Adsorption in heterogeneous porous media: Hierarchical and composite solids

• Experimental and molecular modeling of adsorption on complex heterogeneous solids.
• Adsorption-based characterization of hierarchical and composite porous media.
• Porosity domain analysis in complex materials using superimposition principle.

Experiment and molecular simulation are used to investigate adsorption in heterogeneous porous media consisting of hierarchical solids (combining different porosity scales) or composite solids (such as silver nanoparticles adsorbed at the external surface of zeolite). It is shown that adsorption in such heterogeneous materials can be written as a linear combination of the adsorption isotherms in its different domains (i.e. porosity scales for the hierarchical sample and constituents for the composite sample). In the case of the composite material, we also show that the linear combination can be used with weighing parameters obtained for a different adsorbate. Such a superimposition principle, which is validated using well-characterized experimental samples, is of interest for characterization purpose as well as industrial applications as they can be used to determine accurately the amount of phases in a given sample (volume corresponding to a given porosity scale or constituent). In contrast, significant departure between the experimental adsorption isotherm and the linear combination can be used to detect coupling effects between the different domains or restrained access to a given domain type. Such a characterization strategy of complex heterogeneous media is complementary to other experiments, such as those probing capillary hysteresis shapes, scanning curves and subloops, which allow determining the distribution of domains within the framework of the independent domain theory.

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