Canonical Monte Carlo Simulations Research Articles

In-situ nitrogen-doped porous carbon derived from penicillin mycelial residue for CO2 adsorption and adsorption mechanism investigation

The in-situ nitrogen-doped porous carbon materials was synthesized using penicillin mycelial residue (PMR) as the sole endogenous nitrogen-rich precursor through a two-step process applied for CO2 adsorption. Precise control over the structural properties and CO2 adsorption performance of the porous carbon was achieved by varying the KOH dosage and activation temperature. The porous carbon prepared under mild activation conditions (600 ℃, KOH/BC = 1) exhibited high nitrogen content and abundant ultramicroporous structures (pore size ≤ 0.7 nm), and demonstrated outstanding adsorption performance. The CO2 adsorption capacities reached 6.55 mmol/g at 0 ℃ and 3.48 mmol/g at 25 ℃, showing promising CO2/N2 selectivity and robust cyclic adsorption stability. Quantitative structure–activity relationship analysis displayed a strong correlation between adsorption density, CO2/N2 selectivity, and nitrogen content. Density functional theory calculations further confirmed that nitrogen-doping, especially pyrrolic-N, serves as favorable structural sites for CO2 adsorption. Additionally, Grand Canonical Monte Carlo simulations verified that the ultramicroporous structures significantly enhances CO2 adsorption due to their strong affinity for CO2. This study provides an economic strategy for the preparation of in-situ nitrogen-doped porous carbon with enriched micropore using PMR, highlighting their promising performances in CO2 adsorption, synchronously extending the harmless treatment and resource utilization of PMR to demonstrate a sustainable approach for environmental remediation.

Molecular Simulation of Cyclohexane in Nanoporous Materials: Adsorption of Conformers and Coadsorption with Water and Carbon Dioxide.

Molecular simulation is used to investigate the adsorption of an organic molecule and its different conformers into various nanoporous adsorbents. In more detail, we perform grand canonical Monte Carlo simulations to determine the adsorption isotherms for cyclohexane with its three conformers (chair, boat, and twisted boat) at three different temperatures into a molecular model of active carbons and two metal-organic frameworks (MOFs) (Cu-BTC and Al-Fum). By considering the adsorption of each conformer separately, we show that the adsorption isotherms are weakly dependent on the molecular conformation. When considering the concomitant adsorption of the three conformers under realistic conditions (to verify the population of each conformer in bulk mixtures), we show that the chair conformer is predominantly adsorbed in each material. However, such favorable adsorption mostly reflects the fact that this conformer has the largest population in the bulk mixture under the same thermodynamic conditions. On the contrary, with an increase in the cyclohexane gas pressure, the adsorption of boat and twisted boat conformers increases, while that of the chair conformer decreases as packing (entropy) effects become predominant during confinement. The adsorption of cyclohexane in the active carbon and MOF materials is also considered in the presence of water and CO2 atmospheres. In the case of the Al-fumarate material, the material is found to be strongly organophilic so that water and CO2 adsorption are negligible, and cyclohexane adsorption is not affected by competitive adsorption under any considered thermodynamic condition. On the contrary, for the active carbon and Cu-BTC material, competitive adsorption between cyclohexane and water or CO2 is observed even if cyclohexane adsorption remains preferred. While the different molecules coexist in the adsorbed phase at low pressures, increasing the cyclohexane pressure beyond 103 Pa promotes cyclohexane adsorption concomitantly with water and/or CO2 desorption.

Boron nitride graphite intercalated with carbon-boron-nitride fullerenes for enhanced hydrogen storage

The hydrogen storage of boron nitride graphite intercalated with carbon-boron-nitride fullerenes was investigated using density functional theory and Grand Canonical Monte Carlo simulations. At 77 K and 30 MPa, the optimized structures C14(BN)23-inserted BN graphite material achieved a gravimetric hydrogen adsorption of 6.39 wt% and a volumetric adsorption of 0.07 kg/L, surpassing the U.S. Department of Energy’s targets of 5.5 wt% and 0.04 kg/L, respectively. This improved performance is attributed to the stronger adsorption affinity of BN atoms and the increased lattice constants of the material. These findings offer promising avenues for the development of advanced hydrogen storage materials.

