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Covalent organic frameworks (COFs) represent an emerging class of organic photocatalysts. However, their complicated structures lead to indeterminacy about photocatalytic active sites and reaction mechanisms. Herein, we use reticular chemistry to construct a family of isoreticular crystalline hydrazide-based COF photocatalysts, with the optoelectronic properties and local pore characteristics of the COFs modulated using different linkers. The excited state electronic distribution and transport pathways in the COFs are probed using a host of experimental methods and theoretical calculations at a molecular level. One of our developed COFs (denoted as COF-4) exhibits a remarkable excited state electron utilization efficiency and charge transfer properties, achieving a record-high photocatalytic uranium extraction performance of ~6.84 mg/g/day in natural seawater among all techniques reported so far. This study brings a new understanding about the operation of COF-based photocatalysts, guiding the design of improved COF photocatalysts for many applications. Covalent organic frameworks (COFs) represent an emerging class of organic photocatalysts but it remains challenging to gain insight into photocatalytic active sites and reaction mechanisms. Herein, the authors construct a family of isoreticular crystalline hydrazide-based COF photocatalysts, with the optoelectronic properties and local pore characteristics of the COFs modulated using different linkers

Ammonia is a critical chemical in agriculture and industry that is produced on a massive scale via the Haber–Bosch process 1 . The environmental impact of this process, which uses methane as a fuel and feedstock for hydrogen, has motivated the need for more sustainable ammonia production 2 – 5 . However, many strategies that use renewable hydrogen are not compatible with existing methods for ammonia separation 6 – 9 . Given their high surface areas and structural and chemical versatility, metal–organic frameworks (MOFs) hold promise for ammonia separations, but most MOFs bind ammonia irreversibly or degrade on exposure to this corrosive gas 10 , 11 . Here we report a tunable three-dimensional framework that reversibly binds ammonia by cooperative insertion into its metal–carboxylate bonds to form a dense, one-dimensional coordination polymer. This unusual adsorption mechanism provides considerable intrinsic thermal management 12 , and, at high pressures and temperatures, cooperative ammonia uptake gives rise to large working capacities. The threshold pressure for ammonia adsorption can further be tuned by almost five orders of magnitude through simple synthetic modifications, pointing to a broader strategy for the development of energy-efficient ammonia adsorbents. A three-dimensional metal–organic framework reversibly binds ammonia by cooperative insertion into its metal–linker bonds to form a dense, one-dimensional coordination polymer, enabling high-capacity ammonia uptake with intrinsic thermal management.

Millions of distinct metal-organic frameworks (MOFs) can be made by combining metal nodes and organic linkers. At present, over 90,000 MOFs have been synthesized and over 500,000 predicted. This raises the question whether a new experimental or predicted structure adds new information. For MOF chemists, the chemical design space is a combination of pore geometry, metal nodes, organic linkers, and functional groups, but at present we do not have a formalism to quantify optimal coverage of chemical design space. In this work, we develop a machine learning method to quantify similarities of MOFs to analyse their chemical diversity. This diversity analysis identifies biases in the databases, and we show that such bias can lead to incorrect conclusions. The developed formalism in this study provides a simple and practical guideline to see whether new structures will have the potential for new insights, or constitute a relatively small variation of existing structures. At present there are databases with over 500,000 predicted or synthesized MOF structures, yet a method to establish whether a new material adds new information does not exist. Here the authors propose a machine-learning based approach to quantify the structural and chemical diversity in common MOF databases.

