Diffusion is the random migration of molecules or small particles, arising from motion due to thermal energy [1-19]. It is a general property of matter related to the tendency of systems to occupy all of the accessible states. In a more simple way, diffusion is a spontaneous tendency of all systems to equalize concentration, if any external influence does not impede this process. That is, atoms, molecules, or any particle moves chaotically in the direction where fewer elements of its own type are located. Diffusion of gases in porous materials is a very important topic, since this effect is crucial in catalysis, gas chromatography, and gas separation processes.