MOFs with the Stability for Practical Gas Adsorption Applications Require New Design Rules.

Metal-organic frameworks (MOFs) have been widely studied for their ability to capture and store greenhouse gases. However, most computational discovery efforts study hypothetical MOFs without consideration of their stability, limiting the practical application of novel materials. We overcome this limitation by screening hypothetical ultrastable MOFs that have predicted high thermal and activation stability, as judged by machine learning (ML) models trained on experimental measures of stability. We enhance this set by computing the bulk modulus as a measure of mechanical stability and filter 1102 mechanically robust hypothetical MOFs from a database of ultrastable MOFs (USMOF DB). Grand Canonical Monte Carlo simulations are then employed to predict the gas adsorption properties of these hypothetical MOFs, alongside a database of experimental MOFs. We identify privileged building blocks that lead MOFs in USMOF DB to show exceptional working capacities compared to the experimental MOFs. We interpret these differences by training ML models on CO2 and CH4 adsorption in these databases, showing how poor model transferability between data sets indicates that novel design rules can be derived from USMOF DB that would not have been gathered through assessment of structurally characterized MOFs. We identify geometric features and node chemistry that will enable the rational design of MOFs with enhanced gas adsorption properties in synthetically realizable MOFs.

Constructing CO2 capture nanotraps via tentacle-like covalent organic frameworks towards efficient CO2 separation in mixed matrix membrane

The structure and morphology of fillers are crucial factors in the process of CO2 separation within mixed matrix membranes (MMMs). In this study, the tentacle-like covalent organic framework (COF) as a filler was incorporated into the Pebax matrix for CO2/CH4 separation. The COF has abundant Lewis base sites (O atom and N atom) and porous pores, thereby constructing efficient CO2 capture nanotraps in the MMMs. The CO2 capture nanotraps facilitate the CO2 transport in the MMMs, and this can be verified by the Grand Canonical Monte Carlo (GCMC) simulation and density functional theory (DFT) calculation. Simultaneously, the CO2 capture nanotraps are distributed on the surface of tentacle-like COF, which is conducive to strengthening CO2 capture ability. Therefore, Pebax/COF MMMs exhibited superior CO2 gas separation performance compared to the pure Pebax membrane. Among them, the MMM with 3 wt% COF loading displayed the highest CO2 gas separation performance, exhibiting permeability and selectivity that were 73.4% and 32.9% higher than the pure Pebax membrane, respectively. Thus, CO2 capture nanotraps constructed in the MMMs via tentacle-like COF showed a promising potential for CO2 separation.

A polyhedron-based metal-organic framework with electronegative donor sites for efficient C2H6/C2H4 separation

Ethylene is an important chemical raw material which is widely used in the production of chemical fibers, plastics, rubber, coatings, as well as medicines, and its separation from the gas mixtures of C2H6/C2H4 is of great importance but a challenging task due to their similar physical properties and molecular dimensions. Herein, we report the C2H6 and C2H4 sorption studies on a robust anionic metal-organic framework (MOF) {(NH2Me2)[Co3(μ3OH)(TPT)(TZB)3](H2O)6(DMA)6}n (Co-MOF) formed by connection of Co3(OH) clusters by two N-rich organic ligands 2,4,6-tri(4-pyridyl)-1,3,5-triazine (TPT) and 4-(1H-Tetrazol-5-yl)benzoic acid (H2TZB). The prepared Co-MOF features a polyhedron stacking network structure composed of trigonal bipyramidal cages and octahedral cage which both contribute to a high C2H6 uptake capacity of 6.48 mmol/g and a decent C2H6/C2H4 selectivity of 1.54 at 298 K and 1 bar. Grand Canonical Monte Carlo (GCMC) simulation results agree well with the experimental tendency, both in the adsorption isotherms and sorption heats, which demonstrated that the suitable pore surfaces with electronegative donor sites generated multiple CH ⋅⋅⋅O/N and CH ⋅⋅⋅π interactions between the framework and C2H6, resulting in a stronger interaction between the MOF skeleton and C2H6 molecule than C2H4 as reflected by the dispersion-corrected density functional theory (DFT) calculation.

