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 b y 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 iso therm as if a capillary condensation mechanism occurs for these adsorb ates. However, not all of the isotherms present a hysteresis in the de sorption branch of the isotherm as would be expected for a capillary c ondensation-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 structu ral (neutron diffraction) study however, indicates that the adsorption of deuterium and methane is characterised by short range order, even at 3 K.