Nonlocal Density Functional Theory Research Articles

An analysis of the textural properties of activated carbons obtained from biomass via the LBET, NLDFT and QSDFT methods

This article presents the unique research results of the comprehensive analysis of the porous structure of activated carbons obtained from biomass waste materials from the wood industry during activation in an air atmosphere. The porous structure was analysed on the basis of nitrogen and argon adsorption isotherms via complementary multi-method analysis, i.e. the new numerical clustering-based adsorption analysis, the non-local density functional theory and the quenched solid density functional theory methods. The analytical results for the prepared activated carbons were compared with analogous results obtained for commercial activated carbon. On the basis of the conducted studies it has been determined that the new numerical clustering-based adsorption analysis method gives credible and valuable information on the textural properties of activated carbons which are in strict correlation and mutually complement with the results of the analysis with the use of the quenched solid density functional theory method. The research results obtained in this paper, it has also been shown that from waste materials of the wood industry, in a relatively cheap and cleaner production process, it is possible not only to obtain carbonaceous materials almost comparable to commercial activated carbon, but also to manage the waste in accordance with the principles of a closed-loop economy and sustainable development. The paper pays also attention to the often overlooked economic and ecological aspects, which should nevertheless be taken into account when comparing different adsorbents, rather than their textural properties alone.

Novel hydrogen-bonded organic frameworks-based coating for fat oil and grease deposition control in the sewer system

Hydrogen-bonded organic frameworks (HOFs) represent an emerging class of porous materials characterized by crystalline frame structures, self-assembled from organic molecules through hydrogen bonding. This study demonstrates that HOFs fragment into small particles while preserving their original structure when dispersed in a polymeric epoxy solution. The intermolecular hydrogen bond structural and electronic properties of the LH4-HOF complex as well as the mechanism of HOF incorporated epoxy were extensively studied using density functional theory. LH4-HOF has a high Langmuir surface area of 2758.3 m2/g and nonlocal density functional theory pore size distributions of 3 A°. This unique solution processability enables the fabrication of coatings with high stability in aqueous systems. Results from a 30-day leaching test under controlled conditions (pH 7, temperature 20 ± 2 °C) indicate that LH4-HOF incorporated epoxy-coated concrete samples exhibited an over 85 % reduction in calcium release compared to uncoated samples, with epoxy-coated samples alone showing a 60 % reduction in calcium leaching. Additionally, the LH4-HOF-incorporated epoxy coating reduced the formation of fats, oils and grease deposits by more than 73 % on the concrete samples. Thus, this novel coating approach offers a sustainable solution with significant potential applications in sewer management.

Investigation of Ultramicroporous Structure of One-Dimensional Lepidocrocite Titanates Using Carbon Dioxide and Nitrogen Gases.

The novel material, one-dimensional lepidocrocite (1DL) titanate, is attracting industrial and scientific interest because of its applicability to a wide range of practical applications and its ease of synthesis and scale up of production. In this study, we investigated the CO2 adsorption capability and pore structures of 1DL freeze-dried and lithium chloride washed air-dried powders. The synthesized 1DL was characterized by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Using the constant-volume method, CO2 gas adsorption revealed that the 1DL exhibits type IV adsorption-desorption isotherms. The heats of adsorption obtained from the adsorption branches are lower than those obtained from the desorption branches. Brunauer-Emmett-Teller (BET) analysis, using N2 gas adsorption isotherms at 77 K showed that 1DL possesses 80.2 m2/g of BET specific surface area. Nonlocal density functional theory analysis indicated that two types of pores, meso-pores and ultramicro pores, exist in the 1DL freeze-dried powders. This work provides deep insights into the pore structures and CO2 adsorption mechanisms of 1DL powders.

Evaluation of methane adsorption isotherms at ambient temperature on carbon materials from NLDFT pore size analysis derived from standard N2 and CO2 adsorption isotherms

In this paper, we attempt to estimate CH4 adsorption on carbon materials based on the pore size distribution (PSD) characteristics evaluated from standard nitrogen and carbon dioxide adsorption isotherms using the nonlocal density functional theory (2D-NLDFT), or more precisely, its two-dimensional version (2D-NLDFT). The PSDs of the carbons were calculated by the simultaneous fit of the NLDFT models to the corresponding N2 and CO2 adsorption data. This method is referred to in the literature as the dual gas analysis method which is implemented in SAIEUS software. Essential features of carbon PSDs affecting the shapes of methane adsorption isotherms measured at ambient temperature were identified as related to micro and mesopores carbon structure. We discuss how different micro and mesopore size ranges contribute to methane adsorption isotherms in the low and higher-pressure ranges. The results of the present study may serve as guidance for selecting and designing suitable carbon materials for practical applications such as PSA recovery of CH4 from its mixtures or for methane storage.

