Pore Hierarchy Research Articles

Nanoengineering Multilength-Scale Porous Hierarchy in Mesoporous Metal-Organic Framework Single Crystals.

Developing a reliable method for constructing mesoporous metal-organic frameworks (MOFs) with single-crystalline forms remains a challenging task despite numerous efforts. This study presents a solvent-mediated assembly method for fabricating zeolitic imidazolate framework (ZIF) single-crystal nanoparticles with a well-defined micro-mesoporous structure using polystyrene-block-poly(ethylene oxide) diblock copolymer micelles as a soft-template. The precise control of particle sizes, ranging from 85 to 1200 nm, is achieved by regulating nucleation and crystal growth rates while maintaining consistent pore diameters in mesoporous nanoparticles and a rhombohedral dodecahedron morphology. Furthermore, this study presents a robust platform for nanoarchitecturing to prepare hierarchically porous materials (e.g., core-shell and hollow structures), including microporous ZIF@mesoporous ZIF, hollow mesoporous ZIF, and mesoporous ZIF@mesoporous ZIF. Such a multimodal pore design, ranging from microporous to microporous/mesoporous and further micro-/meso-/macroporous, provides significant evidence for the future possibility of the structural design of MOFs.

Investigating the crystallite morphologies and aggregation in boehmite powders: a combined analysis of two- and three-dimensional electron microscopy with x-ray diffraction

Boehmite (AlOOH) is considered as an important precursor to γ-Al2O3 which when calcinated undergoes topotactic transformation to form the latter. Alumina has extensive applications in fields such as catalysis, abrasives, cosmetics among others. Boehmite falls under the category of hierarchical structures whose structural and textural properties are a result of its compositional and porous hierarchy. Although research has been carried out exquisitely to understand the complete representation of its structure, a true morphological model is an important key to understand and fully explain its transport properties during catalytic processes. 3D electron microscopy helps us to dive deeper into the different hierarchical entities of boehmite, bridging the gaps between the models and assumptions made using some more traditional characterization techniques. We present here deeper insight into the structural and morphological parameters of different commercial boehmites using 3D-TEM. Through the extraction of quantitative descriptors pertaining to hierarchical entities and subsequent comparison with bulk analyses, precise and comprehensive information regarding these microstructures can be obtained. The results of our study indicate that boehmite grades, which appear to be identical in terms of their grades, display discrepancies in the uniformity of particle sizes. Moreover, diverse platelet interactions result in varying types of pores in these grades. Furthermore, it has been observed that the interfacial interactions among various crystallographic planes exhibit variations across different specimens, thereby contributing to the distinctive compositions within the aggregates. The variation in aggregates of different boehmite grades is also reflected in the combination of four distinct quantified morphologies.

Simultaneously enhancing toluene adsorption and regeneration process by hierarchical pore in activated coke: a combined experimental and adsorption kinetic modeling study.

Activated coke is a type of commonly used adsorbent for benzene series VOCs such as toluene, but traditional microporous activated coke usually faces the challenge of poor regeneration performance. Herein, based on self-made activated cokes with typical pore configuration, we found that adsorption and regeneration of toluene can be simultaneously enhanced by constructing hierarchical pore in activated coke. Correlations of pore configuration with toluene adsorption capacity and regeneration efficiency reveal that micropore contributes for strong toluene adsorption; meso-macropore provides mass transfer channel for toluene desorption and regeneration process. Hierarchical porous activated coke prepared from Zhundong subbituminous coal not only achieves the highest toluene adsorption capacity of 340.92mg·g-1, but also can retain more than 90% of initial adsorption capacity after five adsorption-regeneration cycles. By contrast, micropore-dominant activated cokes can only retain 70% of initial adsorption capacity. Adsorption kinetic modelling on adsorption breakthrough curves shows that hierarchical porous activated coke prepared from Zhundong subbituminous coal exhibits high adsorption and diffusion rate constants of 14.39 and 33.45min-1, respectively, much higher than those of micropore-dominant activated cokes. Due to the accelerated surface adsorption and diffusion processes induced by meso-macropore, toluene adsorption and regeneration behavior can be simultaneously improved. Results from this work validated the role of pore hierarchy in toluene adsorption-regeneration process, providing guidance for designing high-performance activated coke with synergistically improved toluene adsorption capacity and regeneration performance.

