Adsorbent density impact on gas storage capacities

In the literature, different approaches, terminologies, concepts and equations are used for calculating gas storage capacities. Very often, these approaches are not well defined, used and/or determined, giving rise to significant misconceptions. Even more, some of these approaches, very much associated with the type of adsorbent material used (e.g., porous carbons or new materials such as COFs and MOFs), impede a suitable comparison of their performances for gas storage applications. We review and present the set of equations used to assess the total storage capacity for which, contrarily to the absolute adsorption assessment, all its experimental variables can be determined experimentally without assumptions, ensuring the comparison of different porous storage materials for practical application. These material-based total storage capacities are calculated by taking into account the excess adsorption, the bulk density (ρbulk) and the true density (ρtrue) of the adsorbent. The impact of the material densities on the results are investigated for an exemplary hydrogen isotherm obtained at room temperature and up to 20 MPa. It turns out that the total storage capacity on a volumetric basis, which increases with both, ρbulk and ρtrue, is the most appropriate tool for comparing the performance of storage materials. However, the use of the total storage capacities on a gravimetric basis cannot be recommended, because low material bulk densities could lead to unrealistically high gravimetric values.

Effect of the porous structure in carbon materials for CO2 capture at atmospheric and high-pressure

Activated carbons prepared from petroleum pitch and using KOH as activating agent exhibit an excellent behavior in CO2 capture both at atmospheric (∼168 mg CO2/g at 298 K) and high pressure (∼1500 mg CO2/g at 298 K and 4.5 MPa). However, an exhaustive evaluation of the adsorption process shows that the optimum carbon structure, in terms of adsorption capacity, depends on the final application. Whereas narrow micropores (pores below 0.6 nm) govern the sorption behavior at 0.1 MPa, large micropores/small mesopores (pores below 2.0–3.0 nm) govern the sorption behavior at high pressure (4.5 MPa). Consequently, an optimum sorbent exhibiting a high working capacity for high pressure applications, e.g., pressure-swing adsorption units, will require a poorly-developed narrow microporous structure together with a highly-developed wide microporous and small mesoporous network. The appropriate design of the preparation conditions gives rise to carbon materials with an extremely high delivery capacity ∼1388 mg CO2/g between 4.5 MPa and 0.1 MPa. Consequently, this study provides guidelines for the design of carbon materials with an improved ability to remove carbon dioxide from the environment at atmospheric and high pressure.

Clay-supported graphene materials: application to hydrogen storage

The present work refers to clay–graphene nanomaterials prepared by a green way using caramel from sucrose and two types of natural clays (montmorillonite and sepiolite) as precursors, with the aim of evaluating their potential use in hydrogen storage. The impregnation of the clay substrates by caramel in aqueous media, followed by a thermal treatment in the absence of oxygen of these clay–caramel intermediates gives rise to graphene-like materials, which remain strongly bound to the silicate support. The nature of the resulting materials was characterized by different techniques such as XRD, Raman spectroscopy and TEM, as well as by adsorption isotherms of N2, CO2 and H2O. These carbon–clay nanocomposites can act as adsorbents for hydrogen storage, achieving, at 298 K and 20 MPa, over 0.1 wt% of hydrogen adsorption excess related to the total mass of the system, and a maximum value close to 0.4 wt% of hydrogen specifically related to the carbon mass. The very high isosteric heat for hydrogen sorption determined from adsorption isotherms at different temperatures (14.5 kJ mol−1) fits well with the theoretical values available for hydrogen storage on materials that show a strong stabilization of the H2 molecule upon adsorption.

Optimizing the performance of catalytic traps for hydrocarbon abatement during the cold-start of a gasoline engine

A key target to reduce current hydrocarbon emissions from vehicular exhaust is to improve their abatement under cold-start conditions. Herein, we demonstrate the potential of factorial analysis to design a highly efficient catalytic trap. The impact of the synthesis conditions on the preparation of copper-loaded ZSM-5 is clearly revealed by XRD, N2 sorption, FTIR, NH3-TPD, SEM and TEM. A high concentration of copper nitrate precursor in the synthesis improves the removal of hydrocarbons, providing both strong adsorption sites for hydrocarbon retention at low temperature and copper oxide nanoparticles for full hydrocarbon catalytic combustion at high temperature. The use of copper acetate precursor leads to a more homogeneous dispersion of copper oxide nanoparticles also providing enough catalytic sites for the total oxidation of hydrocarbons released from the adsorption sites, although lower copper loadings are achieved. Thus, synthesis conditions leading to high copper loadings jointly with highly dispersed copper oxide nanoparticles would result in an exceptional catalytic trap able to reach superior hydrocarbon abatement under highly demanding operational conditions.