Careful choice of carbonyl-containing cmps, and optimisation of Buchwald-Hartwig crosscoupling reaction conditions

In this study, new nitrogen-rich conjugated microporous polymers were synthesized using the Buchwald-Hartwig cross-coupling reaction. The synthesized polymers were characterized using various methods, including X-ray diffraction (XRD), ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, Brunauer-Emmett-Teller (BET) surface area analysis, and pore size distribution (PSD) determination using non-local density functional theory (NLDFT). The results provide important information about the structure and properties of synthesized materials, which contributes to a deeper understanding of their characteristics. The obtained characteristics of the material allow us to judge the quality and purity of the obtained polymers, which is important for further research, as well as serve as a basis for the development of new synthesis methods or improvement of existing technologies.

Assessment of Hydrophilicity/Hydrophobicity in Mesoporous Silica by Combining Adsorption, Liquid Intrusion, and Solid-State NMR Spectroscopy.

We have developed a comprehensive strategy for quantitatively assessing the hydrophilicity/hydrophobicity of nanoporous materials by combining advanced adsorption studies, novel liquid intrusion techniques, and solid-state NMR spectroscopy. For this, we have chosen a well-defined system of model materials, i.e., the highly ordered mesoporous silica molecular sieve SBA-15 in its pristine state and functionalized with different amounts of trimethylsilyl (TMS) groups, allowing one to accurately tailor the surface chemistry while maintaining the well-defined pore structure. For an absolute quantification of the trimethylsilyl group density, quantitative 1H solid-state NMR spectroscopy under magic angle spinning was employed. A full textural characterization of the materials was obtained by high-resolution argon 87 K adsorption, coupled with the application of dedicated methods based on nonlocal-density functional theory (NLDFT). Based on the known texture of the model materials, we developed a novel methodology allowing one to determine the effective contact angle of water adsorbed on the pore surfaces from complete wetting to nonwetting, constituting a powerful parameter for the characterization of the surface chemistry inside porous materials. The surface chemistry was found to vary from hydrophilic to hydrophobic as the TMS functionalization content was increased. For wetting and partially wetting surfaces, pore condensation of water is observed at pressures P smaller than the bulk saturation pressure p0 (i.e., at p/p0 < 1) and the effective contact angle of water on the pore walls could be derived from the water sorption isotherms. However, for nonwetting surfaces, pore condensation occurs at pressures above the saturation pressure (i.e., at p/p0 > 1). In this case, we investigated the pore filling of water (i.e., the vapor-liquid phase transition) by the application of a novel, liquid water intrusion/extrusion methodology, allowing one to derive the effective contact angle of water on the pore walls even in the case of nonwetting. Complementary molecular simulations provide density profiles of water on pristine and TMS-grafted silica surfaces (mimicking the tailored, functionalized experimental silica surfaces), which allow for a molecular view on the water adsorbate structure. Summarizing, we present a comprehensive and reliable methodology for quantitatively assessing the hydrophilicity/hydrophobicity of siliceous nanoporous materials, which has the potential to optimize applications in heterogeneous catalysis and separation (e.g., chromatography).

Advantages of dual CO2 & O2 adsorption model for assessment of micropore development in biochar during two-stage gasification

Residual biochar has the potential to replace commercial carbons even in highly specialised applications, presuming further advances in the engineered biochar production. Optimising biomass conversion requires dynamic feedback on the resultant char porosity, but investigation of pore size distribution (PSD) in pyrogenic carbons is challenging due to their extremely ultramicroporous nature. The most common probe molecule used in gas adsorption methods, N2, is often unable to access the narrowest pores, while CO2 can analyse only pores <10 Å.We propose an approachable way to evaluate PSD of ultramicroporous carbons by simultaneously fitting the 2D-NLDFT (two-dimensional non-local density functional theory) kernels to the CO2 and O2 isotherms. Using O2 as the probe molecule allowed collecting isotherms for the ultramicroporous pyrolytic char, for which a negligible adsorption of N2 was observed. For the more activated, gasification chars, reaching equilibrium for N2 adsorption took up to 8 h for some datapoints. O2 adsorption was much faster, resulting in the total runtime more than two times shorter than for the N2 tests.By analysing chars from the semi-pilot scale gasifier, we confirmed the dependence of the char structure on the process parameters, but we also suspect a strong influence of the reactor design.

