Thermodynamic and structural properties of physisorbed phases within the model mesoporous adsorbent M41S (pore diameter 2.5nm)

An M41S sample with a pore diameter of 2.5 nm has been characterised by the physisorption of various probe molecules (N-2, CO, D-2, CH4, CD4, Ar and Kr). This gives rise to a distinct step in the adsorption isotherm as if a capillary condensation mechanism occurs for these adsorbates. However, not all of the isotherms present a hysteresis in the desorption branch of the isotherm as would be expected for a capillary condensation-type mechanism. Thermodynamic (isothermal microcalorimetry) studies reveal an output of differential enthalpy around \1-2.5kJmol(-1)\ above the enthalpy of liquefaction during this step, Furthermore, in the case of krypton, a ''solidification'' is suspected. A structural (neutron diffraction) study however, indicates that the adsorption of deuterium and methane is characterised by short range order, even at 3 K.