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 (≈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,
εads, 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.
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