Influence of mineral composition and firing temperature on the micro- and mesoporosity of replicate archaeological ceramics

Abstract The role of micro- and mesopores of archaeological ceramics in preserving ancient biomolecules is not well established. To understand the formation of these nano-sized pores in ceramics, reference pottery briquettes were made using two different clay types (illitic and kaolinitic clays), two different tempers (sand and chalk), and two different firing temperatures (600 and 800°C). The mineral content of the briquettes was determined by quantitative X-ray diffraction, and the micro- and mesopores were characterized with the N2 adsorption method. The Brunauer–Emmett–Teller method, the adsorbed volume near liquefaction, and application of non-local density functional theory (NLDFT) were used on the N2 adsorption data to determine specific surface areas, specific pore volumes, and pore-size distributions. Values of the micro- and mesoporosity parameters of most of the briquettes were approximately proportional to the initial clay content and unrelated to temper; the proportionality factors were much larger for illitic clay than for kaolinitic clay. When chalk-tempered briquettes were fired at the higher firing temperature of 800°C, the parameters were no longer proportional to the initial clay content; they decreased in most briquettes formed of illitic clay due to reaction of the clay with lime, and they increased in briquettes formed of kaolinitic clay due mostly to the porosity of unreacted lime. Micropore volumes in briquettes formed mostly of illitic clay were substantial: of the order of 5 mm3 g−1. The work presented here forms a basis for future studies to establish a plausible mechanism of organic residue absorption and preservation in ancient ceramics.

Investigation of CO2 adsorption on nitrogen-doped activated carbon based on porous structure and surface acid-base sites

The purpose of this paper is to reveal the effects of porous structure and surface acid-base sites on CO2 adsorption for those nitrogen-doped activated carbons, which were prepared in a tube furnace with anaerobic conditions (TF) and muffle furnace with oxygen-poor conditions (MF), respectively. Subsequently, the porous structure and surface group distribution of the prepared activated carbons TF and MF were characterized in detail through those methods such as scanning electron microscope (SEM), Brunauer-Emmett-Teller (BET), Barret-Joyner-Halenda method (BJH), Horvath-Kawazoe method (HK), Dubinin-Radushkevich method (DR), mercury intrusion porosimetry (MIP), non-local density functional theory method (NLDFT), (in-situ) Fourier transform infrared spectroscopy (FTIR), Raman spectra (RS), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). Then the thermodynamic and kinetic characteristics of CO2 adsorption such as adsorption capacity, selectivity, and activation energy were systematically investigated based on the static volumetric method and dynamic adsorption method. The results showed that the anaerobic or oxygen-poor preparation atmosphere affected the micropore volume and surface functional group distribution of activated carbons. Thus the activated carbon TF exhibited a higher adsorption capacity of CO2 (7.9 mmol g−1, 100 kPa and 273 K) and longer dynamic breakthrough time due to the higher microporosity (Vmic/Vt = 90 %), while the activated carbon MF exhibited the higher selectivity (14.7, CO2/N2 molar ratio of 1:1) and adsorption rate due to the higher nitrogen and oxygen contents (11.23 %, 13.53 %) in the main forms such as pyrrole nitrogen (N-5), carbony, and carboxyl. Overall, CO2 adsorption on the activated carbons was verified to be a dominant physical adsorption and exothermic process, which was suitable to be described by a quasi-first-order dynamic model. Meanwhile, compared with TF, the activated carbon MF exhibited the higher activation energy of CO2 adsorption (12.98 kJ mol−1 for TF and 17.23 kJ mol−1 for MF), accompanied by the larger rate constants (k1) and diffusion coefficients (D). The higher activation energy for MF meant that the adsorption temperature had a stronger effect on the rate constant during CO2 adsorption.

