High pressure effect in nanoporous carbon materials: Effects of pore geometry

Abundant experimental evidence suggests that adsorbates confined in nanoporous materials exhibit high pressures, such as high pressure crystal structures, high pressure chemical reactions, and the deformation of pore walls due to the adsorbate. We report molecular simulation studies of the pressure tensor for simple adsorbates (e.g. argon) in carbon nanopores of slit, cylindrical and spherical geometries. We find that for modest bulk phase pressures of 1 bar or less, the pressures parallel to the pore walls (tangential pressure) is of the order 104–105 bar, while the pressure normal to the wall is of the order of 103 bar, and can be positive or negative depending on the pore size. Moreover, we find that the pore geometry has a large effect on the structure of the adsorbate and thus on the in-pore pressure because of the curvature that determines the strength of the adsorbate–wall interaction. For the same pore size, temperature and bulk pressure, the in-pore tangential pressure is the largest in spherical pores, followed by that in cylindrical pores and slit pores. We also study the normal pressure of carbon tetrachloride and water confined in activated carbon fibers by molecular simulations and experiments. The pressure acting on the pore wall is found to be of the order of several thousand bar by both methods. Experiments also find that the pore can be expanded or contracted, depending on pore width, as we predict by molecular simulation.