Effect of pretreatment and activation conditions on pore development of coal-based activated carbon

Abundant pore volume and appropriate pore configuration are two crucial characteristics of high-performance carbonaceous adsorbents and catalysts. In this study, influences of pre-treatments (e.g., breaking, sieving and ash removal by acid) and activation conditions (e.g., activation time) on pore parameters (including BET surface area, pore diameter and pore hierarchy degree) of resultant activated carbon were systematically investigated. In addition, the difference between conventional carbonization-activation (i.e., the two-step preparation method) and one-step activation in pore development of activated carbons was revealed. The sample prepared by a 120-minute one-step activation exhibited a BET surface area high to 1038.1 m2/g and a pore hierarchy at 50.7 %. Based on the kinetics analysis using TGA reactor, reaction between the activator (i.e., CO2) and carbon matrix was found enhanced in one-step activation, contributing to the generation of mesopores. Remarkably, a new indicator, Effective Activation Rate (EAR) was proposed to accurately describe the activation rate in one-step synthesis after decoupling the release of moisture and volatile compounds. The above conclusions provide guidance for flexibly regulating the pore configurations of activated carbons derived from natural precursors.

Role of pore hierarchy and Mn incorporation on the electrochemical performance of bimetallic NiMn-LDHs on graphite foil for supercapattery application

In this work, nickel-manganese layered double hydroxides (NiMn-LDHs) are synthesized using hexamethylenetetramine (HMTA) and urea as precipitating agents via hydrothermal synthesis route. The materials have different flower like microstructure and porosity. The effect of microstructures, porosity and mass loading of the LDHs on the overall electrochemical performance is thoroughly studied in this work. It is observed that HMTA assisted NiMn-LDH having open flower like microstructure delivers a high specific capacity of 719 C g−1 at 1 A g–1 and excellent capacity retention of 82.6% and 86% for 10,000 CV and 5000 GCD cycles, respectively. Furthermore, a supercapattery device assembled using NiMn-LDH@HMTA as cathode and activated carbon as anode exhibit high-capacity retention of 80.1% after 5000 GCD cycles along with an energy density of 50 and 34 Wh kg–1 at power densities of 800 and 4000 W kg–1, respectively.

Large Cyclability of Elastocaloric Effect in Highly Porous Ni-Fe-Ga Foams.

Solid-state refrigeration based on elastocaloric materials (eCMs) requires reversibility and repeatability. However, the intrinsic intergranular brittleness of ferromagnetic shape memory alloys (FMSMAs) limits fatigue life and, thus, is the crucial bottleneck for its industrial applications. Significant cyclic stability of elastocaloric effects (eCE) via 53% porosity in Ni-Fe-Ga FMSMA has already been proven. Here, Ni-Fe-Ga foams (single-/hierarchical pores) with high porosity of 64% and 73% via tailoring the material's architecture to optimize the eCE performances are studied. A completely reversible superelastic behavior at room temperature (297 K) is demonstrated in high porosity (64-73%) Ni-Fe-Ga foams with small stress hysteresis, which is greatly conducive to durable fatigue life. Consequentially, hierarchical pore foam with 64% porosity exhibits a maximum reversible ∆Tad of 2.0 K at much lower stress of 45 MPa with a large COPmat of 34. Moreover, it shows stable elastocaloric behavior (ΔTad = 2.0 K) over >300 superelastic cycles with no significant deterioration. The enhanced eCE cyclability can be attributed to the pore hierarchies, which remarkably reduce the grain boundary constraints and/or limit the propagation of cracks to induce multiple stress-induced martensitic transformations (MTs). Therefore, this work paves the way for designing durable fatigue life FMSMAs as promising eCMs by manipulating the material architectures.

