COMPUTER-SIMULATIONS OF THE HIGH-TEMPERATURE ADSORPTION OF METHANE IN A SULFIDED GRAPHITE MICROPORE

Computer simulations of the adsorption isotherms and energies for methane at 300 K in slit pores are reported. The walls of these pores were made to be chemically heterogeneous by introducing sulfur atoms into graphite basal planes. Adsorption in the slit pores with pure graphite walls has been simulated previously. Modifications in the adsorptive behavior produced by the substitution of sulfides for some carbon atoms in the surface planes are shown here. The sulfides form regular lattices, with two levels of substitution studied. In the first case, one carbon atom in fourteen is replaced by sulfur, and in the second, one atom in eight has been replaced by sulfur. The choices of interaction potentials for the systems give rise to a very well defined heterogeneity in the sulfided surfaces. Both the isotherms and the heats show that adsorption is enhanced by the introduction of sulfur. However, a detailed inspection of the interaction potentials indicates that the ''strong sites' for adsorption are not on the sulfides but on the remaining exposed graphite surface between the sulfurs. Distributions of the adsorption energies of the adsorbed atoms have been evaluated. These distributions show that, for such a high temperature, adsorption does not occur by occupation first of the strong sites followed by coverage of the weaker parts of the surface. Instead, the distributions are essentially invariant for coverages up to a large fraction of the monolayer.