Metal-Organic Frameworks for Hydrogen Energy Applications: Advances and Challenges

Magnetic study and DFT analysis of a doubly carboxylato-bridged Co(II) derivative anchored with a 'scorpionate' precursor as a potential electrocatalyst for heterogeneous H2 evolution

A new doubly carboxylato-bridged Co(II) dinuclear complex, [Co(bdtbpza)(NCS)]2 (1), was obtained in a satisfactory yield by employing a 'scorpionate'-type precursor, bdtbpza {bis-(3,5-di-tert-butylpyrazol-1-yl)acetate}, and was then structurally characterized. Single-crystal X-ray diffraction analysis revealed that, in 1, each Co(II) is penta-coordinated, leading to a distorted trigonal-bipyramidal geometry within the coordination environment of N3O2. Weak antiferromagnetic coupling within the Co(II) ions in 1 was found based on the isotropic spin Hamiltonian H = -J(S1·S2) for the Si = 3/2 system. For evaluating the spin density distribution and the mechanism for the magnetic exchange coupling, DFT analysis was performed, with the calculated result agreeing the experimental magnetic data. A study into electrochemical H2 evolution, involving cyclic voltammetry (CV), controlled potential electrolysis (CPE), and gas chromatographic (GC) analyses of the graphite electrode modified with the cobalt complex in a neutral aqueous solution revealed the high catalytic activity of the complex with a low overpotential toward H2O reduction. The faradaic efficiency of the catalyst was found to be 83.7% and the di-cobalt catalyst-modified electrode displayed quite an interesting H2-evolution activity compared with that of bare electrodes. These results are encouraging for the future potential application of 1 in water splitting.

A comprehensive review on unleashing the power of hydrogen: revolutionizing energy systems for a sustainable future

Population growth and environmental degradation are major concerns for sustainable development worldwide. Hydrogen is a clean and eco-friendly alternative to fossil fuels, with a heating value almost three times higher than other fossil fuels. It also has a clean production process, which helps to reduce the emission of hazardous pollutants and save the environment. Among the various production methodologies described in this review, biochemical production of hydrogen is considered more suitable as it uses waste organic matter instead of fossil fuels. This technology not only produces clean energy but also helps to manage waste more efficiently. However, the production of hydrogen obtained from this method is currently more expensive due to its early stage of development. Nevertheless, various research projects are underway to develop this method on a commercial scale.

Unique activity of a Keggin POM for efficient heterogeneous electrocatalytic OER

Polyoxometalates (POMs) have been well studied and explored in electro/photochemical water oxidation catalysis for over a decade. The high solubility of POMs in water has limited its use in homogeneous conditions. Over the last decade, different approaches have been used for the heterogenization of POMs to exploit their catalytic properties. This study focused on a Keggin POM, K6[CoW12O40], which was entrapped in a sol-gel matrix for heterogeneous electrochemical water oxidation. Its entrapment in the sol-gel matrix enables it to catalyze the oxygen evolution reaction at acidic pH, pH 2.0. Heterogenization of POMs using the sol-gel method aids in POM's recyclability and structural stability under electrochemical conditions. The prepared sol-gel electrode is robust and stable. It achieved electrochemical water oxidation at a current density of 2 mA/cm2 at a low overpotential of 300 mV with a high turnover frequency (TOF) of 1.76 [mol O2 (mol Co)-1s-1]. A plausible mechanism of the electrocatalytic process is presented.

Metal-Organic Framework-Based Photocatalysis for Solar Fuel Production

Metal-organic frameworks (MOFs) represent a novel class of crystalline inorganic-organic hybrid materials with tunable semiconducting behavior. MOFs have potential for application in photocatalysis to produce sustainable solar fuels, owing to their unique structural advantages (such as clarity and modifiability) that can facilitate a deeper understanding of the structure-activity relationship in photocatalysis. This review takes the photocatalytic active sites as a particular perspective, summarizing the progress of MOF-based photocatalysis for solar fuel production; mainly including three categories of solar-chemical conversions, photocatalytic water splitting to hydrogen fuel, photocatalytic carbon dioxide reduction to hydrocarbon fuels, and photocatalytic nitrogen fixation to high-energy fuel carriers such as ammonia. This review focuses on the types of active sites in MOF-based photocatalysts and discusses their enhanced activity based on the well-defined structure of MOFs, offering deep insights into MOF-based photocatalysis.

