A Monte Carlo integration method to determine accessible volume, accessible surface area and its fractal dimension

The Analysis of Pore Development and Formation of Surface Functional Groups in Bamboo-Based Activated Carbon during CO2 Activation

Pore development and the formation of oxygen functional groups were studied for activated carbon prepared from bamboo (Bambusa bambos) using a two-step activation with CO₂, as functions of carbonization temperature and activation conditions (time and temperature). Results show that activated carbon produced from bamboo contains mostly micropores in the pore size range of 0.65 to 1.4 nm. All porous properties of activated carbons increased with the increase in the activation temperature over the range from 850 to 950 °C, but decreased in the temperature range of 950 to 1000 °C, due principally to the merging of neighboring pores. The increase in the activation time also increased the porous properties linearly from 60 to 90 min, which then dropped from 90 to 120 min. It was found that the carbonization temperature played an important role in determining the number and distribution of active sites for CO₂ gasification during the activation process. Empirical equations were proposed to conveniently predict all important porous properties of the prepared activated carbons in terms of carbonization temperature and activation conditions. Oxygen functional groups formed during the carbonization and activation steps of activated carbon synthesis and their contents were dependent on the preparation conditions employed. Using Boehm’s titration technique, only phenolic and carboxylic groups were detected for the acid functional groups in both the chars and activated carbons in varying amounts. Empirical correlations were also developed to estimate the total contents of the acid and basic groups in activated carbons in terms of the carbonization temperature, activation time and temperature.

Competitive and Synergistic Adsorption of Mixtures of Polar and Nonpolar Gases in Carbonaceous Nanopores

Most adsorption applications involve mixtures, yet accurate predictions of the adsorption of mixtures remain challenging, in part due to the inability to account for the interplay between adsorbate-adsorbate and adsorbate-adsorbent interactions. This study involves a comprehensive Monte Carlo simulation of the adsorption of two groups of mixtures (namely, supercritical and subcritical ones) in carbon nanopores and quantifies Henry's constants, isotherms, energetics, and density distributions in the pores. When interadsorbate interactions are negligible (e.g., in supercritical mixtures such as mixtures of nonpolar gases), adsorbates behave like ideal gases and the adsorption isotherm can be predicted with the ideal adsorbed solution theory (IAST). However, when interadsorbate interactions become significant, IAST fails. This study reveals that (1) in mixtures of polar and nonpolar gases, the stronger intermolecular interaction for the polar constituent leads to synergistic adsorption that causes the nonpolar adsorbate to desorb and (2) for mixtures of polar gases, such as ethanol and water, the adsorbate-adsorbate interactions are so dominant that the unfavorable adsorbate-adsorbent interactions are overcome, such that water adsorbs onto the hydrophobic adsorbent. The competitive and synergistic interactions highlighted here are expected to be valuable in enhancing gas separations.

The physisorption mechanism of SO2 on graphitized carbon

Sulfur dioxide (SO2) in flue gases emitted from fossil fuel power plants dramatically reduces the CO2 capture efficiency via adsorption, which is due to the potential reaction of SO2 with basic functional groups on the adsorbent. Physisorption rather than chemisorption is preferred, because adsorbents can be more easily regenerated by either reducing the pressure or increasing the temperature. Carbon is a suitable adsorbent for SO2 capture and widely used, and therefore it is important to study SO2 adsorption onto carbon with the Monte Carlo simulation to provide microscopic details to demarcate the roles of the basal plane of the graphene layer and the functional groups in adsorption. SO2 is a polar molecule like water, as they both carry partial charges, but they interact differently with functional groups. Instead of 3D-clusters in the case of water, SO2 is localized around the functional groups and spreads over the basal plane to form 2D-molecular layers because of the strong dispersive interactions with graphite. The results indicate that the functional group has a negligible effect on the enhancement of adsorption and its role is to localize 2D-clusters of SO2 molecules. For non-graphitized carbon, we have found that the greater loadings at low pressure compared to the highly graphitized carbon is due to the presence of defects (crevices) on the basal plane surface. Finally, to describe better the experimental data, we have found that the reduction in the interactions between adsorbed molecules in the first layer is because of the repulsion of their dipoles pointing normal to the surface, a phenomenon called surface mediation and is widely used in the description of gas adsorption on surfaces.

