale channels, rich bifunctional active sites, large surface areas and chemical stability. Moreover, NUC-66a, as a heterogeneous catalyst, could greatly accelerate the Knoevenagel condensation reactions of aldehydes and malononitrile. Hence, this work confirms that the development of rigid nanoporous cluster-based MOFs built on metal ions with a high charge and large radius ratio will be more likely to realize practical applications, such as catalysis, adsorption and separation of gas, etc.

Ir Nanoparticles Anchored on Metal-Organic Frameworks for Efficient Overall Water Splitting under pH-Universal Conditions

The construction of high-activity and low-cost electrocatalysts is critical for efficient hydrogen production by water electrolysis. Herein, we developed an advanced electrocatalyst by anchoring well-dispersed Ir nanoparticles on nickel metal-organic framework (MOF) Ni-NDC (NDC: 2,6-naphthalenedicarboxylic) nanosheets. Benefiting from the strong synergy between Ir and MOF through interfacial Ni-O-Ir bonds, the synthesized Ir@Ni-NDC showed exceptional electrocatalytic performance for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and overall water splitting in a wide pH range, superior to commercial benchmarks and most reported electrocatalysts. Theoretical calculations revealed that the charge redistribution of Ni-O-Ir bridge induced the optimization of H₂ O, OH* and H* adsorption, thus leading to the accelerated electrochemical kinetics for HER and OER. This work provides a new clue to exploit bifunctional electrocatalysts for pH-universal overall water splitting.

Interfacial interaction induced OER activity of MOF derived superhydrophilic Co3O4-NiO hybrid nanostructures

Electrocatalytic water splitting is one of the key technologies for future energy systems envisioned for the storage of energy obtained from variable renewables and green fuels. The development of efficient, durable, Earth-abundant and cheap electrocatalysts for the oxygen evolution reaction is a scorching area of research. The oxygen evolution reaction has huge potential for fuel cell and metal-air battery applications. Herein, we reported interfacially interacted and uniformly decorated Co₃O₄-NiO hybrid nanostructures formed by a metal-organic framework (Co₂-BDC(OH)₂) using BDC as a linker to the metal center. The fine nanosheets of Co₂-BDC(OH)₂ were first uniformly grown over the honeycomb-like structure of nickel foam (NF). After controlled calcination of these nanosheets/NF composites, a uniformly decorated, binder-free Co₃O₄-NiO/NF electrocatalyst was synthesized. The transformation of Co₂-BDC(OH)₂/NF into Co₃O₄-NiO/NF was characterized by several techniques such as powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy, transmission electron microscopy, etc. The catalyst exhibits a low overpotential of 311 mV vs. RHE at 10 mA cm^-2 current density. The catalyst also shows long-term stability (24 h) with a Tafel slope value of 90 mV dec^-1. The obtained experimental results are also in-line with the theoretical data acquired from model systems.

In Situ Synthesis of Surface-Mounted Novel Nickel(II) Trimer-Based MOF on Nickel Oxide Hydroxide Heterostructures for Enhanced Methanol Electro-Oxidation

Engineering the heterogeneous interface fusing MOFs and inorganic active component is an effective strategy to improve the electrochemical performance. Herein, we report a new Ni₃-based MOF (denoted as CTGU-24) with an infrequent two-fold interpenetrating 3D (3,8)-connected network constructed from Ni(II) trimer and mixed tripodal tectonics for the electrocatalytic methanol oxidation reaction (MOR). In order to improve its stability and activities, the heterogeneous hybrid CTGU-24@NiOOH has been fabricated successfully via the first preparation of the NiOOH nanosphere and then in situ formation of CTGU-24 decorated on the NiOOH surface. Moreover, the integration of CTGU-24@NiOOH and different additives [acetylene black (AB) and ketjen black (KB)], resulting in the optimized hybrid sample AB&CTGU-24@NiOOH (4:4). It attains better MOR performance with an area-specific peak current density of 34.53 mA·cm⁻² than pure CTGU-24 (14.99 mA·cm⁻²) and improved durability in an alkali medium. The new findings indicate that the CTGU-24@NiOOH heterostructure formed in situ and the integration of moderate additives are critical to optimizing and improving electrocatalytic activity of pure MOF crystalline material.

NiFe Layered-Double-Hydroxide Nanosheet Arrays on Graphite Felt: A 3D Electrocatalyst for Highly Efficient Water Oxidation in Alkaline Media

It is of great importance to rationally design and develop earth-abundant nanocatalysts for high-efficiency water electrolysis. Herein, NiFe layered double hydroxide was in situ grown hydrothermally on a 3D graphite felt (NiFe LDH/GF) as a high-efficiency catalyst in facilitating the oxygen evolution reaction (OER). In 1.0 M KOH, NiFe LDH/GF requires a low overpotential of 214 mV to deliver a geometric current density of 50 mA cm^-2 (η_{50 mA cm^-2} = 214 mV), surpassing that NiFe LDH supported on a 2D graphite paper (NiFe LDH/GP; η_{50 mA cm^-2} = 301 mV). More importantly, NiFe LDH/GF shows good durability at 50 mA cm^-2 within 50 h of OER catalysis testing and delivers a faradaic efficiency of nearly 100% in the electrocatalysis of OER.

