Hi Saki,

What you need to assess CO2 adsorption is a pcT aka Sieverts apparatus. In principle your instrument that you use to assess the BET works in a pretty similar way. You just (have to) conduct the measurements at 77 K and you use N2. 

If the instrument allows manual mode you can use CO2 and have it operating in another temperature. The keypoint here is whether you can switch your porosimeter to a manual mode/or if the software allows you to use a different gas.

Then you will need to make the adsorption uptake assessment manually. There are different papers around demonstrating how to perform the calculations.

If you do not need high pressure, a TGA can help you assess the CO2 adsorption uptake and kinetics.

Cheers,

Georgios

CO2 adsorption can be investigated with the same sorption apparatus you use normally with N2, provided that the vacuum can be good enough. Some adsorption devices indeed allow measurements of surface area based on single point method only, which is not enough for CO2 adsorption. If your device can measure the full adsorption curve from secondary vacuum to saturation pressure, it'll be perfect.

Alain

CO2 adsorption is very useful for calculation of narrow micropore distribution in microporous materials, such as activated carbon. Quantachrome (and I believe Micromeritics, too) have commercial software for calculation of pore size distribution from CO2 adsorption at 273 K, using NLDFT approximation. This provides information of pores from about 0.35 nm up to about 1 nm (sometimes 1.2 - 1.4 nm). If this is what you want to do, then you may find useful some more  information about experimental setup and procedure:

(1) Switch the gas from N2 to CO2, if your instrument's software allows that.

(2) Use a icy water bath at 273 K in the dewar (rather than liquid N2) and tell the instrument that the new sample temperature is 273 K

(3) Set the Po = 760 torr (rather than using the continuous Po measurement option). This will let the instrument collect data up to atmospheric pressure, and the isotherm will be plotted on the 0 ...1 P/Po scale. The P values will represent pressures in Torr units.

(4) Measure as many points as you can, starting from P/Po ~ 1E-5 or so (when Po is set at 760 Torr). I normally collect a total of ~ 80 isotherm points.

(5) When you calculate the results, if you use the DFT calculation software of the instrument for micropore analysis, correct the Po values for liquid CO2 at 273 K, which is Po = 26400 Torr. The P/Po scale of your isotherm will be adjusted to a maximum of ~ 0.036.

The advantage of using CO2 for pore size distribution measurements on microporous materials is that the adsorption temperature is high (273 K) and thus diffusion is faster and equilibrium is reached faster than when L-N2 is used at 77 K. Also, because of higher Po, the low relative pressures (P/Po) need for micropore analysis (1E-5... 1E-4) corresponds to higher absolute pressures (P, in Torr) which can be more reliably read with pressure gauges normally available with the commercial volumetric gas adsorption instruments.

Be aware that CO2 adsorption data at 273 K cannot be used for BET calculations, because multilayer adsorption does not date place in the P/Po range of CO2 data, and therefore the conditions of the BET equation do not apply.

There is good literature information on using CO2 adsorption for micropore analysis. I recommend  Cazorla-Amoros et at, Langmuir 14 (1998) 4589; Ravikovitch et al, Langmuir 16 (2000) 2311; and Caguiat et al, Carbon 72 (2014) 47.

I hope this helps.

Measure adsorption isotherm

Thank you all for help! Georgios, I had thought on using TGA as I got several papers on CO2 adsorption/desorption but I need high pressure measurement. Alian, can I use physisorption or chemisorption techniques? I will appreciate some application notes. Cristian, I might use CO2 for micropore analysis so let me save this. It's helpful!

You have to use physisorption of CO2 at 0°C (i.e. using a Dewar filled with ice-water mixture). Micromeritics developed a new software for getting the continuous pore-size distribution using CO2 (for very narrow pores, not accessible to N2) and N2 (for all wider micropores and mesopores). Then you'll get the full distribution of pore sizes. You can contact the Micromeritics technical staff for details.

Alain

Thank you Alain! I contacted Micromeritics and they are helping me out. 

Saki, for high pressure measurements we use IGA up to 20 bar (microgravimetric). It's an easy measurement and provides both isotherms and kinetics. You are in Atlanta ? Oak Ridge is at  only 3 hours driving distance.

Cristian ([email protected])

you can use, before using the instrument you should purge the instrument with CO2 gas and also you should use LCO2. in the BET instrument itself, this option in Quanta Chrome Instruments

Forget about CO2 liquid !!

Unless liquid nitrogen, you cannot keep CO2 in liquid state at near room temperature unless you have high pressures (70 bar at 29 oC).

Hi all, 

I am using the BELSORP-MINI instrument and want to measure the CO2 adsorption at room temperature. For that, I am not using any liquid to maintain the temperature. But my problem is how to quantify the amount of CO2 adsorption? Which parameters do i need to consider for the calculation?

Vishewanath, 

I am not familiar with the BELSORP instrument. but the principles should be the same. You need a stable temperature bath, and want to avoid air draft around your sample. I would use a water bath with water at room temperature (in the dewar designed for liquid N2) to make sure you have the temperature right. You will need to know the water bath temperature for calculations. I think the software will ask you what was the sample temperature.

Next, trick the machine and set Po = 760 Torr. This way the readings of P during the run will be directly in Torr. 

Third, you schedule a run with 20 - 25 points up to P/Po very close to 1. The instrument will run, not knowing which gas you used for measurements.

To reduce the data you need to correct for the actual Po at the temperature of the measurement (room temperature, water bath at room temperature). The value for 273 K is 26037 Torr.. This correction will change all P/Po values from 0 ... 1 scale to the correct 0 .... ~0.03 scale.

The software will give you the  adsorbed isotherm (cc/g versus P in Torr) based on (1) correct range of P/Po; (2) correct temperature (in K); (3) amount of sample (in g);  and (4) correct volume calibrations of the instrument. 

If you want to do more calculations (distribution of micropore volumes) you need a specialized software such as DFT.

I hope this hope.