Adsorption of simple gases into the porous glass MCM-41.

Abstract

The porous glass MCM-41 is an important adsorbent to study the process of adsorption of gases onto a cylindrical surface. In this work, we study the adsorption of oxygen, nitrogen, deuterium, and deuteriated methane gases into MCM-41 using a combination of neutron diffraction analysis and atomistic computer modeling to interpret the measured data. Adsorption is achieved by immersing a sample of MCM-41 in a bath of the relevant gas, keeping the gas pressure constant (0.1 MPa), and lowering the temperature in steps toward the corresponding bulk liquid boiling point. All four gases have closely analogous behaviors, with an initial layering of liquid on the inside surface of the pores, followed by a relatively sharp capillary condensation (CC) when the pore becomes filled with dense fluid, signaled by a sharp decrease in the intensity of (100) Bragg diffraction reflection. At the temperature of CC, there is a marked distortion of the hexagonal lattice of pores, as others have seen, which relaxes close to the original structure after CC, and this appears to be accompanied by notable excess heterogeneity along the pore compared to when CC is complete. In none of the four gases studied does the final density of fluid in the pore fully attain the value of the bulk liquid at its boiling point at this pressure, although it does approach that limit closely near the center of the pore, and in all cases, the pronounced layering near the silica interface seen in previous studies is observed here as well.