Template-Induced {Mn2}-Organic Framework with Lewis Acid-Base Canals as a Highly Efficient Heterogeneous Catalyst for Chemical Fixation of CO2 and Knoevenagel Condensation

The target for the self-assembly of functional microporous metal-organic frameworks (MOFs) could be realized by employing ligand-directed and/or template-induced strategies, which prompted us to explore the synthetic technique of d¹⁰ secondary-building-unit-based nanoporous frameworks. Here, the exquisite combination of a paddle-wheel [Mn₂(CO₂)₆(OH₂)₂] cluster and a TDP^6- ligand contributes one robust honeycomb framework of {(Me₂NH₂)₂[Mn₂(TDP)(H₂O)₂]·3H₂O·3DMF}_n (NUC-31; DMF = N,N-dimethylformamide), whose activated state with the removal of associated aqueous molecules characterizes the outstanding physicochemical properties of nanochannels, penta- and tetracoordinated Mn²⁺ serving as highly open metal sites, rich Lewis base sites (rows of C═O groups and N_pyridine atoms), and excellent thermal stability. Moreover, it is worth mentioning that Lewis acid-base sites on the inner surface of the channels in activated NUC-31 successfully form one unprecedented canal-shaped acid-base confined space with evenly distributed open metal sites of Mn²⁺ and N_pyridine atoms as the canal bottom as well as two rows of C═O groups serving as dyke dams. Catalytic experiments displayed that activated NUC-31 could serve as an efficient heterogeneous catalyst for the chemical fixation of CO₂ with epoxides into cyclic carbonates under mild conditions. Furthermore, NUC-31 could effectively catalyze the reaction Knoevenagel condensation, which should be ascribed to the synergistic polarization effect aroused from its plentiful Lewis base sites in the confined channel space. Hence, these results demonstrate that the employment of ligand-directed and template-dependent strategies could overcome the self-assembled barriers of functional microporous MOFs and achieve unexpected frameworks.

Highly Stable Acid-Induced Emission-Enhancing Cd-MOFs: Synthesis, Characterization, and Detection of Glutamic Acid in Water and Fe Ions in Acid

Two novel 3D fluorescent metal-organic frameworks (MOFs), [Cd(L)(bbibp)]_n (1) and [Cd(L)(bbibp)_0.5]_n (2), where H₂L = 4,4'-(4,4'-bipyridine-2,6-diyl)dibenzoic acid and bbibp = 4,4'-bis(benzoimidaz-1-yl)biphenyl, were acquired through a conventional method and characterized via IR spectra, single-crystal X-ray diffraction, elemental analysis, thermogravimetric analysis, powder X-ray diffraction (PXRD), scanning electron microscopy, N₂ adsorption-desorption isotherms, and X-ray photoelectron spectroscopy (XPS). The crystal framework of Cd-MOF 1 remained stable in the range of pH = 1.0-12.0. Interestingly, the emission peak of 1 showed a red shift and exhibited a fluorescence turn-on effect in an acidic environment. X-ray diffraction measurement revealed that the crystal structure of 1 remained unchanged after immersion in a pH = 1.0 solution. In addition, Cd-MOFs 1 and 2 displayed fluorescent quenching to l-glutamic acid with high sensitivity and selectivity. Meanwhile, 1 showed high selectivity in recognizing Fe³⁺ under acidic conditions, which made 1 capable of detecting Fe³⁺ in acidic industrial wastewater. Finally, the fluorescent sensing mechanism was carefully studied by PXRD, transient fluorescence lifetime, XPS, and UV spectroscopy.

Two biologically inspired tetranuclear nickel(II) catalysts: effect of the geometry of Ni4 core on electrocatalytic water oxidation