Droplet‐Directed Anisotropic Assembly of Semifootball‐Like Carbon Nanoparticles with Multimodal Pore Architectures

AbstractHierarchical porous carbon nanoparticles, with tailored asymmetric morphologies and pore structures, have great implications in high‐performance electrode materials. However, the controlled synthesis of anisotropic carbon nanoparticles with tailored multimodal pore structures remains a challenge. Herein, a droplet‐directed anisotropic assembly approach to synthesize asymmetric carbon nanoparticles with macro/mesopores is demonstrated. This synthesis relies on the anisotropic growth of mesoporous polydopamine (PDA) seeds on the emulsion interfaces and the subsequent immersion of 1,3,5‐trimethylbenzene (TMB) droplets into the seeds. The obtained carbon nanoparticles present a semifootball‐shaped morphology with a high surface area (383 m2 g−1), well‐controlled macropores (≈105 nm), and mesopores (≈3.8 nm). By tuning the polarity of the oil phase, the morphologies transform from non‐porous spheres to semifootball‐like architectures and finally to nano‐ellipsoid with meso‐channels. The pore structures are further optimized by ZnCl2 activation, and the semifootball‐like carbon nanoparticles with modulated pore compositions deliver a high reversible capability, excellent rate performance (215 F g−1 at 0.05 A g−1 and 143 F g−1 at 20 A g−1 in organic electrolyte), and enhanced energy density (53.4 Wh Kg−1). Simulation results elucidate the structure–activity relationship between the multistage pore structure and electrochemical performance, i.e., pore hierarchy enhances ion diffusion flux, and large‐mesopore structure facilitates rate performance.

Electrochemical impedance of porous tantalum solids: modeling of frequency response

The paper proposes a new approach to the analysis of electrochemical impedance spectra of porous tantalum bodies, which involves modeling the frequency response via an equivalent circuit that takes into account the pore hierarchy. It was shown that the proposed circuit describes well the experimental data and allows characterization of the porous structure, including the contribution of different types of pores to the total capacitance of the porous body, characteristic relaxation times, and activation frequencies for different type pores. Two types of samples were analyzed: a porous tantalum body obtained by sintering Ta powder and a porous tantalum body covered with a Ta2O5 dielectric layer. Modeling showed a significant redistribution of contributions from pores of different types into the total capacitance after the formation of Ta2O5 due tothe preferential isolation of the smallest pores and/or those difficult to access. The results of modeling of the frequency response of the analyzed samples agree well with the scanning electron microscopy data. The proposed approach has the potential to be advantageous for the technology of tantalum capacitors

3D nitrogen-doped carbon frameworks with hierarchical pores and graphitic carbon channels for high-performance hybrid energy storages.

In principle, hybrid energy storages can utilize the advantages of capacitor-type cathodes and battery-type anodes, but their cathode and anode materials still cannot realize a high energy density, fast rechargeable capability, and long-cycle stability. Herein, we report a strategy to synthesize cathode and anode materials as a solution to overcome this challenge. Firstly, 3D nitrogen-doped hierarchical porous graphitic carbon (NHPGC) frameworks were synthesized as cathode materials using Co-Zn mixed metal-organic frameworks (MOFs). A high capacity is achieved due to the abundant nitrogen and micropores produced by the MOF nanocages and evaporation of Zn. Also, fast ion/electron transport channels were derived through the Co-catalyzed hierarchical porosity control and graphitization. Moreover, tin oxide precursors were introduced in NHPGC to form the SnO2@NHPGC anode. Operando X-ray diffraction revealed that the rescaled subnanoparticles as anodic units facilitated the high capacity during ion insertion-induced rescaling. Besides, the Sn-N bonds endowed the anode with a cycling stability. Furthermore, the NHPGC cathode and SnO2@NHPGC achieved an ultrahigh energy density (up to 244.5 W h kg-1 for Li and 146.1 W h kg-1 for Na), fast rechargeable capability (up to 93C-rate for Li and 147C-rate for Na) as exhibited by photovoltaic recharge within a minute and a long-cycle stability with ∼100% coulombic efficiency over 10 000 cycles.

Developing systems theory in soil agroecology: incorporating heterogeneity and dynamic instability

