Adsorption of Water on Activated Carbons:  A Molecular Simulation Study

Abstract

We report a molecular simulation study for a model of water adsorption on nonporous and porous activated carbons. The grand canonical Monte Carlo method is used, and the temperature is fixed at 300 K. Water molecules are modeled as a Lennard-Jones sphere with four square-well sites to account for the hydrogen bonding. The carbon surfaces consist of planar graphite sheets, with active chemical sites on the surface modeled as square-well sites. The effect of the density and geometric arrangement of the active sites on the surface is studied. Both macroscopic properties (particularly adsorption isotherms) and molecular configurations are obtained. The adsorption mechanism for water on such surfaces is markedly different from that of simple nonassociating molecules such as hydrocarbons or nitrogen. In contrast to the usual buildup of adsorbed layers on the surface, water adsorption is characterized by the formation of peculiar three-dimensional water clusters and networks, whose formation relies on a cooperative effect involving both fluid−fluid interactions and fluid−solid ones with suitably placed active sites. Both the density and arrangement of the sites on the surface have a pronounced effect on the adsorption. Capillary condensation is observed only for low densities of active sites; for higher densities, continuous filling occurs.

Characterization of Single Wall Carbon Nanotubes and Activated Carbon with Water Adsorption in Finite-Length Pore Models

“…A cut-off radius in the calculation of interaction energy is five times the collision diameter of oxygen atom (5σOO), and the long range correction is not considered. Details of this model can be found in [4]. SPCE model consists of an LJ site located at the centre of the oxygen atom and three partial charges: a negative charge is located at the LJ site and two positive charges are located along the oxygen-hydrogen bonds.…”

“…The molecular parameters of the SPCE water model are listed in Table 2 [26]. [4], where εOO and σOO are the energy well depth and the collision diameter of the dispersive site, respectively, k is the Boltzmann's constant, and rOH is the distance between a square well (SW) associating site and the oxygen atom, σHB is the diameter of the associating site to form hydrogen bond (HB) and εHB is the energy well depth for unlike sites to form hydrogen bond. where AB r is the distance between A and B.…”

“…It contains pores of different sizes and shapes. For the simplicity of modeling, AC is assumed to compose of graphitic slit pores of different sizes and early modeling treated these pores having infinite length in extent [3][4][5][6][7][8].…”

“…Molecular simulation has been applied to elucidate the adsorption mechanism of water in hydrophobic SWNTs [7,14,17,20] and carbon slit pores [4][5][6][7][21][22][23][24], and comparison between the simulation results and the experimental data has been made [5,19,22]. To model water, there are broadly two classes of potential models.…”

“…To model water, there are broadly two classes of potential models. One is the point charge models, for example SPC [25], SPCE [26] and ST2 [27] models, while the other is the square-well site models such as the Primitive model (PM) [28] and Müller et al [4][5] models. In our simulation, we chose the SPCE and the Muller et al's model to represent these two classes.…”