Use of Eutectic Mixtures for Preparation of Monolithic Carbons with CO2-Adsorption and Gas-Separation Capabilities

With global warming becoming one of the main problems our society is facing nowadays, there is an urgent demand to develop materials suitable for CO2 storage as well as for gas separation. Within this context, hierarchical porous structures are of great interest for in-flow applications because of the desirable combination of an extensive internal reactive surface along narrow nanopores with facile molecular transport through broad “highways” leading to and from these pores. Deep eutectic solvents (DESs) have been recently used in the synthesis of carbon monoliths exhibiting a bicontinuous porous structure composed of continuous macroporous channels and a continuous carbon network that contains a certain microporosity and provides considerable surface area. In this work, we have prepared two DESs for the preparation of two hierarchical carbon monoliths with different compositions (e.g., either nitrogen-doped or not) and structure. It is worth noting that DESs played a capital role in the synthesis of hierarchical carbon monoliths not only promoting the spinodal decomposition that governs the formation of the bicontinuous porous structure but also providing the precursors required to tailor the composition and the molecular sieve structure of the resulting carbons. We have studied the performance of these two carbons for CO2, N2, and CH4 adsorption in both monolithic and powdered form. We have also studied the selective adsorption of CO2 versus CH4 in equilibrium and dynamic conditions. We found that these materials combined a high CO2-sorption capacity besides an excellent CO2/N2 and CO2/CH4 selectivity and, interestingly, this performance was preserved when processed in both monolithic and powdered form.

Algerian natural montmorillonites for arsenic(III) removal in aqueous solution

The adsorption of As(III) from aqueous solutions using naturally occurring and modified Algerian montmorillonites has been investigated as a function of contact time, pH, and temperature. Kinetic studies reveal that uptake of As(III) ions is rapid within the first 3 h, and it slows down thereafter. Equilibrium studies show that As(III) shows the highest affinity toward acidic montmorillonite even at very low concentration of arsenic. The kinetics of As(III) adsorption on all montmorillonites used is well described by a pseudo-second-order chemical reaction model, which indicates that the adsorption process of these species is likely to be chemisorption. Adsorption isotherms of As(III) fitted the Langmuir and Freundlich isotherm models well. The adsorption of As(III) is pH-dependent obtaining an optimal adsorption at pH 5. From the thermodynamic parameters, it is concluded that the process is exothermic, spontaneous, and favorable. The results suggest that M1, M2, and acidic-M2 could be used as low-cost and effective filtering materials for removal of arsenic from water.

Fate and Transport of Naproxen in a Sandy Aquifer Material: Saturated Column Studies and Model Evaluation

Naproxen-C14H14O3 is a nonsteroidal anti-inflammatory drug which has been found at detectable concentrations in wastewater, surface water, and groundwater. Naproxen is relatively hydrophilic and is in anionic form at pH between 6 and 8. In this study, column experiments were performed using an unconsolidated aquifer material from an area near Barcelona (Spain) to assess transport and reaction mechanisms of Naproxen in the aquifer matrix under different pore water fluxes. Results were evaluated using HYDRUS-1D, which was used to estimate transport parameters. Batch sorption isotherms for Naproxen conformed with the linear model with a sorption coefficient of 0.42 (cm3 g−1), suggesting a low sorption affinity. Naproxen breakthrough curves (BTCs) measured in soil columns under steady-state, saturated water flow conditions displayed similar behavior, with no apparent hysteresis in sorption or dependence of retardation (R, 3.85-4.24) on pore water velocities. Soil sorption did not show any significant decrease for increasing flow rates, as observed from Naproxen recovery in the effluent. Sorption parameters estimated by the model suggest that Naproxen has a low sorption affinity to aquifer matrix. Most sorption of Naproxen occurred on the instantaneous sorption sites, with the kinetic sorption sites representing only about 10 to 40% of total sorption.