Computational methodology for determining textural properties of simulated porous carbons

We have refined and improved the computational efficiency of the TriPOD technique, used to determine the accessible characteristics of porous solids with a known configuration of solid atoms. Instead of placing a probe molecule randomly, as described in the original version of the TriPOD method (Herrera et al., 2011), we implemented a scheme for dividing the porous solid into 3D-grids and computing the solid-fluid potential energies at these grid points. We illustrate the potential of this technique in determining the total pore volume, the surface area and the pore size distribution of various molecular models of porous carbons, ranging from simple pore models to a more complex simulated porous carbon model; the latter is constructed from a canonical Monte Carlo simulation of carbon microcrystallites of various sizes.

Microscopic configurations of methanol molecules in graphitic slit micropores: a computer simulation study

We report a detailed computer simulation study of methanol adsorption in graphitic slit-like micropores to investigate the effects of temperature and pore size on the adsorptive capacity and the configurations of methanol molecules in a confined space. Simulation results show that in the temperature range studied (273-422 K) the amount adsorbed increases gradually with pressure in 0.65 and 0.8 nm pores (where only one molecular layer can be accommodated), while for pores having widths greater than 1.0 nm the adsorption isotherms exhibit a sharp jump at low temperatures which becomes gradual as temperature is increased above the critical pore temperature, which increases with pore width. For a given pore size, the pressure at which a large uptake of adsorption occurs, increases and the excess amount adsorbed, decreases with temperature. The interaction between adsorbate molecules and a pore was studied via the solvation pressure, which exhibits oscillations with pore size. The peaks of this oscillation correspond to pores that have an integer number of layers of methanol molecules. At low loadings snapshots showed methanol molecules in isolated clusters of four or five molecules which maximise the hydrogen bonding within each cluster, in the same way as they do on an open surface. At high loadings, the isolated cluster configuration changes to molecular chains in small pores (0.65 and 0.8 nm), which become more distorted by inter-layer interactions in larger pores. The positions of the first peaks of the O-O and O-H radial distributions for the confined methanol are the same as those for bulk liquid for all pore sizes. However, for confined methanol in pores with an integer number of molecular layers, the amplitude of the first peaks of the O-O and O-H radial distributions are higher than for the bulk liquid, and the positions of the second peaks are slightly shifted to the left. In the incommensurate pores the amplitude of the first peaks and the positions of the second peaks of the O-O and O-H radial distributions are similar to those of liquid methanol. Our simulation results agree well with the experimental results of Ohkubo et al.

Monte Carlo optimization scheme to determine the physical properties of porous and nonporous solids

A new method, based on a Monte Carlo scheme, is developed to determine physical properties of nonporous and porous solids. In the case of nonporous solids, we calculate the surface area. This surface area is found as the sum of areas of patches of different surface energy on the solid, which is assumed to take a patchwise topology (i.e., adsorption sites of the same energy are grouped together in one patch). As a result of this assumption, we derive not only the surface area, but also the accessible volume and the surface energy distribution. In the case of porous solids, the optimization method is used to derive the surface area and the pore size distribution simultaneously. The derivation of these physical properties is based on adsorption data from a volumetric apparatus. We test this novel idea with the inversion problem of deriving surface areas of patches of different energies for a number of nonporous solids. The method is also tested with the derivation of the pore size distribution of some porous solid models. The results are very encouraging and demonstrate the great potential of this method as an alternative to the usual deterministic optimization algorithms which are known to be sensitive to the choice of the initial guess of the parameters. Since the geometrical parameters are physical quantities (i.e., only positive values are accepted), we also propose a scheme to enforce the positivity constraint of the solution.