Two biologically inspired tetranuclear nickel complexes [Ni4(L-H)4(CH3COO)3]·Cl (1) and [Ni4(L-H)4(CH3COO)4]·2CH3OH (2) (L = di(pyridin-2-yl)methanediol) have been synthesized and investigated by a combination of X-ray crystallography, PXRD, electrochemistry, in-situ UV-Vis spectroelectrochemistry and DLS. Both of the two complexes feature a core composed of four Ni(II) ions with the same peripheral ligation provided by the anionic di(pyridin-2-yl)methanediol and MeCOO- ligands. Whereas, complex 1 possesses one distorted cubane-like [Ni4(µ3-O)4] core, while 2 has one extended butterfly-like [Ni4(µ3-O)2] core. The homogeneous electrocatalytic reactivity of the two water-soluble complexes for water oxidation have been thoroughly studied, which demonstrates that both of them can efficiently electrocatalyze water oxidation with high stability under alkaline conditions, at relatively low over-potentials (η) of 420-790 mV for 1 and 390-780 mV for 2, both in the pH range of 7.67-12.32, with the high TOF of about 139 s-1 (1) and 69 s-1 (2) at pH = 12.32, respectively. By a series of comparative experiments for complexes 1 and 2, we proposed that their crystal geometries play an important role in their electrocatalytic reactivity for water oxidation. We verified that biomimetic cubane geometry could promote OER catalysis with two very similar compounds for the first time. Compared with 2, the biomimetic cubane topology of 1 could promote OER catalysis by facilitating efficient charge delocalization and electron-transfer.

V═O Functionalized {Tm2}-Organic Framework Designed by Postsynthesis Modification for Catalytic Chemical Fixation of CO2 and Oxidation of Mustard Gas

In terms of recently documented references, the introduction of V═O units into porous MOF/COF frameworks can greatly improve their original performance and expand their application prospects due to a change in their electronegativity. In this work, by a cation-exchange strategy, a consummate combination of separate 4f [Tm₂(CO₂)₈] SBUs and 3d [V^IVO(H₂O)₂] units generated the functionalized porous metal-organic framework {(Me₂NH₂)₂[VO(H₂O)][Tm₂(BDCP)₂]·3DMF·3H₂O}_n (NUC-11), in which [Tm₂(CO₂)₈] SBUs constitute the fundamental 3D host framework of {[Tm₂](BDCP)₂}_n along with [V^IVO(H₂O)₂] units being further docked on the inner wall of channels by covalent bonds. Significantly, NUC-11 represents the first example of V═O modified porous MOFs, in which uncoordinated carboxylic groups (-CO₂H) further grasp the functional [V^IVO(H₂O)₂] units on the initial basic skeleton along with the formation of covalent bonds as fixed ropes. Furthermore, activated samples of NUC-11 displayed a good catalytic performance for the chemical synthesis of carbonates from related epoxides and CO₂ with high conversion rate. Moreover, by employing NUC-11 as a catalyst, a simulator of mustard gas, 2-chloroethyl ethyl sulfide, could be quickly and efficiently oxidized into low-toxicity products of oxidized sulfoxide (CEESO). Thus, this study offers a brand new view for the design and synthesis of functional-units-modified porous MOFs, which could be potentially applied as an excellent candidate in the growing field of efficient catalysis.

6s-3d {Ba3Zn4}-Organic Framework as an Effective Heterogeneous Catalyst for Chemical Fixation of CO2 and Knoevenagel Condensation Reaction

The exquisite combination of Ba²⁺ and Zn²⁺ with the aid of 2,4,6-tri(2,4-dicarboxyphenyl)pyridine (H₆TDP) under the condition of solvothermal self-assembly generates one highly robust [Ba₃Zn₄(CO₂)₁₂(HCO₂)₂(OH₂)₂]-organic framework of {[Ba₃Zn₄(TDP)₂(HCO₂)₂(OH₂)₂]·7DMF·4H₂O}_n (NUC-27), in which adjacent 2D layers are interlaced via hydrogen-bonding interactions to form a 3D skeleton with peapod-like channels and nano-caged voids. It is worth emphasizing that both Ba²⁺ and Zn²⁺ ions in NUC-27 display the extremely low coordination modes: hexa-coordinated [Ba(1)] and tetra-coordinated [Ba(2), Zn(1), and Zn(2)]. Furthermore, to the best our knowledge, NUC-27 is one scarcely reported 2D-based nanomaterial with an unprecedented Z-shaped hepta-nuclear heterometallic cluster of [Ba₃Zn₄(CO₂)₁₂(HCO₂)₂(OH₂)₂] as SBUs, which not only has plentiful low-coordinated open metal sites but also has the excellent physicochemical properties including omni-directional opening pores, ultrahigh porosity, larger specific surface area, and the coexistence of Lewis acid-base sites. Just as expected, thanks to its rich active metal sites and pyridine groups as strong Lewis acid-base roles, completely activated NUC-27 displays high catalytic efficiency on the chemical transformation of epoxides with CO₂ into cyclic carbonates under mild conditions and effectively accelerates the reaction process of Knoevenagel condensation.

