Bimodal Pore Size Distribution Research Articles

Methane adsorption on microporous carbon adsorbent with wide pore size distribution

Methane adsorption on a microporous carbon adsorbent with a bimodal pore size distribution is studied at temperatures of 303–333 K at pressures up to 30 MPa. The total micropore volume of the adsorbent, as determined by the Dubinin method, is as large as 1.02 cm3/g. Maximum values of methane adsorption of ≈18 mmol/g are attained at a temperature of 303 K and a pressure of 30 MPa. Methane adsorption isosteres are plotted based on experimental data, and adsorption equilibria at low temperatures are calculated using the linearity of the plots. Experimental isotherms of methane adsorption are compared with the isotherms calculated by the Dubinin–Nikolaev equation with variations in parameters E and n. Temperature dependences of these parameters are determined. Specific characteristics of methane adsorption accumulation are calculated.

Pt–CeO2nanoporous spheres – an excellent catalyst for partial oxidation of methane: effect of the bimodal pore structure

Bimodal pore size distribution played the most important role for the catalyst's superior activity during POM.

Adsorption of Cd(II) and Pb(II) ions from aqueous solutions using mesoporous activated carbon adsorbent: Equilibrium, kinetics and characterisation studies

In this study, cadmium and lead ions removal from aqueous solutions using a commercial activated carbon adsorbent (CGAC) were investigated under batch conditions. The adsorbent was observed to have a coarse surface with crevices, high resistance to attrition, high surface area and pore volume with bimodal pore size distribution which indicates that the material was mesoporous. Sorption kinetics for Cd(II) and Pb(II) ions proceeded through a two-stage kinetic profile-initial quick uptake occurring within 30min followed by a gradual removal of the two metal ions until 180min with optimum uptake (qe,exp) of 17.23mgg−1 and 16.84mgg−1 for Cd(II) and Pb(II) ions respectively. Modelling of sorption kinetics indicates that the pseudo first order (PFO) model described the sorption of Pb(II) ion better than Cd(II), while the reverse was observed with respect to the pseudo second order (PSO) model. Intraparticle diffusion modelling showed that intraparticle diffusion may not be the only mechanism that influenced the rate of ions uptake. Isotherm modelling was carried out and the results indicated that the Langmuir and Freundlich models described the uptake of Pb(II) ion better than Cd(II) ion. A comparison of the two models indicated that the Langmuir isotherm is the better isotherm for the description of Cd(II) and Pb(II) ions sorption by the adsorbent. The maximum loading capacity (qmax) obtained from the Langmuir isotherm was 27.3mgg−1 and 20.3mgg−1 for Cd(II) and Pb(II) ions respectively.

Fabrication of Solid‐State Flexible Fiber Supercapacitor Using Agave Americana Derived Activated Carbon and Its Performance Analysis at Different Conditions

AbstractFlexible fiber symmetric supercapacitors were fabricated using activated carbon derived from Agave americana having uniform pores because of chemical activation using two different activating agents, KOH (K‐AC) and NaOH (Na‐AC). The X‐ray diffraction analysis revealed the formation of amorphous carbon. The Fourier transform infrared and X‐ray photoelectron spectroscopies infers the presence of oxygen functional groups. The N2 adsorption/desorption study revealed a bimodal type pore size distribution in both the samples. A maximum specific surface area of 1122.7 m2 g−1 is obtained for K‐AC. The fabricated symmetric supercapacitor (K‐AC∥PVA+H2SO4∥K‐AC) exhibits a maximum length specific capacitance of 18 mF cm−1 with good capacitance retention and cycle life. Subsequently, the effects of temperature, device length, and flexibility of the device were studied by fabricating different shapes (normal, zigzag, and spiral). Finally, the device was tested in a practical application in a flexible mode, which was integrated with an energy harvesting system, indicating good reliability of the device.

Pore-size distribution of a compacted silty soil after compaction, saturation, and loading

Soil microstructure is an important feature that controls soil behaviour at the macroscale. This microstructure depends on the sample preparation method and evolves during saturation and loading. This paper investigates the effects of compaction water content, drying techniques adopted prior to performing porosimetry, saturation and loading on the evolution of the microstructure of a silty soil. Mercury intrusion porosimetry is used to obtain the pore-size distribution at different states of soil presenting single and double porosity. It is observed that in presence of aggregates (obtained from a compaction on the dry side of optimum), the bimodal pore-size distribution is not significantly affected by the saturation process at zero stress. This feature of behaviour is quite specific to low plasticity silty soil. Conversely, loading under saturated conditions has a more significant effect on the pore-size distribution. This bimodal distribution converges into a single mode of pore size when the stress is significantly greater than the apparent preconsolidation pressure. The observed experimental results are interpreted in the light of pore-size distribution evolution during saturation and loading.

