Determination of specific surface area and pore size distribution by isothermal adsorption and desorption of nitrogen - Master's thesis - Dissertation

â—†The six types of adsorption isotherms are mentioned in almost every similar reference book. The first five are BDDT (Brunauer-Deming-Deming-Teller) classification. The four people first classify a large number of isotherms into five types, stepped. The sixth category is Sing. Each type will have a set of arguments. In fact, it can be understood that the relative pressure is the X axis, the nitrogen adsorption amount is the Y axis, and the X axis relative pressure is roughly divided into the low pressure (0.0-0.1) and the medium pressure (0.3- 0.8), high pressure (0.90-1.0) three segments. Then the adsorption curve is at:
The Y-axis of the low-pressure end indicates that the material has strong interaction with nitrogen (I type, II type, IV type). When there are more micropores, due to the strong adsorption potential in the micropores, the adsorption curve starts at the beginning; The end-off X-axis description is weak with the material force (? type, V type).
The medium pressure end is mostly the condensation accumulation of nitrogen in the pores of the material. The mesoporous analysis is derived from this data, including the pores generated by the accumulation of sample particles, and the pores in the ordered or gradient mesopores. The BJH method is based on the aperture data obtained from this section;
The high-pressure section can roughly see the degree of particle accumulation. If the type is finally raised, the particles may not be uniform. The usual total pore volume is usually the condensation value of the nitrogen adsorption amount when the relative pressure is about 0.99.
â—† a few constants
1. When the liquid nitrogen temperature is 77K, the liquid nitrogen hexagonal densely packed nitrogen molecular cross-sectional area is 0.162 square nanometer. When the monolayer is spread, the monolayer thickness is considered to be 0.354nm.
2. Under the condition (STP), the volume of 1 mL of nitrogen is agglomerated (assuming the same density of cohesiveness) is 0.001547 mL.
For example, if the nitrogen adsorption capacity is about 400 mL when the adsorption curve p/p0 is the maximum in the drawing, the total pore volume is 400*0.001547=400/654=about 0.61mL.
3. STP per mL of nitrogen molecules laid into a monolayer occupied area of ​​4.354 square meters. The specific surface area obtained by the BET method is S / (square meters per gram) = 4.354 * Vm, where Vm is treated by the BET method to know that Vm = 1 / (slope + intercept)
Take the adsorption isotherm of SBA-15 molecular sieve as an example to illustrate that this isotherm belongs to type IV and H1 hysteresis loop in IUPAC classification. It can be seen from the figure that the adsorption amount increases gently in the low pressure section. At this time, the N2 molecules are adsorbed on the inner surface of the mesopores from a single layer to a plurality of layers, and the relative pressure is taken when the specific surface area is calculated by the BET method for the ordered mesoporous materials. /p0 = 0.10~0.29 is more suitable. There is a sudden increase in the adsorption amount at p/p0 = 0.5~0.8. The position of the segment reflects the size of the sample pore size, and its variation width can be used as a basis for measuring the uniformity of the mesopores. At higher p/p0, there is sometimes a third rise in the sample, which can reflect the large holes or particle accumulation holes in the sample. The specific surface area, pore volume and pore size distribution can be determined by N2-sorption and isotherm. The analysis of its specific surface area generally employs the BET (Brunauer-Emmett-Teller) method. The pore size distribution is usually in the BJH (Barrett-Joiner-Halenda) model.
â—†Kelvin equation
The Kelvin equation is the basis of the BJH model. The diameter derived from the Kelvin equation plus the liquid film thickness is the pore diameter. The radius of curvature of the curved surface is R'=2γVm/[RT*ln(p0/p)]. If the aperture R generated by the curved liquid surface is to be calculated, then R'Cosθ=R, because the contact angle θ of different materials is different, The figure shows the curvature radius R' and the relative pressure p/po corresponding to the contact angle without considering the contact angle:

â—†hysteresis loop
1. The reason for the hysteresis loop is that the capillary condensation causes the N2 molecules to condense and fill the mesoporous pores below the normal pressure, and the capillary condensation occurs at the liquid surface of the annular adsorption membrane on the pore walls. The desorption is started from the spherical meniscus of the orifice, so that the adsorption and desorption isotherms do not coincide, often forming a hysteresis loop. There is another saying that the contact angle between liquid nitrogen entering the channel and the material during adsorption is the advancing angle, and the retreat is the receding angle. These two angles lead to differences in the Kelvin equation. Of course, there may be a combination of the two, individuals tend to identify the former, at least intuitively (Xuan Ji?) the former can make sense.
