Piotr A. Gauden

Estimation of the pore-size distribution function from the nitrogen adsorption isotherm. Comparison of density functional theory and the method of Do and co-workers

A comparative analysis of the results for the estimation of the pore-size distribution based on the methods of Do and co-workers (ND) and density functional theory (DFT) is given. A new algorithm (ASA, Adsorption Stochastic Algorithm) is adopted and it is shown that this algorithm can be successfully applied for the determination of the PSD curve from the ND method. The obtained results show that, generally, the ND and DFT methods lead to almost the same PSD curves and this similarity is observed for carbons of different origin and possessing different pore structures. However, if the contribution of micropores to the PSD increases, the differences in the fit of ND and DFT to the experimental data are more pronounced.

What kind of pore size distribution is assumed in the Dubinin-Astakhov adsorption isotherm equation?

Abstract Two most sophisticated methods of carbon porosity characterization (high resolution αs-plot and the procedure proposed by Nguyen and Do, (ND)) were utilized for the assessment of porosity from the series of numerically generated adsorption isotherms. Basing on the Dubinin–Astakhov (DA) adsorption isotherm equation, two series of adsorption isotherms of nitrogen (T=77.5 K) were generated for constant E0 and different n values, and for constant n and different E0. They were described by the both above-mentioned methods. The types of obtained αs-plots as well as the pore size distribution curves (PSD) lead to suggestions about the basic features of the DA and the meaning of the parameters of this adsorption isotherm equation.

Developing the solution analogue of the Toth adsorption isotherm equation

The well-known Toth adsorption isotherm equation developed formerly for adsorption of vapors is converted into its solution analogue. It is shown that this equation can be successfully applied to the description of adsorption data of organics on activated carbons.

Can carbon surface oxidation shift the pore size distribution curve calculated from Ar, N2 and CO2 adsorption isotherms? Simulation results for a realistic carbon model

Using the virtual porous carbon model proposed by Harris et al, we study the effect of carbon surface oxidation on the pore size distribution (PSD) curve determined from simulated Ar, N(2) and CO(2) isotherms. It is assumed that surface oxidation is not destructive for the carbon skeleton, and that all pores are accessible for studied molecules (i.e., only the effect of the change of surface chemical composition is studied). The results obtained show two important things, i.e., oxidation of the carbon surface very slightly changes the absolute porosity (calculated from the geometric method of Bhattacharya and Gubbins (BG)); however, PSD curves calculated from simulated isotherms are to a greater or lesser extent affected by the presence of surface oxides. The most reliable results are obtained from Ar adsorption data. Not only is adsorption of this adsorbate practically independent from the presence of surface oxides, but, more importantly, for this molecule one can apply the slit-like model of pores as the first approach to recover the average pore diameter of a real carbon structure. For nitrogen, the effect of carbon surface chemical composition is observed due to the quadrupole moment of this molecule, and this effect shifts the PSD curves compared to Ar. The largest differences are seen for CO(2), and it is clearly demonstrated that the PSD curves obtained from adsorption isotherms of this molecule contain artificial peaks and the average pore diameter is strongly influenced by the presence of electrostatic adsorbate-adsorbate as well as adsorbate-adsorbent interactions.

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

Fractal dimension of microporous carbon on the basis of Polanyi–Dubinin theory of adsorption. Part IV. The comparative analysis of two alternative solutions of the overall adsorption isotherm equation for microporous fractal solids

Abstract The comparative analysis of two alternative adsorption isotherm equations, i.e. approximate (proposed by Avnir and Jaroniec) and analytical solutions (developed by us and called FRDA equation) of the global adsorption isotherm on microporous fractal solids is presented. The general conclusion is that if adsorption proceeds by a micropore filling mechanism and the pore size distribution is assumed to be Pfeifer and Avnir function, not only the adsorption value but also the process energetics cannot be correctly described by simpler—however generating some erroneous results—approximated isotherm equations and the corresponding thermodynamical formulas, proposed by Avnir, Jaroniec et al.

