Physisorption (physical adsorption) is used to measure specific surface area and pore size. As gas molecules are introduced into a sorption chamber, adsorbate molecules form a thin layer on the adsorbent surface (monolayer). Using the Brunauer, Emmett and Teller (B.E.T.) theory, the number of molecules required to cover the adsorbent surface with a monolayer of adsorbed molecules, Nm, can be calculated. Multiplying Nm by the cross-sectional area of an adsorbate molecule produces the sample's surface area. As more gas molecules are added on top of the monolayer, multiple layers form. This process occurs in parallel to capilliary condensation.
Capilliary condensation can be described by the proportionality between residual/equilibrium gas pressure and the size of capillaries capable of condensing gas within them. Computational methods such as Barrett, Joyner and Halenda (BJH), or Density Function Theory (DFT) allow the determination of pore sizes from equilibrium gas pressures. This information can be used to create isotherms which link adsorbed gas volumes with relative saturation pressures at equilibrium. These can be converted to cumulative or differential pore size distributions.
In house: Gemini VII 2390 Micromeritics and Autosorb-1 Quantachrome
Combines gas detection by high sensitivity, thermal conductivity detector (TCD) for TPR/TPD/TPO analysis and automatic physisorption and chemisorption by precise vacuum volumetric method for analysis of BET surface area, meso- and micropore size distribution, active surface area, degree of metal dispersion, heats of adsorption, etc. Temperature programmed techniques (TPR/TPD/TPO) automatically performed via user programmable pretreatment and analysis procedures including multi-step temperature ramping up to 1,100 deg C, automatic gas switching and automated data acquisition.
Gas volume range: 1 x10-3 cc to > 1 x102 cc
Reproducibility: typically better than 0.5%
Sensitivity: 10 positions (dynamic range: 512) plus user selectable detector current
TPR/TPD heating rates: 1°C/min to 100°C/min (up to 500°C); 1°C/min to 50°C/min (up to 750°C); 1°C/min to 30°C/min (up to 1000°C); 1°C/min to 20°C/min (up to 1100°C)
Furnace controller: PID, featuring auto-tuning
Sample thermocouple: positioned alongside sample
TCD filament: tungsten-rhenium, diffusion-type, oxidation resistant
Operating environment: 20 - 80% humidity, 15° to 35° C