Nanochannel-based heterometallic {ZnIIHoIII}-organic framework with high catalytic activity for the chemical fixation of CO2

The exquisite combination of ZnII and HoIII generated the highly robust [ZnHo(CO2)6(OH2)]-based heterometallic framework of {[ZnHo(TDP)(H2O)]·5H2O·3DMF} n (NUC-30, H6TDP = 2,4,6-tri(2',4'-dicarboxyphenyl)pyridine), which featured outstanding physicochemical properties, including honeycomb nanochannels, high porosity, large specific surface area, the coexistence of highly open Lewis acid-base sites, good thermal and chemical stability, and resistance to most organic solvents. Due to its extremely unsaturated metal tetra-coordinated Zn(ii) ions, hepta-coordinated Ho(iii) and high faveolate void volume (61.3%), the conversion rate of styrene oxide and CO2 into cyclic carbonates in the presence of 2 mol% activated NUC-30 and 5 mol% n-Bu4NBr reached 99% under the mild conditions of 1.0 MPa and 60 °C. Furthermore, the luminescence sensing experiments proved that NUC-30 could be used as a fast, sensitive and highly efficiency sensor for the detection of Fe3+ in aqueous solution. Therefore, these results prove that nanoporous MOFs assembled from pyridine-containing polycarboxylate ligands have wide applications, such as catalysis and as luminescent materials.

A binuclear Co-based metal-organic framework towards efficient oxygen evolution reaction

The search for low-cost and high-performance electrocatalysts for oxygen evolution reaction (OER) has aroused enormous research interest in the last few years. Reported herein is the topotactic construction of a binuclear Co-based metal-organic framework (Co2-tzpa) using a solvothermal reaction. Prominently, as a porous catalyst, Co2-tzpa holds its activity for at least 25 hours and exhibits low OER overpotentials of 336 and 396 mV to achieve the current density of 10 mA cm-2 in 1 M KOH and 0.1 M KOH, respectively. The excellent OER performance should be attributed to each cobalt site coordinated with two tetrazolate N atoms.

Versatility of Amide-Functionalized Co(II) and Ni(II) Coordination Polymers: From Thermochromic-Triggered Structural Transformations to Supercapacitors and Electrocatalysts for Water Splitting

The new 2D coordination polymers (CPs) [M(L)₂(H₂O)₂]_n [M = Co^II (1) and Ni^II (2); L = 4-(pyridin-3-ylcarbamoyl)benzoate] were synthesized from pyridyl amide-functionalized benzoic acid (HL). They were characterized by elemental, Fourier transform infrared, thermogravimetric, powder X-ray diffraction (PXRD), and single-crystal X-ray diffraction (XRD) structural analyses. Single-crystal XRD analysis revealed the presence of a 2D polymeric architecture, and topological analyses disclose a 2,4-connected binodal net. A thermochromic effect leads to the production of two new CPs, 1' and 2', by heating at ca. 220 °C, accompanied by a color change from orange to purple in the case of 1 and from blue to green in the case of 2. The transformation of 1 to 1' takes place through an intermediate (1a) with a different twist of the L^- ligand, leading to the formation of a 1D polymeric architecture, as proven by single-crystal XRD analysis. The addition of water or keeping 1' or 2' in air for several days leads to regeneration of 1 or 2, respectively. The thermochromic-triggered structural transformations of 1 and 2 were further substantiated by PXRD and UV-vis ground-state diffuse-reflectance absorption studies. The supercapacitance ability of the CPs 1 and 2 and a Ni-Co composite (made from mixing the CPs 1 and 2) was investigated by electroanalytical techniques, such as cyclic voltammetry and electrochemical impedance spectroscopy. The CP 2 exhibits the highest specific capacity of 273.8 C g^-1 at an applied current density of 1.5 A g^-1. These newly developed CPs further act as electrocatalysts for the water-splitting reaction.

Recent advances in metal-organic frameworks for electrocatalytic hydrogen evolution and overall water splitting reactions

Rational design and synthesis of efficient metal-organic frameworks (MOFs) as electrode modifiers for energy-related electrocatalytic applications are crucial for the development of clean-energy technologies. The present review focuses on recent work on robust earth-abundant heterogeneous catalysts based on pristine MOFs for the hydrogen evolution reaction (HER) and overall water splitting. These catalysts have been extensively studied as alternatives for noble metal-based ones, demonstrating "hydrogen economy" development prospects. In addition, novel strategies to enhance the conductivity, chemical stability and efficiency of MOF-based electrocatalysts are discussed. The best electrocatalysts even surpass the achievements of the platinum group of metals and MOF-derived catalysts in catalytic performance. The electrolytic cells with MOF-modified electrodes demonstrated excellent catalytic activity and can deliver a high current density at a voltage lower than that using the precious metal-based Pt/C cathodes and IrO2 anodes. In this review article, current approaches to design such MOF and MOF-modified electrode materials are summarized and analyzed.

Evolution of metal organic frameworks as electrocatalysts for water oxidation

The development of metal organic framework based water oxidation catalysts is discussed here in connection with various design strategies.