Photocatalytic CO2 -to-Syngas Evolution with Molecular Catalyst Metal-Organic Framework Nanozymes

Enhanced Electrocatalytic Carbon Dioxide Reduction Activity via Local Charge Environment Regulation of Active Sites with Rational Functionalization

Covalent organic frameworks (COFs) are a new emerging class of electrochemical catalysts for the CO2 reduction reaction (CO2RR) with fascinating structural tunability. In this work, to dig more detailed information about the effect of local charge environment regulation of active sites via structure modification on the catalytic performance of COFs for CO2RR, the Gibbs free-energy change (ΔG) of each elementary reaction step involved in the CO2RR and competitive hydrogen evolution reaction (HER) on COF366-Co and its derivatives were examined theoretically. It is observed that the valence band maximum (VBM) energy level of the COFs is increased by incorporation of electron-donating groups, and then the charge distribution on the Co center of COF366-Co is increased due to the increased charge-transfer amount from the electron-occupied N sp2 hybrid orbitals to the empty Co3d orbitals. For incorporating electron-withdrawing groups, the VBM energy level and the d-band center (ξd) of the Co atom are downshifted, and the d-band center gets closer to the occupied VBM energy level as the VBM is decreased to a larger extent than the ξd. As a result, electrosorption of the intermediate is facilitated and the CO2RR performance is enhanced by such a linker functionalization strategy, especially for electron-withdrawing groups. Our study highlights the key role that controlled local electrical environment via chemical structure modification of COFs can play in regulating the catalytic activity for its CO2RR applications.

Dual-Action NO Delivery from One Mixed Metal Metal-Organic Framework

Owing to their varied and controllable reticular chemistry, metal-organic frameworks (MOFs) represent excellent, structurally precise material platforms for creating multifunctional devices. This flexibility allows the design of MOFs to feature two different delivery mechanisms for the medicinal gas nitric oxide (NO) within one structure: a rapid, kinetic release of preadsorbed NO and subsequent continuous, catalytic generation of NO on exposure to a suitable substrate. In our approach, this was realized by preparing mixed metal MOF-74 analogues (also known as CPO-27) combining two metals, namely, nickel and copper. The introduction of 10 atom % nickel into Cu-MOF-74 provides a material platform with a number of excellent properties: (i) a high capacity of NO can be released with a moisture-triggered mechanism; (ii) the available copper sites allow the (potentially indefinite) catalytic generation of NO in the presence of S-nitrosoglutathione (GSNO), a store of NO in the human body; and (iii) it features prolonged material stability on exposure to phosphates. This material therefore shows great promise as part of the next generation of multifunctional MOF-based medicinal devices.

Engineering ZnIn2S4 with efficient charge separation and utilization for synergistic accelerate dual-function photocatalysis

Combining photocatalytic reduction with organic synthetic oxidation in the same photocatalytic redox system can effectively utilize photoexcited electrons and holes from solar to chemical energy. Here, we stabilized 0D Au clusters on the substrate surface of Zn vacancies modified 2D ZnIn2S4 (ZIS-V) nanosheets by chemically bonding Au-S interaction, forming surfactant functionalized Au/ZIS-V photocatalyst, which can not only synergistic accelerate the selective oxidation of phenylcarbinol to value-added products coupled with clean energy hydrogen production but also further drive photocatalytic CO2-to-CO conversion. An internal electric field of Au/ZIS-V ohmic junction and Zn vacancies synchronously promote the photoexcited charge carrier separation and transfer to optimized active sites for redox reactions. Compared with CO2 reduction in water and the pristine ZnIn2S4, the reaction thermodynamics and kinetics of CO2 reduction over the Au/ZIS-V were simultaneously improved about 11.09 and 45.51 times, respectively. Moreover, the photocatalytic redox mechanisms were also profoundly studied by 13CO2 isotope tracing tests, in situ electron paramagnetic resonance (in situ EPR), in situ X-ray photoelectron spectroscopy (in situ XPS), in situ diffuse reflection infrared Fourier transform spectroscopy (in situ DRIFTS) and density functional theory (DFT) characterizations, etc. These results demonstrate the advantages of vacancies coupled with metal clusters in the synergetic enhancement of photocatalytic redox performance and have great potential applications in a wide range of environments and energy.

