Direct Measurement of Microporosity and Molecular Accessibility in Stöber Spheres by Adsorption Isotherms

Abstract

The microporous nature of monodisperse Stöber silica spheres is demonstrated in the literature, although usually via indirect evidence. Contradictorily, there also exist numerous reports of nonporosity based on conventional N2 adsorption isotherms, leading to a confusing scenario and questioning the evaluation methodology. Thus, there is the strong need of straight measure of microporosity in Stöber spheres, at best by available adsorption techniques, which must be further directly confronted with the standard nitrogen method. Here, for the first time, microporosity detection by N2 and CO2 adsorption is compared in Stöber spheres. We demonstrate that CO2 isotherms at 273 K allow direct detection and quantification of the microporosity (about 0.1 cm3/g in our samples), while N2 at 77 K cannot probe adequately the internal volume. We also show that a large amount of water fills the micropores under usual ambient conditions, also revealing the presence of small mesoporosity. Thus, the porous nature of Stöber spheres is investigated by a simple combination of adsorption isotherms, and the different accessibility of N2, CO2, and H2O molecules is discussed. We emphasize the inadequacy of standard N2 isotherms for micropore detection in Stöber silica, as the access of nitrogen molecules at cryogenic temperatures is kinetically restricted and may lead to erroneous conclusions. Instead, we propose CO2 isotherms as a simple and direct means for evaluation of microporosity.