Packing effects on argon and methanol adsorption inside graphitic cylindrical and slit pores: A GCMC simulation study

Performance evaluation of activated carbon with different pore sizes and functional groups for VOC adsorption by molecular simulation

Volatile organic compounds (VOCs) are growing pollutants now that cause air pollution and threaten human health. In this paper, the Grand Canonical Monte Carlo was used to simulate the adsorption performance of activated carbon on VOCs (benzene, toluene, acetone and methanol). After simulating different pore sizes (0.902 nm,1.997 nm,3 nm and 4 nm) adsorption performances of activated carbon, activated carbon with a pore size of 1.997 nm was selected to further study the influence of functional groups (carboxyl, amino, hydroxyl and hydrogen), and the capillary condensation was explained by the Kelvin equation. Furthermore, effects of functional groups under saturated vapor pressure (P₀) of VOCs that range from 0 to 0.1 P₀ were explained by the accessible volume and intermolecular interaction potential, respectively. Under pressure range of 0-0.1 P₀, at the beginning of adsorption of acetone and methanol, carboxyl and amino groups could reduce the threshold pressure while hydroxyl and hydrogen have the opposite effect. For benzene and toluene, all functional groups have little effect on the threshold pressure, and they reduce the adsorption capacity instead. It could be concluded that the activated carbon could achieve the best adsorption effect on acetone and methanol, on the contrary, the addition of functional groups on benzene and toluene will weaken their adsorption performance.

Molecular dynamics simulations of nano-confined methanol and methanol-water mixtures between infinite graphite plates: Structure and dynamics

Molecular dynamics simulations are used to investigate microscopic structures and dynamics of methanol and methanol-water binary mixture films confined between hydrophobic infinite parallel graphite plate slits with widths, H, in the range of 7-20 Å at 300 K. The initial geometric densities of the liquids were chosen to be the same as bulk methanol at the same temperature. For the two narrowest slit widths, two smaller initial densities were also considered. For the nano-confined system with H = 7 Å and high pressure, a solid-like hexagonal arrangement of methanol molecules arranged perpendicular to the plates is observed which reflects the closest packing of the molecules and partially mirrors the structure of the underlying graphite structure. At lower pressures and for larger slit widths, in the contact layer, the methanol molecules prefer having the C-O bond oriented parallel to the walls. Layered structures of methanol parallel to the wall were observed, with contact layers and additional numbers of central layers depending on the particular slit width. For methanol-water mixtures, simulations of solutions with different composition were performed between infinite graphite slits with H = 10 and 20 Å at 300 K. For the nanoslit with H = 10 Å, in the solution mixtures, three layers of molecules form, but for all mole fractions of methanol, methanol molecules are excluded from the central fluid layer. In the nanopore with H = 20 Å, more than three fluid layers are formed and methanol concentrations are enhanced near the confining plates walls compared to the average solution stoichiometry. The self-diffusion coefficients of methanol and water molecules in the solution show strong dependence on the solution concentration. The solution mole fractions with minimal diffusivity are the same in confined and non-confined bulk methanol-water mixtures.

Dynamic behavior of a rotary nanomotor in argon environments

When argon is used as a protecting gas in the fabrication or working environment of a nanodevice, absorption of some argon atoms onto the surface of the device lead to different responses. In this work, the rotation of the rotor in a carbon nanotube (CNT)-based rotary nanomotor in argon environment is investigated. In the rotary nanomotor, two outer CNTs act as the stator and are used to constrain the inner CNT (i.e., the rotor). The rotor is driven to rotate by the stator due to their collision during thermal vibration of their atoms. A stable rotational frequency (SRF) of the rotor occurs when the rotor reaches a dynamic equilibrium state. The value of the SRF decreases exponentially with an increase in the initial argon density. At dynamic equilibrium date, some of the argon atoms rotate synchronously with the rotor when they are absorbed onto either internal or external surface of the rotor. The interaction between the rest of the argon atoms and the rotor is stronger at higher densities of argon, resulting in lower values of the SRF. These principles provide insight for future experimentation and fabrication of such rotary nanomotor.

Understanding adsorption of CO2, N2, CH4 and their mixtures in functionalized carbon nanopipe arrays

Effects of functionalization on excess adsorption of CO₂ (a) and selectivity of equimolar mixtures in 5% COOH functionalized CMK-5 (b).

Adsorption of Water and Methanol on Highly Graphitized Thermal Carbon Black and Activated Carbon Fibre

Adsorption of water and methanol on different carbonaceous solids was carried out to investigate the roles of porous structure and functional groups on the adsorption of associating fluids. A highly graphitized thermal carbon black, non-porous Carbopack F, was chosen to study the effects of functional groups and their concentration, and two samples of porous activated carbon fibre (ACF), microporous A-5 and micro-mesoporous A-15, were used to investigate the interplay between the functional groups and confinement. On Carbopack F, adsorption of water at 298 K is not experimentally detectable until the relative pressure reaches about 0.9, and the adsorption isotherm exhibits a large hysteresis loop spanning a very wide range of pressure; by contrast methanol adsorption at the same temperature shows an onset of adsorption at a lower relative pressure of 0.2 and the isotherm has a very small hysteresis loop. This early onset, compared with water, is due to the dispersion interaction between the methyl group and the graphene surface; an interaction which is absent in water. For the porous ACF samples, the onset of water uptake shifts from a relative pressure of 0.9; as observed for Carbopack F, to the much lower values, depending on pore size, of 0.3 for microporous A-5 and 0.5 for micro-mesoporous A-15.

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.