Desiccant capability of organic xerogels: Surface chemistry vs porous texture

Resorcinol-Formaldehyde xerogels are organic polymers that can be easily tailored to have specific properties. These materials are composed of carbon, hydrogen and oxygen, and have a surface that is very rich in oxygen functionalities, and is therefore very hydrophilic. Their most interesting feature is that they may have the same chemical composition but a different porous texture. Consequently, the influence of porous characteristics, such as pore volume, surface area or pore size can be easily assessed. In this work, a commonly used desiccant, silica gel, is compared with organic xerogels to determine their rate and capacity of water adsorption, and to evaluate the role of surface chemistry versus porous texture. It was found that organic xerogels showed a higher rate of moisture adsorption than silica gel. Pore structure also seems to play an important role in water adsorption capacity. The OX-10 sample, whose porosity was mainly composed of micro-mesoporosity displayed a water adsorption capacity two times greater than that of the silica gel, and three times higher than that of the totally macroporous xerogel OX-2100. The presence of feeder pores (mesopores) that facilitate the access to the hydrophilic surface was observed to be the key factor for a good desiccant behaviour. Neither the total pore volume nor the high surface area (i.e. high microporosity) of the desiccant sample, is as important as the mesopore structure.

Removal of antibiotic compounds by adsorption using glycerol-based carbon materials

This study is focused on the synthesis and application of glycerol-based carbon materials (GBCM200, GBCM300 and GBCM350) as adsorbents for the removal of the antibiotic compounds flumequine and tetracycline from aqueous solution. The synthesis enrolled the partial carbonization of a glycerol-sulfuric acid mixture, followed by thermal treatments under inert conditions and further thermal activation under oxidative atmosphere. The textural properties were investigated through N2 adsorption–desorption isotherms, and the presence of oxygenated groups was discussed based on zeta potential and Fourier transform infrared (FTIR) data. The kinetic data revealed that the equilibrium time for flumequine adsorption was achieved within 96 h, while for tetracycline, it was reached after 120 h. Several kinetic models, i.e., pseudo-first order, pseudo-second order, fractional power, Elovich and Weber–Morris models, were applied, finding that the pseudo-second order model was the most suitable for the fitting of the experimental kinetic data. The estimated surface diffusion coefficient values, Ds, of 3.88 and 5.06 10 14 m2 s 1, suggests that the pore diffusion is the rate limiting step of the adsorption process. Finally, as it is based on SSE values, Sips model well-fitted the experimental FLQ and TCN adsorption isotherm data, followed by Freundlich equation. The maximum adsorption capacities for flumequine and tetracycline was of 41.5 and 58.2 mg g 1 by GBCM350 activated carbon.

Ge/Si ratios in marine siliceous microfossils

A procedure is presented to separate diatoms and radiolaria from marine sediments and from each other, to purify them of elements associated with other phases, and to dissolve them to determine their elemental composition. The cleaning procedure eliminates artifacts due to the presence of detrital clays and the high sorption capacity of hydrated silica. The concentration of trace elements (Al, Fe, Mg, and Ba) that we find in alkaline dissolutions of clean diatoms are at least an order of magnitude lower than previously reported. The overall long-term precision in the determination of Ge/Si in a sub-standard of clean diatoms is ±0.024 * 10**-6 (1 sigma). Ge/Si measured in diatoms and radiolaria from core tops indicates that high-latitude Holocene diatoms accurately record the present-day oceanic Ge/Si, while radiolarian ratios are systematically lower and display more scatter. Evaluation of Ge/Si in diatoms and radiolaria from Hole DSDP 265 (Plio-Pleistocene) suggests that post-depositional alteration of the ratio does not occur at this site, but the average ratio carried by diatoms over this time interval was lower than that in the present ocean.

The comparative characterization of structural heterogeneity of mesoporous activated carbon fibers (ACFs)

The comparative analysis of the most widely used methods of mesoporosity characterization of two activated carbon fibers is presented. Not only the older methods are used, i.e. Barrett, Joyner and Halenda (BJH), Dubinin (the so-called first variant-D-1ST and the so-called second variant-D-2ND), Dollimore and Heal (DH), and Pierce (P) but the recently developed ones, i.e. the method of Nguyen and Do (ND) and that developed by Do (Do) are also applied. Additionally, the method of the characterization of fractality is put to use (fractal analog of FHH isotherm). The results are compared and discussed. (C) 2002 Elsevier Science B.V. All fights reserved.

