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A non-equilibrium thermodynamics model for combined adsorption and diffusion processes in micro- and nanopores

A non-equilibrium thermodynamics model able to analyze the combined effect of diffusion and adsorption in porous materials is proposed. The model considers the coupled dynamics of the diffusive phase, described by a diffusion type equation, and the adsorbed phase governed by a generalized Langmuir equation. It is shown that the combination of diffusion and adsorption can be treated as diffusion in an effective medium, with an effective diffusion coefficient depending on the characteristics of the adsorption kinetics. The interplay between these effects leads to the appearance of different regimes in which the effective diffusion coefficient exhibits peculiar behaviours such as a decrease due to the presence of the adsorption process and a non-monotonous behavior resulting from particle interactions. We outline applications of the model developed to diffusion in porous materials such as zeolites and bentonites, and to engineered highly porous materials.