Effect of Facet on Local Electron Density of Oxygen Vacancy in Catalysts for Nitrogen Electro-Reduction.

Oxygen-vacancy (Ov) engineering is an effective strategy to manipulate the electronic configuration of catalysts for electrochemical nitrogen reduction reaction (eNRR). The influence of the stable facet on the electronic configuration of Ov is widely studied, however, the effect of the reactive facet on the local electron density of Ov is unveiled. In this work, an eNRR electrode R(111)-TiO2/HGO is provided with a high proportion exposed reactive facet (111) of rutile-TiO2 (denoted as R(111)-TiO2) nanocrystals with Ov anchored in hierarchically porous graphite oxide (HGO) nanofilms. The R(111)-TiO2/HGO exhibits excellent eNRR performance with an NH3 yield rate of 20.68 µg h-1 cm-2, which is ≈20 times the control electrode with the most stable facet (110) exposed (R(110)-TiO2/HGO). The experimental data and theoretical simulations reveal that the crystal facet (111) has a positive effect on regulating the local electron density around the oxygen vacancy and the two adjacent Ti-sites, promoting the π-back-donation, minimizing the eNRR barrier, and transforming the rate determination step to *NNH→*NNHH. This work illuminates the effect of crystal facet on the performance of eNRR, and offers a novel strategy to design efficient eNRR catalysts.

A 3D Lithiophilic Host for Dendrite-Free Lithium Metal Anode via One-Step Carbonization of an Energetic Metal-Organic Framework.

Low Coulombic efficiency (CE) and safety issues are huge problems that hinder the practical application of Li metal anodes. Constructing Li host structures decorated with functional species can restrain the growth of Li dendrites and alleviate the great volume change. Here, a 3D porous carbonaceous skeleton modified with rich lithiophilic groups (Zn, ZnO, and Zn(CN)2 ) is synthesized as a Li host via one-step carbonization of a triazole-containing metal-organic framework. The nano lithiophilic groups serve as preferred sites for Li nucleation and growth, regulating a uniform Li+ flux and uniform current density distribution. In addition, the 3D porous network functions as a Li reservoir that provides rich internal space to store Li, thus alleviating the volumetric expansion during Li plating/stripping process. Thanks to these component and structural merits, an ultra-low overpotential for Li deposition is achieved, together with high CE of over 99.5% for more than 500 cycles at 1 mA cm-2 and 1 mAh cm-2 in half cells. The symmetric cells exhibit a prolonged cycling of 900 h at 1 mA cm-2 . The full cells by coupling Zn/ZnO/Zn(CN)2 @C-Li anode with LiFePO4 cathode deliver a high capacity retention of 94.3% after 200 cycles at 1 C.

Simulation of Hydrogen Adsorption in Hierarchical Silicalite: Role of Electrostatics and Surface Chemistry.

Adsorption in nanoporous materials is one strategy that can be used to store hydrogen at conditions of temperature and pressure that are economically viable. Adsorption capacity of nanoporous materials depends on surface area which can be enhanced by incorporating a hierarchical pore structure. We report grand canonical Monte Carlo (GCMC) simulation results on the adsorption of hydrogen in hierarchical models of silicalite that incorporate 4 nm wide mesopores in addition to the 0.5 nm wide micropores at 298 K, using different force fields to model hydrogen. Our results suggest that incorporating mesopores in silicalite can enhance adsorption by at least 20% if electrostatic interactions are not included and up to 100% otherwise. Incorporating electrostatic interactions results in higher adsorption by close to 100% at lower pressures for hierarchical silicalite whereas for unmodified silicalite, it is less significant at all pressures. Hydroxylating the mesopore surface in hierarchical silicalite results in an enhancement in adsorption at pressures below 1 atm and suppression by up to 20% at higher pressures. Temperature dependence at selected pressures exhibits expected decrease in adsorption amounts at higher temperatures. These findings can be useful in the engineering, selection, and optimization of nanoporous materials for hydrogen storage.

