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This work analyses the formation of the Knudsen layer in micro/nanoscale flows by linking a rough wall collision model to a continuum flow model via asymptotic matching. Expressions for the accommodation coefficients in terms of the surface characteristics are derived, allowing for boundary layer analysis of rarefied flows without the use of prior determined accommodation coefficients. This derived model, through use of the Lennard–Jones parameters for a nanoscale system, allows for a prediction of the the effective Tangential Momentum Accommodation Coefficient (TMAC) in flows against ordered nanoscale surfaces.

The quest to reduce transport resistance in separations using nanomaterials has led to considerable interest in nanoscale adsorbents and ultrathin membranes. It is now established that interfacial resistance limits the performance of such nanosized materials; however, the origin of this resistance is uncertain. While it is associated with surface pore blockages and distortions in some materials, its existence even in ideal materials is largely putative. Here, we report equilibrium molecular dynamics (EMD) simulations with ideal zeolite-based nanosheets, indicating the transport resistance to be entirely distributed within the solid, without contribution from an interfacial effect. We demonstrate the presence of an internal entry region over which fluid decorrelation occurs, and in which the local transport coefficient inside the crystal is nonuniform and position-dependent, increasing to the uniform value in the bulk material at larger distances. Our EMD-based diffusivity profiles within the nanomaterial enable us to unequivocally determine the entry length, and reveal an internal excess resistance, frequently assumed to be an interfacial resistance, due to significant reduction of the internal transport coefficient in the entrance and exit regions. A decrease in the entry length with loading in PON zeolite nanosheets is seen. We demonstrate a reduction in external resistance in the external bulk chambers used in simulations, triggered by the interplay of incomplete decorrelation in the nanosheet and periodic boundary conditions imposed on the system comprising the nanosheet and surrounding bulk reservoirs when the nanosheet thickness is less than the entry length. Our analysis of the transport dynamics within the nanosheet demonstrates that, at least for ideal systems, decomposition of the inhomogeneous diffusivity-based internal resistance into an interfacial and a uniform transport coefficient-based internal contribution is not appropriate for finite-sized systems. Our results will enable the improved design of nanoscale membranes and materials for applications in separation and other processes.

Fluid transport in finite-sized nanoporous materials is critically affected by apparent interfacial barriers, which severely restrict efficiency improvement on reduction in system size; however, th...

We develop novel methods to estimate the gas permeation within the mixed matrix membranes (MMMs) fabricated by polycarbonate (PC), p-nitroaniline (pNA), and zeolite 4A. To this end, molecular simulation including molecular dynamics and grand ganonical Monte Carlo methods were utilized to determine the diffusivity and solubility coefficients of H-2, CH4, CO2, O-2, and N-2 gas molecules within the PC/pNA/zeolite 4A membranes. Coupled with other analysis such as X-ray diffraction, fractional free volume, and radial distribution function, structural and separation features like glass-transition temperature, mean square displacement, density, and adsorption isotherms define the solution-diffusion separation mechanism. Two state-of-the-art and accurate artificial intelligence (AI) models, viz ant colony optimization-adaptive neuro-fuzzy inference system (ACO-ANFIS) and genetic programming (GP) are developed to estimate the permeability of mentioned gas molecules within the MMMs. The R-2 value for ACO-ANFIS and GP was obtained as 0.97 and 0.96, respectively. The mean squared error value for ACO-ANFIS and GP was obtained as 0.41 and 0.51, respectively. Although the accuracy of ACO-ANFIS is higher, both models can be regarded as efficient models. The findings of the study demonstrate that the AI models are accurate. In addition, the effects of various key parameters like zeolite and pNA loading and kinetic diameter of gases are investigated. Also, it was concluded that the simple PC membrane indicated the best performance compared to other prepared MMMs.

The high conductivity of nanoprous carbons has a significant effect on adsorption of polar molecules; however, the mechanisms underlying this effect are not well-understood, and this effect is generally not considered in adsorption modeling. To investigate the impact of high host conductivity on the adsorption properties of vertical stacks of the graphene sheet, phenol vapor has been chosen as a simple polar adsorptive for our case studies. The influence of high surface conductivity of graphene is taken into account during Grand Canonical Monte Carlo (GCMC) simulations by considering the resulting image charges, and using the corresponding analytical solution to compute the electrostatic energy of the polar molecules confined between two infinite parallel conducting planes. We find that the consideration of the high conductivity affects the atomic configuration of the adsorbed molecules, based on our results for two different pore widths of 1 nm and 0.65 nm. Allowing for the high conductivity results in more stable energy levels, greater heat of adsorption and smaller distances between phenol molecules and the graphene sheet. There results are shown to be in accord with electronic density functional theory calculations, and literature experimental data. The contributions of different guest–guest (phenol–phenol) and guest-host (phenol and graphene) interactions between molecules are studied.