The structural characterization of carbon molecular sieve membrane (CMSM) via gas adsorption

The microstructure of a carbon molecular sieve membrane (CMSM) is characterized using adsorption equilibrium information. The pore size distributions of the CMSM derived from N-2 and CH4 adsorption isotherm are found to be consistent with each other and in agreement with the results of gas permeation experiments as well as the general characteristics of such molecular sieve materials. (C) 2003 Elsevier B.V. All rights reserved.

Analysis of adsorption of gases and vapors on nonporous graphitized thermal carbon black

In this paper we analyzed the adsorption of a large number of gases and vapors on graphitized thermal carbon black. The Henry constant was used to determine the adsorbate-adsorbent interaction energy, which is found to be a modest decreasing function of temperature. Analysis of the complete adsorption isotherm over a wider range of pressure yields information on the monolayer coverage concentration and the adsorbate-adsorbate interaction energy. Among the various equations tested, the Hill-de Boer equation accounting for BET-postulated multilayer formation describes well the adsorption isotherms of all adsorbates. On average, the adsorbate-adsorbate interaction energy in the adsorbed phase is less than that in the bulk phase, suggesting that the distance between adsorbed molecules in the first layer of the adsorbed phase is slightly less than the equilibrium distance between two adsorbate molecules in the bulk phase. This suggests that the first layer is in a compressed state, which is due to the attraction of the adsorbent surface. The monolayer concentration as determined from the fitting of the Hill-de Boer equation with experimental data is slightly larger than the values calculated from the molecular projection area, suggesting that molecules can be oriented such that a larger number of molecules can be accommodated on the carbon black surface. This further supports the shorter distance between adsorbate molecules in the adsorbed phase.

The plasticisation effect of glycerol and water on the gelatinisation of wheat starch

In the extrusion manufacture of starch-based thermoplastics, such as biodegradable packaging materials, glycerol is an effective additive as a plasticiser, that is, to diminish the brittle nature of the product and provide the desired extent of flexibility. However, the addition of glycerol may also affect the gelatinisation behaviour of the starch-water mixture, and hence the required processing conditions for producing a homogeneously gelatinised starch-based material. The effect of glycerol on the gelatinisation of wheat starch was studied using differential scanning calorimetry (DSC). Mixtures of starch, water and glycerol were investigated with a water content ranging from 12 - 40% and a glycerol concentration up to 75%. Dependent on composition, the enthalpy of gelatinisation ranged from 1.7 - 12.6 J/g (on a dry starch basis), while the onset and peak temperatures varied from 54 to 86 degreesC and 60 to 90 degreesC, respectively. As expected, water acted as a plasticiser in that the onset temperature for gelatinisation (TO) decreased with increasing moisture content. Glycerol, however, increased To. It is shown that the T-0 of starch-glycerol-water mixtures may be predicted on the basis of the effective moisture content of the starch fraction of these mixtures resulting from the relative speed of moisture absorption by glycerol and starch, respectively. Moisture sorption kinetics of wheat starch and glycerol in 100% relative humidity were determined and used to predict the preferential water absorption by glycerol in starch-glycerol-water mixtures and hence the resulting T-0 of the system.

Microstructure of MmM(5)/Mg multi-layer hydrogen storage films prepared by magnetron sputtering

Multi-layer hydrogen storage thin films with Mg and MmNi(3.5)(CoAlMn)(1.5) (here Mm denotes La-rich mischmetal) as alternative layers were prepared by direct current magnetron sputtering. Transmission electron microscopy investigation shows that the microstructure of the MmNi(3.5)(CoAlMn)(1.5) and Mg layers are significantly different although their deposition conditions are the same. The MmNi(3.5)(CoAlMn)(1.5) layer is composed of two regions: one is an amorphous region approximately 4 nm thick at the bottom of the layer and the other is a nanocrystalline region on top of the amorphous region. The Mg layer is also composed of two regions: one is a randomly orientated nanocrystalline region 50 nm thick at the bottom of the layer and the other is a columnar crystallite region on top of the nanocrystalline region. These Mg columnar crystallites have their [001] directions parallel to the growth direction and the average lateral size of these columnar crystallites is about 100 nm. A growth mechanism of the multi-layer thin films is discussed based on the experiment results. Wiley-Liss, Inc.