Micelles Cascade Assembly to Tandem Porous Catalyst for Waste Plastics Upcycling.

Catalytic upcycling of polyolefins into high-value chemicals represents the direction in end-of-life plastics valorization, but poses great challenges. Here, we report the synthesis of a tandem porous catalyst via a micelle cascade assembly strategy for selectively catalytic cracking of polyethylene into olefins at a low temperature. A hierarchically porous silica layer from mesopore to macropore is constructed on the surface of microporous ZSM-5 nanosheets through cascade assembly of dynamic micelles. The outer macropore arrays can adsorb bulky polyolefins quickly by the capillary and hydrophobic effects, enhancing the diffusion and access to active sites. The middle mesopores present a nanoconfinement space, pre-cracking polyolefins into intermediates by weak acid sites, which then transport into zeolites micropores for further cracking by strong Brønsted acid sites. The hierarchically porous and acidic structures, mimicking biomimetic protease catalytic clefts, ideally match the tandem cracking steps of polyolefins, thus suppressing coke formation and facilitating product escape. As a result, light hydrocarbons (C1-C7) are produced with a yield of 443 mmol gZSM-5 -1, where 74.3 % of them are C3-C6 olefins, much superior to ZSM-5 and porous silica catalysts. This tandem porous catalyst exemplifies a superstructure design of catalytic cracking catalysts for industrial and economical upcycling of plastic wastes.

Shape-Controlled Synthesis of Covalent Organic Frameworks Enabled by Polymerization-Induced Phase Separation.

The shape and morphology modulations of covalent organic frameworks (COFs) are both difficult, but are of significance to tackle to realize high-performance and practical applications. Here, a two-step method is reported that separates the phase separation and crystallization processes for the shape-controlled synthesis of COFs. The insight into the polymerization-induced phase separation (PIPS) allows for the flexible shaping of COFs into column, rod, film and others, as well as for constructing hierarchically porous structure. The as-synthesized COF monoliths are crack-free, no powder detaching, and show 214 MPa of compressive modulus. The resulting good permeability and mechanical flexibility enable COF films to apply for flow-through adsorption and extraction of pollutants at high flow rates. This work successfully resolves the contradiction between PIPS and crystallization, offering a general approach for scalable production of COFs with desired shapes, sizes, and morphologies.

Red Phosphorus Anchored on Nitrogen-Doped Carbon Bubble-Carbon Nanotube Network for Highly Stable and Fast-Charging Lithium-Ion Batteries.

A nitrogen-doped carbon bubble-carbon nanotube@red phosphorus (N-CBCNT@rP) network composite is fabricated, featuring an rP film embedded in a highly N-doped CBCNT network with hierarchical pores of different sizes and interior void spaces. Highly N-doped CBCNT with an optimized structure is utilized to achieve an ultrahigh rP content of 53 wt% in the N-CBCNT@rP composite by the NP bond, which shows a record rP content for rP-carbon composites by the vaporization-condensation process. When tested as an anode for lithium-ion batteries, the N-CBCNT@rP composite exhibits an ultrahigh initial Coulombic efficiency of 87.5%, high specific capacity, outstanding rate performance, and superior cycling stability at a high current density (capacity decay of 0.011% per cycle over 1500 cycles at 5 A g-1 ), which is the lowest capacity fading rate of those previously reported for rP-based electrodes. The superior lithium-ion storage performance of the N-CBCNT@rP composite electrode is primarily attributed to its structure. The 3D hierarchical conducting network of the N-CBCNT@rP composite with abundant N-P bonds endows the entire electrode with maximized conductivity for superior ion and electron transfer kinetics. Moreover, N-CBCNT networks with hierarchical pores of different sizes can fix the location of rP, prevent agglomeration, and avoid volume expansion of rP.