Photocatalytic Semi-Hydrogenation of Acetylene to Polymer-Grade Ethylene with Molecular and Metal-Organic Framework Cobaloximes

The semi-hydrogenation of acetylene in ethylene-rich gas streams is a high-priority industrial chemical reaction for producing polymer-grade ethylene. Traditional thermocatalytic routes for acetylene reduction to ethylene, despite progress, still require high temperatures and high H2 consumption, possess relatively low selectivity, and use a noble metal catalyst. Light-powered strategies are starting to emerge, given that they have the potential to use directly the abundant and sustainable solar irradiation, but are ineffective. Here an efficient, >99.9% selective, visible-light powered, catalytic conversion of acetylene to ethylene is reported. The catalyst is a homogeneous molecular cobaloxime that operates in tandem with a photosensitizer at room temperature and bypasses the use of non-environmentally friendly and flammable H2 gas feed. The reaction proceeds through a cobalt-hydride intermediate with ≈100% conversion of acetylene under competitive (ethylene co-feed) conditions after only 50 min, and with no evolution of H2 or over-hydrogenation to ethane. The cobaloxime is further incorporated as a linker in a metal-organic framework; the result is a heterogeneous catalyst for the conversion of acetylene under competitive (ethylene co-feed) conditions that can be recycled up to six times and remains catalytically active for 48 h, before significant loss of performance is observed.

Engineering the electron localization of metal sites on nanosheets assembled periodic macropores for CO2 photoreduction

Photocatalytic conversion of CO2 into syngas is highly appealing, yet still suffers from the undesirable product yield due to the sluggish carrier transfer and the uncontrollable affinity between catalytic sites and intermediates. Here we report the fabrication of Co sites with tunable electron localization capability on two dimensional (2D) nanosheets assembled three dimensional (3D) ordered macroporous framework (3DOM-NS). The as-prepared Co-based 3DOM-NS catalysts exhibit attractive photocatalytic performances toward CO2 reduction, among which the cobalt sulfide one (3DOM Co-SNS) shows the highest syngas generation rate up to 347.3 μmol h-1 under the irradiation of visible light and delivers a remarkable catalytic activity (1150.7 μmol h-1) in a flow reaction system under natural sunlight. Mechanism studies reveal that the high electron localization of metal sites in 3DOM Co-SNS strengthens the interaction between Co and HCOO* via the orbital interactions of dyz/dxz-p and s-s, thus facilitating the cleaving process of C-O bond. Additionally, the ordered macroporous framework with nanosheet subunits elevates the transfer efficiency of photoexcited electrons, which contributes to its high activity.

Reticular Materials for Photocatalysis

Photocatalysis leverages solar energy to overcome the thermodynamic barrier, enabling efficient chemical reactions under mild conditions. It can greatly reduce reliance on traditional energy sources and has attracted significant research interest. Reticular materials, including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), represent a class of crystalline materials constructed from molecular building blocks linked by coordination and covalent bonds, respectively. Reticular materials function as heterogeneous catalysts, combining well-defined structures and high tailorability akin to homogeneous catalysts. In this review, the regulation of light absorption, charge separation, and surface reactions in the photocatalytic process through precise molecular-level design based on the features of reticular materials is elaborated. Notably, for MOFsmicroenvironment modulation around catalytic sites affects photocatalytic performance is delved, with emphasis on their unique dynamic and flexible microenvironments. For COFs, the inherent excitonic effects due to their fully organic nature is discussed and highlight the strategies to regulate excitonic effects for charge- and/or energy-transfer-mediated photocatalysis. Finally, the current challenges and future directions in this field, aiming to provide a comprehensive understanding of how reticular materials can be optimized for enhanced photocatalysis is discussed.

A Change of Pace: Record Photoresponse through Spirooxazine Confinement in a Metal-Organic Matrix

Modern and upcoming high-speed optoelectronics as well as secure data storage or solar energy harvesting technologies integrating stimuli-responsive materials fully rely on the fundamental concept of rapid transitions between discrete states possessing different properties. Relatively slow transition kinetics between those states for commonly used classes of photochromic compounds in solution or bulk solids severely restrict the applicability of stimuli-responsive materials for device development. Herein, we report a multivariate strategy based on a photochromic spirooxazine derivative, coordinatively integrated in the solvent-free confined space of a solid-state matrix, such as a metal-organic framework (MOF), for the first time, resulting in the fastest photoresponse reported for any solid-state material to date. The photoisomerization rate for the developed photochromic material was estimated to be 126 s-1, surpassing any literature reports to the best of our knowledge. We also shed light on the fundamentals of the correlation between framework topology, the nature of organic linkers, and the presence/absence of organic solvent within the scaffold voids on the material photoresponse using a series of isoreticular frameworks. Overall, the presented conceptual approach allows for tailoring the isomerization kinetics of photochromic molecules in the solid state over a range of 4 orders of magnitude-an unprecedented span that provides a pathway for addressing challenges associated with the response rate and photoisomerization, which are key criteria in stimuli-responsive material development.