Discovery and In Situ Crystallization Studies of Cerium-Based Metal-Organic Frameworks with V-Shaped Linker Molecules.

We report the discovery and characterization of two porous Ce(III)-based metal-organic frameworks (MOFs) with the V-shaped linker molecules 4,4'-sulfonyldibenzoate (SDB2-) and 4,4'-(hexafluoroisopropylidene)bis(benzoate) (hfipbb2-). The compounds of framework composition [Ce2(H2O)(SDB)3] (1) and [Ce2(hfipbb)3] (2) were obtained by using a synthetic approach in acetonitrile that we recently established. Structure determination of 1 was accomplished from 3D electron diffraction (3D ED) data, while 2 could be refined against powder X-ray diffraction (PXRD) data using the crystal structure of an isostructural La-MOF as the starting model. Their framework structures consist of chain-like inorganic building units (IBUs) or hybrid-BUs that are interconnected by the V-shaped linker molecules to form framework structures with channel-type pores. The composition of both compounds was confirmed by PXRD, elemental analysis, as well as NMR and IR spectroscopy. Interestingly, despite the use of (NH4)2[CeIV(NO3)6] in the synthesis, cerium ions in both MOFs occur exclusively in the + III oxidation state as determined by X-ray absorption near edge structure (XANES) and X-ray photoelectron spectroscopy (XPS). Thermal analyses reveal remarkably high thermal stabilities of ≥400 °C for the MOFs. Initial N2 sorption measurements revealed the peculiar sorption behavior of 2 which prompted a deeper investigation by Ar and CO2 sorption experiments. The combination with nonlocal density functional theory (NL-DFT) calculations adds to the understanding of the nature of the different pore diameters in 2. An extensive quasi-simultaneous in situ XANES/XRD investigation was carried out to unveil the formation of Ce-MOFs during the solvothermal syntheses in acetonitrile. The crystallization of the two Ce(III)-MOFs presented herein as well as two previously reported Ce(IV)-MOFs, all obtained by a similar synthetic approach, were studied. While the XRD patterns show time-dependent MOF crystallization, the XANES data reveal the presence of Ce(III) intermediates and their subsequent conversion to the MOFs. The addition of acetic acid in combination with the V-shaped linker molecule was identified as the crucial factor for the formation of the crystalline Ce(III/IV)-MOFs.

Acid-activated natural zeolite and montmorillonite as adsorbents decomposing metsulfuron-methyl herbicide

Modern technologies of intensive crop production demand a wide application of various types of pesticides. The adverse effect of cropping intensification is that toxic organic compounds, in particular herbicides of the sulfonylurea family, are accumulated in the soil. The herbicide metsulfuron-methyl is soluble in water and accumulates in soil moisture, causing a toxic effect on plant development. The work examines the issue of the chemical stability of metsulfuron-methyl in a model aqueous environment for the targeted selection of a mineral that adsorbs the herbicide and simultaneously catalyzes the process of herbicide hydrolysis. Degradation processes of sulfonylurea herbicide metsulfuron-methyl in aqueous media in the presence of natural zeolite, acid-activated natural zeolite, and montmorillonite К10 were investigated. Under conditions of static contact of the adsorbents with an herbicide aqueous solution, montmorillonite was more active in the sorption and degradation of metsulfuron-methyl. During the first 1–2 days the herbicide concentration decreased by a factor of 5. An almost complete degradation of metsulfuron-methyl in aqueous dispersions of the acid-activated zeolite and montmorillonite was attained within 20 days. Pore size distribution functions (Barrett-Joyner-Halenda and standard non-local density functional theory methods) were evaluated. The acid treatment of the natural zeolite changed its porous structure - its specific surface area grew by a factor of 5. Three fractions of mesopores 4–10, 10–20, 20–30 nm were revealed in the original zeolite, and one fraction of pores 2.0–3.5 nm was found in the acid-activated zeolite. In the montmorillonite, only one group of 3–15 nm pores was detected. Studies of structural changes in natural zeolite after acid modification were carried out using the methods of X-ray fluorescence and X-ray phase analysis, scanning electron microscopy and infrared spectroscopy. The acid-activated montmorillonite and natural zeolite show promise for land reclamation work on detoxification of soils contaminated by sulfonylurea herbicides.

