Subtracting Pore Effect Research Articles

The subtracting pore effect method for an accurate and reliable surface area determination of porous carbons

The Ar (87.3 K) and N2 (77.4 K) adsorption isotherms of six non-graphitized carbon blacks are compared to choose the best reference material for an accurate determination of surface area of porous carbons with subtracting pore effect (SPE) method using high resolution αs-plot. Microporous carbon blacks cannot be used as references for SPE method since they underestimate the surface area. On the other hand, reference carbon blacks exhibiting capillary condensation underestimate external surface areas. The total surface area from SPE method using Ar adsorption at 87.3 K has a stronger dependence on the microporosity of reference carbon blacks than that using N2 adsorption at 77.4 K. The surface area of carbons whose pore widths are >1 nm result more affected by the reference carbon black than those with widths <1 nm. The optimized reference data and calculation programs are provided in an open repository (https://osf.io/zjg8a) for quick and easy use of the SPE method to characterize the porous carbons precisely.

Surface modified mechanism of activated carbon fibers by thermal chemical vapor deposition and nitrate adsorption characteristics in aqueous solution

In this study, commercial activated carbon fibers (ACFs) KF1500 was modified by thermal chemical vapor deposition (CVD) method using acetonitrile and steam to adsorb nitrate ions in aqueous solution. Physical properties and surface chemical characteristics of each modified ACF were examined to reveal the surface modification mechanism of modified ACFs. Adsorption kinetics and influence of solution pH were also examined to investigate the adsorption properties. The sample of KF-8ST10-8AN20-9.5HT30-8ST30 showed maximal nitrate adsorption capacity of 0.74 mmol/g at solution pH 3, and surface area was calculated to be 1358 m2/g and 1483 m2/g by Brunauer-Emmett-Teller (BET) method and subtracting pore effect (SPE) method using αs plots, respectively. By means of modification, the quaternary N (or graphitic N) (N–Q) configuration, which is a strong site to adsorb nitrate ion, represented nearly 5 times greater value than that of KF1500. The N–Q configuration reached up to 1.86% in total elemental composition from 0.4% of KF1500. The adsorptive kinetic data of KF-8ST10-8AN20-9.5HT30-8ST30 ACFs were highly well described with pseudo-second-order (R2 = 0.998) model. The adsorption capacity of nitrate decreased rapidly with the increase of the alkalinity in nitrate solution, which indicated that hydroxyl groups rather than nitrate ions in the aqueous solution were more attractive and competitive to the N-Q and other adsorptive sites on the surface of ACFs.

Nitrogen Adsorption on Graphene Sponges Synthesized by Annealing a Mixture of Nickel and Carbon Powders

Abstract Adsorption by graphene sponge (GS) manufactured by annealing nickel-carbon powder mixture in inert atmosphere has been studied. By determining the specific surface area (SSA) for the GS sample, it has been found that Brunauer, Emmett, Teller method (BET) of approximation of experimental isotherms gives wrong results in the pressure range of 0.025–0.12 because adsorption in this pressure region is affected by walls of ampoule. Real SSA value has been found by subtracting pore effect method (SPE) or by BET approximation in a low range of relative pressure of 0.0004–0.002.

Correlation in structure and properties of highly-porous graphene monoliths studied with a thermal treatment method

We prepared high surface area nanoporous graphene by reduction and KOH activation of graphene oxides and then heat-treated the nanoporous graphene up to 3073 K in Ar. The surface area of thus-prepared samples decreased remarkably from 1560 m2 g−1 to 10 m2 g−1 according to the subtracting pore effect (SPE) method. The transmission electron microscopic (TEM), X-ray diffraction (XRD), Raman spectroscopy and N2 adsorption isotherms at 77.4 K clearly illustrate the evolution of morphology, crystallinity and porosity during the graphitization process with increasing the heating temperature. The high temperature treatment shows a clear segregation of the disordered sp3 carbon frames in addition to the growth of graphitic structure in the porous graphene monoliths, although overall changes in properties and structure of porous graphene monoliths with high temperature treatment are similar to those of other porous carbons. The electrical conductivity measurements indicate that heat-treated samples have an imperfect semimetallic property due to the crystallite boundaries. The cyclic voltammograms measurements indicate that the optimized pore size range of nanoporous graphene for specific capacitance with 2 M KCl aqueous solution as electrolyte is of 0.7 nm < w < 2 nm.

