Adsorption behaviors of nitrogen on Ti doped SBA-15

Thermodynamic properties of nitrogen on Ti doped santa barbara amorphous No. 15 samples having large pore sizes in an order of a few micrometers were studied by analyzing the adsorption isotherm data obtained near the triple point (63.15 K). A series of adsorption data as a function of the normalized pressure revealed that nitrogen forms two distinctive isotherm steps. The first step represents the adsorption behavior on surface while the second step is believed to be responsible for the interaction of gas molecules in a capillary, so called a 'capillary condensation.' The existence of a hysteresis measured between adsorption and desorption isotherms at 77.3 K evidenced that gas molecules fill the pores in the sample. Plots of 2-dimensional compressibility values that were calculated in terms of change of adsorbed molecules with respect to the change of measured pressure exhibit a prominent peak near the second isotherm step supporting that the pores are filled with adsorbed molecules. A careful examination of the second isotherm step revealed that the amount of adsorbed molecules increased at temperatures below the triple point while it decreased above the triple point, and such a phenomenon is responsible for the geometrical confinement of gas molecules in porous materials.