Geometric and Electronic Effects in the Binding Affinity of Imidazole-Based N-Heterocyclic Carbenes to Cu(100)- and Ag(100)-Based Pd and Pt Single-Atom Alloy Surfaces.

We have conducted nonlocal periodic density functional theory (DFT) calculations of N-heterocyclic carbenes (NHCs) adsorbed to Pd/Cu(100), Pt/Cu(100), Pd/Ag(100), and Pt/Ag(100) single atom alloys (SAAs) utilizing the nonlocal optPBE-vdW functional. NHCs with electron donating groups (EDGs) are predicted to bind more strongly to the SAA surface compared to NHCs functionalized with electron withdrawing groups (EWGs). Our calculations show that NHCs typically bind to SAA geometries containing a small space between the heteroatom sites for the SAAs considered. Generally, this pattern is predicted to persist for a single NHCs or for a pair of NHCs bound to the SAA surfaces. Approximate linear relationships between NMR-based parameters and NHC-SAA binding energies are uncovered. We predict that the binding of NHCs to SAA surfaces is composition-dependent and heteroatom geometry dependent.

Accurate quantification of the stability of the perylene-tetracarboxylic dianhydride on Au(111) molecule–surface interface

Studying inorganic/organic hybrid systems is a stepping stone towards the design of increasingly complex interfaces. A predictive understanding requires robust experimental and theoretical tools to foster trust in the obtained results. The adsorption energy is particularly challenging in this respect, since experimental methods are scarce and the results have large uncertainties even for the most widely studied systems. Here we combine temperature-programmed desorption (TPD), single-molecule atomic force microscopy (AFM), and nonlocal density-functional theory (DFT) calculations, to accurately characterize the stability of a widely studied interface consisting of perylene-tetracarboxylic dianhydride (PTCDA) molecules on Au(111). This network of methods lets us firmly establish the adsorption energy of PTCDA/Au(111) via TPD (1.74 ± 0.10 eV) and single-molecule AFM (2.00 ± 0.25 eV) experiments which agree within error bars, exemplifying how implicit replicability in a research design can benefit the investigation of complex materials properties.

Gas adsorption isotherm, pore size distribution, and free volume fraction of polymer-polymer mixed matrix membranes before and after thermal rearrangement

In this work, CO2 adsorption at 273.15 K and N2 adsorption at 77 K of mixed matrix membranes has been studied, as a method to directly determine their fractional free volume (FFV). These membranes consist of a continuous phase of copoly(o-hydroxyamide)s (HPA) or copoly(o-hydroxyamide-amide)s (PAA) and a relatively highly porous polymer network filler (PPN1). Both the pure copolymers and the mixed matrix membranes (MMMs) have been analyzed before and after a thermal rearrangement (TR) process.The CO2 adsorption results have allowed characterizing the pore size distribution of the studied membranes in the 3–15 Å range, by using the Non-Local Density Functional Theory (NLDFT). Whereas the N2 adsorption has allowed determining the pore size distributions in the range between 20 and 250 Å.The experimental determination of the pore volume and the density allows the direct calculation of the membranes’ free volume fractions, which were in good agreement with the most usual FFV evaluation methods. In addition, part of the pore volume detected by N2 adsorption was associated with defects and poor integration of the membrane components. This correction has allowed us to make a new evaluation of the density of these materials.

Influence of Nanoporous Carbon Structures on High-Pressure Adsorption Separation of Light Hydrocarbons