Comparative pore structure analysis of highly porous graphene monoliths treated at different temperatures with adsorption of N2 at 77.4 K and of Ar at 87.3 K and 77.4 K

We prepared nanoporous graphene monolith of different porosity by high temperature treatment up to 2073 K in Ar. The porosity is comparatively evaluated with N2 adsorption isotherms at 77.4 K and Ar adsorption isotherms at 87.3 K and 77.4 K. N2 adsorption at 77.4 K shows an excess adsorption amount below 3 × 10−3 of the relative pressure which is caused by the quadrupole moment of an N2 molecule. This effect doesn't give significant influence on the determination of the total surface area from subtracting pore effect (SPE) method, the micropore volume from Dubinin-Radushkevich (DR) method and the total pore volumes from the Gurvitch rule. However, the peak of the micropore size distribution determined by Horvath–Kawazoe (HK) method from N2 adsorption at 77.4 K shifts to a smaller size than that from Ar adsorption at 87.3 K by 0.05–0.09 nm.

Absolute assessment of adsorption-based porous solid characterization methods: comparison methods.

The design and use of microporous solids depends on having access to characteristics such as the pore volume and surface area. Comparison methods such as the alpha(s) method are one of the most widely used means of determining these parameters. An assessment of this group of methods was undertaken by comparing estimates obtained from them using adsorption isotherms generated by grand canonical Monte Carlo simulation on a selection of model nanoporous solids with exactly known surface areas and pore volumes. Conclusions are drawn from this absolute assessment in regard to the validity of the alpha(s) method for determining the micropore volume, the mesopore surface area, and the separation of pore groups based on the concept of primary and cooperative filling, the subtracting pore effect (SPE) method, and the required character of the reference surface.

Micropore Size Distribution of Activated Carbon Fiber Using the Density Functional Theory and Other Methods

The pore-size distribution of a series of pitch-based activated carbon fiber samples (A5, A10, A15, and A20) has been determined from the nitrogen adsorption isotherms at 77 K, with the nonlocal density functional theory (DFT), the Dubinin−Stoeckli (DS) method, and the subtracting pore effect (SPE) method using the high-resolution αS plot. The pore-size distributions of all the samples from DFT show a distinguishable peak at small pore widths around 0.6 nm and a broad peak at bigger micropore size. The mean pore widths obtained from DFT and DS methods are similar for samples whose pores are narrow and different for wider ones. The use of the SPE method shows that the mean pore widths are slightly smaller than those obtained by DFT and DS methods.

Simulation study on the relationship between a high resolution αs-plot and the pore size distribution for activated carbon

The effects of the micropore structure of activated carbons on the high resolution α s-plot for nitrogen adsorption isotherms were examined with the grand canonical Monte Carlo simulation. Adsorption isotherms of nitrogen were simulated in graphitic slit pores at 77 K as a function of the slit width ( w). As no pore effect was observed below P/ P 0=0.6 for w=3.5 nm, α s-plots for the simulated adsorption isotherms were constructed using the standard isotherm simulated for w=3.5 nm. The simulated α s-plots had filling and cooperative swings which were experimentally shown in the previous works, and the shape of the simulated α s-plot varied with the micropore structure. As the subtracting pore effect (SPE) method for the specific surface area (SSA) determination using the α s-plot was proposed in the previous experimental works, the theoretical ground for the SPE method was discussed. The best evaluation method of SSA using the α s-plot was shown, almost agreeing with the SPE method. This simulation study showed clearly that the SPE method is available for pore systems of w≥0.7 nm, whereas even the SPE method underestimates the SSA of the pores of w≤0.6 nm. The observed swings of the α s-plot were simulated using the different micropore size distribution. The bimodal micropore size distribution lead to both of filling and cooperative swings, while a single pore size distribution <0.9 nm gave only the filling swing. Thus, four representative types of the α s-plot for activated carbons were proposed and it was shown how to understand the micropore size distribution through the α s-plot.