Hydrologic response of engineered media in living roofs and bioretention to large rainfalls: experiments and modeling

AbstractThe hydrologic response of engineered media plays an important role in determining a stormwater control measure's ability to reduce runoff volume, flow rate, timing, and pollutant loads. Five engineered media, typical of living roof and bioretention stormwater control measures, were investigated in laboratory column experiments for their hydrologic responses to steady, large inflow rates. The inflow, medium water content response, and outflow were all measured. The water flow mechanism (uniform flow vs. preferential flow) was investigated by analyzing medium water content response in terms of timing, magnitude, and sequence with depth. Modeling the hydrologic process was conducted in the HYDRUS‐1D software, applying the Richards equation for uniform flow modeling, and a mobile–immobile model for preferential flow modeling. Uniform flow existed in most cases, including all initially dry living roof media with bimodal pore size distributions and one bioretention medium with unimodal pore size distribution. The Richards equation can predict the outflow hydrograph reasonably well for uniform flow conditions when medium hydraulic properties are adequately represented by appropriate functions. Preferential flow was found in two media with bimodal pore size distributions. The occurrence of preferential flow is more likely due to the interaction between the bimodal pore structure and the initial water content rather than the large inflow rate.

Pore size distribution of MCM-41-type silica materials from pseudomorphic transformation - A minimal input data approach based on excess surface work

A collection of porous silica based materials have been synthesized by pseudomorphic transformation of silica-gel, commonly used in chromatography, to an ordered MCM-41 type material. The modified materials exhibit a bimodal pore size distribution (4 nm of MCM-41 and 20 nm for silica gel) with slit type geometry of the unmodified silica material and a cylindrical pore geometry of the MCM-41 type structure of the fully transformed material. Based on the Derjaguin-Broekhoff-de Boer (DBdB) theory and the previously published Excess Surface Work (ESW) approach we propose a method to derive the pore size distribution directly from the experimental isotherm without the need for a reference isotherm. Combining the ESW and the disjoining pressure approach an expression for a critical width is derived. This in turn relates the critical width to the relative pressure in a range where capillary condensation occurs. The method is simple, yet it provides information comparable to the standard NLDFT (non-local density functional theory) approach. Due to the absence of an interaction model the method is applicable to cases where interaction parameters are not available. We demonstrate the utility of the method comparing the results of characterization of the bimodal biphasic silica materials with the commonly used NLDFT and the BJH (Barrett-Joyner-Halenda) approach.

Morphological analysis of differently sized highly porous poly(ether imide) microparticles by mercury porosimetry

Highly porous poly(ether imide) (PEI) microparticles prepared by a spraying/coagulation process are discussed as candidate adsorber materials for apheresis applications, i.e. removal of uremic toxins from the blood of renal failure patients. PEI particles obtained by the aforementioned procedure can have a broad size distribution with particle diameters ranging from 20 to 800 µm. In order to further estimate the adsorption behavior of PEI microparticles packed in application relevant apheresis modules, a quantitative information about the relation between particle size and pore morphology is required. In this study, we explored whether the intraparticle porosity of PEI microparticles varies with altering the diameter of the particulate adsorbers. By an analytical wet sieving procedure, the obtained PEI microparticles were separated into five size fractions, which were analyzed by mercury intrusion porosimetry, nitrogen adsorption, and scanning electron microscopy. Mercury intrusion porosimetry revealed for all size fractions high porosity values in the range from 78% to 84% with pore diameters in the range from 10 to 1000 nm. A bimodal pore size distribution was found having a first peak at around 100 nm, while a second pronounced peak maximum was found at higher pore sizes that increased with raising particle diameter from 300 nm for the smallest particle size fraction (50–100 µm) to 700 nm for particles with a diameter of 200 to 250 µm.Based on these findings, it can be assumed that the main PEI particle size fraction (200–250 µm) should exhibit the highest adsorption capacity in an apheresis module. Copyright © 2016 John Wiley & Sons, Ltd.

