I want to find out the surface area of my samples, but they are in the form of monoliths. For doing BET measurements, I have to crush the sample into powdered form using pestle and mortar. Is that the correct way to do it?
Also am I remarkably changing the pore hierarchy and structure by crushing my sample or the change is acceptable?
In general it depends on the equipment you will use, more specifically on the sample tube: Micromeritics uses larger diameter tubes compared to Quantachrome or BEL. You should try to test your sample in one of these.
In case it is still small, I would try two different measurements by breaking the sample to different degrees (i.e. first breaking it just enough to fit inside, and second experiment, crushing the sample a little bit more after this). If two isotherms are similar, then I would not worry about porous structure changes.
You don't have to crush the sample, but break it frankly into smaller pieces so that the material can enter the sample holder. Biberon-like samples holders indeed exist, but there is no porous jacket for a good homogeneity of temperature. Therefore, their use is uneasy, and moreover they are really justified for materials of very low surface area, for which you need a high amount of material. If your material has a not so low surface area, a few pieces broken in a neat way, not grinded, shoudl work perfectly.
Alain
Large samples with low areas can be measured accurately with Kr. If you expect micropores to be present in your large sample, it might be best to measure with very long equilibration times, or test the effect of equilibration time on the isotherm. If possible, I try to cut large samples into pieces that fit the tubes, rather than crushing them, hoping to alter the pore system less.
It depends on what pore sizes you are predicting in your sample. In materials containing small pores lower than 2nm down to 1 nm BET-N2 surface area is overestimated as a result of capillary condensation at low relative pressures. In contrast, when pores are smaller than 0.5 nm, the pore volume is hugely underestimated. As a guideline, whenever a type III isotherm is observed during BET experiment, it represents an exteremly weak adsrobate-adsrbent interaction. Furthermore, Adsorption measurements using nitrogen at 77.4 K is difficult, because the filling of 0.5 - 1 nm pores occurs at P/Po of 10-7 to 10-5, where the rate of diffusion and adsorption equilibration is very slow.
I suggest methylene blue (MB) dye adsorption in aqueous solution which is usually used to determine the surface area. Titration method by MB has been used for clay minerals and activated carbon generally by industries for several decades.
Check the attached paper page 18 onwards under the title surface area easurements
If your sample has a low surface area and you crush it, the external surface area created will not be negligible compared to the initial one. Therefore, the measured surface area will not be representative of the sample before crushing. On the contrary, if the initial surface area is high, due to the existence of micropores and narrow mesopores, the additional surface due to crushing will be negligible.
You should always measure more than 10m2 when using N2 adsorption, i.e.. if your sample has 100 m2/g unless 0.1 g should be used for measuring; If you have low surface areas or low sample amount better use Kr adsorption to calculate the SBET. Only with N2, you will have adsorption-desoption isotherms, whose shape gives information on the porosity.
Using large amount of sample or big sizes requires using nonstandard ‘milk-bottle’ sample containers that you have to fabricate with the help of a glass maker.
I have done quite a bit of testing with low area materials, and do not fully agree with Vanessa Fierro's reply.
If you measure on cm-sized fragments, the 'fracture' surface area created by crushing (couple of cm2/g) is generally well below the detection limit of BET from N2 adsorption, so it only becomes significant when your sample surface area is too low to measure.
The limit of 10m2 depends on the device you are using. I tested this with standards on a Micromeritics Gemini VII 2390t. The manual states that a minimal absolute area of only 0.1m2 is required, for BET, and I think it can even do a little better. It requires a bit more time and work than a routine analysis, because the procedure is a bit different, but it seems to give proper results.
regards,
Pieter
Because the sample can´t be milled to the nanometric scale you are not modifying the pore structure at this scale by milling. This means that measurements can be valid only crusing the monolith just to enter in the tube or in a powder forms. You can check both posibilities and compare the isotherms obtained. Of course, adsorption/desorption processes are slower as smaller the pore diameter, therefore, be carefull with the presence of ultramicropores.
Moreover, we are assuming that your monolhits are homogeneous. Large monoliths, depending on the synthesis procedure, can present different porosity along the monolith diameter.
Micromeritics had a special cell for the ASAP family that allowed the introduction of monoliths up to half an inch in diameter and one inch high. The bulb, in which the monolith is got, was constructed with a ground glass joint allowing opening to enter the entire monolith.
Based on this design, we build a little bigger cell that allows to enter our standard monoliths of 16 mm diameter by 30 mm high. It is essential to use a high quality vacuum grease to resist heating during degassing and liquid nitrogen during measurement without making difficult the subsequent opening of the cell at the end of the measurement. In our experience (over 15 years regularly using these cells, more than 1000 isotherms of monoliths) the best vacuum grease is the Dow Corning High Vacuum grease, which can be purchased at any supplier of laboratory chemicals.
It is also important to use a closed tube within the cell stem to reduce the dead volume of the cell.
Exceptionally, we also built a larger cell to measure monoliths of one inch in diameter, which worked without problems, but in this case it is necessary to change the original Dewar because the diameter of the mouth is too small for that cell.
If anyone needs more details, you can email me and I will send some photos of our cells.
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