Soils are increasingly acknowledged as complex systems, with potential non-linear behaviors having important implications for ecosystem and Earth system dynamics, but soil models could improve adoption of analytical tools from the broader interdisciplinary field of complex systems. First- and new-generation soil models formulate many soil pools using first-order decomposition, which tends to generate simpler yet numerous parameters. Systems or complexity theory, developed across various scientific and social fields, may help improve robustness of soil models, by offering consistent assumptions about system openness, potential dynamic instability and distance from commonly assumed stable equilibria, as well as new analytical tools for formulating more generalized model structures that reduce parameter space and yield a wider array of possible model outcomes, such as quickly shrinking carbon stocks with pulsing or lagged respiration. This paper builds on recent perspectives of soil modeling to ask how various soil functions can be better understood by applying a complex systems lens. We synthesized previous literature reviews with concepts from non-linear dynamical systems in theoretical ecology and soil sciences more broadly to identify areas for further study that may help improve the robustness of soil models under the uncertainty of human activities and management. Three broad dynamical concepts were highlighted: soil variable memory or state-dependence, oscillations, and tipping points with hysteresis. These themes represent possible dynamics resulting from existing observations, such as reversibility of organo-mineral associations, dynamic aggregate- and pore hierarchies, persistent wet-dry cycles, higher-order microbial community and predator-prey interactions, cumulative legacy land use history, and social management interactions and/or cooperation. We discuss how these aspects may contribute useful analytical tools, metrics, and frameworks that help integrate the uncertainties in future soil states, ranging from micro-to regional scales. Overall, this study highlights the potential benefits of incorporating spatial heterogeneity and dynamic instabilities into future model representations of whole soil processes, and contributes to the field as a modern synthetic review that connects existing similar ideas across disciplines and highlights their implications for future work and potential findings. Additionally, it advocates for transdisciplinary collaborations between natural and social scientists, extending research into anthropedology and biogeosociochemistry.

Physisorption of SO2 by carbonaceous model adsorbents with tunable hierarchical pore configurations: Experiment and simulation

Adsorption removal of SO2 using activated carbons has attracted enormous attention due to its relatively low water consumption and environmental friendliness. The physisorption of SO2 is regarded as the prerequisite step of SO2 chemisorption (i.e., SO2 reacting with oxygen and moisture in the flue gas). In this study, a series of carbonaceous model adsorbents (i.e., activated carbons, ACs) with various pore hierarchy degrees and similar surface chemistry were synthesized by simply regulating the conditions of catalytic activation. The physical and chemical characteristics of the ACs were elucidated by N2 physisorption, XPS, TEM and SEM. Both dynamic breakthrough performances and isotherms of SO2 adsorption on ACs were investigated, presenting a negative correlation between pore hierarchy and SO2 saturated adsorption capacity. Compared with the fitting of semi-empirical isotherm models, classical molecular dynamic (MD) simulation can better demonstrate the spatial distribution and the local density of SO2 molecules in nanopores of ACs with diameters <2 nm.

Potential of 3D Hierarchical Porous TiO2-Graphene Aerogel (TiO2-GA) as Electrocatalyst Support for Direct Methanol Fuel Cells

Fuel cells have already demonstrated their potential for green energy generation. However, the low reaction performance becomes an obstacle in terms of large-scale commercial manufacturing. Accordingly, this work focuses on a new unique fabrication of three-dimensional pore hierarchy TiO2-graphene aerogel (TiO2-GA) supporting PtRu catalyst for anodic catalyst direct methanol fuel cell, which is facile, ecologically benign, and economical. In this work, a hydrothermal technique was used, followed by a freeze-drying technique and a microwave-assisted ethylene reduction technique. The structural properties of the studied materials were confirmed by UV/visible spectroscopy, XRD, Raman spectrum, FESEM TEM, and XPS. Based on existing structural advantages, the performance of PtRu/TiO2-GA has been investigated on DMFC anode catalysts. Furthermore, electrocatalytic stability performance with the same loading (~20%) was compared to commercial PtRu/C. Experimental outcomes show that the TiO2-GA support offered a significantly high surface area value of 68.44 m2g−1, mass activity/specific activity (608.17 mAmg−1/0.45 mA/cm2PtRu) that is higher than commercial PtRu/C (79.11 mAmg−1/0.19 mA/cm2PtRu). In passive DMFC mode, PtRu/TiO2-GA showed a maximum power density of 3.1 mW cm−2, which is 2.6 times higher than that of the PtRu/C commercial electrocatalyst. This suggests that PtRu/TiO2-GA has a promising possibility for methanol oxidation and may be used as an anodic element in DMFC.

