Effects of potential models on the adsorption of ethane and ethylene on graphitized thermal carbon black. Study of two-dimensional critical temperature and isosteric heat versus loading

Adsorption Mechanism and Characteristic Temperatures of the Monolayer Adsorption of CO2 on Graphite: The Role of Graphene Dimensions

Although simulation results for gaseous adsorption on a surface of infinite extent, modeled with periodic conditions at the boundaries of the simulation box, agree with experimental data at high temperatures, simulated isotherms at temperatures below the triple point temperature show unphysical substeps because of the compromise of interactions within the box and interactions between the box and its mirror image boxes. This has been alleviated with surfaces of finite dimensions (Loi, Q. K.; Colloids Surf., A 2021, 622, 126690 and Castaño Plaza, O.; Langmuir 2023, 39 (21), 7456-7468) to account for free boundaries at the adsorbate patch on the surface, and the critical parameter of this model substrate is the size of the finite surface. If it is too small, the adsorbate patch does not model the physical reality; however, if it is too large, the computation time is excessive, making the simulation impractical. In this study, we used carbon dioxide/graphite as the model system to explore the effects of finite dimensions on the description of experimental data of Terlain, A.; Larher, Y. Surf. Sci. 1983, 125 (1), 304-311, especially for temperatures below the bulk triple point temperature. With the appropriate choice of graphene size, we derived the 2D triple point and 2D critical point temperatures of the monolayer, and most importantly, for temperatures below the 2D critical point temperature, the adsorption mechanism for the formation of the monolayer is due to the interplay between the boundary growth process and the vacancy filling. The extent of this interplay is found to depend on the fractional coverage of the surface.

The role of adsorbate size on adsorption of Ne and Xe on graphite

We have carried out an extensive grand canonical Monte Carlo simulation to investigate the adsorption of neon and xenon on graphite. The adsorbate collision diameters of neon and xenon are smaller and greater respectively, than the commensurate graphite lattice spacing λ=3×3R30⁰ of 0.426 nm. Simulated isotherms and isosteric heats were obtained using a graphite model that has been shown to describe successfully the adsorbate transitions for krypton, methane and nitrogen by Prasetyo et al. (2017), which have collision diameters close to λ. Neon does not exhibit commensurate (C) packing because the gain in the intermolecular potential interactions in the incommensurate (IC) packing when molecules move away from carbon hexagon centres, does not compensate for the increase in the solid-fluid potential energy. Xenon, on the other hand, exhibits IC packing because its molecular size is greater than λ. Nevertheless, at a sufficiently high chemical potential, the first layer of xenon changes from the IC to C packing (in contrast to what is observed for krypton, nitrogen and methane). This transition occurs because the decrease in the xenon intermolecular interactions is sufficiently compensated by the increase in the solid-fluid interaction, and the increase in the fluid-fluid interactions between molecules in the first layer and those in the second layer. This finding is supported by the X-ray diffraction study by Mowforth et al. (1986) and Morishige et al. (1990).

Ethane selective IRMOF-8 and its significance in ethane-ethylene separation by adsorption

The separation of ethylene from ethane is one of the most energy-intensive single distillations practiced. This separation could be alternatively made by an adsorption process if the adsorbent would preferentially adsorb ethane over ethylene. Materials that exhibit this feature are scarce. Here, we report the case of a metal-organic framework, the IRMOF-8, for which the adsorption isotherms of ethane and ethylene were measured at 298 and 318 K up to pressures of 1000 kPa. Separation of ethane/ethylene mixtures was achieved in flow experiments using a IRMOF-8 filled column. The interaction of gas molecules with the surface of IRMOF-8 was explored using density functional theory (DFT) methods. We show both experimentally and computationally that, as a result of the difference in the interaction energies of ethane and ethylene in IRMOF-8, this material presents the preferential adsorption of ethane over ethylene. The results obtained in this study suggest that MOFs with ligands exhibiting high aromaticity character are prone to adsorb ethane preferably over ethylene.

Capture of trace sulfur gases from binary mixtures by single-walled carbon nanotube arrays: a molecular simulation study

Adsorption of H(2)S and SO(2) pure gases and their selective capture from the H(2)S-CH(4), H(2)S-CO(2), SO(2)-N(2), and SO(2)-CO(2) binary mixtures by the single-walled carbon nanotubes (SWNT) are investigated via using the grand canonical Monte Carlo (GCMC) method. It is found that the (20, 20) SWNT with larger diameter shows larger capacity for H(2)S and SO(2) pure gases at T = 303 K, in which the uptakes reach 16.31 and 16.03 mmol/g, respectively. However, the (6,6) SWNT with small diameter exhibits the largest selectivity for binary mixtures containing trace sulfur gases at T = 303 K and P = 100 kPa. By investigating the effect of pore size on the separation of gas mixtures, we found that the optimized pore size is 0.81 nm for separation of H(2)S-CH(4), H(2)S-CO(2), and SO(2)-N(2) binary mixtures, while it is 1.09 nm for the SO(2)-CO(2) mixture. The effects of concentration and temperature on the selectivity of sulfide are also studied at the optimal pore size. It is found that the concentration (ppm) of sulfur components has little effect on selectivity of SWNTs for these binary mixtures. However, the selectivity decreases obviously with the increase of temperature. To improve the adsorption capacities, we further modify the surface of SWNTs with the functional groups. The selectivities of H(2)S-CO(2) and SO(2)-CO(2) mixtures are basically uninfluenced by the site density, while the increase of site density can improve the selectivity of H(2)S-CH(4) mixture doubly. It is expected that this work could provide useful information for sulfur gas capture.