Selective Separation of C8 Aromatics by an Interpenetrating Metal-Organic Framework Material.

O-xylene (OX) is an important chemical raw material, but it is often produced in mixtures with other C8 aromatics. Similar physicochemical properties of the C8 isomers make their separation and purification very difficult and energy intensive. There is an unmet need for an adsorbent that would be effective for the separation of OX from the other C8 isomers. This work reports a three-dimensional interpenetrated metal-organic framework, SYUCT-110, that interacts with each of the single-component C8 isomers to form. The selectivity of C8 aromatic hydrocarbons was determined through liquid-phase batch uptake experiments. The results revealed that the selectivity order was OX > PX > MX > ethylbenzene (EB). The selectivity values were found to be 2.63, 1.58, 5.51, 3.71, 1.86, and 3.02 for OX/MX, OX/PX, OX/EB, PX/MX, MX/EB, and PX/EB, respectively. The adsorption capacity of OX was 71 mg/g. Grand Canonical Monte Carlo simulations were used to study the C8 adsorption sites, revealing that π···π interactions are the main reason for the observed adsorption selectivity. The adsorption energy calculation results also verified the selectivity of SYUCT-110 for the synthesis of OX.

Elucidating the separation role of C3N slit-pore on N2/CH4 in coalbed methane

A hybrid approach integrating molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) simulations was employed to investigate the separation efficacy of N2/CH4 across various C3N slit apertures. MD simulations enabled the determination of separation selectivity and diffusion coefficients of N2/CH4, thereby aiding in the elucidation of the separation mechanism. On the other hand, GCMC simulations provided insights into the adsorption isotherms and isosteric heats of N2 and CH4 within the C3N slit. These findings reveal that the highest selectivity of N2 over CH4 occurs at the zigzag-edge C3N with a pore size of 6.2 Å. However, this selectivity is accompanied by the smallest diffusion coefficient, attributable to size sieving effects. As the pore diameter increases, more gas molecules can permeate into the pores, leading to a rise in both the maximum adsorption capacity and the isosteric heat of methane adsorption. With increasing pressure, gas molecules tend to reach saturation at higher pressure levels. Our study primarily centers on examining the separation of N2/CH4 across various C3N slit-pores, encompassing both adsorption and diffusion phenomena. These findings offer theoretical insights and comprehension for the experimental separation of coalbed methane utilizing cheaper carbon-based materials.

PyMSER─An Open-Source Library for Automatic Equilibration Detection in Molecular Simulations.

Automated molecular simulations are used extensively for predicting material properties. Typically, these simulations exhibit two regimes: a dynamic equilibration part, followed by a steady state. For extracting observable properties, the simulations must first reach a steady state so that thermodynamic averages can be taken. However, as equilibration depends on simulation conditions, predicting the optimal number of simulation steps a priori is impossible. Here, we demonstrate the application of the Marginal Standard Error Rule (MSER) for automatically identifying the optimal truncation point in Grand Canonical Monte Carlo (GCMC) simulations. This novel automatic procedure determines the point at which a steady state is reached, ensuring that figures of merit are extracted in an objective, accurate, and reproducible fashion. In the case of GCMC simulations of gas adsorption in metal-organic frameworks, we find that this methodology reduces the computational cost by up to 90%. As MSER statistics are independent of the simulation method that creates the data, this library is, in principle, applicable to any time series analysis in which equilibration truncation is required. The open-source Python implementation of our method, pyMSER, is publicly available for reuse and validation at https://github.com/IBM/pymser.