A molecular simulation study on empirical determination method of pore structures of activated carbons

A validity of the subtracting pore effect (SPE) method for the specific surface area (SSA) determination of activated carbons was investigated by the grand canonical Monte Carlo (GCMC) simulation of nitrogen adsorption isotherms on a model graphite slit-pore at 77K. The simulated adsorption isotherm changed sensitively with the micropore width. The αs-plot was constructed for the simulated adsorption isotherm and it showed the filling-and cooperative-swings, which were assigned to the enhanced surface-molecule and intermolecular interactions, respectively. The simulated as-plot gave a theoretical basis for the SPE method of the correct SSA determination for activated carbons. Also it was given that the shape analysis of the αs-plot can provide the pore size distribution.

Surface and physical properties of microporous carbon spheres

The surface area of activated mesocarbon microbeads (activated MCMBs) was determined by N 2 adsorption using the subtracting pore effect (SPE) method. The microcrystalline graphitic structure of activated MCMBs was determined by X-ray diffraction. Electron spin resonance (ESR) spectra and magnetic susceptibility were measured for analysis of structure and magnetic properties. Spin concentration and spin-lattice relaxation time ( T 1) were determined from ESR data. The spin concentration of activated MCMB increased with the increase of surface area. The higher the surface area of activated MCMB, the longer the T 1. The longer T 1 indicates that micrographitic units of activated MCMB of higher surface area should be more isolated. A single electron spin was presumed to be situated on each micrographitic unit. The isolated spin system gave a random magnetism of a long magnetic relaxation (>3000 s).

Structural Characterization of Heat-Treated Activated Carbon Fibers

Pitch (PIT) and cellulose (CEL)-based activated carbon fibers (ACFs) were heated at 1473–3173 K under an Ar atmosphere. The N2 adsorption, X-ray diffraction (XRD), and magnetic susceptibility of the heat-treated ACFs were measured. The specific surface area of ACF samples was determined by the subtracting pore effect (SPE) analysis using the N2 adsorption isotherm.

A Molecular Simulation Study on Empirical Determination Method of Pore Structures of Activated Carbons

A validity of the subtracting pore effect (SPE) method for the specific surface area (SSA) determination of activated carbons was investigated by the grand canonical Monte Carlo (GCMC) simulation of nitrogen adsorption isotherms on a model graphite slit-pore at 77K. The simulated adsorption isotherm changed sensitively with the micropore width. The αs-plot was constructed for the simulated adsorption isotherm and it showed the filling-and cooperative-swings, which were assigned to the enhanced surface-molecule and intermolecular interactions, respectively. The simulated as-plot gave a theoretical basis for the SPE method of the correct SSA determination for activated carbons. Also it was given that the shape analysis of the αs-plot can provide the pore size distribution.

Origin of superhigh surface area and microcrystalline graphitic structures of activated carbons

The microcrystalline graphitic structures of microporous carbon fibers determined by x-ray diffraction, high-resolution transmission electron microscopy, and magnetic susceptibility measurements were determined. The relationship between the microcrystalline graphitic structures and specific surface area was geometrically derived on the basis of the structural data. A simple new method for determining specific surface area for microporous carbons, which is designated the subtracting pore effect (SPE) method, was proposed. The surface area by the SPE method was compared with that by the BET method for high surface area-activated carbon fibers (ACFs); the BET surface area of more than 2000 m 2g −1, which is determined by the data in the relative pressure of 0.1-0.3, was overestimated by 40%–50%. The valid relative pressure region of the BET plot for microporous carbons was suggested. The micrographitic structural changes of ACFs with water adsorption were examined by in situ x-ray diffraction and in situ small angle x-ray scattering. Water adsorption led to marked compression of the micrographitic pore walls, accompanying the clear swelling and development of the micropore.