Capillary condensation of nitrogen in MCM-41 and SBA-15

Abstract

We performed measurements of a series of adsorption–desorption isotherms of nitrogen into MCM-41 and SBA-15 mesoporous molecular sieves with different pore sizes in a wide temperature range between a bulk triple point (Tt) and a bulk critical point (Tc). The hysteresis loop shrinks with increasing temperature and eventually disappears at the hysteresis critical temperature (Tch), in accord with the appearance of metastable states in a single pore. We estimated the exact pore sizes of our samples from comparing the experimental equilibrium pressure at 77 K with the NLDFT isotherms, where the condensation pressures at 77 K were determined by extrapolating a plot of the equilibrium pressure versus temperature measured above Tch. The pore critical temperature (Tcp) was also determined from the inflection point in a plot of the inverse slope of the adsorption step against temperature. Tch is always lower than Tcp. Both plots of (Tc−Tch)/Tc versus d/rp and (Tc−Tcp)/Tc versus d/rp form straight lines passing through the origin, where d is the molecular diameter, although the former plot deviate from the linear relationship at large pores; Tch approaches Tcp in large pores. This strongly suggests that in large pores the energy barrier from the metastable state to the stable state is too large to be overcome within the period of observations even at the high temperatures just below Tcp.