Sorption and pore condensation behavior of nitrogen, argon, and krypton in mesoporous:: MCM-48 silica materials

We present results of a systematic study on the sorption- and phase behavior of nitrogen, argon, and krypton at 77 and 87 K in different pristine mesoporous MCM-48 silica phases (BJH pore diameters 2-3 nm) and MCM-38 silica/iron(III) host compound, which consists of two interwoven, but unconnected three-dimensional pore systems. Different methods (e.g., BET, BJH, SF) were used to analyze nitrogen (77 K) and argon (87 K) sorption isotherms in order to characterize the MCM-48 silica materials with respect to surface area, pare volume and pore diameter. In contrast to nitrogen, argon, and krypton sorption isotherms on all MCM-48 silica materials reveal phase transitions accompanied by sorption hysteresis of type H1 (IUPAC classification) down to temperatures T well below the bulk triple-point temperature T-R, i.e., 87 K (T-R - T approximate to 28.5 K) in the case of krypton and 77 K (T-R - T approximate to 6.5 K) in the case of argon. Details of the sorption hysteresis depend on temperature and pore size, e.g., with increasing temperature and decreasing pore size a shrinkage of the hysteresis loops is observed. In contrast to the argon/MCM-48 silica system, argon sorption isotherms at T-R - T approximate to 6.5 K in controlled-pore glass of BJH pore diameter ca. 16 nm reveal no pore condensation and hysteresis, indicating that in such wide pores the pore condensation line does not extend down to this temperature for pressures up to the corresponding bulk sublimation pressure.