Recent development of organic-inorganic hybrid photocatalysts for biomass conversion into hydrogen production

Over the last few years, photocatalysis using solar radiation has been explored extensively to investigate the possibilities of producing fuels. The production and systematic usage of solar fuels can reduce the use of fossil-based fuels, which are currently the primary source for the energy. It is time for us to exploit renewable sources for our energy needs to progress towards a low-carbon society. This can be achieved by utilizing green hydrogen as the future energy source. Solar light-assisted hydrogen evolution through photocatalytic water splitting is one of the most advanced approaches, but it is a non-spontaneous chemical process and restricted by a kinetically demanding oxidation evolution reaction. Sunlight is one of the essential sources for the photoreforming (PR) of biomass waste into solar fuels, or/and lucrative fine chemicals. Hydrogen production through photoreforming of biomass can be considered energy neutral as it requires only low energy to overcome the activation barrier and an alternate method for the water splitting reaction. Towards the perspective of sustainability and zero emission norms, hydrogen production from biomass-derived feedstocks is an affordable and efficient process. Widely used photocatalyst materials, such as metal oxides, sulphides and polymeric semiconductors, still possess challenges in terms of their performance and stability. Recently, a new class of materials has emerged as organic-inorganic hybrid (OIH) photocatalysts, which have the benefits of both components, with peculiar properties and outstanding energy conversion capability. This work examines the most recent progress in the photoreforming of biomass and its derivatives using OIHs as excellent catalysts for hydrogen evolution. The fundamental aspects of the PR mechanism and different methods of hydrogen production from biomass are discussed. Additionally, an interaction between both composite materials at the atomic level has been discussed in detail in the recent literature. Finally, the opportunities and future perspective for the synthesis and development of OIH catalysts are discussed briefly with regards to biomass photo-reforming.

Switching on photocatalytic NO oxidation and proton reduction of NH2-MIL-125(Ti) by convenient linker defect engineering

Photocatalysis technology has been widely adopted to abate typical air pollutants. Nevertheless, developing photocatalysts aimed at improving photocatalytic efficiency is a challenge. Herein, the linker-defect NH₂-MIL-125(Ti) photocatalyst was synthesized through a convenient one-step heating-stirring method (just adjusting multiple temperatures) to firstly realize efficient photocatalytic performances of NO removal and hydrogen evolution. The optimal sample (named 65-NMIL) with a linker-defect content of 32.08% exhibited a NO removal ratio of 65.49%, which was 37.57% higher than that of pristine NH₂-MIL-125(Ti), and displayed better H₂-production activity. Through ESR, it was confirmed that 65-NMIL can generate more •O₂^- and •OH under visible light, and the radical trapping experiment further proved that •O₂^- played a more important role in photocatalytic activity. Moreover, the photocatalytic NO oxidation process was also monitored by in situ DRIFTS, it was found that the defective samples could promote the oxidation of NO and intermediates to the final product (NO₃^-). On the basis of the above-mentioned photocatalytic experimental results and characterization, a possible mechanism or pathway was proposed and illustrated. This work can provide a new strategy for the subsequent defect engineering for photocatalytic MOFs materials to further solve environmental and energy crises.

Synthesis, Structural Characterization, and Biological Activities of Organically Templated Cobalt Phosphite (H2DAB)[Co(H2PO3)4]·2H2O