Evaluation and screening of multivariate metal-organic frameworks for hydrogen storage

Multivariate metal-organic frameworks (MTV-MOFs) are characterized by their unique structural feature of incorporating multiple organic linkers and metal ions into a unified framework. This composite construction bestows MTV-MOFs with a broader spectrum of diverse and intricate properties compared to traditional single-component MOFs. In this study, the performance of 560 MTV-MOFs as hydrogen storage adsorbents has been thoroughly investigated. Firstly, the key structural parameters of materials, including density, pore occupied accessible volume, accessible surface area, and porosity were computed. Then, high-throughput Grand Canonical Monte Carlo (GCMC) simulations were employed to calculate hydrogen adsorption capacity of MTV-MOFs under hydrogen pressures of 100 bar at both 77 K and 298 K. Based on simulated results, the deliverable hydrogen capacity of these MTV-MOFs were deduced under both room temperature conditions (298 K, 97.2 bar → 298 K, 5 bar) and low-temperature conditions (77 K, 100 bar → 160 K, 5 bar). Through high-throughput screening, we identified top ten promising MTV-MOFs with the highest hydrogen storage capacity and conducted in-depth studies on their hydrogen adsorption properties. Furthermore, we developed a multivariate linear regression model to quantitatively predict the relationship between hydrogen adsorption capacity and their structural parameters for these MTV-MOFs. The predictions of this model align closely with the outcomes derived from GCMC simulations. The present study highlights the potential of MTV-MOFs as promising candidates for hydrogen storage applications, thereby providing valuable theoretical insights into the exploration of high-capacity hydrogen storage materials.

The utility of MOF-based materials in direct air capture (DAC) application to ppm-level CO2

Metal-organic frameworks (MOFs) are well-suited materials for CO2 removal and have robust capture capacity and selectivity. Although the adsorption of CO2 in MOFs has been studied, the implementation of ppm-level CO2 uptake in MOFs and the effects of the pore size and charge have not been fully explored. We performed grand canonical Monte Carlo (GCMC) simulations combined with the Density Functional Theory plus U (DFT + U) charge method to investigate MOF screening for ppm-level CO2 uptake and its application in a direct air capture (DAC) system. Three types of MOFs containing eight members were studied: i.e., ZIF-68, 69, 70; UiO-66, 67, 68; CAU-10; and MIL-125. The pore landscape characterization, electrostatic field-induced enhancement, and preferential binding sites of these MOFs were examined for CO2 capture. MOFs with pore limited diameters (PLD) 1.5 times the size of CO2 molecules and with large cavity diameters (LCD) smaller than 10 Å exhibit robust confinement capacity. Polar functional groups and metal ions dominate the electrostatic contributions and subsequently enhance the surface adhesion of CO2 molecules. For a given framework, favorable CO2 binding occurs in the following order: small pores/cages > polar functional group/metal ions > larger pores/cages. ZIF-69 which comprises smaller pores (7.5 Å) and robust polar functional groups (–Cl) collectively enhances CO2 capture; thus, ZIF-69 outperforms other MOFs; the performance of ZIF-69 is followed by that of CAU-10 which has an optimal pore size of 6 Å. These findings are of fundamental and practical importance for the application of MOFs in DAC technologies for CO2 removal.

Introducing Noble Gas as Space Holder under High Pressure to Design Porous Titanium Carbides with Open Metal Sites for Hydrogen Storage at Near-Ambient Conditions.

It has been a long-standing challenge to develop high-performance solid-state hydrogen storage materials operated under near-ambient conditions. In this work, we propose a new strategy of using noble gases for space holding to design porous titanium carbides with abundant open metal sites for hydrogen storage. By using machine learning and graph theory-assisted universal structure searching methods, we obtain 28 porous titanium carbides from three precursors (TiC dimer, C atom, and Kr atom) under 30 GPa of pressure. The stability and hydrogen storage performance of the resulting structures are further assessed and validated through density function theory and grand canonical Monte Carlo simulations with a DFT-fitted force field. Finally, p-TiC2 is identified as a promising quasi-molecular hydrogen storage material with capacity of 4.0 wt % and 106.0 g/L at 230 K and 16 bar.