Four activated carbons (ACs) prepared from polymers (PMB, P-300), wood waste (S-1), and silicon carbide (AUK) by chemical and physical activation were investigated as potential adsorbents for ethane/methane mixture separation. Their textural properties were compared by performing scanning electron microscopy, X-ray, and CHNS/O elemental analysis. N2 and CO2 adsorption interpreted in terms of the theory of volume filling of micropores, non-local density functional theory (NLDFT), and Brunauer–Emmett–Teller (BET) models revealed very different porosity features of these ACs. The adsorption of pure ethane and methane onto all of the ACs was measured at pressures up to 50 bar and temperatures of 303–333 K in order to evaluate their adsorption selectivity for a methane/ethane mixture using the IAST method. The calculations indicated that ACs with the largest mean pore sizes, P-300 and PBM, are least suitable for adsorption-based separation of the ethane/methane mixture since they demonstrated a relatively strong dependence of selectivity coefficient (SF) on pressure and temperature. AUK showed the highest value of SF for ethane at low pressures (up to 5 bar), reaching 18 at 303 K for the mixture 95/5% CH4/C2H6. Finally, S-1 was found to be the most efficient adsorbent for the ethane/methane separation since it demonstrated a relatively high separation performance in a pressure range of 2–50 bar with almost no reduction in separation performance. In general, the SF value of ACs for ethane correlated with the difference between the initial differential isosteric heats of ethane and methane adsorption as well as with the presence of high-energy adsorption centers as narrow micropores with size less than 1 nm and heteroatoms (N,O). Molecular dynamics simulations of the adsorption of the methane/ethane mixture onto the model carbon adsorbents also indicated the influence of pore width on selective capacity.

Influence of nanopores volumes in hydrogen absorption properties of B4C and WC carbide-derived carbon nanomaterials

Sustainable energy development is needed to meet rising energy demands and reduce fossil fuel risks. Hydrogen generation is a crucial study area for sustainable energy. This study examines hydrogen adsorption on carbide-derived carbon nano-porous (CDC nP) nanomaterials. B4C CDC nP and WC-CDC nP Nanomaterials were produced by reacting chlorine gas with boron carbide (B4C) and tungsten carbide (WC) at 1200 °C and 980 °C in a high-pressure quartz stationary bed reactor. A CDC nP Nanomaterials was activated by heating it to 900 °C for 1 h in hydrogen to produce A-B4C-CDC nP Nanomaterials, which were characterized using Small-angle X-ray scattering (SAXS), nonlocal density functional theory (NLDFT), X-ray diffractometers (XRD), transmission electron microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), energy dispersive spectroscopy (EDS), and Scanning Electron Microscopy. Before treatment, the BET (Brunauer, Emmett, and Teller) surface area of activated B4C CDC nP material was 2240 m2/g, non-activated WC-CDC nP was 1240 m2/g, and A-B4C-CDC Nanomaterials was 1580 m2/g. This novel study examines CDC pore size and absorption on CDCs. Nitrogen adsorption isotherms showed better pore volume and distribution than raw material. High linearity allows an incompressible adsorbed hydrogen phase at ambient temperature. The isotherms match IUPAC (International Union of Pure and Applied Chemistry) Type I isotherms and confirm nano-porous materials. The equal temperature matches IUPAC Type I isotherms, indicating nano-porous material. H2 adsorption of Sample A peaked at 6.75 wt% at pressures above 125 MPa. Since all three carbide-derived carbons had the same maximum adsorbate density, overall pore volume is a major determinant.

Phase transitions and droplet shapes above a wetting stripe

We consider fluid adsorption on a planar wall patterned by a single stripe of width L of a wet material imbedded within a partially wet surface. If L were infinite we suppose that the wall would exhibit a first-order wetting transition at temperature Tw, and an associated line of pre-wetting transitions extending off bulk-coexistence. For finite widths L, we a) determine the finite-size scaling shift of the wetting temperature Tw(L), b) derive an equation, analogous to the Kelvin equation for capillary condensation, for the shift of the pre-wetting line but involving boundary tensions and a boundary contact angle and c) highlight the scaling and conformally invariant properties of condensed liquid drops lying above the stripe at bulk coexistence. We point out analogies between these predictions and those for capillary condensation and criticality in a parallel plate geometry and test them against numerical results obtained from a microscopic non-local classical density functional theory.