We engineer the interfacial characteristics of a ZIF-8–6FDA–durene polyimide (PI) polymer mixed matrix membrane in silico by using an ionic liquid (IL) at the polymer–ZIF interface for improved CO2...

The drive for enhancing process efficiency by decreasing the system length scale to reduce transport resistance in nanomaterials brings into prominence the interfacial resistance, the relative impo...

This paper dwells into the diffusional transport of water inside a pelleted zeolites wherein the transport behavior in binder phase as well as in the zeolite phase is explored. It is noted that both the phases contributes towards the separation process. A new variant of bi-disperse diffusion model is proposed for the water transport inside the adsorbent pellets. The equilibrium uptake of water is separately encountered in the binder and zeolite phases using a dual site Henry-Langmuir isotherm and a thermodynamically consistent correlation is developed for the boundary layer concentration of the zeolite phase with respect to the concentration within the binder phase. Adsorption kinetics measurements are reported for various adsorbent sizes and process temperatures, and their influence on transport diffusivity is explored. The proposed model prediction is in good agreement with the experimental observation in which the pellet diffusion controls the overall transport behaviour.

An understanding of interfacial barriers underpinning fluid transport in nanoporous membranes is critical for the design of efficient next generation membranes, to harness their potential for indus...

We extend the Maxwell-Stefan (M-S) formulation of irreversible thermodynamics to multicomponent transport in mixed-matrix membranes (MMMs), using a simulation-based rigorous modeling approach (SMA) through finite-element method (FEM) solution of the three-dimensional (3-d) transport problem in full-scale MMMs. In the new approach, we generalize the dual-mode/partial immobilization (DM/PI) theory for the local permeability in glassy polymers to describe multicomponent permeation in pure glassy polymer membranes and MMMs, by reformulating the M-S constitutive equations in the Onsager formalism considering concentration-dependent transport diffusivities and non-uniform concentration gradients across the MMM. In this way, the new M-S formulation explicitly considers effects of intrinsic MMM features such as finite filler particle size and isotherm nonlinearity in the MMM constituent phases, as well as mixture-related effects, such as competitive adsorption and friction amongst permeants, on the calculation of the mixture fluxes (permeabilities). This is achieved without introduction of empirical fitting parameters in the MMM permeability calculation and only requiring single-gas experimental or simulation-based adsorption and permeation data on the individual MMM materials to predict the mixture perm-selectivity in the MMM as a whole. Further, we validate the new approach by using available experimental permeation data for the separation of an equimolar binary mixture of propylene ( C 3 H 6 ) and propane ( C 3 H 8 ) in ZIF‑8/PIM-6FDA-OH MMMs, with the rigorous simulation results showing very good agreement with both experimental single and mixed-gas permeabilities and perm-selectivities.

We have used equilibrium molecular dynamics (EMD) simulations to investigate the temperature dependence of Maxwell–Stefan (MS) diffusivities of a pure component as well as an equimolar mixture of C...

Nanodiamonds (ND) have recently emerged as excellent candidates for various applications including membrane technology due to their nanoscale size, non-toxic nature, excellent mechanical and thermal properties, high surface areas and tuneable surface structures with functional groups. However, their non-porous structure and strong tendency to aggregate are hindering their potential in gas separation membrane applications. To overcome those issues, this study proposes an efficient approach by decorating the ND surface with polyethyleneimine (PEI) before embedding it into the polymer matrix to fabricate MMMs for CO2/N2 separation. Acting as both interfacial binder and gas carrier agent, the PEI layer enhances the polymer/filler interfacial interaction, minimising the agglomeration of ND in the polymer matrix, which is evidenced by the focus ion beam scanning electron microscopy (FIB-SEM). The incorporation of PEI into the membrane matrix effectively improves the CO2/N2 selectivity compared to the pristine polymer membranes. The improvement in CO2/N2 selectivity is also modelled by calculating the interfacial permeabilities with the Felske model using the gas permeabilities in the MMM. This study proposes a simple and effective modification method to address both the interface and gas selectivity in the application of nanoscale and non-porous fillers in gas separation membranes.

This work investigates the effect of condensable Hg vapour on the transport of N-2 gas across cobalt oxide silica (CoOxSi) membranes. Experimental results suggest that Hg significantly affects N-2 permeation at 100 and 200 degrees C, though this effect is negligible at 300 degrees C. This effect was found to have a correlation with Hg adsorption on CoOxSi xerogels. In order to understand the Hg effect in the transport phenomena of N-2 permeation, the oscillator model was used to model gas transport through pores with different sizes. By including effective medium theory (EMT), the oscillator model fitted well the experimental results and gave good prediction of mass transfer in ultra-microporous materials with a tri-modal pore size distribution, such as silica membranes. It is postulated that Hg seeks lower level potentials in micro-pores, and therefore Hg molecules tend to block small pores (2.5-4 angstrom from 2.9 angstrom), or reduce the average pore size of larger pores (6.7-7.8 angstrom and 12-14 angstrom). Although N-2 permeation decreased with the presence of Hg, it did not decrease when the Hg load was increased by a factor of ten; this strongly suggests the adsorption of Hg molecules in the smaller pores (2.5-4.0 angstrom), or along the pore wall for the larger pore ranges (6.7-7.8 angstrom and 12-14 angstrom). (C) 2018 Elsevier Ltd. All rights reserved.

