硅铝粉的法
English
Share
Overview
资料来源: 凯瑞先生和迈克尔 g. 本顿, 路易斯安那州立大学化学工程系, 巴吞鲁日, LA
表面面积和孔径分布是吸附剂和催化剂制造商和用户使用的属性, 以确保质量控制和确定产品何时结束其有用的生命。多孔固体的表面积直接与其吸附能力或催化活性有关。吸附剂或催化剂的孔径分布是受控的, 这样毛孔大到足以容易地承认分子的兴趣, 但小到足以提供一个高的表面积每质量。
通过等温氮气吸附/解吸技术, 可以测量表面面积和孔径分布。在本试验中, 将采用氮 porosimeter 测量二氧化硅/氧化铝粉末的表面面积和孔径分布。
Principles
Procedure
Results
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 calculated total pore volume, using the ideal gas law and the molar volume of liquid N2, is 0.63 cm3/g.
Figure 2. Volume adsorbed (gas phase basis) vs. relative pressure (the isotherm) for N2 adsorption on silica-alumina S/N 3001.
Two explanations for this effect have been proposed.1 During adsorption, multilayers build up on pore walls, but a complete meniscus is not formed until saturation is reached. Therefore, the surface area for adsorption (the wall surfaces) exceeds that of desorption (meniscus only) in the capillary condensation region. The adsorption branch of the isotherm is therefore governed by a multilayer isotherm analogous to the BET equation, but desorption in the capillary condensation region is governed by the Kelvin equation (Equation 5). The differences in the branches could also arise from a difference in the shape of the meniscus. During adsorption the pore fills radially and a cylindrical meniscus is formed. During desorption, the meniscus is hemispherical and the Kelvin equation applies. By either argument, only the desorption isotherm should be used to calculate the pore size distribution in the hysteresis region, although neither argument is entirely correct. Disagreements from theory arise from deviations in the shape of the pores from simple cylindrical geometry and from the fact that transmission electron microscopy and other techniques suggest that BOTH explanations are partly correct. In particular, the physics of adsorption dictates that there must be some adsorbed material in the pores, clinging to the walls even below the Kelvin Pv/P0 = P/P0. The space occupied by this multilayer adsorbed material must be corrected for in the Kelvin pore size calculation – this is called a "t-plot correction" in the literature. Various theoretical equations can be used to calculate this adsorbed layer thickness (= t), as a function of P/P0. For our system, the Halsey-Faas correction to obtain t is used automatically in the machine software, and the pore size distribution for both branches computed automatically.
The BET equation yields a straight line when plotted linearly (see Equation 3), to give Vm and c from the slope and intercept. The specific surface area, A, is found assuming the average area occupied by one molecule of adsorbate (am) is known (for N2, 0.1620 nm2):1
(6)
where Vm [=] cm3/g, A [=] m2/g, and L = Avogadro's number.
A typical BET plot (Figure 3) showing data and regression fit is shown below. The value of R2 (correlation coefficient) and the average relative deviation of the fit are reported. The confidence limits on the slope and intercept from the linear regression can be used to estimate the confidence limit on A, from propagation of error theory. The regressed (predicted) values are: c = 139, Vm = 49.3 cm3/g STP, A = 214 m2/g, R2 = 0.9998, ARD = 0.59%.
Figure 3: BET plot for sample silica-alumina S/N 3001.
A typical pore size distribution for the same sample is shown in Figure 4. For this sample, the average pore D from the distribution was calculated as 8.6 nm, while the cylindrical pore estimate (4 PV/A) was 8.0 nm (PV is pore volume/mass). This is fairly good agreement, suggesting that these pores are roughly cylindrical. Using both the adsorption and desorption dV/dD the average pore diameter can be determined from the properties of a probability distribution. Note that (dV/dD) x (dD) is the probability of adsorbed volume, expressed as a gas phase volume, at D. The desorption average, D, is always smaller than the adsorption average, as predicted by Equation 5. This is because, as shown in Figure 2, its fugacities (Pv, Pv = P) at a given Vads are smaller.
Figure 4. Calculated pore-size distribution of the data in Figure 2, desorption branch.
Applications and Summary
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 information on pore shape.
Commercial catalysts and adsorbents are often manufactured to tight pore size specifications. The fastest way to determine if the correct morphology is present is to measure the pore size distribution. For example, uneven temperature control in the calcining (heat treatment) step during manufacture can greatly alter the distribution. For many catalysts, lifetimes are greatly shortened if larger pores are not present, even if the surface area is still high, because these large pores often serve as gateways for the removal of oligomeric carbon residues ("coke") that otherwise would poison many active sites.
For cylindrical pores the average pore diameter, D, should also equal 4PV/A (PV is pore volume/mass). The software reports the cylindrical estimates for both adsorption and desorption branches, and the magnitude of their differences from the average diameters calculated from the distributions themselves gives an idea of the porous material's deviation from perfectly cylindrical pores. Some solids have pores that are slit-like, with a critical short dimension (h) analogous to the diameter of 2 PV/A for very long and wide slit pores. Find both the adsorption and desorption estimates of the average, and then determine if the porous solid is more slit-like, comparing the different branch estimates to more exact values generated from the probability distributions. If both cylindrical and slit estimates are significantly in error, what could this mean? Similar calculations could be done to test for other pore shapes.
Porosimeters can be easily adapted to measure surface areas as small as 0.01 m2/g (e.g., in concretes, although Kr or Xe are used instead of N2) and pore sizes less than 1 nm (e.g., in zeolites, although Ar is used and special procedures necessary). While it is true that zeolites are important catalysts and commercial adsorbents, their primary use is in detergents, where they can bind almost all the grime removed from laundry.
Additionally, it is also important to know the morphological properties of tableting materials, such as excipients (lubricants) and binders, to control the pill tableting process and the dissolution and degradation of outer shells to ensure controlled release of the active pharma ingredient in vivo.
References
- 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.
- J. Amer. Chem. Soc., 60, 309-319 (1938).
Transcript
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.