Simulating Physisorption in Porous Solids

The physisorption of non-polar molecules within mieroporous solids, where no electron transfer is involved, determines adsorptive and dilfusional characteristics. These are also important in their catalytic performance, which depends at least in part on molecular access to and from the active catalytic sites. Simulations that account for atom-atom interactions through pair potential methods can be used to determine accurately the energy of a physisorbed molecule, but they must include routines that enable a search through the available space to find the distribution of adsorption energies and also the global energy minimum. 

Such searches use Monte Carlo techniques, in which adsorbate molecules are inserted with random positions and orientations into the porous host structure. For complex molecular guests, a series of low-energy configurations of 

Investigation of the motion of adsorbed molecules, which give mechanisms and rates of re-orientation and diffusion, require alternative approaches. For systems that contain highly mobile species. Molecular Dynamics (MD) techniques are widely used. However, for many adsorbates the timescales of motion are much longer than can feasibly be simulated, so that MD is only relevant either for small molecules or at high temperatures. In order to simulate slower diffusion, the process must be considered in terms of rare events with significant activations. The activated processes are then usefully treated by transition state theory, and the associated processes treated over extended timescales and volumes by, for example, Kinetic Monte Carlo (KMQ techniques. 

1 Monte Carlo Methods Grand Canonical Monte Carlo (GCMC) 

Monte Carlo methods can be used to simulate the adsorption of molecules as a function of temperature and the pressure of the gas in contact with the porous solid. In this way adsorption isotherms of single or multiple component gas 

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