Porosimetry of a Silica Alumina Powder
Source: Kerry M. Dooley and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA
Surface area and pore s…
1. Starting the porosimeter
- Start the porosimeter and allow it to stabilize.
- Weigh the plastic tube holder, sample tube, glass insert, and plastic valve that screws into the top of the tube.
- 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.
- Using the software for the porosimeter, initialize a new sample by clicking 'File' followed by 'New Sample' and select the appropriate method. Enter both weights (apparatus and apparatus + sample) into the program and rename the sample.
- Load the sample and O-ring into the degas port and adjust the degas conditions as needed. The following program should be followed: the sample should first be heated and evacuated to 12 μm of Hg at a low temperature (90 °C) during the 'Evacuation Phase'. Then, ramp to the desired final temperature (usually 300 °C for inorganic materials and carbons) and hold for the desired time (the "Heating Phase").
- Load the sample tube and O-ring into the sample port. Push upwards on the tube slightly before turning the nut to engage the pin that opens the plastic valve can engage. Make sure to hold the tube vertically.
- Place a heating mantle under the bulb holding the sample tube, and support the heating mantle with a lab jack. Do not wiggle the bulb - hold the mantle firmly.
- Show the degas schematic by clicking under "Degas" and selecting "Show degas schematic". Select "Unit 1", then "Start Degas." Click "Browse" to select your sample file(s), then "Start". The degas phase removes all traces of water and CO2 from the sample before the adsorption experiment. N2 cannot displace both water and CO2! Once the desired pressure and temperature have been reached, degas steps can be skipped.
- When the degas phase reaches the "cool down" step, lower the heating mantle and let the sample tube cool to room temperature.
- Backfill the sample with helium. If the pressure does not approach 800 mm Hg, then the sample tube may have popped out of the fitting. If necessary, hold in place with your hands.
- Once degassing is complete, weigh the sample and apparatus and edit the mass in the sample file.
2. Porosimetry measurement
- Fill the porosimeter's Dewar flask with liquid N2.
- Put the plastic jacket on the sample tube, and load the sample and O-ring in the port above the Dewar flask. Attach the plastic insulating cover on the Dewar near the sample port.
- Click 'Unit 1', then 'Sample Analysis'. Browse for the degassed sample file, then click 'Start' to begin taking measurements.
- Ensure the initial evacuation completes successfully. If that fails, try re-setting the tube in the port (check the O-ring and re-tighten the nut). Measurements will be collected automatically over several hours. Results can be downloaded into an Excel spreadsheet.
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.
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Q1: What is physisorption and how does it work in porosimetry?
Physisorption is the process where gas molecules adhere to a solid surface through weak intermolecular interactions created by delocalizing valence electrons. As more gas molecules physisorb, they form layers and deposit in micropores, mesopores, and capillaries, greatly increasing available surface area. This equilibrium process increases with pressure and reverses into desorption as pressure decreases.
Q2: How does the BET isotherm model describe gas adsorption at low pressures?
The BET isotherm relates adsorbed gaseous volume to monolayer volume and adsorption energy. At low pressures, it assumes gas molecules form sequential monolayers on the solid surface. Above one-third the critical pressure, the adsorbate condenses and is better modeled by the Kelvin Equation for more accurate pore size calculations.
Q3: What are the main components and function of a nitrogen porosimeter?
A porosimeter consists of two chambers connected by a valve: one with a flow-controlled gas inlet and pressure transducer, and another holding the sample cooled by liquid nitrogen. Both connect to a vacuum pump. The device measures nitrogen adsorption at increasing pressures to generate adsorption isotherms, then measures desorption by partial evacuation to calculate surface area and pore size distribution.
Q4: Why is the degas procedure necessary before measuring surface area?
Degassing removes water and carbon dioxide previously adsorbed on the sample surface. Nitrogen cannot adsorb on surfaces already occupied by these molecules, so degassing is essential to expose clean active sites. The procedure involves heating the sample under vacuum at controlled temperatures, typically ramping from 90 degrees Celsius for inorganic materials.
Q5: What does hysteresis in an adsorption isotherm indicate about pore structure?
Hysteresis, where adsorption and desorption curves differ, suggests either meniscus formation late in the adsorption cycle that reduces available surface area, or different meniscus geometries between adsorption and desorption. Differential analysis using the cylindrical Kelvin Equation on hysteresis data reveals pore size distribution and confirms whether pores are cylindrical or another geometry.
Q6: How is surface area calculated from nitrogen adsorption data?
In the low-pressure region where the BET isotherm applies, molar adsorption as a function of pressure is multiplied by the average area occupied by a single nitrogen molecule. Regressing these data according to the BET equation yields the total surface area of the sample, a critical parameter for quality control in catalyst and adsorbent manufacturing.
Q7: What are practical applications of porosimetry in materials science and chemical engineering?
Porosimetry measures surface area and pore characteristics of carbon aerogel foams for catalyst supports and supercapacitors, and evaluates naturally occurring carbonate rocks for carbon capture and storage research. It ensures quality control for materials produced by advanced techniques like sol-gel synthesis and helps determine when adsorbent and heterogeneous catalytic reactor materials reach end-of-life.