Self-assembly of one-dimensional nitrogen-doped hollow carbon nanoparticle chains derived from zinc hexacyanoferrate coordination polymer for lithium-ion capacitors

One-dimensional nitrogen-doped hollow carbon nanoparticle chains (CNCs) featuring bimodal pore size distribution were obtained by direct thermal pyrolysis of a three-dimensional cyanide-bridged coordination polymer precursor (zinc hexacyanoferrate) without the need for additional carbon, nitrogen, and catalyst sources. Nitrogen doping turned out to play the key role in the formation of mesoporous compartment layers and structural defects in the CNCs. Small mesopores in the walls provided high surface area for charge storage and allowed the migration of electrolyte into the compartments. Large mesopores in the hollow compartments accommodated electrolyte for easy transport of lithium ions. The commercial multiwalled carbon nanotube (CNT) electrode stored lithium ions primarily through the intercalation process, while the CNC electrode stored lithium ions through both the intercalation and adsorption processes. Thus, the CNC electrode exhibited superior supercapacitive performance than the CNT electrode. The CNC electrode with low internal resistance could deliver a high capacitance of 680Fg−1 at 1Ag−1 in the working potential range of 0.01-3.50V vs. Li/Li+, which was much better than the commercial CNT electrode (252Fg−1).

Mixed matrix carbon stainless steel (MMCSS) hollow fibres for gas separation

Abstract This work reports the preparation and investigation of novel mixed matrix carbon stainless steel (MMCSS) membranes. The study involves the production of MMCSS hollow fibres using SS particles of 6, 10, 16 and 45 μm in diameter, polyetherimide as a polymeric binder and pyrolysis using a N2 inert atmosphere. As a result, the binder pyrolysed to carbon was retained in the hollow fibre structure, filling the voids between the SS particles. Smaller SS particles (6 μm) yielded a bi-modal pore size distribution and superior mechanical properties. An interesting morphological feature was the formation of honeycomb-like carbon structures between the SS particles, attributed to the densification of the hollow fibre during pyrolysis at 1050 °C. The MMCSS hollow fibres (6 μm) delivered almost pure N2 for the separation of a synthetic flue gas composition (13% CO2 and 87% N2). It was found that CO2 had a strong affinity to the surface of the MMCSS materials (isosteric heat of adsorption of 38 kJ mol−1) whilst N2 was a non-absorbing gas. Therefore, CO2 permeation was controlled by surface diffusion whilst N2 was controlled by the faster Knudsen diffusion mechanism. For CO2 feed concentrations in excess of 13%, the CO2 diffusion increased as the excess CO2 could not adsorb on the fully saturated surface of the MMCSS hollow fibres, thus slightly reducing the N2 purity in the permeate stream.

Effect of Porosity on Structure, Young's Modulus, and Thermal Conductivity of SiC Foams by Direct Foaming and Gelcasting

The study demonstrates the aqueous processing of solid‐state‐sintered SiC foams by gelcasting technique. Aside from increasing strength of green bodies, gelcasting monomers were the source of carbon additive which helped in sintering of SiC foams. Sintered foams with the relative density (RD) between 0.44 and 0.11 were processed by direct foaming of SiC slurries followed by gelcasting and sintering. Structural analysis by X‐ray tomography showed the presence of spherical pores with bimodal pore size distribution and the proportion of large size cell and their interconnectivity increased in low RD foams. SEM study revealed that decreased RD resulted in gradual changes in the strut microstructure from the grains with faceted interface to smooth interfaced grains. The analysis of changes in Young's modulus and thermal conductivity with RD were in agreement with the Ashby model for open cell foams.

Catalytic pyrolysis of waste tire using HY/MCM-41 core-shell composite

The catalytic pyrolysis of waste tire was investigated using core-shell composite of HY and MCM-41 as a catalysts in a bench-scale reactor ramped from room temperature to 500°C (pyrolysis zone) and 350°C (catalyst bed), aiming to enhance the formation of petrochemicals by transformation of the bulky aromatics to valuable aromatics. The core-shell composite of HY and MCM-41 were synthesized by growing of MCM-41 over HY zeolite particles. The gaseous products were analyzed using GC-FID, whereas the GCxGC-TOF/MS and SIMDIST-GC were used for analysis of waste tire-derived oil. Moreover, the sulfur content in oils was determined by S-analyzer. It was found that The MCM-41 shell thickness was not uniform, which varied in the range of 50–100nm. Nevertheless, the core-shell composite catalyst exhibited the great catalytic behavior in the enhancement of quality of waste tire-derived oil. The oil produced from the composite catalyst contained a higher amount of gasoline and valuable aromatics, especially ethylbenzene and toluene than the pure HY and MCM-41 catalysts, indicating the existence of bimodal pore size distribution and good balance of acidity between micropore and mesopore layers of core-shell composite exhibits a higher cracking activity and better petrochemical selectivity than pure HY and MCM-41 catalyst. Furthermore, the core-shell catalysts also provided lower sulfur content in oil than the both HY and MCM-41 catalyst. Therefore, the core-shell composite of HY and MCM-41 has a great potential in producing petrochemical-rich oil with low sulfur content from waste-tire pyrolysis.