We extend effective medium theory (EMT) to composite hollow fiber mixed matrix membranes, considering the asymmetric filler volume fraction profile arising from finite system size. This volume fraction profile leads to strong variation of the driving force (i.e. pseudo-bulk concentration gradient) in the regions adjacent to the composite ends, and to sensitivity of the effective permeability of the composite to the geometrical configuration. The new theory is validated against rigorous simulations of the transport in mixed-matrix membranes (MMMs) using both concentration-independent and concentration-dependent diffusivities in the MMM constituent phases. Both theory and simulations show that flat mixed-matrix membranes (F-MMMs) have higher effective overall permeability than hollow fiber mixed-matrix membranes (HF-MMMs), upon comparison of systems having identical operating conditions, filler phase loading and particle size. Furthermore, we show here that the sensitivity to the geometry vanishes with increase of inner radius of the hollow fiber membrane at fixed thickness, and the effective permeability of a HF-MMM is found to asymptotically approach that of a F-MMM.

We present a new theory to estimate fluxes and effective transport conductances of binary mixtures through a membrane comprising a nonuniform porous medium with both pore size and pore length distributions, using the Onsager formulation at the single pore level. The theory defines a conductance of each species that is dependent on the concentration gradients of the various species, and on using effective medium theory determines the fluxes and concentration profiles self-consistently in the porous medium. The transport of CH4/H2 mixtures in a silica membrane having a known pore size distribution is examined using this theory, and the results compared with those from rigorous simulations, showing good agreement. It is found that an optimal network coordination number exists at which species fluxes are a maximum, due to the opposing effects of increasing porosity and mean pore length with increase in coordination number. Further, network fluxes decrease with increase in pore dispersion, indicating that uniform pore size is optimal. A species and pressure-dependent optimal temperature is also predicted, due to the competing effects of increase in diffusivity and decrease in adsorption on increasing temperature. It is seen that the CH4 selectivity is very sensitive to temperature, and undergoes a cross-over, with the membrane being more selective to CH4 at low temperature and to H2 at high temperature. In general, the selectivity is very sensitive to the distribution of pore volume, and for bimodal pore networks, undergoes a sharp transition at the percolation threshold, when the smaller pore size is impermeable to the larger species, CH4. The approach offers a convenient adaption of effective medium theory to multicomponent systems with nonlinear isotherms, overcoming drawbacks of existing theory.

For the calcination of CaCO3 under CO2-containing atmospheres, a mathematical model taking into account the CO2-catalyzed sintering of the CaO product layer is developed. In this model, a modified shrinking core model is coupled with a population balance-based sintering model. By comparing model predictions with experimental data, it is found that CO2 strongly affects the overall calcination rate both at high temperature and CO2 partial pressure, since under these conditions CaO densification considerably reduces the effective diffusivity of CO2 within product layer. It is observed that for large particles, the CO2-catalyzed sintering of CaO can produce the die off phenomenon, which takes place when the reaction stops due to the blockage of pores within product layer. Finally, it was determined that limestone impurities do not significantly affect the calcination reaction under atmospheres containing CO2, because CO2 causes a much greater increase of the CaO sintering rate than limestone impurities do. (c) 2018 American Institute of Chemical Engineers AIChE J, 64: 3638-3648, 2018

Water displays anomalous fast diffusion in narrow carbon nanotubes (CNTs), a behavior that has been reproduced in both experimental and simulation studies. However, little is reported on the effect of bulk water-CNT interfaces, which is critical to exploiting the fast transport of water across narrow carbon nanotubes in actual applications. Using molecular dynamics simulations, we investigate here the effect of such interfaces on the transport of water across arm-chair CNTs of different diameters. Our results demonstrate that diffusion of water is significantly retarded in narrow CNTs due to bulk regions near the pore entrance. The slowdown of dynamics can be attributed to the presence of large energy barriers at bulk water-CNT interfaces. The presence of such intense barriers at the bulk-CNT interface arises due to the entropy contrast between the bulk and confined regions, with water molecules undergoing high translational and rotational entropy gain on entering from the bulk to the CNT interior. The intensity of such energy barriers decreases with increase in CNT diameter. These results are very important for emerging technological applications of CNTs and other nanoscale materials, such as in nanofluidics, water purification, nanofiltration, and desalination, as well as for biological transport processes.

