Comparing Laser Diffraction and BET surface area measurements

BET surface area, or gas absorption, measurement techniques are used to measure the surface area and porosity of the particles present in a sample. Molecules of an adsorbate gas are physically adsorbed onto the particle surfaces, including the internal surfaces of any pores, under controlled conditions within a vacuum chamber. An adsorption isotherm is obtained by measuring the pressure of the gas above the sample as a function of the volume of gas introduced into the chamber. The linear region of the adsorption isotherm can then be used to determine the volume of gas required to form a monolayer across the available particle surface area, using BET theory, as described by the following equation:

where ν is the volume of gas, P is the pressure, P0 is the saturation pressure, νm is the volume of gas required to form a monolayer and c is the BET constant. Plotting relative pressure, φ (=P/P0), and volume, as represented in Figure 1, allows the volume of a monolayer to be determined from the gradient and intercept of the line. The specific surface area can then be calculated using the cross sectional area of the gas molecules, the molecular volume of the gas and the weight of the sample.

Figure 1: BET plot

The specific surface area can also be calculated from the results of a laser diffraction measurement using the following equation:

where Vi is the relative volume in class i with a mean class diameter of d_i, ρ is the density of the material, and D[3,2] is the surface area weighted mean diameter. This can be carried out automatically within the laser diffraction system software, providing a means of rapidly estimating the particle surface area. In carrying out this calculation, it is assumed that the particles are perfectly smooth, solid spheres.

The difference between the surface area measured by BET surface area measurements and calculated from laser diffraction will depend on the porosity and surface roughness of the particles. If the particles are porous, or have a rough surface structure, the BET surface area will be greater than that calculated by laser diffraction. This is demonstrated in Figure 2, where BET and laser diffraction for a range of catalyst support materials are compared. Although the values obtained are different, it is clear that a good correlation exists between the two techniques. As such laser diffraction can be used to predict changes in the SSA during product processing.

Laser diffraction will also allow the relationship between the particle size distribution and surface area to be investigated. As the particle size distribution moves to finer particle sizes, so the specific surface area of the particles will increase. This is shown clearly in Figure 3, where the Dv90 (a measure of the upper limit of the size distribution) measured by laser diffraction is plotted against the BET surface area.

Figure 2: Comparison of SSA measured by BET and Laser diffraction

Figure 3: SSA measured by BET vs Dv90 measured by laser diffraction