Charge regulation of nanoparticles in the presence of multivalent electrolytes.

We explore the charge regulation (CR) of spherical nanoparticles immersed in an asymmetric electrolyte of a specified pH. Using a recently developed reactive canonical Monte Carlo (MC) simulation method, titration isotherms are obtained for suspensions containing monovalent, divalent, and trivalent coions. A theory based on the modified Poisson-Boltzmann approximation, which incorporates the electrostatic ion solvation free energy and discrete surface charge effects, is used to compare with the simulation results. A remarkably good agreement is found without any fitting parameters, both for the ion distributions and titration curves, suggesting that ionic correlations between coions and hydronium ions at the nanoparticle surface play only a minor role in determining the association equilibrium between hydroniums and the functional sites on the nanoparticle surface. On the other hand, if suspension contains multivalent counterions, we observe a large deviation between theory and simulations, showing that the electrostatic correlations between counterions and hydronium ions at the nanoparticle surface are very significant and must be properly taken into account to correctly describe CR for such solutions.

A Novel Composite Material UiO-66-Br@MBC for Mercury Removal from Flue Gas: Preparation and Mechanism.

To reduce the mercury content in flue gas from coal-fired power plants and to obtain high-performance, low-cost mercury adsorbents, a novel composite material was prepared by structural design through the in situ growth method. Functionalization treatments such as the modification of functional groups and multilayer loading of polymetallic were conducted. These materials include the MOF material UiO-66 and modified biochar doped with Fe/Ce polymetallic, both of which contain unsaturated metal centrals and oxygen-containing functional groups. On the basis of obtaining the effects of adsorption temperature and composite ratio on the Hg0 removal characteristics, coupling and synergistic mechanisms between the various types of active centers included were investigated by using a variety of characterization and analysis tools. The active adsorption sites and oxidation sites were identified during this process, and the constitutive relationship between the physicochemical properties and the performance of Hg0 removal was established. The temperature-programmed desorption technique, Grand Canonical Monte Carlo simulation, and adsorption kinetic model were employed to reveal the mechanism of Hg0 removal. The results showed that the UiO-66-Br@MBC composite adsorbent possessed an excellent Hg0 removal performance at adsorption temperatures ranging from 50 to 250 °C, and targeted construction of adsorption and oxidation sites while maintaining thermal stability. The Hg0 removal by the composites is the result of both adsorption and oxidation. The micropores and small pore mesopores in the samples provide physical adsorption sites. The modified biochar acts as a carrier to facilitate the full exposure of the central metal zirconium ions, the formation of more active sites, and the process of electron transfer. The doping modification of the Br element can enhance the overall redox ability of the sample, and the introduced Fe and Ce polymetallic ions can work in concert to promote the oxidation process of Hg0. The excellent regulation of the ratio between adsorption and oxidation sites on the surface of the composite material finally led to a significant boost in the samples' capacity to remove Hg0.

Removal of tricyclazole residues from water by sorption on metal organic frameworks (MOFs): A theoretical insight of the experimental data

All over the world, there are stringent regulations in place concerning the acceptable amount of pesticide residues in various food commodities, including cereals. The majority of the water tainted with pesticides comes from agricultural drainage, making it imperative that this water be cleansed of any lingering traces of the chemicals. The presence of unacceptable levels of pesticides residues is another issue hindering India's ability to export Basmati rice and other commodities. Tricyclazole, a fungicide has been found in rice in recent years, leading to the rejection of a number of bulk consignments. Therefore, three metal organic frameworks (MOFs) viz., MIL-53(Al), Fe-BTC, and ZIF-8, based on three different metal ions (aluminium, iron, and zinc) and three different organic "struts" were investigated for the removal of TRCZ from water. Interestingly, TRCZ was found to be selective towards both metal ions and linkers. The maximum adsorption capacities of TRCZ onto MIL-53(Al), Fe-BTC and ZIF-8 were 980.0, 896.7 and 904.6 μg g−1 (@1 ppm aqueous solution), respectively. Adsorption followed pseudo-second-order kinetics and best fit the Sips isotherm. The adsorption of TRCZ molecules onto the surface of the MOFs were confirmed using Fourier Transform infrared (FTIR) and X-Ray Diffraction (XRD) spectroscopy. MIL-53(Al) showed the best regeneration capacity (> 90 % efficiency after the 4th cycle). The adsorption of pesticide molecules onto MOF surfaces was studied (in vacuo and in aqueous) circumstances using Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) simulations revealing the prime competitive role of water molecules aided by π-π stacking and π-lone pair interactions during adsorption. Metal organic frameworks have the potential to replace costly commercial activated carbons in the future to treat pesticide-contaminated water.

