Source: Kerry M. Dooley and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA

Surface area and pore size distribution are attributes used by adsorbent and catalyst manufacturers and users to ensure quality control and to determine when products are at the end of their useful lives. The surface area of a porous solid is directly related to its adsorption capacity or catalytic activity. The pore size distribution of an adsorbent or catalyst is controlled such that pores are large enough to easily admit molecules of interest, but small enough to provide a high surface area per mass.

Surface area and pore size distribution can be measured by the technique of isothermal nitrogen adsorption/desorption. In this experiment, a nitrogen porosimeter will be used to measure the surface area and pore size distribution of a silica/alumina powder.

1. Starting the porosimeter

1. Start the porosimeter and allow it to stabilize.
2. Weigh the plastic tube holder, sample tube, glass insert, and plastic valve that screws into the top of the tube.
3. Then load the sample in the tube and re-weigh. When loading, try to get at least 20 m² of total surface area in the tube. Look up a typical surface area range for the type of solid you are using. However, never use less than 50 mg of sample.
4. Using the software for the

In the capillary condensation region, the isotherm generally shows hysteresis so that the apparent equilibrium pressures observed in adsorption and desorption experiments are different (Figure 2). The desorption branch is always at lower fugacity and pressure. The hysteresis begins at P/P₀ = ~0.6, where capillary condensation begins to dominate the adsorption process, although the pore size distribution algorithm uses the entire isotherm. The calcu

The method of measurement and calculation presented here is the gold standard in porosimetry. The mercury porosimetry technique is an alternative, but its high pressures and possibility of exposure to mercury are disadvantages. Better pressure transducers, vacuum pumps, and software have greatly extended the utility of N₂ porosimetry, and the method gives all 3 key adsorbent or catalyst morphological measurements (A, pore volume, pore size distribution) in one experiment. It also provides info

1. Gregg and K.S.W. Sing, Adsorption, Surface Area, & Porosity, 2nd Ed., Academic, 1982 , and D. Ruthven, Principles of Adsorption and Adsorption Processes, Wiley, New York, 1984.
2. J. Amer. Chem. Soc., 60, 309-319 (1938).