In our lab we use TGA for CO2 adsorption but, we want to compare the physisorption with BET method.
I am wondering why you don't run BET instrument with CO2 as sorption gas (instead of N2)?!
Dear Pramod,
A good point!, However, it is possible to use CO2 as soprtion gas. Of course it depends on the adsorbent reactivity, which can be checked with a cyclic adsorption/desorption.
Dear Vishwa
Analysis of adsorption of CO2 by BET (adsorption isotherm) and TG/TDA (Desorption) are different techniques and cannot be compared quantitatively to each other. For example there can be formation of stable carbonates which can result in unusual desorption or adsorption profile of the isotherm. Also if formation of stable carbonates (Eg. BaCO3 stable >800 C) occur, they has to be desorbed at higher temperature (1000 C). Heating a material at such high temperature have other limitations of oxygen desorption and phase change.
It depends on your material and what data you looking for... only adsorption profile, CO2 capture/storage, energetics of adsorption? all the best brother..
If CO2 adsorbs to the surface of your sample only by physical forces (and no chemical interactions) the amount of CO2 adsorbed is a function of pressure and temperature. The appropriate pressure range (or more accurately speaking the relative pressure range) at any given temperature needs to be used in order to calculate the monolayer uptake from which the BET surface area is calculated.
CO2 physisorption is normally done at 0degC, at which temperature its saturation pressure, Po is approximately 26,000 torr. Therefore, in a normal sub-atmospheric pressure (i.e. vacuum manometric) sorption analyzer, the maximum relative pressure (P/Po) achievable is barely 0.03 which mans that other measurable points are even lower. This usually means that the linear BET range is not reachable (it would require higher pressures at this temperature). In that case, one must reduce the temperature even lower to reduce the absolute pressure range so that the full BET P/Po range can be measured. One might use dry-ice/acetone for example.
To make equivalent measurements in a TGA apparatus one must be able to vary the concentration of CO2 in the gas phase (by pressure if you have a vacuum TGA with a sufficiently precise pressure gauge) or by dilution with non-adsorbing helium in an atmospheric pressure rig to the appropriate concentration, and to be able to take the temperature down to 0degC or lower. Without controlling pressure (concentration) and temperature - one cannot know what the adsorbed amount corresponds to... monolayer only (for surface area), or multilayer, chemisorption, etc. Isothermal desorption (by reducing pressure/concentration) will indicate how reversible the adsorption is... any irreversibility suggests chemisorbed species.
An introduction to gas physisorption can be found here:(http://sol.rutgers.edu/~aneimark/PDFs/IUPAC_Report_PAC_2015.pdf)
http://sol.rutgers.edu/~aneimark/PDFs/IUPAC_Report_PAC_2015.pdf
Why using CO2 vs using N2? Is it better to use CO2 if no chemical bonding occurs?
Lisandro Cunci Because CO2 and N2 probe different sizes of the pores!
N2 (77K) probes micro- to mesopores from 1-2 nm to 2-50 nm, while CO2 adsorption (273K) probes ultramicroporosity 0.3-0.7 nm sized pores.
CO2 can penetrate narrow pores with higher kinetic energy than N2 or Ar adsorptions.
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