Pressure enhancement in carbon nanopores: a major confinement effect

Phenomena that only occur at high pressures in bulk phases are observed in nanopores because of a pressure enhancement effect.

Some remarks on the calculation of the pore size distribution function of activated carbons

Different authors investigated the effects of geometric and energetic heterogeneities on adsorption and on carbon characterization methods. In most theoretical studies carbon structure is modeled as parallel infinite graphite walls that form ideal slit-shaped pores of the fixed widths. In the literature there is the lack of systematic studies showing the influence of pore structural and Lennard-Jones (LJ) potential parameters on the pore-size distribution functions. Moreover, the parameters characterizing the properties of the adsorbed phase and the heterogeneity of the adsorbent surface should be taken into account. The Nguyen and Do method with proposed by us ASA algorithm, were utilized for the assessment of the porosity from the series of almost few thousands numerically generated local adsorption isotherms. The values of the mentioned-above parameters are varied over the wide range (ca. +/-20%) of the reference ones. Different types of the theoretical and experimental adsorption isotherms (nitrogen at 77 K) were taken into account as the global ones. They were related to the mechanism of the primary, secondary or mixed micropore filling. The variations in some above-mentioned parameters have significant effects only for PSDs (and for average pore widths) corresponding to the primary micropore filling mechanism. On the other hand, for the process of the secondary micropore filling, the influence of these parameters (without the BET coefficient for adsorption on a "flat" surface, c(s,B)) is rather insignificant. Nevertheless the differences between local and global adsorption isotherms (in the whole range of relative pressures) the absence of micropores having pore half width equal to ca. 1 nm on PSDs was observed for studied adsorbate-adsorbent systems with exceptions of the strictly microporous adsorbents and/or the low values of c(s,B). Comparison of the experimental data with the generated theoretical isosteric enthalpy of adsorption indicates that the phenomenal uptake observed from experiment can be explained in terms of the reasonable solid-fluid interaction parameters. Therefore, we varied the heterogeneity of the adsorbent surface via the strength and the range of the solid-fluid potential and the parameter c(s,B) in order to reproduce the experimental data of enthalpy of adsorption. Note that similar procedure was applied by Wang and Johnson to reproduce some hydrogen adsorption data measured for carbon nanofibres. The analysis of the obtained results shows that the selection of the values of the parameters of the intermolecular interactions and the quantities characterizing the properties of the adsorbed phase and the heterogeneity of the adsorbent walls for molecular simulations should be made with care and the influence of possible errors should be considered.

Raman spectroscopic investigation of gas interactions with an aligned multiwalled carbon nanotube membrane

Raman spectroscopy has been used to investigate ethane, propane, and SF6 interactions with an aligned multiwalled carbon nanotube (MWNT) membrane. Pressures of 7.5-9.3 atm and temperatures of 293-333 K were examined for propane and SF6, whereas slightly lower temperatures (263-293 K) and pressures (6.7-7.5 atm) were used for ethane. Red-shifting and broadening is seen for the C-C stretching vibrations of the two hydrocarbons, as well as for the A1g symmetric vibration (nu1) of SF6. These spectral features indicate that the interaction between the gas and the nanotube membrane is capable of perturbing molecular vibrations and creating red-shifted features. Control experiments done on polycrystalline graphite and a polystyrene blank indicate that this spectral behavior is unique to gases interacting with the nanotubes in the membrane.

Storage of Hydrogen at 303 K in Graphite Slitlike Pores from Grand Canonical Monte Carlo Simulation

Grand canonical Monte Carlo (GCMC) simulations were used for the modeling of the hydrogen adsorption in idealized graphite slitlike pores. In all simulations, quantum effects were included through the Feynman and Hibbs second-order effective potential. The simulated surface excess isotherms of hydrogen were used for the determination of the total hydrogen storage, density of hydrogen in graphite slitlike pores, distribution of pore sizes and volumes, enthalpy of adsorption per mole, total surface area, total pore volume, and average pore size of pitch-based activated carbon fibers. Combining experimental results with simulations reveals that the density of hydrogen in graphite slitlike pores at 303 K does not exceed 0.014 g/cm(3), that is, 21% of the liquid-hydrogen density at the triple point. The optimal pore size for the storage of hydrogen at 303 K in the considered pore geometry depends on the pressure of storage. For lower storage pressures, p < 30MPa, the optimal pore width is equal to a 2.2 collision diameter of hydrogen (i.e., 0.65 nm), whereas, for p congruent with 50MPa, the pore width is equal to an approximately 7.2 collision diameter of hydrogen (i.e., 2.13 nm). For the wider pores, that is, the pore width exceeds a 7.2 collision diameter of hydrogen, the surface excess of hydrogen adsorption is constant. The importance of quantum effects is recognized in narrow graphite slitlike pores in the whole range of the hydrogen pressure as well as in wider ones at high pressures of bulk hydrogen. The enthalpies of adsorption per mole for the considered carbonaceous materials are practically constant with hydrogen loading and vary within the narrow range q(st) congruent with 7.28-7.85 kJ/mol. Our systematic study of hydrogen adsorption at 303 K in graphite slitlike pores gives deep insight into the timely problem of hydrogen storage as the most promising source of clean energy. The calculated maximum storage of hydrogen is equal to approximately 1.4 wt %, which is far from the United States Department of Energy (DOE) target (i.e., 6.5 wt %), thus concluding that the total storage amount of hydrogen obtained at 303 K in graphite slitlike pores of carbon fibers is not sufficient yet.