Currently, there is considerable interest in developing advanced rechargeable batteries that boast efficient distribution of electricity and economic feasibility for use in large-scale energy storage systems. Rechargeable aqueous zinc batteries are promising alternatives to lithium-ion batteries in terms of rate performance, cost, and safety. In this investigation, we employ Cu 3 (HHTP) 2 , a two-dimensional (2D) conductive metal-organic framework (MOF) with large one-dimensional channels, as a zinc battery cathode. Owing to its unique structure, hydrated Zn 2+ ions which are inserted directly into the host structure, Cu 3 (HHTP) 2 , allow high diffusion rate and low interfacial resistance which enable the Cu 3 (HHTP) 2 cathode to follow the intercalation pseudocapacitance mechanism. Cu 3 (HHTP) 2 exhibits a high reversible capacity of 228 mAh g −1 at 50 mA g −1 . At a high current density of 4000 mA g −1 (~18 C), 75.0% of the initial capacity is maintained after 500 cycles. These results provide key insights into high-performance, 2D conductive MOF designs for battery electrodes. Aqueous zinc batteries are promising candidates for large scale energy storage systems but development of the cathode material remains a challenge. Here, the authors show a conductive 2D metal-organic framework involving intercalation pseudocapacitance mechanism for enhanced rate capability.

Theoretical and experimental results have revealed that the lithium-ion storage capacity for nitrogen-doped graphene largely depends on the nitrogen-doping level. However, most nitrogen-doped carbon materials used for lithium-ion batteries are reported to have a nitrogen content of approximately 10 wt% because a higher number of nitrogen atoms in the two-dimensional honeycomb lattice can result in structural instability. Here we report nitrogen-doped graphene particle analogues with a nitrogen content of up to 17.72 wt% that are prepared by the pyrolysis of a nitrogen-containing zeolitic imidazolate framework at 800 °C under a nitrogen atmosphere. As an anode material for lithium-ion batteries, these particles retain a capacity of 2,132 mA h g −1 after 50 cycles at a current density of 100 mA g −1 , and 785 mAh g −1 after 1,000 cycles at 5 A g −1 . The remarkable performance results from the graphene analogous particles doped with nitrogen within the hexagonal lattice and edges. Nitrogen-doped graphene can be used for lithium storage, but the nitrogen content is often quite low. Here, the authors synthesize graphene analogous with high nitrogen content using a zeolitic imidazolate framework, which show exceptional battery performances.

Cobalt coordinated covalent organic frameworks have attracted increasing interest in the field of CO 2 photoreduction to CO, owing to their high electron affinity and predesigned structures. However, achieving high conversion efficiency is challenging since most Co related coordination environments facilitate fast recombination of photogenerated electron-hole pairs. Here, we design two kinds of Co-COF catalysts with oxygen coordinated Co atoms and find that after tuning of coordination environment, the reported Co framework catalyst with Co-O 4 sites exhibits a high CO production rate of 18000 µmol g −1 h −1 with selectivity as high as 95.7% under visible light irradiation. From in/ex-situ spectral characterizations and theoretical calculations, it is revealed that the predesigned Co-O 4 sites significantly facilitate the carrier migration in framework matrixes and inhibit the recombination of photogenerated electron-hole pairs in the photocatalytic process. This work opens a way for the design of high-performance catalysts for CO 2 photoreduction. A class of inexpensive aminoanthraquinone organic dyes are shown to facilitate visible-light-driven CO 2 reduction. Overall reaction efficiencies were found to be optimal when both electron donating and accepting groups were on a single dye molecule.

Two-dimensional metal-organic frameworks represent a family of materials with attractive chemical and structural properties, which are usually prepared in the form of bulk powders. Here we show a generic approach to fabricate ultrathin nanosheet array of metal-organic frameworks on different substrates through a dissolution–crystallization mechanism. These materials exhibit intriguing properties for electrocatalysis including highly exposed active molecular metal sites owning to ultra-small thickness of nanosheets, improved electrical conductivity and a combination of hierarchical porosity. We fabricate a nickel-iron-based metal-organic framework array, which demonstrates superior electrocatalytic performance towards oxygen evolution reaction with a small overpotential of 240 mV at 10 mA cm −2 , and robust operation for 20,000 s with no detectable activity decay. Remarkably, the turnover frequency of the electrode is 3.8 s −1 at an overpotential of 400 mV. We further demonstrate the promise of these electrodes for other important catalytic reactions including hydrogen evolution reaction and overall water splitting. Metal-organic frameworks are generally considered to be inert catalysts for many electrochemical reactions, however this is not always the case. Here the authors fabricate an ultrathin nanosheet array of metal-organic frameworks exhibiting enhanced performance toward electrocatalytic water splitting.

