Simulating CO2 adsorption and diffusion on a graphite surface

We performed classical molecular dynamics (MD) simulation to understand the mechanism of CO2 adsorption and transport on graphite surface. The temperature of the system in our simulation was in the range 300-500K. The simulation data show that there are two layers of CO2 molecules absorbed on the surface. These two layers have a different behavior. The first CO2 layer is isolated as it does not exchange molecules with the second layer and is liquid-like, while the second layer exchanges molecules with the gas phase. The layers are separate thermodynamic systems. We use the simple Langmuir model to fit the adsorption isotherm for the second layer. The enthalpy of adsorption is calculated ?H0 = -16 kJ/mol. This value is in good agreement with experimental data of adsorption of CO2 on activated carbon. Along the graphite surface, the diffusion coefficient of CO2 in the first layer and the second layer are roughly of 10-11 m2/s, 10-10 m2/s respectively. These values are much smaller than for H2