Synergetic effect of carbon nanopore size and surface oxidation on CO2 capture from CO2/CH4 mixtures

We have studied the synergetic effect of confinement (carbon nanopore size) and surface chemistry (the number of carbonyl groups) on CO2 capture from its mixtures with CH4 at typical operating conditions for industrial adsorptive separation (298 K and compressed CO2-CH4 mixtures). Although both confinement and surface oxidation have an impact on the efficiency of CO2/CH4 adsorptive separation at thermodynamics equilibrium, we show that surface functionalization is the most important factor in designing an efficient adsorbent for CO2 capture. Systematic Monte Carlo simulations revealed that adsorption of CH4 either pure or mixed with CO2 on oxidized nanoporous carbons is only slightly increased by the presence of functional groups (surface dipoles). In contrast, adsorption of CO2 is very sensitive to the number of carbonyl groups, which can be examined by a strong electric quadrupolar moment of CO2. Interestingly, the adsorbed amount of CH4 is strongly affected by the presence of the co-adsorbed CO2. In contrast, the CO2 uptake does not depend on the molar ratio of CH4 in the bulk mixture. The optimal carbonaceous porous adsorbent used for CO2 capture near ambient conditions should consist of narrow carbon nanopores with oxidized pore walls. Furthermore, the equilibrium separation factor was the greatest for CO2/CH4 mixtures with a low CO2 concentration. The maximum equilibrium separation factor of CO2 over CH4 of ~18-20 is theoretically predicted for strongly oxidized nanoporous carbons. Our findings call for a review of the standard uncharged model of carbonaceous materials used for the modeling of the adsorption separation processes of gas mixtures containing CO2 (and other molecules with strong electric quadrupolar moment or dipole moment).

Cryogenic separation of hydrogen isotopes in single-walled carbon and boron-nitride nanotubes: Insight into the mechanism of equilibrium quantum sieving in quasi-one-dimensional pores

Quasi-one-dimensional cylindrical pores of single-walled boron nitride and carbon nanotubes efficiently differentiate adsorbed hydrogen isotopes at 33 K. Extensive path integral Monte Carlo simulations revealed that the mechanisms of quantum sieving for both types of nanotubes are quantitatively similar; however, the stronger and heterogeneous external solid-fluid potential generated from single-walled boron nitride nanotubes enhanced the selectivity of deuterium over hydrogen both at zero coverage and at finite pressures. We showed that this enhancement of the D(2)/H(2) equilibrium selectivity results from larger localization of hydrogen isotopes in the interior space of single-walled boron nitride nanotubes in comparison to that of equivalent single-walled carbon nanotubes. The operating pressures for efficient quantum sieving of hydrogen isotopes are strongly depending on both the type as well as the size of the nanotube. For all investigated nanotubes, we predicted the occurrence of the minima of the D(2)/H(2) equilibrium selectivity at finite pressure. Moreover, we showed that those well-defined minima are gradually shifted upon increasing of the nanotube pore diameter. We related the nonmonotonic shape of the D(2)/H(2) equilibrium selectivity at finite pressures to the variation of the difference between the average kinetic energy computed from single-component adsorption isotherms of H(2) and D(2). In the interior space of both kinds of nanotubes hydrogen isotopes formed solid-like structures (plastic crystals) at 33 K and 10 Pa with densities above the compressed bulk para-hydrogen at 30 K and 30 MPa.

The new correlation between microporosity of strictly microporous activated carbons and fractal dimension on the basis of the Polanyi–Dubinin theory of adsorption

Abstract Adsorption on microporous fractal carbons is investigated in terms of their microporous structure. The structure is characterised by the micropore-size distribution (MSD) proposed by Pfeifer and Avnir, and the considerations are limited only to the range of micropores, following the IUPAC classification. Beginning with numerical studies performed for model solids with different microporosity, a new relationship between the parameters of the Dubinin–Astakhov equation (adsorption energy, E0, and parameter, nDA) and the fractal dimension (D) is derived. The obtained results are compared with those published by Jaroniec et al. and Ehrburger-Dolle. The general conclusion is that if adsorption proceeds by a micropore filling mechanism and the pore size distribution function is assumed to be the Pfeifer and Avnir one, the relationship between D and parameters of the Dubinin–Astakhov equation is more complicated than has been presumed up to the present.

Equilibrium clusters in concentrated lysozyme protein solutions

We have studied the structure of salt-free lysozyme at 293 K and pH 7.8 using molecular simulations and experimental SAXS effective potentials between proteins at three volume fractions, ϕ=0.012, 0.033, and 0.12. We found that the structure of lysozyme near physiological conditions strongly depends on the volume fraction of proteins. The studied lysozyme solutions are dominated by monomers only for ϕ≤0.012; for the strong dilution 70% of proteins are in a form of monomers. For ϕ=0.033 only 20% of proteins do not belong to a cluster. The clusters are mainly elongated. For ϕ=0.12 almost no individual particles exits, and branched, irregular clusters of large extent appear. Our simulation study provides new insight into the formation of equilibrium clusters in charged protein solutions near physiological conditions.