Mechanistic effects of graphitization and oxygen functional groups on benzene competitive adsorption of porous carbon under high humidity conditions

Porous carbon is an ideal adsorbent for the elimination of volatile organic compounds (VOCs) attributed to their price, availability and excellent adsorption capacity. However, there are significant variation in the adsorption capacity and adsorption selectivity of different precursors for VOCs under humid conditions after modification because of the differences in their own structural properties. In this study, graphitized hydrophobic porous carbon with excellent specific surface area was successfully synthesized by selecting bamboo fiber, maize, coconut husk and coal as precursors and potassium hydroxide as an activator. Among them, the dynamic adsorption capacity of benzene in the sample with bamboo fiber as precursor was maintained at 74.7% under the high humidity condition (RH90%) compared with the dry state (RH0%). Grand Canonical Monte Carlo (GCMC) simulations and weak interaction force analyses showed that enhanced dispersion attraction between graphitized carbon surfaces and benzene molecules in comparison to disordered carbon surfaces. The existence of oxygen functional groups enhanced the interaction between porous carbon and water through hydrogen bonding leading to preferential adsorption of water molecules, which resulting in the decrease of their adsorption performance in wet condition. This research provided theoretical and experimental views of the effects of graphitization and oxygen functional groups on the competitive adsorption capture of benzene and water by porous carbon, which further provided a reliable approach to synthesizing porous carbon with high adsorption capture for benzene removal in high humidity environments.

Impact of aminopropyl units on physisorption in mesostructured silica is larger for CH4 than CO2 and depends on pore curvature

Mesoporous silica-based materials attract great interest for diverse applications, including capture, storage, and catalysis due to their large surface area, uniform pore size distribution, and customizable surface. The regular 2D hexagonal pore symmetry of MCM-41 makes it an ideal model. Amine groups are introduced to enhance CO2 adsorption, but the interplay of factors influencing overall performance, including pore size, remains unclear. This computational study explores the physisorption of CO2 and CH4, shedding light on the microscopic details underlying adsorption capacity and selectivity. Three aminopropyl-functionalized MCM-41 models with different pore size are compared through Grand Canonical Monte Carlo and Molecular Dynamics simulations. Aminopropyl chains affect CO2 adsorption, but its affinity for the preserved silanol groups is larger. In addition, the competitive interaction between amine and silanol groups emerged, and it is pore curvature dependent. Surprisingly, CH4 physisorption is influenced even more by surface functionalization. Results show a non-monotonic trend as a function of pore size. Local density of the alkyl chains plays the major role. Overall, the intermediate pore size demonstrates the best performance, thanks to the balance among the various effects. The study emphasizes the importance of understanding the interplay between material texture and gas interactions to optimize adsorption performance.

Theoretical investigation of mixed-metal metal-organic frameworks as H2 adsorbents: insights from GCMC and DFT simulations

