Gas Adsorption for Surface Area (BET) determination and Pore Size Analysis (BJH)

Gas Adsorption Test for Surface Area and Pore Size Analysis

Gas Adsorption using BET (after Brunauer, Emmett and Teller) and BJH (after Barrett, Joyner, and Halenda) methods is used to obtain specific surface area, pore size distribution and pore volume of adsorbents. The method is based upon analyzing the physical adsorption and desorption isotherms of a suitable gas in monolayer and multilayer adsorption regions. The most common gas used is Nitrogen though Argon or Krypton may be used, in case more sensitive measurements are needed.

The experiment is commenced by completely removing contamination from the active site by applying heat and vacuum. The adsorbent is placed in an analysis tube under vacuum and cooled to the temperature of liquid nitrogen (77 K). Nitrogen gas is charged to the solid in controlled increments, starting from low pressures. After each increment, the system is allowed to reach steady state and the volume of adsorbed gas is measured. Pressure increments and equilibrium measurements are continued till the saturation pressure is reached and adsorption ceases. An isotherm is plotted showing volume uptake versus relative pressure (ratio of actual to saturation pressures). The low pressure region corresponds to monolayer adsorption while the higher pressure regions show a steep rise in uptake and represent multilayer adsorption. The monolayer segment of the isotherm is used to determine the surface area by the BET method. The BET equation provides a theoretical relationship between volume of adsorbed gas, volume of gas required to form a monolayer and the system pressure. Using the measurements of adsorbed volume in the monolayer segment, the BET surface area can be calculated, knowing Nitrogen molecular size and the BET monolayer volume. The actual surface area correlates very well with the BET surface area.

The BJH method is a procedure for calculating pore volumes and pore size distributions from experimental isotherms, using the Kelvin equation.  It applies only to the mesopore and small macropore size range. The Kelvin equation provides a correlation between pore diameter and pore condensation pressure.

To determine the BJH pore volume and pore size distribution, the Nitrogen pressure is increased incrementally followed by measurement of adsorbed volume, in the multilayer adsorption region. At higher relative pressures, capillary condensation occurs and the pores get filled with liquid Nitrogen. The process is then reversed by reducing the relative pressure and measuring desorption equilibria. The adsorption and desorption curves may not coincide and this is termed hysteresis. For mesopores (2 nm to 50 nm diameter), the pressure where the condensation occurs depends on the pore diameter. The pore size distribution is calculated from the desorption isotherm.

Video 01: Introduction to gas adsorption analysis, pore size, and BET surface area measurement