A novel hybrid cobalt phosphite, (H2DAB)[Co(H2PO3)4] 2H2O, was synthesized by using a slow evaporation method in the presence of cobalt nitrate, phosphorous acid, and 1,4-diaminobutane (DAB = 1,4-diaminobutane) as a structure-directing agent. Single-crystal X-ray diffraction analysis showed that the compound crystallizes in the triclinic system (space group P-1(n.2)) with the following unit cell parameters (Å, °) a = 5.4814 (3), b = 7.5515 (4), c = 10.8548 (6), α = 88.001 (4), β = 88.707 (5), γ = 85.126 (5), and V = 447.33 (4) Å3. The crystal structure is built up from corner-sharing [CoO6] octahedra, forming chains parallel to [001], which are interconnected by H2PO3− pseudo-tetrahedral units. The diprotonated cations, residing between the parallel chains, interact with the inorganic moiety via hydrogen bonds, thus leading to the formation of the 3D crystal structure. The Fourier transform infrared spectrum showed characteristic bands corresponding to the phosphite group and the organic amine. The thermal behavior of the compound mainly consisted of the loss of its organic moiety and the water molecules. The biological tests exhibited significant activity against Candida albicans and Escherichia coli strains at different concentrations, while less inhibitory activity was pronounced against Staphylococcus epidermidis and Saccharomyces cerevisiae, and in the case of multi-cellular organisms, no activity against the nematode model Steinernema feltiae was detected.

Splitting of multiple hydrogen molecules by bioinspired diniobium metal complexes: a DFT study

Splitting of molecular hydrogen (H₂) into bridging and terminal hydrides is a common step in transition metal chemistry. Herein, we propose a novel organometallic platform for cleavage of multiple H₂ molecules, which combines metal centers capable of stabilizing multiple oxidation states, and ligands bearing positioned pendant basic groups. Using quantum chemical modeling, we show that low-valent, early transition metal diniobium(ii) complexes with diphosphine ligands featuring pendant amines can favorably uptake up to 8 hydrogen atoms, and that the energetics are favored by the formation of intramolecular dihydrogen bonds. This result suggests new possible strategies for the development of hydrogen scavenger molecules that are able to perform reversible splitting of multiple H₂ molecules.

Advanced Electrocatalysts Based on Metal-Organic Frameworks

In recent years, metal-organic frameworks (MOFs) have been wildly studied as heterogeneous catalysts due to their diversity of structures and outstanding physical and chemical properties. Meanwhile, MOFs have also been regarded as promising templates for the synthesis of conductive and electrochemically active catalysts. However, in most of the studies, high-temperature annealing is needed to transform nonconductive or low-conductive MOFs to conductive materials for electrocatalyis, during which the unique structures and intrinsic active sites in MOFs can be easily destroyed. Therefore, in recent years, different strategies have been developed for improving the catalytic performances of MOF-based composites for electrochemical reactions with no need of post-treatment. This mini-review highlights the recent advances on MOF-based structures with improved conductivities and electrochemical activities for the application in electrocatalysis. Overall, the advanced MOF-based electrocatalysts include the highly conductive and electrochemically active pristine MOFs, MOFs combined with conductive substrates, and MOFs hybridized with active materials. Finally, we propose the direction for future works on MOF-based electrocatalysts.

Transfer hydrogenation of alkynes into alkenes by ammonia borane over Pd-MOF catalysts

Ammonia borane with both hydridic and protic hydrogens in its structure acted as an efficient transfer hydrogenation agent for selective transformation of alkynes into alkenes in non-protic solvents.

Structural variety of aluminium and gallium coordination polymers based on bis-pyridylethylene: from molecular complexes to ionic networks

Diverse molecular (0D and 1D) as well as ionic (0D, 1D, 2D, mixed 1D–2D) crystal structures of complexes of aluminium and gallium trihalides with bis(4-pyridylethylene) were obtained by solvent-free melt reactions.

Recent advances in the mechanisms of the hydrogen evolution reaction by non-innocent sulfur-coordinating metal complexes

This review comments on the homogeneous HER mechanisms for catalysts carrying S-non-innocent ligands in the light of experimental and computational data.

UiO-66 as a catalyst for hydrogen production via the hydrolysis of sodium borohydride

The exploration of a highly efficient catalyst for the hydrolysis of sodium borohydride (NaBH₄) is a valuable step toward a hydrogen economy.

Mini-review on an engineering approach towards the selection of transition metal complex-based catalysts for photocatalytic H2 production

Advances in transition-metal (Ru, Co, Cu, and Fe) complex-based catalysts since 2000 are briefly summarized in terms of catalyst selection and application for photocatalytic H₂ evolution.