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Porosimetry is a technique to measure the surface areas and pore sizes of porous solids. It is commonly used in materials science. For instance, in ceramic manufacture, the surface of both precursor powders and finished pieces exert a strong influence on physical properties. Porosimetry is also useful in chemical engineering. Supported heterogeneous catalysts require large surface area-to-volume ratios to optimize reaction speeds. And adsorbent materials need large surface areas to perform separations. This video illustrates the principles of porosimetry, demonstrates a procedure for surface area and pore size measurements, and discusses related applications.
Adsorption is the process by which fluid molecules adhere and concentrate on the surface of a solid. One type of adsorption, known as physisorption, begins with a gas molecule, the adsorbate, contacting the solid surface, the adsorbent. The valence electrons of the gas atoms delocalize into the orbitals of the solid atoms, creating a weak intermolecular interaction. As more gas molecules physisorb to the surface, they form layers. The adsorbate cannot penetrate the solid, but it can deposit in the micropores, mesopores and capillaries, which greatly increase the surface area available for adsorption. Physisorbtion is an equilibrium phenomenon that increases with pressure and reverses into desorption as pressure decreases. A graph of adsorption as a function of pressure at constant temperature is known as an Adsorption Isotherm. Gasses are best described using the BET Isotherm. It relates the adsorbed gaseous volume to the volume of a gaseous monolayer and a function of the energy released through adsorption. At low pressures, the BET model assumes gas molecules form sequential monolayers on the solid surface. However, above 1/3 the critical pressure, the adsorbate condenses and is better modeled by the Kelvin Equation. Now that we've seen how adsorption works, let's see how it is applied in a porosimeter.
A porosimeter is an analytical device capable of highly automated surface area and pore size measurements. It consists of two chambers connected by a valve. The first chamber contains a flow-controlled gas inlet and a pressure transducer. The second holds the sample of adsorbent and is cooled by liquid nitrogen. Both chambers connect to a vacuum pump. Initially, the chambers are evacuated and the connecting valve closed. Nitrogen gas passes through an inlet and into the first chamber. The molar quantity of nitrogen is determined from the pressure measurement. Next, the valve between the two chambers is opened, and the nitrogen molecules begin adsorbing on the solid. The pressure correspondingly decreases until equilibrium is reached, and the molar adsorption is calculated. Then more nitrogen gas is added to the first chamber, and the cycle repeats. The molar adsorption measurements are then plotted to generate Adsorption Isotherms. To calculate the desorption isotherm, the vacuum pump is used to partially evacuate the chamber, effectively reversing the process. Those are the principles. Now let's examine the operating procedure in the lab.
In this experiment, the surface area and pore size distribution of a silica alumina powder will be measured using a nitrogen porosimeter. Begin by starting the porosimeter and allowing it to stabilize. The sample holder consists of four components. A sample tube. A tube holder. A glass insert. And a plastic valve. Weigh the assembly. Then load the sample into the tube. Use at least 50 milligrams of sample and enough to provide at least 20 square meters of surface area. Seal the sample and weigh it again. Using the control software, initialize a new sample and select a method. Enter the empty and loaded sample holder weights. Apply an O-ring to the sample tube and load the sample into the degas port. The degas steps are needed because nitrogen cannot adsorb on a surface that has already adsorbed water or carbon dioxide. Set the degas vacuum and temperature set points to typical values for inorganic materials, such as a vacuum of 12 microtorr with temperature ramping from 90 degrees Celsius to the desired final temperature. Place a heating mantle under the bulb holding the sample tube and support the heating mantle with a lab jack. Enter the degas schematic. Click unit one. Start degas. Select the sample file and begin. When the degas procedure reaches its cool-down phase, lower the heating mantle holding the sample tube in place, if necessary, and allow the sample tube to cool to room temperature. The degas concludes with the sample tube being back-filled with helium. Weigh the sample tube after degassing is complete. Enter the mass data into the sample file. Using cryogenic safety equipment, fill the porosimeter's Dewar with liquid nitrogen, and attach the plastic insulating cover. Keeping the tube vertical, load the sample tube and O-ring into the sample port until the plastic valve engages. Click unit one, sample analysis. Browse for the sample file for the degassed sample and click start. Ensure the initial evacuation completes successfully. The unit may then be left unattended until measurements are complete.
In this demonstration, nitrogen was adsorbed and desorbed on a silica alumina adsorbent. The isotherms demonstrate hysteresis. This suggests either the formation of a meniscus late in the adsorption cycle that reduces the surface area available for desorption, or different meniscus geometries for the adsorption and desorption cycles. In the low pressure region where the BET Isotherm applies, the molar adsorption as a function of pressure is multiplied by the average area occupied by a single nitrogen molecule to obtain surface area. Regressing these data, according to the BET equation, yields the surface area of the sample. Differential analysis using the cylindrical form of the Kelvin Equation, yields the pore size distribution and suggests the pore geometry is indeed cylindrical.
Porosimetry is routinely used in material science and specialty chemical manufacture. Carbon aerogel foams are highly porous, three-dimensional carbon networks, suitable for catalyst supports and super capacitors. Research is proceeding into new manufacturing techniques, such as sol gel synthesis, which allow high control over surface area. Porosimetry is a necessary part of quality control for the resulting materials. Naturally occurring sub-surface carbonate rocks exhibit surface porosity and adsorb carbon dioxide. However, the adsorption process is affected by the presence of high-pressure fluid in several phases. Porosimetry is used to measure surface area, while x-ray tomography is used to non-invasively study the adsorption process. These studies are needed for the development of carbon capture and storage technologies.
You've just watched Jove's Introduction to Porosimetry. You should now be familiar with the adsorption process, a procedure for measuring surface area, and some applications. As always, thanks for watching.