Novel model for the sintering of ceramics with bimodal pore size distributions: Application to the sintering of lime

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. © 2016 American Institute of Chemical Engineers AIChE J, 63: 893–902, 2017

Synthesis and characterization of novel triptycene dianhydrides and polyimides of intrinsic microporosity based on 3,3ʹ-dimethylnaphthidine

Abstract Two intrinsically microporous polyimides were obtained by high-temperature, one-pot polycondensation reaction of novel triptycene-based dianhydrides containing dimethyl- or diisopropyl-bridgehead groups with a commercially available highly sterically hindered 3,3ʹ-dimethylnaphthidine (DMN) diamine monomer. The dimethyl bridgehead groups in the triptycene building block provided the DMN-based polyimide (TDA1-DMN) with larger surface area (760 m 2 g -1 ) than the diisopropyl-based polyimide (TDA i 3-DMN) (680 m 2 g -1 ), greater fraction of ultramicroporosity, as observed from N 2 and CO 2 NLDFT adsorption analysis, and higher gas permeability and selectivity. Wide-angle X-ray diffraction (WAXD) measurements demonstrated that TDA1-DMN and TDA i 3-DMN exhibited a bimodal pore size distribution, where TDA1-DMN showed smaller d-spacing values and broader intensity peaks. Both TDA-DMN-based polyimides showed very high gas permeabilities with moderate selectivities. For example, fresh TDA1-DMN exhibited an O 2 permeability of 783 Barrer coupled with an O 2 /N 2 selectivity of 4.3 and H 2 permeability of 3050 Barrer with H 2 /N 2 selectivity of 16.7, values that surpassed the 2008 Robeson permeability/selectivity upper bounds. Physical aging of the TDA-DMN polyimide films over a period of 250 days showed relatively small changes in permeability (∼20%) and selectivity (∼5%).

Gas-diffusion-layer structural properties under compression via X-ray tomography

There is a need to understand the structure properties of gas-diffusion layers (GDLs) in order to optimize their performance in various electrochemical devices. This information is important for mathematical modelers, experimentalists, and designers. In this article, a comprehensive study of a large set of commercially available GDLs' porosity, tortuosity, and pore-size distribution (PSD) under varying compression is presented in a single study using X-ray computed tomography (CT), which allows for a noninvasive measurement. Porosities and PSDs are directly obtained from reconstructed stacks of images, whereas tortuosity is computed with a finite-element simulation. Bimodal PSDs due to the presence of binder are observed for most of the GDLs, approaching unimodal distributions at high compressions. Sample to sample variability is conducted to show that morphological properties hold across various locations. Tortuosity values are the lowest for MRC and Freudenberg, highest for TGP, and in-between for SGL papers. The exponents for the MRC and Freudenberg tortuosity demonstrate a very small dependence on compression because the shapes of the pores are spherical indicating minimal heterogeneity. From the representative-elementary-volume studies it is shown that domains of 1 × 1 mm in-plane and full thickness in through-plane directions accurately represent GDL properties.

Facile fabrication of nanoporous gold with bimodal pore structure

A hierarchical nanoporous gold (HNPG) membrane with a bimodal pore size distribution was fabricated by a simple one-step dehydration reaction of H3BO3, which is different from the complex methods of multi-cyclic electrochemical co-alloying/dealloying. The HNPG exhibits a wide pore distribution from 10 to 120nm. The special architecture of HNPG significantly enhances the Raman signal to crystal violet and electrocatalytic activity toward the oxidation of methanol. The formation mechanism of the bimodal structure is discussed.