Hierarchically Porous Poly(ionic liquid) - Organic Cage Composite Membrane for Efficient Iodine Capture.

The effective capture of iodine with high volatility and poisonousness is significant for reprocessing the spent nuclear fuel. In this article, we report a hierarchically porous poly(ionic liquid)-organic cage composite membrane (PIL@CC3) possessing a gradient content distribution of CC3 cage crystals throughout the membrane to capture iodine vapor. The introduction of microporous CC3 can significantly enhance the uptake capacity of iodine up to 980 mg g-1 , which is superior to that of a pristine PIL membrane carrying large meso- and macropores (99 mg g-1 ), and CC3 crystalline powder (662 mg g-1 ). Such enhanced performance benefits from the micro-meso-macroporous structure of the PIL@CC3 membrane in which the large meso- and macropores facilitate the mass transfer of iodine molecules from the external environment into the surface of the CC3 crystal, followed by diffusion of iodine molecules from the CC3 surface into the interior and exterior pores of the CC3 crystal. In addition, the asymmetric distribution of CC3 crystals across the PIL@CC3 membrane also displays its advantage in intercepting trace iodine, revealing its great potential for practical application. This study provides an idea for constructing hierarchically porous membrane composites for the removal of toxic vapors.

Three-Dimensional Reticulated, Spongelike, Resilient Aerogels Assembled by SiC/Si3N4 Nanowires.

For nanofibrous aerogels, a three-dimensional porous structure with interwoven nanofibers as a pore wall has become an urgent demand, and it remains to be a challenge to ensure the mechanical stability and thermal insulation. Other than the reported nanofiber as raw materials to generate three-dimensional cellular nanofibrous aerogels, an alternative low-cost and facile procedure has been proposed here via tactfully utilizing polymer sponge as a template attached with reactive particles, followed by a carbothermal reduction process to realize nanowire growth and their replacement of the original framework. The resulting spongy aerogels with numerous interlaced SiC/Si3N4 nanowires as a skeleton exhibit an ultrahigh porosity of 99.79%. Meanwhile, compressive elasticity after a compression at strain of 35% for 400 cycles, a low thermal conductivity of 23.19 mW/(m K), an excellent absorption capacity of 33.9-95.3 times for varied organic solvents removal, along with flexibility in shape design favored by the initial organic sponge make this nanofibrous aerogel an ideal material for heat shielding, absorption, or catalyst support.

Confined Thermolysis for Oriented N-Doped Carbon Supported Pd toward Stable Catalytic and Energy Storage Applications.

Carbon-based nanomaterials have been widely utilized in catalysis and energy-related fields due to their fascinating properties. However, the controllable synthesis of porous carbon with refined morphology is still a formidable challenge due to inevitable aggregation/fusion of resulted carbon particles during the high-temperature synthetic process. Herein, a hierarchically oriented carbon-structured (fiber-like) composite is fabricated by simultaneously taking advantage of a confined pyrolysis strategy and disparate bond environments within metal-organic frameworks (MOFs). In the resultant composite, the oriented carbon provides a fast mass (molecule/ion/electron) transfer efficiency; the doping-N atoms can anchor or act as active sites; the mesoporous SiO2 (mSiO2 ) shell not only effectively prevents the derived carbon or active metal nanoparticles (NPs) from aggregation or leaching, but also acts as a "polysulfide reservoir" in the Li-S batteries to suppress the "shuttle" effect. Benefiting from these advantages, the synthesized composite Pd@NDHPC@mSiO2 (NDHPC means N-doped hierarchically porous carbon) exhibits extremely high catalytic activity and stability toward the one-pot Knoevenagel condensation-hydrogenation reaction. Furthermore, the oriented NDHPC@mSiO2 manifests a boosted capacity and cycling stability in Li-S batteries compared to the counterpart that directly pyrolyzes without silica protection. This report provides an effective strategy of fabricating hierarchically oriented carbon composites for catalysis and energy storage applications.

