bet presentatie 2008-final

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1. Welcome everybody to this lecture about BET. As this is a basic talk about what you can do with it, I want to ask Bert to put this lecture on the web, under my name. So that you can have a look at it any time. I prefer questions after the talk, as the different subjects overlap things might become clear later. Also at my room I’m always willing to answer questions whenever they come up. Welcome everybody to this lecture about BET. As this is a basic talk about what you can do with it, I want to ask Bert to put this lecture on the web, under my name. So that you can have a look at it any time. I prefer questions after the talk, as the different subjects overlap things might become clear later. Also at my room I’m always willing to answer questions whenever they come up.

2. BET is a method that is mainly used to calculate the Surface Area of a compound. This is an important value because it says something about the properties of a sample. The method can be extended so that eventually more properties can be detect. These are the topics I want to tell about. And more or less in this order. BET is a method that is mainly used to calculate the Surface Area of a compound. This is an important value because it says something about the properties of a sample. The method can be extended so that eventually more properties can be detect. These are the topics I want to tell about. And more or less in this order.

3. BET: Principle I will start telling about the principle of this method. The sample is put in a tube and first of all it has to be degassed to get rid of all things that are adsorbed already, like for instance water. Degassing is done at a high temperature, mostly 300 degrees Celsius during a certain time. Subsequently the tube is connected to the analysis station and the measurement starts by cooling down and evacuating of the system. Then the measurement starts: A certain amount of nitrogen is added and the system waits until equilibrium. Then the pressure is measured. Another amount is added and again the pressure is measured. As long as the pressure doesn’t increase it means that the nitrogen adsorbs on the surface of the sample. When the pressure increases it means that surface is covered completely with nitrogen. After some time atmospheric pressure is reached and than the procedure is repeated the other way around. So nitrogen comes off the sample and pressure is measured after equilibrium If the complete system is vacuum again the measurement is finished. It ends up with a graph that looks like this. The x-ax shows the relative pressure and the y-ax the quantity that is adsorbed. I will start telling about the principle of this method. The sample is put in a tube and first of all it has to be degassed to get rid of all things that are adsorbed already, like for instance water. Degassing is done at a high temperature, mostly 300 degrees Celsius during a certain time. Subsequently the tube is connected to the analysis station and the measurement starts by cooling down and evacuating of the system. Then the measurement starts: A certain amount of nitrogen is added and the system waits until equilibrium. Then the pressure is measured. Another amount is added and again the pressure is measured. As long as the pressure doesn’t increase it means that the nitrogen adsorbs on the surface of the sample. When the pressure increases it means that surface is covered completely with nitrogen. After some time atmospheric pressure is reached and than the procedure is repeated the other way around. So nitrogen comes off the sample and pressure is measured after equilibrium If the complete system is vacuum again the measurement is finished. It ends up with a graph that looks like this. The x-ax shows the relative pressure and the y-ax the quantity that is adsorbed.

4. BET: Brunauer, Emmett, Teller And this is the equipment we use. Here’s the degas system: temperature can be increased until 400 degrees Celsius while nitrogen is flushed. It can handle 6 samples at the same time The analyzing station can do 3 samples in one run. And this is the equipment we use. Here’s the degas system: temperature can be increased until 400 degrees Celsius while nitrogen is flushed. It can handle 6 samples at the same time The analyzing station can do 3 samples in one run.

5. Pores are classified so that everybody talks about the same: Micro pores are smaller than 2 nm; macro pores are bigger than 50 nm; in between are mesopores Schematically it looks like this: Micro, meso and macro pores as well as ink bottle pores. Between particles there are interparticle voids, shown here With our equipment all pores are measured, also the interparticle voids, as long as the size is within the measured range that can be measured. Pores are classified so that everybody talks about the same: Micro pores are smaller than 2 nm; macro pores are bigger than 50 nm; in between are mesopores Schematically it looks like this: Micro, meso and macro pores as well as ink bottle pores. Between particles there are interparticle voids, shown here With our equipment all pores are measured, also the interparticle voids, as long as the size is within the measured range that can be measured.

