Monte Carlo simulations of krypton adsorption in nanopores: Influence of pore-wall heterogeneity on the adsorption mechanism

We present molecular simulation results of the adsorption of krypton in a model of the mesoporous material MCM-41. The adsorption isotherm and adsorption enthalpies are studied at 77 K. Comparison of the experimental and simulation data allows us to analyze how the available interaction models (Kr-Kr and Kr-walls) are able to reproduce the experimental situation. The role of the heterogeneous interactions versus the homogenous model is studied and compared with the previous simulation results of nitrogen adsorption in MCM-41. The results show that a model of ideal cylindrical pores gives qualitatively and quantitatively different results. A distribution of the adsorption sites must exist to explain the loading at low pressure (below capillary condensation). Such a distribution in MCM-41 is a consequence of inhomogenous walls that contain a wide variety of attractive sites ranging from weakly attractive silica-type to highly attractive regions. In our simulations the MCM-41 structure is modeled as an amorphous array of oxygen and silicon atoms, each one interacting with an adsorptive atom via the atom-atom potential. The distribution of the adsorption sites is merely a consequence of the local atomic structure. Such a model of the wall reproduces the smooth increase in loading seen experimentally. (C) 2003 American Institute of Physics.