Multiple Enzyme-Mimicking CuMOF-Driven Self-Cascading Antioxidant Reaction for Synergistic Electrochemiluminescence Modulation in Ultrasensitive Biosensing

Developing nanozyme-based free radical scavenging is a promising signal modulation approach for ECL sensing. Nevertheless, the relatively low antioxidant activity and inherent pro-oxidant activity of numerous nanozymes have significantly hindered the development of this strategy. Here a biofunctional copper-based metal-organic framework (CuMOF) with multiple enzyme-mimicking activities was employed for the modulation of the ECL immunosensor, guided by the self-cascade antioxidant reaction. The inherent SOD, CAT, and the capacity to eliminate ·OH endow CuMOF with powerful synergistic antioxidant effects while little pro-oxidant activities were displayed, enabling efficient scavenging of the O2·- produced during the electrochemical oxidation of H2O2. Subsequently, the nanoconfinement effect of the layered double hydroxide was introduced to ensure a steady ECL signal. The suggested ECL immunosensor, using aflatoxin B1 as a proof-of-concept target, demonstrated a detection range spanning from 0.001 pg/mL to 10 ng/mL, with the detection limit calculated to be 0.18 fg/mL. This exceptional achievement greatly broadens the range of possible uses for nanozyme-based radical scavenging modulated ECL analysis.

Photochargeable Mn-Based Metal-Organic Framework and Decoupled Photocatalysis

Designing multifunctional materials that mimic the light-dark decoupling of natural photosynthesis is a key challenge in the field of energy conversion. Herein, we introduce MnBr-253, a precious metal-free metal-organic framework (MOF) built on Al nodes, bipyridine linkers and MnBr(CO)3(bipyridine) complexes. Upon irradiation, MnBr-253 colloids demonstrate an electron photocharging capacity of ~42 C ⋅ g-1 MOF, with state-of-the-art photocharging rate (1.28 C ⋅ s-1 ⋅ g-1 MOF) and incident photon-to-electron conversion efficiency of ~9.4 % at 450 nm. Spectroscopic and computational studies support effective electron accumulation at the Mn complex while high porosity and Mn loading account for the notable electron storage performance. The charged MnBr-253 powders were successfully applied for hydrogen evolution under dark conditions thus emulating the light-decoupled reactivity of photosynthesis.

Heterogeneous CuS QDs/BiVO4@Y2O2S Nanoreactor for Monitorable Photocatalysis

Exploration of multifunctional integrated catalysts is of great significance for photocatalysis toward practical application. Herein, a 1D confined nanoreactor with a heterogeneous core-shell structure is designed for synergies of efficient catalysis and temperature monitoring by custom encapsulation of Z-scheme heterojunction CuS quantum dots/BiVO4 (CuS QDs/BiVO4) and Y2O2S-Er, Yb. The dispersed active sites created by the QDs with high surface energy improve the mass transfer efficiency, and the efficient electron transport channels at the heterogeneous interface extend the carrier lifetime, which endows the nanoreactor with excellent catalytic performance. Meanwhile, real-time temperature monitoring is realized based on the thermally coupled levels 2H11/2/4S3/2→4I15/2 of Er3+ using fluorescence intensity ratio, which enables the monitorable photocatalysis. Furthermore, the nanoreactor with a multidimensional structure increases effective intermolecular collisions to facilitate the catalytic process by restricting the reaction within distinct enclosed spaces and circumvents potential unknown interaction effects. The design of multi-space nanoconfined reactors opens up a new avenue to modulate catalyst function, providing a unique perspective for photocatalytic applications in the mineralization of organic pollutants, hydrogen production, and nitrogen fixation.

Covalent Organic Framework with Donor1-Acceptor-Donor2 Motifs Regulating Local Charge of Intercalated Single Cobalt Sites for Photocatalytic CO2 Reduction to Syngas

Covalent organic framework (COF) has attracted increasing interest in photocatalytic CO2 reduction, but it remains a challenge to achieve high conversion efficiency owing to the insufficient active site and fast charge recombination. Rationally optimizing the electronic structures of COF to regulate the local charge of active sites precisely is the key point to improving catalytic performance. Herein, intercalated single Co sites coordinated by imine-N motifs have been designed by using trinuclear copper-based imine-COFs with distinct electronic moieties via a molecular engineering strategy. It is confirmed that the charge delivery property and local charge distribution of these heterometallic frameworks can be profoundly influenced by electronic structures. Among these featured structures with mixed-state copper clusters, Co/Cu3-TPA-COF stands out for an exceptional photocatalytic CO2 reduction activity and tunable syngas (CO/H2) ratio by changing various bipyridines. Experimental and theoretical results indicate that interlayer Co-imine N motifs on the donor1-acceptor-donor2 structures facilitate the formation of a highly separated electron-hole state, which effectively induces the oriented electron transfer from dual electron donors to Co centers, achieving an enhanced CO2 activation and reduction. This work opens up an avenue for the design of high-performance COF-based catalysts for photocatalytic CO2 reduction.