ABSTRACT Molecular hydrogen ( H 2 ) is a renewable energy carrier, however, its practical applications are limited due to the challenges of developing safe and efficient H 2 storage devices. Metal Organic Frameworks (MOFs) containing at least two different metal ions in their structures are called as mixed-metal MOFs (MM-MOFs) and they could adsorb H 2 in higher amounts compared to structures containing single metal nodes. We theoretically examined the H 2 storage capacities of 26 MM-MOFs having various physical and chemical properties applying Grand Canonical Monte Carlo (GCMC) and Density Functional Theory (DFT) simulations. H 2 adsorption isotherms were calculated using a five-site anisotropic H 2 model. QIXSOG, YOMVIG, OSOYUR, Cu-Mg-BTC, Fe-Mg-BTC, and Cr-Mg-BTC were selected as top-performing MM-MOFs maximising H 2 adsorption gravimetrically and volumetrically at near-ambient conditions (233 K and 100 bar), approaching the DOE targets. YOMVIG has the largest H 2 adsorption enthalpy, calculated as − 9.93 kJ/mol at 233 K and 100 bar. DFT simulations have been conducted to analyse preferable H 2 adsorption sites as well as identify guest-host interactions. Electron density difference analysis showed that adsorbed H 2 molecules in the OSOYUR, Cr-Mg-BTC, Cu-Mg-BTC, and Fe-Mg-BTC are polarised. Our study challenges existing literature by identifying promising MM-MOFs as potential next-generation hydrogen storage adsorbents at near-ambient conditions.

Effect of surface roughness on methane adsorption in shale organic nanopores from the perspective of molecular simulations

Understanding the shale gas adsorption behavior within organic nanopores is crucial for its development. The pore surfaces commonly exhibit random inherent roughness, which can significantly impact gas adsorption, while it’s still in debate about whether the roughness could enhance the gas adsorption or not. To clarify the underlying mechanisms of the effect of surface roughness, in this study, the Grand Canonical Monte Carlo and Molecular Dynamic simulations are used to investigate the methane adsorption in wrinkled graphene nanochannels with various roughness. The heterogeneity of the adsorption layer along the rough topology is described in detail. The adsorption is enhanced near the concave region and weakened near the convex region. Additionally, increased surface roughness shifts the gas molecules’ aggregation modes in the concave region from a gradual gradient to oscillatory “adsorption points”. We observe that wall roughness due to the wrinkle can enhance the adsorption amount overall, attributed to the increased surface area and the increased unit adsorption capacity due to potential energy overlap. However, the contributions of both factors differ under various pressure and roughness conditions. Then a new mathematical method to estimate the gas adsorption amounts is provided and aligns well with molecular simulation results for a wide range of pressure and roughness conditions. And a potential answer the current debate about the positive or negative impact of adsorption due to roughness is promoted. The key may lie in the dominant role of the co-existed roughness caused by the fragment basal or edge surface. Our work could shed light on the underlying effect of surface roughness on the gas adsorption in shale, and have significant implications for the reserve estimation of shale gas formations.

Combined experimental and grand canonical Monte Carlo simulation study of CO2 capture in nitrogen and sulfur co-doped biochar derived from biowaste: Cost analysis, kinetics, and equilibrium

Nitrogen (N) and sulfur (S) co-doped biochar was produced from an agricultural waste. Heat treatment was combined with the use of dopants including thiourea, CO2, and air. The results show that the modified N and S co-doped biochar with the use of CO2 gas agent at 1123 K for 2 hr in the modification process, had the highest CO2 adsorption capacity of 4.36 and 3.05 mmol/g at 273 and 298 K at 1 bar among others in literature even it had lower surface area or pore volume. This is due to the containing of N and S on the solid surface and superior pore size of 1.0 nm. Moreover, the modified biochar had superior kinetics and CO2/N2 selectivity and was more economical than its unmodified biochar. To support experimental observation and better understand individual characteristics of functionalized carbons, a grand canonical Monte Carlo simulation was carried out to systematically illustrate the effect of N and S functional groups, pore size and pore volume on CO2 adsorption. The surface chemistry was the minor factor and played the key role in CO2 adsorption capacity at low pressures until it became saturated by adsorbate molecules at 0.3 bar. S was stronger than N for enhanced CO2 adsorption capacity. Whereas the pore size was the major factor in both low and moderate pressures (< 23 bar). The optimum pore size was found in a range of 0.66⎼4.0 nm which was pressure-dependent. The pore volume became the main factor at higher pressures (> 23 bar).