Current models for transport in dispersions, while grounded in well-established effective medium theory (EMT), rely on the assumption of uniformity of the driving force. As consequence, theoretical approaches cannot accommodate driving force inhomogeneities as well as variations over the space occupied by the dispersed phase particles, and existing EMT-based models therefore fail to represent finite particle size effects. Moreover, because transport coefficients are generally considered uniform, such models largely pertain to the Henry's law region. Here, using the context of permeation in mixed-matrix membranes (MMMs), we introduce a self-consistent theory for transport in dispersion-based composites, which captures effects of isotherm nonlinearity and dispersant size without introducing fitting parameters, and accurately predicts concentration-dependent permeabilities. The model is validated against rigorous 3d simulations of transport in filler-polymer composite MMMs, with excellent agreement between theoretical results and those from simulation. Both model and simulations confirm isotherm nonlinearities to have a very significant effect on effective MMM permeability, which is found to be more sensitive to isotherm nonlinearity in the filler phase than in the continuous phase. These effects disappear when the filler phase is much more permeable than the continuous phase, although additional system size effects related to exclusion regions at the ends due to finite particle size lead to decrease in permeability with increase in particle size even for linear isotherms.

We investigate the structure of polyimide (PI) at the surface of a silicalite zeolite (MFI), as part of a model hybrid organic-inorganic mixed matrix membrane system, through equilibrium molecular dynamics simulations. Furthermore, we report a comparison of the adsorption and transport characteristics of pure components CO2 and CH4 in PI, MFI, and PI-MFI composite membranes. It is seen that incorporation of MFI zeolite into PI results in the formation of densified polymer layers (rigidified region) near the surface, having thickness around 1.2 nm, before bulklike behavior of the polymer is attained, contrary to empirical fits suggesting the existence of an approximately 1 μm thick interface between the polymer and filler. This region offers an extra resistance to gas diffusion especially for the gas with a larger kinetic diameter, CH4, thus improving the CO2/CH4 kinetic selectivity in the PI-MFI composite membrane. Furthermore, we find that the kinetic selectivity of CO2 over CH4 in the rigidified region increases with temperature and that additivity of transport resistances in MFI, interfacial layer, and bulklike region of the polymer satisfactorily explains transport behavior in the composite sandwich investigated. The gas adsorption isotherms are extracted considering the dynamics and structural transitions in the PI and PI-MFI composite upon gas adsorption, and it is seen that the rigidified layer affects the gas adsorption in the polymer in the PI-MFI hybrid system. A significant increase in CO2/CH4 selectivity as well as gas permeability is observed in the PI-MFI composite membrane compared to that in the pure PI polymer membrane, which is correlated with the high selectivity of the rigidified interfacial layer in the polymer. Thus, while enhancing transport resistance, the rigidified layer is beneficial to membrane selectivity, leading to improved performance based on the Robeson upper bound plot for polymers.

A series of Li4SiO4 was synthesized using LiNO3 and six different silicon precursors. The precipitated-silica-derived Li4SiO4 presented the highest CO2 capacity in a 10 h sorption test, and ZSM-5-derived Li4SiO4 demonstrated the most rapid CO2 sorption. The CO2 sorption kinetics predominantly followed the nucleation mode and could be accurately described by the Avrami-Erofeev model. The Avrami-Erofeev model provided an in-depth analysis of correlation between sorption performance and material properties. Both the nucleation speed and nucleation dimensionality affected the overall sorption kinetics. The kinetics and pore-size distribution suggest that the sorption kinetics was dependent on the quantity of ∼4 nm-pores which favors nucleation dimensionality. For the cyclic tests, the precipitated-silica-derived sample presented the poorest performance with the capacity decreasing from 31.33 wt% at the 1st cycle to only 11.52 wt% at the 30th cycle. However, the sample made from fumed silica displayed an opposite trend with the capacity increasing from 19.90 wt% at the 1st cycle to 34.23 wt% at the 30th cycle. The radically distinct behaviour of samples during cycles was on account of the alternation of sorption kinetics. The decrease in ∼4 nm-pores after cycles was responsible for the decrease of nucleation dimensionality for the precipitated-silica-derived sample. The rearrangement during cycles could enrich the pores of ∼4 nm for the fumed silica-derived sample, which improved the nucleation growth, thus enhancing the kinetics with cycles.

