Porous Structure of Ordered Silica with Cagelike Pores Examined by Successive Adsorption of Water and Nitrogen

To examine the porous structure of ordered silica with cagelike pores, we measured successive adsorption of water at 283 K and nitrogen at 77 K on four kinds of SBA-16 and four kinds of KIT-5. For materials with smaller necks, we found the amount of nitrogen condensed inside the large cavities decreased rapidly with an increase in water filling and reached a small value at water filling, just before the onset of pore condensation of water inside the spherical cavities. This clearly indicates that most of the cavities are isolated from bulk nitrogen gas with water frozen in the necks. A comparison with the accessibility curve of voids expected for a random distribution of neck sizes throughout an ordered structure indicates the presence of correlation in the spatial distribution of neck sizes in the material. In addition, the relative pressure range of water, in which the amount of nitrogen condensed in the large cavities considerably drops, gave information about pore size distribution of the necks. However, for materials with larger necks, the amount of nitrogen condensed inside the large cavities decreased gradually with an increase in water filling because pore condensation of water occurs simultaneously in the necks and cavities.