Defect-Rich Hierarchical Porous UiO-66(Zr) for Tunable Phosphate Removal.

The introduction of defects into hierarchical porous metal-organic frameworks (HP-MOFs) is of vital significance to boost their adsorption performance. Herein, an advanced template-assisted strategy has been developed to fine-tune the phosphate adsorption performance of HP-MOFs by dictating the type and number of defects in HP-UiO-66(Zr). To achieve this, monocarboxylic acids of varying chain lengths have been employed as template molecules to fabricate an array of defect-rich HP-UiO-66(Zr) derivatives following removal of the template. The as-prepared HP-UiO-66(Zr) exhibits a higher sorption capacity and faster sorption rate compared to the pristine UiO-66(Zr). Particularly, the octanoic acid-modulated UiO-66(Zr) exhibits a high adsorption capacity of 186.6 mg P/g and an intraparticle diffusion rate of 6.19 mg/g·min0.5, which are 4.8 times and 1.9 times higher than those of pristine UiO-66(Zr), respectively. The results reveal that defect sites play a critical role in boosting the phosphate uptake performance, which is further confirmed by various advanced characterizations. Density functional theory (DFT) calculations reveal the important role of defects in not only providing additional sorption sites but also reducing the sorption energy between HP-UiO-66(Zr) and phosphate. In addition, the hierarchical pores in HP-UiO-66(Zr) can accelerate the phosphate diffusion toward the active sorption sites. This work presents a promising route to tailor the adsorption performance of MOF-based adsorbents via defect engineering.

Nitrogen-Doped Carbon Composites with Ordered Macropores and Hollow Walls.

Metal-organic frameworks provide versatile templates for the fabrication of various metal/carbon materials, but most of the derived composites possess only microspores, limiting the accessibility of embedded active sites. Herein, we report the construction of cobalt/nitrogen-doped carbon composites with a three-dimensional (3D) ordered macroporous and hollow-wall structure (H-3DOM-Co/NC) using a single-crystal ordered macropore (SOM)-ZIF-8@ZIF-67 as precursor. During the pyrolysis, the interconnected macroporous structure of SOM-ZIF-8@ZIF-67 is mostly preserved, whereas the pore wall achieves a solid-to-hollow transformation with Co nanoparticles formed in the hollow walls. The 3D-ordered macroporous carbon skeleton may effectively promote long-range mass transfer and the hollow wall can facilitate local accessibility of active sites. This unique structure can greatly boost its catalytic activity in the selective hydrogenation of biomass-derived furfural to cyclopentanol, much superior to its counterparts without this well-designed hierarchically porous structure.

Hierarchically Porous Organic Cages.

Imparting mesopores to organic cages of an intrinsic microporous nature to build up hierarchically porous cage soft materials is a grand challenge and will reshape the property and application scope of traditional organic cage molecules. Herein, we discovered how to engineer mesopores into microporous organic cages via their host-guest interactions with long chain ionic surfactants. Equally important, the ionic head of surfactants equips the supramolecularly assembled porous structures with charge-selective uptake and release function in solution. Interestingly, such hierarchically porous organic cage can serve as a nanoreactor once trapping enzymes within the cavity, which show 5-fold enhanced activity of enzymatic catalysis when compared with the free enzymes.

Tailoring Hierarchically Porous Nitrogen-, Sulfur-Codoped Carbon for High-Performance Supercapacitors and Oxygen Reduction.