We use atomistic simulations of CH4, H2 and CO2 transport to elucidate the effect of internal interfacial barriers on gas transport within nanoscale zeolite crystals of different morphology; such barriers are shown here to have critical significance for the performance of ultrathin membranes and nanoscale mixed matrix composite membranes for gas separation. These interfacial resistances are seen to be confined to a narrow region within the zeolite, adjacent to the phase boundary, and are distinct from the external fluid phase resistance. Our results indicate that interfacial barriers depend on the nature of the zeolite pore network, and significantly hinder gas transport in nanocrystals. We show that interaction with dense surrounding media such as a polymer enhances the interfacial resistance internal to the zeolite, and further attenuates gas transport. The relative contribution of the interfacial resistance to the gas transport resistance is more significant at lower temperatures, and this resistance is larger than the external gas phase resistance by an order of magnitude. We determine the critical membrane thickness, below which these interfacial barriers are pronounced, and find that interfacial barriers can be significant up to a thickness of about 0.1 μm. Furthermore, these interfacial barriers to gas transport are significantly higher for a gas with a kinetic diameter comparable to that of the limiting pore diameter of the zeolite (methane in this case), thus improving the H2/CH4 kinetic selectivity in finite crystals. We show that small finite crystals of SAS type zeolite are selective for H2 over CH4, while large crystals are selective for CH4 over H2.

Owing to the geometrical features of 2D graphene intercalated into 3D mesoporous silica, CO2 sorption increased by 51% and the heat of sorption reduced by up to 27% as compared to a pure 3D mesoporous silica sorbent without graphene. The superior performance of the amine-functionalised 2D/3D hybrid sorbent was attributed to the enhanced porosity and graphene physisorption at the expense of amine chemisorption.

Grand Canonical Monte Carlo (GCMC) simulations have been conducted to investigate the interaction between phenol and a commercial activated carbon cloth KIP1200 in aqueous solution. The nanostructure of KIP1200 has been extracted using Hybrid Reverse Monte Carlo (HRMC) simulations based on experimental measurements of XRD-based structure factor, and shows the presence of pore spaces inaccessible to both phenol and nitrogen. The impact of empirical force fields (EDIP and Erhart-Albe) on the structural parameters and adsorption capacity, as well as pore accessibility is investigated via molecular simulations and materials informatics employing lattice and Voronoi decomposition algorithms. While the Erhart-Albe force field leads to more amorphous structures and inaccessible pores, the EDIP force field creates more lamellar structures with sharp edges and narrow pore width. The molecular state of adsorbed phenol in these structures is markedly different, with the narrower pore spaces of the EDIP-based structure providing stronger interaction between phenol molecules and carbon structure than for water. The calculated X-ray intensities and adsorption isotherms for the simulation-based structure are in good agreement with corresponding experimental results. The simulated solvation state of phenol has been verified by experiments, and demonstrates the existence of inter-layer spaces inaccessible to phenol for both force fields.

The paper deals with the understanding of the CO2 capacity of adsorption through the anhydrite cement compound, i.e., CaO·SiO2 structure. The Wollastonite structure is prepared with various CaO/SiO2 ratios for the atomistic simulations. Subsequently, simulations of binary adsorption of H2O and CO2 have been performed to arrive at C–S–H structure. The CaO/SiO2 ratio is varied from 0.66 to less than 1.5 in the present study. The interlayer spacing between CaO sheets is considered as 13.5 A. The impact of the electrostatic charges has been explored using available CLAY_FF and CSH_FF potentials. The outcomes have been successfully examined and compared with experimental results. To understand the capacity adsorption of CO2 and selectivity effect on C–S–H forming, numerical experiments have been performed using CO2 concentrations from 10 ppm up to 1,000,000 ppm at 20 °C. These numerical experiments allow understanding of the selectivity of H2O and CO2 in binary mixture adsorption, and the capacity of CO2 adsorption in the Wollastonite atomic structure. According to the physisorption simulations, the CO2 adsorption capacity depends not only on CO2 concentration but also the Wollastonite structure. The possible CaCO3 precipitation and C–S–H structure are addressed.

We use grand canonical Monte Carlo simulation to investigate the adsorption of a CO2/N2 mixture in neutral and charged (7, 7) carbon nanotube (CNT) arrays. It is found that both the adsorption of CO2, and the CO2/N2 selectivity are either enhanced or reduced when the charges are positive or negative. The CO2/N2 selectivity in a CNT bundle carrying +0.05e charge with intertube distance of 0.335 nm exceeds 1000 for pressures up to 15 bar, which is remarkably high. It is seen that strong electrostatic interactions from neighbouring CNTs enhance the adsorption of CO2 over N2, and while the adsorption of CO2 has complex dependence on intertube distance, the CO2/N2 selectivity decreases with intertube spacing. We propose a quantitative performance coefficient as an aid to assessing the efficiency of CNT bundles to separate CO2 from flue gas, and show that a +0.05e charged bundle with intertube distance of 0.335 nm provides the best performance. Further, it is found that water vapor in flue gas imposes negligible effect on the adsorption of CO2 and its selectivity over N2 in the neutral and positively charged (7, 7) CNT bundles, but dramatically reduces the adsorption of CO2 and N2 in negatively charged bundles.