2. Types of Hysteresis Loops The characteristics of the hysteresis loop correspond to specific pore structure information, and this comparison is tested to understand the Kelvin equation.
H1 is a uniform hole model that can be considered as a straight hole for easy understanding. However, some students will say that the S1-15 has an ordered hexagonal mesoporous structure from the H1 hysteresis loop, which is a mistake. The H1 type hysteresis loop can be seen as an ordered mesopores, but it is not known whether it is hexagonal, tetragonal, or triangular. The hexagonal is what the small angle XRD sees. This is the obvious Zhang Guan Li Dai;
H2 is more difficult to explain. It is generally considered to be caused by porous adsorbate or uniform particle packing pores. It is considered to be “ink bottle”. After liquid nitrogen desorption in a small pore size bottleneck, the liquid nitrogen gas bound in the bottle will suddenly escape. ;
H3 has a larger adsorption amount at the high pressure end than H4, and is considered to be a slit hole formed by the accumulation of flaky particles;
H4 is also a slit hole, which is different from particle stacking and is a hole similar to that produced by a layered structure.
3. When the medium pressure part has a large adsorption amount but does not produce a hysteresis loop, when the relative pressure is about 0.2-0.3, according to the Kelvin equation, the pore radius is small, and the effective pore radius is only a few adsorbent molecules. Capillary agglomeration occurs, and the adsorption and desorption isotherms coincide. When the pore size of MCM-41 is 2 or 3 nm, there is no hysteresis loop in the ordered mesoporous adsorption and desorption.
â—†The mesoporous analysis usually adopts the BJH model (Barrett-Joiner-Halenda), which is the application of the Kelvin equation in the cylinder model. It is suitable for the mesoporous range, and the obtained result is smaller than the actual one.
KJS (Kruk-Jaroniec-Sayari) with higher precision for MCM-41 and SBA-15 pore structure analysis and its modification method. When KJS came out, the highly ordered MCM41 was used as the material for pore analysis, combined with XRD results. There is a higher precision KJS equation than BJH, and the applicable pore size analysis range is between 2 and 6.5 nm. Later, it was promoted to make it have a larger scope of application, which can be used for the characterization of SBA-15 pore structure (4.6-30nm).
◆About the t-Plot and αs methods is the treatment method for the whole adsorption or desorption curve. t-Plot can be understood as the thickness pattern method, and the adsorption amount of the monolayer is plotted by the nitrogen adsorption amount, and the adsorption film formed during the aggregation is formed. The average thickness is the average number of adsorbed layers multiplied by the thickness of the monolayer (0.354 nm), specific surface area = 0.162 * monolayer adsorption amount * Avogadro constant. When the sample is a non-porous material, t-Plot is a straight line passing through the origin. When the sample contains micropores, mesopores, and large pores, the straight line becomes several fold lines, which need to be analyzed separately. The subscript in the αs method is standard. Sing proposes to use the amount of adsorption at a relative pressure of 0.4 instead of the amount of adsorption of the monolayer, and then to map, using this method to specify a standard or to do it on the instrument. A standard, processing method and graphical interpretation are similar. The two can be transformed into each other, t=0.538αs
â—†Microporous analysis Microporous analysis of microporous materials has different requirements for vacuum degree, control system and temperature sensor. The test time is also longer, and the time may be ten times or even twenty times that of ordinary samples. Due to the limited difference between micropore size and probe molecular size, some microporous probe molecules can not enter, the analytical method should be determined according to different samples. If necessary, refer to relevant literature methods for reference, and then do a batch of samples. It is an analytical method, and the trend of the results is mostly correct. It is still difficult to analyze the pore size distribution of all ranges with a model. The theory of nonlinear density generalized theory (NLDFT) is ok, but it is used less in the paper.