Heteroatom-doped carbon materials are intensively studied in supercapacitors and fuel cells, because of their great potential for sustainably bearing on the energy crisis and environmental pollution. Although enormous efforts are put in material perfection with a hierarchically porous microstructure, the simultaneous optimization of both porous structures and surface functionalities is hard to achieve due to inevitable concurrent dopant leaching effect and structural collapse under required high pyrolysis temperature. In this study, an in situ dehalogenation polymerization and activation protocol is introduced to synthesize nitrogen- and sulfur-codoped carbon materials (NS-PCMs) with hierarchical pore distribution and abundant surface doping, which endows them with good conductivity, abundant accessible active sites, and efficient mass transport. As a result, the as-prepared carbon materials (NS-a-PCM-1000) show an excellent mass specific capacitance of 461.5 F g-1 at a current density of 0.1 A g-1 , long cycle life (>23 k, 10 A g-1 ), and high device energy and power density (17.3 Wh kg-1 , 250 W kg-1 ). Significantly, NS-a-PCM-1000 also exhibits one of the highest oxygen reduction reaction activities (onset potential of 1.0 V vs reversible hydrogen electrode) in alkaline media among all reported metal-free catalysts.

Preparation of hydrophobic hierarchical pore carbon-silica composite and its adsorption performance toward volatile organic compounds.

Carbon-silica materials with hierarchical pores consisting of micropores and mesopores were prepared by introducing nanocarbon microspheres derived from biomass sugar into silica gel channels in a hydrothermal environment. The physicochemical properties of the materials were characterized by nitrogen physical adsorption (BET), scanning electron microscopy (SEM), and thermogravimetric (TG), and the adsorption properties of various organic waste gases were investigated. The results showed that microporous carbon materials were introduced successfully into the silica gel channels, thus showing the high adsorption capacity of activated carbon in high humidity organic waste gas, and the high stability and mechanical strength of the silica gel. The dynamic adsorption behavior confirmed that the carbon-silica material had excellent adsorption capacity for different volatile organic compounds (VOCs). Furthermore, the carbon-silica material exhibited excellent desorption characteristics: adsorbed toluene was completely desorbed at 150°C, thereby showing superior regeneration characteristics. Both features were attributed to the formation of hierarchical pores.

Rapid Generation of Hierarchically Porous Metal-Organic Frameworks through Laser Photolysis.

Hierarchically porous metal-organic frameworks (HP-MOFs) facilitate mass transfer due to mesoporosity while preserving the advantage of microporosity. This unique feature endows HP-MOFs with remarkable application potential in multiple fields. Recently, new methods such as linker labilization for the construction of HP-MOFs have emerged. To further enrich the synthetic toolkit of MOFs, we report a controlled photolytic removal of linkers to create mesopores within microporous MOFs at tens of milliseconds. Ultraviolet (UV) laser has been applied to eliminate "photolabile" linkers without affecting the overall crystallinity and integrity of the original framework. Presumably, the creation of mesopores can be attributed to the missing-cluster defects, which can be tuned through varying the time of laser exposure and ratio of photolabile/robust linkers. Upon laser exposure, MOF crystals shrank while metal oxide nanoparticles formed giving rise to the HP-MOFs. In addition, photolysis can also be utilized for the fabrication of complicated patterns with high precision, paving the way towards MOF lithography, which has enormous potential in sensing and catalysis.

Harnessing Bottom-Up Self-Assembly To Position Five Distinct Components in an Ordered Porous Framework.

Positioning a diverse set of building blocks in a well-defined array enables cooperativity amongst them and the systematic programming of functional properties. The extension of this concept to porous metal-organic frameworks (MOFs) is challenging since the installation of multiple components in a well-ordered framework requires careful design of the lattice topology, judicious selection of building blocks, and precise control of the crystallization parameters. Herein, we report how we met these challenges to prepare the first quinary MOF structure, FDM-8, by bottom-up self-assembly from two metals, ZnII and CuI , and three distinct carboxylate- and pyrazolate-based linkers. With a surface area of 3643 m2 g-1 , FDM-8 contains hierarchical pores and shows outstanding methane-storage capacity at high pressure. Furthermore, functional groups introduced on the linkers became compartmentalized in predetermined arrays in the pores of the FDM-8 framework.

Generation of Hierarchical Porosity in Metal-Organic Frameworks by the Modulation of Cation Valence.