We present a novel theory for estimation of the effective permeability of pure gases in flat mixed-matrix membranes (MMMs), in which effective medium theory (EMT) is extended to systems with finite filler size and membrane thickness. We introduce an inhomogeneous filler volume fraction profile, which arises due to depletion of the filler in regions adjacent to the membrane ends, into the MMM permeation model. In this way, the effective medium approach (EMA) can still be applied to systems where the dispersant size is not small in comparison to the membrane thickness, and for which a permeability profiles arises in the MMM that is dependent on both filler size and membrane thickness, besides the filler-polymer equilibrium constant. It is found that increase in particle size reduces the effective membrane permeability at fixed membrane thickness, and that the effective membrane permeability increases with increase of the membrane thickness to asymptotically reach the value predicted by existing models. The present theory is validated against detailed simulations of the transport in MMMs, and theoretical predictions are found to be in agreement with those obtained from the exact calculations. Further, comparison of the exact effective permeability at different filler volume fractions is made for different packing configurations, showing variations in dispersant packing structure to have only a very weak effect on MMM performance.

The adsorption of CO2 and H2O on divacanacy (DV) defected graphene cluster, and its bilayer counterpart is investigated using first-principles calculations. Both single and bilayer DV graphene cluster, are functionalised with H and F atoms. On these sheets the gas molecules are physisorbed, and the divacancy defect effectively improves the adsorption of CO2, while fluorination enhances the hydrophobicity of the graphene cluster. Among the convex and concave curvature regions induced due to the DV defect, the adsorption of the gas molecules on the concave meniscus is more favourable. Fluorine termination induces 73% reduction in Henry law constants for H2O, while for the CO2 molecule it increases by 8%, which indicates the DV defective sheet is a better candidate for CO2 capture compared to the STW defective sheet. Besides, both AA and AB divacant defect bilayer sheets are equally stable, wherein AA stacking results in a cavity between the sheets, while in AB stacking, the layers slide one over the other. Nevertheless, both these bilayer sheets are comparatively stabler than the monolayer. However, intercalation of lithium decreases the interlayer separation, particularly in AA stacking, which enhances the CO2 adsorption, but in the Bernal stacking enhances it hydrophobicity.

A grain model based on population balances for redox reactions of copper-based oxygen carriers is developed. It considers the effects of grain size distribution and sintering, and a new expression for the aggregation frequency, which takes into account the sintering mechanisms and synthesis method, is deduced. In order to validate the model, six copper-based oxygen carriers were synthesized by excess wet impregnation and by incipient impregnation. They were characterized by BET surface area, BJH porosimetry, and SEM microscopy. Model results were compared with experimental data acquired by thermogravimetry during several redox cycles and the model was able to predict the conversion drop over the course of redox cycles due to sintering. It was found that copper-based oxygen carriers synthesized by incipient impregnation are more strongly affected by sintering than those prepared via excess wet impregnation, and finally it is shown that metallic copper sinters during oxidation, resolving a controversy in this regard that has existed in the literature.

A comprehensive mathematical model for the CO-catalyzed sintering of CaO is proposed. It takes into account the mechanisms of surface diffusion and grain boundary diffusion, catalyzed by CO chemisorption and dissolution, respectively. In addition, the model proposed here considers the change in pore size distribution during sintering, grain growth, and the densification by lattice diffusion, which is the intrinsic sintering mechanism of the CaO. Model predictions are validated using experimental data on the sintering of two CaO samples, one of them derived from pure CaCO and the other from limestone. It is found that impurities in limestone-derived CaO do not significantly affect the CO dissolution or chemisorption processes; however, they strongly increase the rate of sintering by lattice diffusion. It is also established that low temperatures and CO partial pressures promote the coarsening by surface diffusion, whereas high temperatures and CO partial pressures favor densification.

Fluid transport through nanoporous membranes is subject to additional resistance at the membrane interface, a large part of which is due to the difference in thermodynamic states of the fluid inside and outside the membrane. The state of the fluid confined within a membrane depends on the size of the nanopores, which results in a corresponding dependence of the interfacial resistance. We investigate here the dependence of the thermodynamic resistance on the radius of the nanopore and the thickness of the pore wall, considering the transport of carbon dioxide and methane through carbon nanotubes of radii between 4 A and 50 A at room temperature, and a wide range of pressures. We find that the thermodynamic resistance strongly depends on the state of the fluid adsorbed in the membrane, which is determined by the size of the pores and the external pressure. In particular, for narrow micropores the thermodynamic resistance has two pressure regimes, being constant at low pressures and increasing gradually at high pressures. Furthermore, moderate and wide pores allow presence of multiple fluid phases with distinct condensation. In the corresponding pressure range the thermodynamic resistance is subjected to large fluctuations, which are not observed for small pores. Furthermore, our results reveal strong dependence of the thermodynamic resistance on the pore radius for very narrow pores and large pressures, when the state of the fluid inside of the membrane is most different from that of the external bulk fluid, with the resistance increasing with decrease in pore radius. Our results also indicate that analyzing the pore size dependence of the interfacial resistance makes it possible to distinguish the contribution of the thermodynamic resistance from the other sources of resistance to fluid flow through the membrane, in particular, the hydrodynamic resistance and the internal resistance.