Ionic Liquid Modified Fe-Porphyrinic Metal-Organic Frameworks as Efficient and Selective Photocatalysts for CO2 Reduction

Porphyrin-based metal-organic frameworks (MOFs) are ideal platforms for heterogeneous photocatalysts toward CO2 reduction. To further explore photocatalytic MOF systems, it is also necessary to consider their ability to fine-tune the microenvironments of the active sites, which affects their overall catalytic operation. Herein, a kind of ionic liquid (IL, here is 3-butyric acid-1-methyl imidazolium bromide, BAMeImBr) was anchored to iron-porphyrinic Zr-MOFs with different amounts to obtain ILx@MOF-526 (MOF-526 = Zr6O4(OH)4(FeTCBPP)3, FeTCBPP = iron 5,10,15,20-tetra[4-(4'-carboxyphenyl)phenyl]-porphyrin, x = 100, 200, and 400). ILx@MOF-526 series was designed to investigate the effects of the microenvironmental and electronic structural modification on the efficiency and selectivity of the photochemical reduction of CO2 after introducing IL fragments. Compared to parent MOF-526, the production and selectivity of CO were greatly improved in the absence of any photosensitizer under visible light by the ILx@MOF-526 series. Among them, the CO yield of IL200@MOF-526 was up to 14.0 mmol g-1 within 72 h with a remarkable CO selectivity of 97%, which is superior to that of MOF-526 without BAMeIm+ modification and other amounts of BAMeIm+ loaded. Furthermore, density functional theory calculations were performed to study the mechanism of the CO2 reduction.

Using Cu-Based Metal-Organic Framework as a Comprehensive and Powerful Antioxidant Nanozyme for Efficient Osteoarthritis Treatment

Developing nanozymes with effective reactive oxygen species (ROS) scavenging ability is a promising approach for osteoarthritis (OA) treatment. Nonetheless, numerous nanozymes lie in their relatively low antioxidant activity. In certain circumstances, some of these nanozymes may even instigate ROS production to cause side effects. To address these challenges, a copper-based metal-organic framework (Cu MOF) nanozyme is designed and applied for OA treatment. Cu MOF exhibits comprehensive and powerful activities (i.e., SOD-like, CAT-like, and •OH scavenging activities) while negligible pro-oxidant activities (POD- and OXD-like activities). Collectively, Cu MOF nanozyme is more effective at scavenging various types of ROS than other Cu-based antioxidants, such as commercial CuO and Cu single-atom nanozyme. Density functional theory calculations also confirm the origin of its outstanding enzyme-like activities. In vitro and in vivo results demonstrate that Cu MOF nanozyme exhibits an excellent ability to decrease intracellular ROS levels and relieve hypoxic microenvironment of synovial macrophages. As a result, Cu MOF nanozyme can modulate the polarization of macrophages from pro-inflammatory M1 to anti-inflammatory M2 subtype, and inhibit the degradation of cartilage matrix for efficient OA treatment. The excellent biocompatibility and protective properties of Cu MOF nanozyme make it a valuable asset in treating ROS-related ailments beyond OA.

Advances in heterogeneous single-cluster catalysis

Heterogeneous single-cluster catalysts (SCCs) comprising atomically precise and isolated metal clusters stabilized on appropriately chosen supports offer exciting prospects for enabling novel chemical reactions owing to their broad structural diversity with unparalled opportunities for engineering their properties. Although the pioneering work revealed intriguing performance trends of size-selected metal clusters deposited on supports, synthetic and analytical challenges hindered a thorough understanding of surface chemistry under realistic conditions. This Review underscores the importance of considering the cluster environment in SCCs, encompassing the development of robust metal-support interactions, precise control over the ligand sphere, the influence of reaction media and dynamic behaviour, to uncover new reactivities. Through examples, we illustrate the criticality of tailoring the entire catalytic ensemble in SCCs to achieve stable and selective performance with practically relevant metal coverages. This expansion in application scope transcends from model reactions to complex and technically relevant reactions. Furthermore, we provide a perspective on the opportunities and future directions for SCC design within this rapidly evolving field.