In-depth study of surface resistances in nanoporous materials by microscopic diffusion measurement

•Surface resistances are indicated by differences in micro- and macro-diffusivities.•Their quantitation is possible by micro-imaging via IR or interference microscopy.•The mechanisms of resistance may be revealed by comparison with intra-diffusivities.•Existence of surface barriers allows a meaningful definition of sticking probabilities.

Experimental evidence leading to our present view on transport resistances on the external surface of nanoporous materials (“surface barriers”) is reviewed. First substantial arguments for the existence of surface resistances was provided by the direct measurement of intracrystalline diffusion, enabled by the application of the pulsed field gradient (PFG) technique of NMR to sufficiently large zeolite crystallites. With the advent of the techniques of micro-imaging and the thus established avenue towards monitoring transient guest concentrations, first in-depth studies of surface barriers, based on the measurement of their permeabilities, have become possible. Highlights among these studies were the detection of surface barriers formed by impermeable layers with dispersed holes, giving rise to proportionality between surface permeation and intracrystalline diffusion, and the determination of “sticking factors” which, in the present context, refer to the probability that, after colliding with the external surface, a molecule of the gas phase will surmount the surface resistance and get into the genuine pore space. The formation of surface barriers is, in conclusion, shown to be a rather complex phenomenon whose in-depth exploration necessitates efforts comprising a large spectrum of activities over essentially all fields of zeolite research and technology.

Graphical abstractAfter colliding with the external surface, only a fraction of molecules is able to surmount the surface barrier and to get into the genuine pore space.Figure optionsDownload full-size imageDownload as PowerPoint slide