Herein, we demonstrate that a porphyrin interspersed graphene-oxide framework with a d-spacing of ∼7.67 A can significantly enhance the cycling stability of graphene-based anodes in sodium-ion batteries. These robust electrodes can deliver a reversible capacity of ∼200 mA h g−1 at a current density of 100 mA g−1 in the 20th cycle with negligible capacity fading over 700 cycles. In addition to the superior rate tolerance, the specific capacity was stable even after a resting time of one month. The excellent performance may be nested in the larger interlayer spacing, and rich nitrogen content along with the defect sites available for sodium interaction. Experimental studies and density functional theory calculations presented in this work give insights into the structure–property relationship of porphyrin–graphene oxide frameworks and their electrochemical performance.

We develop a new approach for modeling titanium carbide derived-carbon (TiC-CDC) systems with residual titanium by the generation of modified atomistic structures based on a silicon carbide derived-carbon (SiC-CDC) model and the application of weighted combinations of these structures. In our approach, the original SiC-CDC structure is modified by (i) removing carbon, (ii) adding carbon and (iii) adding titanium. The new atomic scale carbide-derived carbon (CDC) structures are investigated using classical molecular dynamics simulations, and their pure CO2 adsorption isotherms are calculated using grand canonical Monte Carlo simulations. The system of TiC-CDC with residual titanium is modeled as weighted combinations of pure carbon CDC structures, CDC structures with titanium and a TiC crystalline structure. Our modeling is able to produce both structural properties and adsorption isotherms in accordance with experimental data. The fraction of different models in the systems successfully reflects the structural differences in various experimental TiC-CDC samples. The modeling also suggests that in partially etched TiC-CDC systems, the titanium that may be accessible to CO2 gas at the transitional interface may provide significant interaction sites for CO2 and may lead to more efficient overall gas adsorption.

The molecular orientation of permeating molecules crucially affects the gas transfer behavior in kinetic molecular sieving, particularly if the molecules are linear; however, this effect has been largely overlooked in the literature. To investigate this effect here we induce specific unfavourable molecular orientation of several linear gases based on adsorption effects in a hollow fiber NaA zeolite membrane, by anchoring a 3-dimensional alkali lignin molecule on the hydroxyls of the membrane surface. The results indicated that although the permeance was systemically reduced for all the gases investigated due to unfavourable orientation, the ideal selectivity for H2 relative to the linear molecules was consistently enhanced due to the presence of the alkali lignin molecule. On the other hand the permeation behavior for the spherically symmetric CH4 molecule was unaffected by the alkali lignin molecule, with the H2/CH4 unchanged and nearly constant over the operating temperature range. In addition to the defect healing, the enhancement in selectivity for H2 over the linear gases is strongly attributed to the normal orientation of the linear molecule relative to the pore axis on entry, induced by the adsorption force of the alkali lignin, leading to a repulsive potential at the pore mouth of the zeolite membrane. To validate the induced orientation effect of the linear molecules, ab initio method was employed to estimate the interaction energies between the pore mouth and linear molecules oriented parallel and normal to the pore axis. The computational results showed that the interaction energies were consistently higher for each specie when oriented normal to the pore axis (i.e. parallel to the pore cross-section), so that the tilted molecular orientation experienced stronger energy barriers; this is in accord with the experimental observation of increased selectivity for the more weakly-adsorbed H2 over the linear molecules. These findings can provide scientific guidance and facilitate new solutions to extend the separation boundaries of nanoporous membrane among gases.

Electrode mass ratio is one of the important parameters that influence capacitance, cycle life, and operating voltage of an electrochemical supercapacitor; however, molecular level understanding of the consequences of inappropriate electrode mass ratio on the overall performance of a supercapacitor is still lacking. Here, we performed constant voltage Gibbs ensemble based grand canonical Monte Carlo simulations on different combinations of microporous carbon electrodes of known atomic structure, and room temperature ionic liquid as electrolyte. Our results indicate that the optimum mass ratio depends not only on the symmetry of electrodes, but also on the size symmetry of the respective counterions. There is an enhancement of 5% in capacitance from the usual mass ratio of 1.0 to the optimum of ∼0.7–0.8; however, the imbalance in the masses of electrodes causes overloading of the lighter electrode, causing excluded volume interactions to dominate. We anticipate that highly repulsive interactions and large ...

