Methane Adsorption in Model Mesoporous Material, SBA-15, Studied by Small-Angle Neutron Scattering

The understanding of methane adsorption is important for many industrial applications, especially for the shale gas production, where it is critical to understand the adsorption/desorption of methane in pores even as small as a few nanometers. Using small-angle neutron scattering (SANS), we have studied the adsorption of deuterated methane (CD4) into one model mesoporous material, SBA-15, with pore diameter approximately 6.8 nm at the temperature range from 20 to 295 K at low pressure (approximate to 100 kPa). A new scattering model is developed to analyze the SANS patterns of gas adsorption in SBA-15. The surface roughness of the SBA-15 matrix is estimated. The gas adsorption behaviors on the surface regions are extracted from the fitting. The rough surface of the pores is found to retain a large amount of CD4 at the temperature above the capillary condensation temperature (T-c). At temperatures below T-c, the confined liquid and solid methane are estimated to be less dense than the corresponding bulk liquid and solid methane. Detailed theoretical analysis and experimental verification also show that SANS patterns at temperatures higher than T-c are much more sensitive to the change of the excess adsorption, epsilon(ads), rather than the average density of adsorbed layers commonly used in many studies. The model we establish can be used to analyze future SANS/SAXS data for gas confined in similar model porous materials.