Hierarchically porous metal-organic frameworks (HP-MOFs) have attracted great attention owing to their advantages over microporous MOFs in some applications. Despite many attempts, the development of a facile approach to generate HP-MOFs remains a challenge. Herein we develop a new strategy, namely the modulation of cation valence, to create hierarchical porosity in MOFs. Some of the CuII metal nodes in MOFs can be transformed into CuI via reducing vapor treatment (RVT), which partially changes the coordination mode and thus breaks coordination bonds, resulting in the formation of HP-MOF based on the original microporous MOF. Both the experimental results and the first-principles calculation show that it is easy to tailor the amount of CuI and subsequent hierarchical porosity by tuning the RVT duration. It is found that the resultant HP-MOFs perform much better in the capture of aromatic sulfides than the original microporous MOF.

Hierarchical Porous Zr-Based MOFs Synthesized by a Facile Monocarboxylic Acid Etching Strategy.

A new monocarboxylic acid etching (MAE) strategy was developed for transforming chemically stable Zr-based metal-organic frameworks (MOFs) of UiO-66 to their hierarchical porous counterpart. The key design element was based on the incomplete replacement of bridging ligands in MOFs by monocarboxylic acids (MAs), leading to the departure of partial ligands and metal clusters to create mesopores in MOFs. A series of MAs with different acidity and carbon chain length were tested, and propionic acid (PA) was screened to be the suitable choice. The textural features including pore size distribution, specific surface area, and pore volume of the obtained products can be controlled by adjusting the MA concentration and reaction temperature. The obtained hierarchical porous MOFs inherited excellent stability from their parent materials. Additionally, the MAE strategy was universal to construct hierarchical porous Zr-based MOFs, and it was expanded to etch UiO-66 derivatives. The excellent adsorption behavior of the resultant hierarchical porous Zr-based MOFs over two enzymes with different size was also successfully exemplified, demonstrating their application potentials with bulky molecules involved.

Composition Tailoring via N and S Co-doping and Structure Tuning by Constructing Hierarchical Pores: Metal-Free Catalysts for High-Performance Electrochemical Reduction of CO2.

A facile route to scalable production of N and S co-doped, hierarchically porous carbon nanofiber (NSHCF) membranes (ca. 400 cm2 membrane in a single process) is reported. As-synthesized NSHCF membranes are flexible and free-standing, allowing their direct use as cathodes for efficient electrochemical CO2 reduction reaction (CO2 RR). Notably, CO with 94 % Faradaic efficiency and -103 mA cm-2 current density are readily achieved with only about 1.2 mg catalyst loading, which are among the best results ever obtained by metal-free CO2 RR catalysts. On the basis of control experiments and DFT calculations, such outstanding CO Faradaic efficiency can be attributed to the co-doped pyridinic N and carbon-bonded S atoms, which effectively decrease the Gibbs free energy of key *COOH intermediate. Furthermore, hierarchically porous structures of NSHCF membranes impart a much higher density of accessible active sites for CO2 RR, leading to the ultra-high current density.

Pyrolysis of Animal Bones with Vitamin B12: A Facile Route to Efficient Transition Metal-Nitrogen-Carbon (TM-N-C) Electrocatalysts for Oxygen Reduction.

By pyrolyzing cattle bones, hierarchical porous carbon (HPC) networks with a high surface area (2520 m(2) g(-1) ) and connected pores were prepared at a low cost and large scale. Subsequent co-pyrolysis of HPC with vitamin B12 resulted in the formation of three-dimensional (3D) hierarchically structured porous cobalt-nitrogen-carbon (Co-N-HPC) electrocatalysts with a surface area as high as 859 m(2) g(-1) as well as a higher oxygen reduction reaction (ORR) electrocatalytic activity, better operation stability, and higher tolerance to methanol than the commercial Pt/C catalyst in alkaline electrolyte.