Polyacrylonitrile-Based Composite Carbon Nanofibers with Tailored Microporosity

Carbon nanofibers are currently used in many applications including electrochemical power sources, particularly, fuel cells. Their properties are highly dependent on the micro- and mesoporous structure. Here we provide a porosimetric analysis of the polyacrylonitrile-based electrospun composite Zr- and Ni-containing carbon nanofiber mats by N2 and CO2 adsorption methods for the first time. It was found that pyrolysis temperature affects specific surface area and volume: the values increase for the sample pyrolyzed at 900 °C compared with the initial stabilized nanofibers (300 °C, air) according to the Dubinin --- Radushkevich, non-local density functional theory (NLDFT) and grand canonical Monte-Carlo methods (GCMC). For higher pyrolysis temperatures (1000 and 1200 °C), the porosimetric parameters decrease compared with the one pyrolyzed at 900 °C. According to the NLDFT and GCMC pore size distribution, the difference for pyrolyzed samples is mostly related to a sharp decrease in the specific surface area for pores with a size of ~ 0.5 nm and an increase for pores at 0.55--0.8 nm compared with the initial stabilized sample. The study demonstrates a way to adjust porosimetric parameters depending on the pyrolysis conditions of the nanofiber mats, since it can improve characteristics of such type of carbon materials in electrochemical devices

Selection Effect of Liquid Nitrogen Freeze-Thaw Cycles on Full Pore Size Distribution of Different Rank Coals.

In China, the capacity to produce coalbed methane and extract underground gas is restricted by the prevalence of low-permeability coal seams. Liquid nitrogen fracturing is a new low temperature-high-pressure anhydrous fracturing technology that uses low temperature and high frost heave forces to increase coal permeability. To better understand the liquid nitrogen fracturing effect on coal, we conduct the liquid nitrogen freeze-thaw cycle (LNCFT) experiments on different rank coals from Qinghai, Shanxi, and Shaanxi provinces. We combined the low-pressure nitrogen and carbon dioxide adsorption experiment with the non-local density functional theory model and mercury injection porosimetry with compressibility corrections to examine the full pore size distributions of untreated and water-saturated samples before and after LNCFT. The results found that LNCFT can effectively increase the pore volume (PV) and specific surface area of the water-saturated coal sample. Compared with the raw coal, the increased ratio of the full pore size PV is 70.41-100.17%. However, the scale-selective transformation effect on pores during liquid nitrogen fracturing is noticeable. Under the same conditions, LNCFT can significantly increase the pore volume of micropores (>2 nm) and macropores (>50 nm), and the increase ratios are 24.40-44.16 and 103.55-327.93%, respectively. The PV of mesopores (2-50 nm) shows a slightly increasing trend with the increase in metamorphic degree, and the increase ratio is between 8.7 and 56%. Comparing the full pore size distribution curves before and after LNCFT, it is found that the alteration of high-volatile bituminous coal (BLT coal) and anthracite (SH coal) has more significance in the range of less than 2 and 50-20,000 nm, while middle-volatile bituminous coal (YJL coal) varies between 50 and 2000 nm. Meanwhile, the ratio of micropore and mesopore PV to the total decreased gradually before and after LNCFT, while the proportion of macropores increased, indicating that small-scale pores would intersect and connect to form larger-scale pores during the fracturing. The combined effects of temperature gradient, water-ice phase transition, and heat transmission rate are the key factors that determine the impact of LNCFT on pore size distribution. Our results provide new information for enhancing the permeability of low-permeability coal seams of different ranks.

Pore structure evolution during lean-oxygen combustion of pyrolyzed residual from low-rank coal and its effect on internal oxygen diffusion mechanism

The lean-oxygen combustion of incompletely pyrolyzed char dominates at the combustion front of coalfield fires. Using non-local-density functional theory (NLDFT) and Barrette-Joynere-Halenda (BJH) analysis based on N2 and CO2 composited adsorption isotherms, the pore (0.4–300 nm) structure characteristics of pyrolyzed char during lean-oxygen combustion were obtained. The results indicate that the main pore morphology changes from irregular ink-bottle shape of raw coal to cylindrical shape of pyrolyzed char, then to slit-shaped and wedge-shaped pores under the effect of oxidative expansion, and finally coalesce into amorphous macropores during combustion. For the weak-caking coal, softened colloids diffuse and block the mesopores, leading to the reverse development of mesopores into smaller-sized pores. The pore development changes from the generation of secondary micropores in pyrolysis stage to the progressive expansion mode of mesopores in initial combustion stage, then to the free coalescence mode of macropores in late combustion stage. The dominant internal oxygen diffusion mechanism transforms from Knudsen diffusion to transition diffusion at 650 °C, and the carbon-oxygen reaction interfaces shift from the inner surface of deep micropores to the shallow mesopores and macropores. Finally, 650 °C is determined as the early-warning temperature for the accelerating spread of coalfield fires.