6. Pore filling Pore filling works like this: At low pressure the sample begins to adsorb nitrogen at higher pressure first a monolayer is formed after that a multi layer starts to appear at a certain moment all pores are filled It gives a graph like this. To make this complete I draw a desorption branch that arises by evacuating the system again In practice the filling of the pores is not so ideal as it is shown here. A second layer starts before the area is completely covered The BET equation, which is developed by the gentlemen Brunauer, Emmett and Teller, enables to calculate a Surface Area in m2/g which has become known as the BET Surface Area. Pore filling works like this: At low pressure the sample begins to adsorb nitrogen at higher pressure first a monolayer is formed after that a multi layer starts to appear at a certain moment all pores are filled It gives a graph like this. To make this complete I draw a desorption branch that arises by evacuating the system again In practice the filling of the pores is not so ideal as it is shown here. A second layer starts before the area is completely covered The BET equation, which is developed by the gentlemen Brunauer, Emmett and Teller, enables to calculate a Surface Area in m2/g which has become known as the BET Surface Area.

7. It’s this equation which results in the graph below Practically I’ll show here a BET graph of the silica alumina reference sample Using the BET equation the Surface Area is calculated, from this range of relative pressure. The BET surface area comes with a value of 215 m2/g Notice the C-value; this should not be negative!! In fact this C-value tells also something about the sample: It’s this equation which results in the graph below Practically I’ll show here a BET graph of the silica alumina reference sample Using the BET equation the Surface Area is calculated, from this range of relative pressure. The BET surface area comes with a value of 215 m2/g Notice the C-value; this should not be negative!! In fact this C-value tells also something about the sample:

8. A high C-value means a strong interaction from N2 with surface and a low C –value represent a weak interaction However: it never can be negative. If it shows negative, the reason can be that the BET range is not chosen well Another possibility is that there is no multilayer, but monolayer gas adsorption, which is described by the Langmuir model In the next slides I will explain what consequences this has on the basis of some examples. A high C-value means a strong interaction from N2 with surface and a low C –value represent a weak interaction However: it never can be negative. If it shows negative, the reason can be that the BET range is not chosen well Another possibility is that there is no multilayer, but monolayer gas adsorption, which is described by the Langmuir model In the next slides I will explain what consequences this has on the basis of some examples.

9. C-value Here’s a BET plot with a very high negative C-value. In this case it is possible to adjust the BET range by leaving out the highest point of the pressure range And this ends up with a high positive C-value Calculating the Surface area doesn’t show such a big difference; it’s within the error of the measurement Here’s a BET plot with a very high negative C-value. In this case it is possible to adjust the BET range by leaving out the highest point of the pressure range And this ends up with a high positive C-value Calculating the Surface area doesn’t show such a big difference; it’s within the error of the measurement

10. What to do now? In this example there’s a rather big negative intercept. Leaving out some points doesn’t make it positive. This behavior is best described by the Langmuir monolayer adsorption. The problem is, that this Surface Area is far not the same as the BET Surface Area So actually the Langmuir Surface Area is the best value. However, in literature mostly BET values are used. For me it’s best to give both calculations and values, so that you can decide for yourself what is best to use 3 point for BET results in a BET SA 423.7 m2/g; however C-value is still negative (sample = 2007-287)In this example there’s a rather big negative intercept. Leaving out some points doesn’t make it positive. This behavior is best described by the Langmuir monolayer adsorption. The problem is, that this Surface Area is far not the same as the BET Surface Area So actually the Langmuir Surface Area is the best value. However, in literature mostly BET values are used. For me it’s best to give both calculations and values, so that you can decide for yourself what is best to use 3 point for BET results in a BET SA 423.7 m2/g; however C-value is still negative (sample = 2007-287)

11. Now back to the complete isotherm Other calculations can be performed as well First: The start of an isotherm says something about micro pores: are they present or not. If the starting point is high, definitely they are present. Although this equipment cannot distinguish between micro and mesopores, valuable information about micropores can be achieved from this part of the isotherm. This part of the isotherm is used to extrapolate to the lower pressure range and in that way it’s possible to have an idea about the micropores Now back to the complete isotherm Other calculations can be performed as well First: The start of an isotherm says something about micro pores: are they present or not. If the starting point is high, definitely they are present. Although this equipment cannot distinguish between micro and mesopores, valuable information about micropores can be achieved from this part of the isotherm. This part of the isotherm is used to extrapolate to the lower pressure range and in that way it’s possible to have an idea about the micropores

