(109d) Theoretical and Experimental Studies of Scanning Adsorption-Desorption Isotherms On Mesoporous Materials

Although the phenomenon of adsorption hysteresis has been attracting a lot of attention among both experimental and theoretical communities for several generations of researchers, its adequate description is still lacking.  Indeed, the interplay between the thermodynamic and geometrical factors gives rise to distinct features of hysteresis in different materials. Experimentally, these distinct features are prominently displayed in the behavior of scanning isotherms, which provide additional information about the pore network geometry, including its connectivity and pore size distribution, which cannot be revealed from the main adsorption and desorption branches. Theoretically, it has been well understood since the seminal works of Everett [1] that the description of scanning isotherms cannot be achieved based on the models of adsorption in individual pores; it is necessary to take into account a cooperative nature of capillary condensation and desorption processes in three dimensional pore networks with distributed geometrical parameters of individual pores. In this work, we present high resolution adsorption and desorption scanning isotherms on porous materials of different yet well characterized structure: regular hexagonal arrays of pore channels in SBA-15 silica, cubically ordered three dimensional gyroid structure in KIT-6 silica, and ordered three dimensional network of spheroidal cages in 3DOm carbons. The classical system of Xenon adsorption on disordered pore network of Vycor porous glass [1] is served as a reference system. An original theoretical model is presented, which distinguish the materials, at which the scanning hysteresis can be described based on the assumption of individual pore domains, and the materials, at which the pore blocking effects are significant. For the latter case, a percolation model is suggested, which is capable of a quantitative description of the scanning isotherms and revealing the pore network connectivity and the size distribution of constrictions (necks) from the experimental data. At the same time, our research revealed more questions than answers that requires further studies.

1. Everett, D. H. In The Solid-Gas Interface; Flood, E. A.; Ed.; Decker: New York, 1967; Vol. 2, pp 1055-1113.