THERMAL OSCILLATION ARISING IN A HEAT SHOCK OF A POROUS HIERARCHY AND ITS APPLICATION

A building or a bridge might collapse after a heat shock. This paper shows that a porous hierarchy of a coating can effectively prevent a building or a bridge from such damage. A cocoon’s geometrical structure is studied and its resistance to the heat shock is revealed by a thermal oscillator. The theoretical model reveals an extremely low frequency of the thermal oscillator, which is very important for cocoons’ biomechanism, especially in the heat insulation function. At the same time, it shows that the cocoons have the best thickness to protect the pupa from the environment. In addition, surface temperature measurement of hierarchical mulberry leaves is performed. This work provides new insights into biomimetic design of the protective building and coatings.

SAPO-containing alumina CoMoNi-catalysts for hydrotreatment of heavy oil: Pore hierarchy as a key parameter for catalyst stabilization

This paper is devoted to the study of activity and lifetime of CoMoNi-catalyst containing one-dimensional SAPOs in heavy oil hydrotreatment. A special attention is paid to porous structure of SAPOs and catalyst supports. Porosity of supports is designed by two hard templates. Pore structure containing narrow and wide mesopores and macropores is referred as high-resolved pore hierarchy. In turn, pore structure containing narrow mesopores and macropores is referred as bimodal pore hierarchy. Zeolite-like SAPOs also differ in their own pore hierarchy and acidity. Catalysts have been characterized with XRD, nitrogen adsorption-desorption, Hg-intrusion, Raman and XP spectroscopies and SEM. Catalytic tests for 100 h for each catalysts have been shown that support pore hierarchy should be designed depending on pore hierarchy and morphology of zeolite-like material. In general, high-resolved pore hierarchy improves mass transfer, makes CoMoNi sites accessible and creates gradual pore channel expansion and, as a result, prevents deactivation of zeolite-like containing catalysts.

Synthesis of carbon energized materials with directed regulation of specific surface and pore structure as potential adsorbent for methane mitigation

In this study, nanoporous carbon material (NCM) specimens were synthesized from polymer raw materials (a mixture of furfural, hydroquinone, and urotropine). The influence of the conditions and parameters of the synthesis method (temperature, weight ratio of the activating agent to the precursor and contact time) on the NCM structure was examined. The effect of the weight ratio of the initial components on the properties of the resulting NCM was investigated, and the optimal composition of the ternary system of furfural, hydroquinone and urotropine was found. The results of nitrogen adsorption-desorption showed that NCM specimens obtained by varying the activation temperature were microporous but with different micropore volumes and the same pore size distribution. Varying the weight ratio of the activating agent (KOH) to the precursor made it possible to obtain microporous, micromesoporous, and mesoporous NCMs. NCM specimens with different pore hierarchy and different specific surface area were also obtained, including microporous NCMs (SBET: 1546–3030 m3/g; Vmic/Vt: 76.0–100%), micro- mesoporous NCMs (SBET: 3204–3516 m3/g; Vmic/Vt: 41.2–57.4% Vmes/Vt: 42.6–58.8%) and mesoporous NCMs (SBET: 3641–3649 m3/g; Vmes/Vt: 74.0–83.8%). Further, a mechanism for the development of pores during KOH activation of polymeric carbon precursor was proposed. The outcomes of this study signify that the NCM5/750/1 has a high adsorption capacity for CH4 and has good prospects for monitoring CH4 emissions and reducing greenhouse gas emissions.

Fabrication and microstructure evolution of monolithic bridged polysilsesquioxane-derived SiC ceramic aerogels

SiC aerogels are representative high-temperature ceramic aerogels that have demonstrated extensive potential utility in the thermal insulation field under extreme conditions. Although the efficient fabrication of monolithic and highly crystalline SiC aerogels is crucial, it remains highly challenging. Herein, a one-step pyrolysis strategy of a bridged polysilsesquioxane (BPSi) aerogel precursor is reported. The preceramic BP aerogel was prepared via a sol-gel method followed by vacuum drying, and the subsequent pyrolysis process conveniently converted the BPSi aerogel precursor into a SiC aerogel. This circumvents the need for harsh synthetic conditions, high-cost noble metal catalysts, special drying, and additional calcination processes. Furthermore, a competitive mechanism between the “gas-escape” caused by the volatilization of low molecular compounds and carbothermal reduction reaction, and the “volume-shrinkage” resulting from high-temperature sintering was proposed to explain the evolution of phase composition and pore hierarchy of BPSi-derived ceramic aerogels.