12. Now something about the T-plot, that says something about the micropores. I choosed this sample because it contains micro as well as meso pores The BET Surface Area Value (14.5 cm2/g) represents all pores together Performing the T-plot it’s possible to distinguish between micro and meso pores: a calculation is performed in which an extrapolation to this micro area is done. A tangent is drawn like this and the intercept is a measure for the micro pores present. In the list now these both values are seen: in this case about 3 m2/g for the micropores and about 11 for the mesopores. Together it’s 14 m2/g, The value from the BET area Now something about the T-plot, that says something about the micropores. I choosed this sample because it contains micro as well as meso pores The BET Surface Area Value (14.5 cm2/g) represents all pores together Performing the T-plot it’s possible to distinguish between micro and meso pores: a calculation is performed in which an extrapolation to this micro area is done. A tangent is drawn like this and the intercept is a measure for the micro pores present. In the list now these both values are seen: in this case about 3 m2/g for the micropores and about 11 for the mesopores. Together it’s 14 m2/g, The value from the BET area

13. Next I want to tell about pore size distribution I’ll show again the ideal Silica-Alumina reference material The best result for the distribution plots is given by the desorption branch, Next I want to tell about pore size distribution I’ll show again the ideal Silica-Alumina reference material The best result for the distribution plots is given by the desorption branch,

14. And here are the distribution spectra. The main purpose of the red graph is the value from the top: this represents the area of most of the pores. In this case 83 Angstrom The green graph shows the cumulative pore area The same you can tell from the pore volume. So in this case is the diameter of most of the pores about 83 Angstrom, again the value from the X-ax. The volume is given by the area under the peak And the cumulative pore volume is about 0.6 m3/g And here are the distribution spectra. The main purpose of the red graph is the value from the top: this represents the area of most of the pores. In this case 83 Angstrom The green graph shows the cumulative pore area The same you can tell from the pore volume. So in this case is the diameter of most of the pores about 83 Angstrom, again the value from the X-ax. The volume is given by the area under the peak And the cumulative pore volume is about 0.6 m3/g

15. Another example is this SiO2 sample: In the isotherm you can see that there are big pores as the hysterese is near to relative pressure = 1 The pore volume plot shows that most of the pores are around 500 Angstrom Big pores mostly go together with small area, which is seen in this graph: goes to smaller than 20 Angstrom Another example is this SiO2 sample: In the isotherm you can see that there are big pores as the hysterese is near to relative pressure = 1 The pore volume plot shows that most of the pores are around 500 Angstrom Big pores mostly go together with small area, which is seen in this graph: goes to smaller than 20 Angstrom

16. Although it’s preferable to take the desorption branche of an isotherm, caution has to be taken when a sample contains so called ink-bottle pores. If the pressure becomes lower the whole pores are emptied at once which causes a sudden drop of the adopted amount There’s a sudden closure of the hysteresis because of the adsorbed quantity. This causes an artifact in the desorption pore volume and area graphs. Actually I see that quite often This peak always appears at around 40 Angstrom. So if you see a peak in the desorption spectra, that does not appear in the adsorption spectra, it’s mostly due to this fact. In that case one can better take the adsorption spectra or any how, look at both spectra. Although it’s preferable to take the desorption branche of an isotherm, caution has to be taken when a sample contains so called ink-bottle pores. If the pressure becomes lower the whole pores are emptied at once which causes a sudden drop of the adopted amount There’s a sudden closure of the hysteresis because of the adsorbed quantity. This causes an artifact in the desorption pore volume and area graphs. Actually I see that quite often This peak always appears at around 40 Angstrom. So if you see a peak in the desorption spectra, that does not appear in the adsorption spectra, it’s mostly due to this fact. In that case one can better take the adsorption spectra or any how, look at both spectra.

17. Isotherm MgO A good example is this sample of MgO Here you see the sudden drop in the desorption branch A good example is this sample of MgO Here you see the sudden drop in the desorption branch

18. First the adsorption spectra And subsequently the desorption spectra The sharp signals around 40 angstrom are obvious. But this value doesn’t say anything about the pore diameter. A careful look shows that the graph can be drawn like this as well. And then they look more like the adsorption graphs In this case it’s all very obvious and can be seen immediately. Next is another example First the adsorption spectra And subsequently the desorption spectra The sharp signals around 40 angstrom are obvious. But this value doesn’t say anything about the pore diameter. A careful look shows that the graph can be drawn like this as well. And then they look more like the adsorption graphs In this case it’s all very obvious and can be seen immediately. Next is another example

19. This isotherm looks Ok, but a better look shows the same drop This isotherm looks Ok, but a better look shows the same drop

20. And here again the adsorption and desorption spectra, in which the same happens as with the MgO sampleAnd here again the adsorption and desorption spectra, in which the same happens as with the MgO sample

21. At the end of all the graphs a summary report is given, with all calculated values on one page. At the end of all the graphs a summary report is given, with all calculated values on one page.

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