Few hydrogen adsorbents balance high usable volumetric and gravimetric capacities. Although metal-organic frameworks (MOFs) have recently demonstrated progress in closing this gap, the large number of MOFs has hindered the identification of optimal materials. Here, a systematic assessment of published databases of real and hypothetical MOFs is presented. Nearly 500,000 compounds were screened computationally, and the most promising were assessed experimentally. Three MOFs with capacities surpassing that of IRMOF-20, the record-holder for balanced hydrogen capacity, are demonstrated: SNU-70, UMCM-9, and PCN-610/NU-100. Analysis of trends reveals the existence of a volumetric ceiling at ∼40 g H 2  L −1 . Surpassing this ceiling is proposed as a new capacity target for hydrogen adsorbents. Counter to earlier studies of total hydrogen uptake in MOFs, usable capacities in the highest-capacity materials are negatively correlated with density and volumetric surface area. Instead, capacity is maximized by increasing gravimetric surface area and porosity. This suggests that property/performance trends for total capacities may not translate to usable capacities. Considering the large number of existing synthesised and hypothesised metal-organic frameworks, determining which materials perform best for given applications remains a challenge. Here, the authors screen the usable hydrogen uptake capacities of nearly 500,000 MOFs, and find that three frameworks outperform the current record-holder.

It is known that the main-group metals and their related materials show poor catalytic activity due to a broadened single resonance derived from the interaction of valence orbitals of adsorbates with the broad sp-band of main-group metals. However, Mg cofactors existing in enzymes are extremely active in biochemical reactions. Our density function theory calculations reveal that the catalytic activity of the main-group metals (Mg, Al and Ca) in oxygen reduction reaction is severely hampered by the tight-bonding of active centers with hydroxyl group intermediate, while the Mg atom coordinated to two nitrogen atoms has the near-optimal adsorption strength with intermediate oxygen species by the rise of p-band center position compared to other coordination environments. We experimentally demonstrate that the atomically dispersed Mg cofactors incorporated within graphene framework exhibits a strikingly high half-wave potential of 910 mV in alkaline media, turning a s/p-band metal into a highly active electrocatalyst. Although magnesium-based cofactors are highly active in biochemical reactions, magnesium-based materials generally exhibit poor catalytic activity for oxygen reduction. Here the authors enhance electrocatalytic activity of magnesium through atomic dispersion with a graphene framework.

Carbon-defect engineering in metal single-atom catalysts by simple and robust strategy, boosting their catalytic activity, and revealing the carbon defect-catalytic activity relationship are meaningful but challenging. Herein, we report a facile self-carbon-thermal-reduction strategy for carbon-defect engineering of single Fe-N 4 sites in ZnO-Carbon nano-reactor, as efficient catalyst in Fenton-like reaction for degradation of phenol. The carbon vacancies are easily constructed adjacent to single Fe-N 4 sites during synthesis, facilitating the formation of C-O bonding and lowering the energy barrier of rate-determining-step during degradation of phenol. Consequently, the catalyst Fe-NCv-900 with carbon vacancies exhibits a much improved activity than the Fe-NC-900 without abundant carbon vacancies, with 13.5 times improvement in the first-order rate constant of phenol degradation. The Fe-NCv-900 shows high activity (97% removal ratio of phenol in only 5 min), good recyclability and the wide-ranging pH universality (pH range 3-9). This work not only provides a rational strategy for improving the Fenton-like activity of metal single-atom catalysts, but also deepens the fundamental understanding on how periphery carbon environment affects the property and performance of metal-N 4 sites. Designing carbon-defect engineering is significant for boosting the catalytic performance of metal single-atom sites catalysts. Herein, the authors report a self-carbon-thermal-reduction strategy for carbon defect engineering to boost the Fenton-like activity of single Fe-N 4 sites.