Study of repellence on polymeric surfaces with two individually adjustable pore hierarchies

Highly repellent surfaces offer many interesting applications as oil-repellent textiles and self-cleaning coatings, making these surfaces highly sought-after. Engineering these repellent surfaces requires the combination of a low surface energy material with special surface geometries with multiple levels of hierarchy in roughness. Yet, the current approaches for the fabrication of these surfaces have many drawbacks (complex fabrication techniques, fragile nature of the surface features with low abrasion resistances) that preventing their wider application. Here, we present a study of a new double–hierarchical Fluoropor which allows creating a bulk-porous fluorinated polymer foam with two individually tunable pore hierarchies. We combined our previous structuring approach for the micro /nanoscale (addition of an adaptable porogen mixture to the monomer) with sacrificial templating utilizing spherical polystyrene particles to obtain a second micropore hierarchy, resulting in a novel double-hierarchical material. We found that a micro-/nanoporosity with a larger pore structure in combination with a higher volumetric loading of structuring particles was beneficial to increase the repellence. We found the highest repellence for double–hierarchical Fluoropor with a bulk–porosity with 259 nm median pore diameter structured with 50 vol% of 40 µm sized microspheres. These superhydrophobic surface showed high static contact angles for ethylene glycol of 164° with a roll–off angle of 13° and a static contact angle for n–hexadecane of 145°. The material was covalently coated onto technical substrates (glass, copper, steel) and demonstrated good self-clean abilities for inorganic as well for organic contaminants. Abrasion test with 1000 cycles showed the repellence was not affected and persisted.

Sapo-Containing Alumina Comoni-Catalysts for Hydrotreatment of Heavy Oil: Pore Hierarchy as a Key Parameter for Catalyst Stabilization

Sapo-Containing Alumina Comoni-Catalysts for Hydrotreatment of Heavy Oil: Pore Hierarchy as a Key Parameter for Catalyst Stabilization

Highly Enhanced Catalytic Stability of Copper by the Synergistic Effect of Porous Hierarchy and Alloying for Selective Hydrogenation Reaction

Supported copper has a great potential for replacing the commercial palladium-based catalysts in the field of selective alkynes/alkadienes hydrogenation due to its excellent alkene selectivity and relatively high activity. However, fatally, it has a low catalytic stability owing to the rapid oligomerization of alkenes on the copper surface. In this study, 2.5 wt% Cu catalysts with various Cu:Zn ratios and supported on hierarchically porous alumina (HA) were designed and synthesized by deposition–precipitation with urea. Macropores (with diameters of 1 μm) and mesopores (with diameters of 3.5 nm) were introduced by the hydrolysis of metal alkoxides. After in situ activation at 350 °C, the catalytic stability of Cu was highly enhanced, with a limited effect on the catalytic activity and alkene selectivity. The time needed for losing 10% butadiene conversion for Cu1Zn3/HA was ~40 h, which is 20 times higher than that found for Cu/HA (~2 h), and 160 times higher than that found for Cu/bulky alumina (0.25 h). It was found that this type of enhancement in catalytic stability was mainly due to the rapid mass transportation in hierarchically porous structure (i.e., four times higher than that in bulky commercial alumina) and the well-dispersed copper active site modified by Zn, with identification by STEM–HAADF coupled with EDX. This study offers a universal way to optimize the catalytic stability of selective hydrogenation reactions.

Hierarchical pore construction of alumina microrod supports for Pt catalysts toward the enhanced performance of n-heptane reforming

Regulation of pore hierarchy has been an important subject in the design of industrial catalysts. Synthesis of alumina without template/additive often results in poor control of morphologies, porosities, and metal dispersion thereafter. Herein, alumina microrods with a tailored pore hierarchy (Rod2) were fabricated without template/additive by tuning the pore structure of the synthetic precursors, which were used as supports of Pt for n-heptane reforming. Improved toluene selectivity was observed at low space velocities as a result of promoted dehydroisomerization of pentacyclic hydrocarbons, presumably due to a combined effect of the desired pore hierarchy and rod morphology. The Pt/Rod2 also showed better resistance to carbon deposition while maintaining good metal dispersion, as compared to alumina supports with spherical morphology or uncontrolled pore hierarchy. The simulation results revealed that the improved toluene selectivity of Pt/Rod2 is attributed to the proper pore combination of meso-pore1 (6–40 nm) and meso-pore 2 (40–300 nm).