We investigate the transport diffusion of methane at 300 K in a series of short (10, 10) carbon nanotubes with length of up to 100 nm, using a novel equilibrium molecular dynamics simulation (EMD) method. The calculated transport diffusivities for methane in the short CNTs were validated by gravity-driven nonequilibrium molecular dynamics (NEMD) simulations. Because of the dominant interfacial resistance, which collectively accounts for the entrance and exit interfacial barriers, the transport diffusivities of methane in the finite CNTs are generally 2–3 orders of magnitude lower than those in an infinitely long CNT and depend significantly on the tube length. The EMD simulations show that interfacial resistance is the major source of resistance to transport, and that the ratio of the interfacial resistance to the overall resistance decreases with increase in the length of the CNTs. Significant correlation between the motion of methane near the interfaces and that deep inside the CNT was found, showing th...

The performance of few-layered metal-reduced graphene oxide (RGO) as a negative electrode material in sodium-ion battery was investigated. Experimental and simulation results indicated that the as-prepared RGO with a large interlayer spacing and disordered structure enabled significant sodium-ion storage, leading to a high discharge capacity. The strong surface driven interactions between sodium ions and oxygen-containing groups and/or defect sites led to a high rate performance and cycling stability. The RGO anode delivered a discharge capacity of 272 mA h g−1 at a current density of 50 mA g−1, a good cycling stability over 300 cycles and a superior rate capability. The present work provides new insights into optimizing RGOs for high-performance and low-cost sodium-ion batteries.

A mathematical model for the sintering of ceramics with bimodal pore size distributions at intermediate and final stages is developed. It considers the simultaneous effects of coarsening by surface diffusion, and densification by grain boundary diffusion and lattice diffusion. This model involves population balances for the pores in different zones determined by each porosimetry peak, and is able to predict the evolution of pore size distribution function, surface area, and porosity over time. The model is experimentally validated for the sintering of lime and it is reliable in predicting the so called “initial induction period” in sintering, which is due to a decrease in intra-aggregate porosity offset by an increase inter-aggregate porosity. In addition, a novel methodology for determination of mechanisms based on the analysis of the pore size distribution function is proposed, and with this, it was demonstrated that lattice diffusion is the controlling mechanism in the CaO sintering.

International audience; We report the effect of fluorine doping on hydrophobicity of microporous silicon carbide-derived carbon (SiCDC), exploring water vapor adsorption in virgin and fluorinated samples experimentally and using semiempirical isotherm models. The surface chemistry of the virgin carbon and samples fluorinated to three different levels is characterized by X-ray photoelectron spectroscopy and 19F nuclear magnetic resonance analysis techniques. Besides having different textural features such as surface area and pore size distribution, the virgin and fluorinated SiCDCs show different surface chemistries in terms of the nature and quantity of the C–F bonds. The comparison of the characterization results of the samples and model parameters is used as the methodology to understand the water adsorption mechanism in virgin and fluorinated SiCDCs. We demonstrate that with increasing fluorination level the hydrophobic character of the low and medium fluorinated SiCDC samples remains almost constant while the highly fluorine-doped SiCDC is less hydrophobic. We identify a pore accessibility problem for argon in the fluorine-doped SiCDCs, demonstrating that fluorine functionalization blocks the carbon pores for the nonpolar argon while allowing polar H2O molecules to form clusters that enter the internal volume of the micropores. These findings provide important new understanding of the mechanism of water adsorption in microporous carbons and their fluorinated counterparts.

We develop an atomistic model of disordered Ti3SiC2 carbide-derived carbon (Ti3SiC2-DC) through hybrid reverse Monte Carlo simulation, and validate it against experimental adsorption data of Ar and CO2 using grand canonical Monte Carlo (GCMC) simulation. While supporting the atomistic model, the GCMC simulations reveal inadequate accessibility of narrow micropores, leading to small deviation between experimental and simulated isotherms at low pressure. It is found that the Ti3SiC2-DC structure is lamellar and highly anisotropic, with a percolating path in only one direction, which is parallel to the lamellae, leading to anisotropic diffusion. The energy barriers for diffusion in this anisotropic structure are found to be smaller, and the diffusion coefficient larger, than in the more disordered but isotropic SiC-derived carbon, despite the larger pore volume of the latter. These findings, based on molecular dynamics simulations are confirmed by analysis of the free energy landscape, showing larger free energy barriers for SiC-derived carbon. Our findings suggest that diffusion in isotropic carbon structures is hindered by higher energy barriers, arising from greater short-range disorder, in comparison to highly anisotropic structures, consistent with recent literature observations of larger pore wall-mediated scattering in isotropic structures.