Soil structure changes induced by tillage systems

Structure represents one of the main soil physical attributes indicators. The soil porous system (SPS) is directly linked to the soil structure. Water retention, movement, root development, gas diffusion and the conditions for all soil biota are related to the SPS. Studies about the influence of tillage systems in the soil structure are important to evaluate their impact in the soil quality. This paper deals with a detailed analysis of changes in the soil structure induced by conventional (CT) and no-tillage (NT) systems. Three different soil depths were studied (0–10, 10–20 and 20–30cm). Data of the soil water retention curve (SWRC), micromorphologic (impregnated blocks) (2D) and microtomographic (μCT) (3D) analyses were utilized to characterize the SPS. Such analyses enabled the investigation of porous system attributes such as: porosity, pore number and shape, pore size distribution, tortuosity and connectivity. Results from this study show a tri-modal pore size distribution (PSD) at depths 0–10 and 10–20cm for the soil under CT and a bi-modal PSD for the lower layer (20–30cm). Regarding the soil under NT, tri-modal PSDs were found at the three depths analyzed. Results based on the micromorphologic analysis (2D) showed that the greatest contribution to areal porosity (AP) is given by pores of round (R) shape for CT (52%: 0–10cm; 50%: 10–20cm; 67%: 20–30cm). Contrary to the results observed for CT, the soil under NT system gave the greatest contribution to AP, for the upper (0–10cm) and intermediate (10–20cm) layers, due to the large complex (C) pore types. For the μCT analysis, several types of pores were identified for each soil tillage system. Small differences in the macroporosity (MAP) were observed for the 0–10 and 20–30cm between CT and NT. A better pore connectivity was found for the 0–10cm layer under NT.

Influence of catalyst pore network structure on the hysteresis of multiphase reactions

The effects of the catalyst pore network structure on multiphase reactions in catalyst pellets are investigated by using the experimentally validated pore network model proposed in our recent work (AIChE J, 62, 451, 2016). The simulations display hysteresis loops of the effectiveness factor. The hysteresis loop area becomes significantly larger, when having small volume‐averaged pore radius, wide pore‐size distribution, and low pore connectivity; however, the loop area is insensitive to pellet size, even though it affects the value of the effectiveness factor. The hysteresis loop area is also strongly affected by the spatial distribution of the pore size, in particular for a bimodal pore‐size distribution. The pore network structure directly influences mass transfer, capillary condensation, and pore blocking, and subsequently passes these influences on to the hysteresis loop of the effectiveness factor. Recognizing these effects is essential when designing porous catalysts for multiphase reaction processes. © 2016 American Institute of Chemical Engineers AIChE J, 63: 78–86, 2017

Multi-Scale Correlative Tomography of a Li-Ion Battery Composite Cathode.

Focused ion beam/scanning electron microscopy tomography (FIB/SEMt) and synchrotron X-ray tomography (Xt) are used to investigate the same lithium manganese oxide composite cathode at the same specific spot. This correlative approach allows the investigation of three central issues in the tomographic analysis of composite battery electrodes: (i) Validation of state-of-the-art binary active material (AM) segmentation: Although threshold segmentation by standard algorithms leads to very good segmentation results, limited Xt resolution results in an AM underestimation of 6 vol% and severe overestimation of AM connectivity. (ii) Carbon binder domain (CBD) segmentation in Xt data: While threshold segmentation cannot be applied for this purpose, a suitable classification method is introduced. Based on correlative tomography, it allows for reliable ternary segmentation of Xt data into the pore space, CBD, and AM. (iii) Pore space analysis in the micrometer regime: This segmentation technique is applied to an Xt reconstruction with several hundred microns edge length, thus validating the segmentation of pores within the micrometer regime for the first time. The analyzed cathode volume exhibits a bimodal pore size distribution in the ranges between 0–1 μm and 1–12 μm. These ranges can be attributed to different pore formation mechanisms.

Adsorption of pyridine from aqueous solutions by polymeric adsorbents MN 200 and MN 500. Part 2: Kinetics and diffusion analysis

The adsorption kinetics of pyridine adsorption on Macronet adsorbents MN 200 and MN 500 from aqueous solution was investigated at various initial pyridine concentrations and temperatures. The Weber-Morris plots revealed the influence of both external film diffusion and intraparticle diffusion resistances. The two linear regions in Weber-Morris plots were attributed to macropore and micropore diffusion, respectively, which was associated to the bimodal pore size distribution of the adsorbents. New insights into the diffusion mechanisms were highlighted, with the proposed internal film diffusion resistance dominating into the macropore region, whereas homogeneous particle diffusion resistance describes diffusion in the micropore region. The importance of pore and surface diffusion in the micropores was noted in contributing to the observed diffusion kinetics. The pore diffusion coefficient was estimated from PFG (pulsed-field gradient) parameter and molecular diffusion coefficient of pyridine in bulk liquid. A greater contribution of the surface diffusion to the overall diffusion kinetics was found for MN 500 as inferred from a proposed calculation method, which agrees with its better adsorption performance. The overall findings highlight the effect of pore structure onto the diffusion mechanisms inside the pores and help to gain a better understanding into the adsorption kinetics of these Macronet adsorbents which are promising materials for the removal of N-heterocyclic compounds from waste water.