1. BY D.K. SHARMA KUNASH INSTRUMENTS
SOME IMPORTANT ASPECTS OF PHYSISORPTION EXPERIMENT TO STUDY MESO- AND MICRO-POROUS MATERIALS BY
D.K. SHARMA
KUNASH INSTRUMENTS

2. GENERAL PURPOSE OF PHYSISORPTION STUDIES
# SURFACE AREA
# PORE STRUCTURE
# ADSORPTION CAPACITIES

3. GENERALISED SCHEME OF ADSROPTION
N2 Gas
Vacuum
EVACUATE SAMPLE
DOSE N2 GAS
MEASURE EQUILIBRIUM PRESSURE
CALCULATE VOLUME ADSORBED
REPEAT STEP 2 TO 4
XCR
Sample tube

4. REQUIREMENTS SAMPLE PRESSURE MEASUREMENT
SATURATION PRESSURE MEASUREMENT
VOLUME- ADSORBED CALCULATED FROM
SAMPLE PRESSURE
FREE SPACE ( COLD AS WELL AS WARM )
MANIFOLD PRESSURE AND VOLUME
USING IDEAL GAS LAWS
ALL ABOVE MEASUREMENT AT EQUILIBRIUM CONDITIONS
LONG DURATION OF EXPERIMENTS

6. NH3 Adsorption on MFI Ammonia chemisorption
Automated sequence to collect the isotherms

9. LOW SURFACE AREA SAMPLES
“Free-space”
Physical volume
that the gas occupies
in the sample cell.
Vtube-Vsample
Free space

10. High ads. Vs. low ads. High ads. Low ads. N N N N N N N N N N N N N N

11. EFFECT OF FREE SPACE MEASUREMENT ----- ON MESOPOROUS MATERIAL CHARACTERISATION:
MEASURED FREE SPACE VALUES
WARM COLD
ENTERED FREE SPACE VALUES
WARM COLD
BET SURFACE AREA (sqm / g)
928
925
TOTAL PORE VOLUME
(single point)
2.0294
2.025
AVERAGE PORE DIAMETER (B.J.H)
77.326
76.106
7.3843
7.1000
Adsorption measurements are less sensitive to Free-Space errors
Note:

12. Adsorption measurements are less sensitive to Free-Space errors
Contd…EFFECT OF FREE SPACE MEASUREMENT ON MICROPOROUS MATERIAL CHARACTERISATION:
MEASURED FREE SPACE VALUES
WARM COLD
ENTERED FREE SPACE VALUES
WARM COLD
LANGMUIR SURFACE AREA (sqm/ g)
1166.7
1164.1
TOTAL PORE VOLUME
(single point)
0.4471
0.4442
AVERAGE PORE DIAMETER (B.J.H)
28.014
27.773
7.0499
6.8499
Note:
Adsorption measurements are less sensitive to Free-Space errors

13. Adsorption measurements are highly sensitive to Free-Space errors
Contd…EFFECT OF FREE SPACE MEASUREMENT ON NON-POROUS MATERIAL CHARACTERISATION:
MEASURED FREE SPACE VALUES
WARM COLD
ENTERED FREE SPACE VALUES
WARM COLD
BET SURFACE AREA (sqm/ g)
0.1465
0.2127
TOTAL PORE VOLUME (single point) -
AVERAGE PORE DIAMETER (B.J.H)
5.3433
5.4433
MEASURED FREE SPACE VALUES
WARM COLD
ENTERED FREE SPACE VALUES
WARM COLD
BET SURFACE AREA (sqm/ g)
5.12
6.69
TOTAL PORE VOLUME (single point)
0.0977
0.1000
AVERAGE PORE DIAMETER (B.J.H)
689
510
7.0592
7.1592
MEASURED FREE SPACE VALUES
WARM COLD
ENTERED FREE SPACE VALUES
BET SURFACE AREA (sqm/ g)
6.64
5.22
TOTAL PORE VOLUME (single point)
0.0986
0.0964
AVERAGE PORE DIAMETER (B.J.H)
573
879
7.5184
7.4184
Adsorption measurements are highly sensitive to Free-Space errors
Note:

16. SURFACE AREA CALCULATION
REWRITING BET EQUATION WITH ADSORBED AMOUNT EXPRESSED IN TERMS OF VOLUME (STP)
p / p = C – 1
V [ 1 – p/p0 ] VmC VmC
WHERE,
V = VOLUME AT STP ADSORBED AT PRESSURE p
p0 = SATURATION PRESSURE ( VAPOUR PRESSURE OF
LIQUID GAS AT ADSORPTION TEMP)
Vm = VOLUME OF GAS (STP) REQUIRED TO FORM ONE
MONOLAYER
C = CONSTANT RELATED TO ENERGY OF ADSORPTION
p/ p0

23. BET SURFACE AREA CALCULATION
WHAT TO LOOK FOR……….
a) BET Transform plot linearity (Correlation coefficient > 99.99)
b) Y-Intercept ve
c) ‘C’ value > 10 < 300
d) Vm or Qm value within the plotted range
e) Aim for uncertainty of Surface area value < 1%
NOTE: THE PRESSURE POINTS SHOULD BE SELECTED FOR BET CALCULATIONS IN SUCH A MANNER THAT ALL ABOVE (A TO E) ARE SATISFIED. IF NOT, GO FOR LANGMUIR APPLICATION

24. RESULTS PRESENTATION ISOTHERM STUDIES: Sample source identification
Pre-treatment conditions
Nature of Adsorptive used
Temperature of Adsorption
Mass of the sample used
Value of Saturation pressure
Type of Isotherm
Graphical output

25. Contd……….. Sample source Pre-treatment conditions
SURFACE AREA CALCULATIONS – BET METHOD
Sample source
Pre-treatment conditions
Nature of Adsorptive and Temperature of Adsorption
Applicable pressure range
‘C’ value
Value of cross sectional area of Adsorptive
Note: ENSURE THAT – Y-Intercept is positive
–‘C’ value is positive
– ‘C’ value is > 10 and < 300
– Linearity coefficient is close to or >99.99

26. PORE SIZE DISTRIBUTION CALCULATIONS
1- Sufficient number of points in the region of pore activity (between 0.4 – p/po )
2- Selection of t-plot
3- Adsorption branch is to be used for pore size distribution calculations, specially, if networking in pore structure is present (this is many a times identified by Steep desorption branch at the closure point of the Loop

27. PORE SIZE CALCULATION
In addition to all the requirements under “Isotherm studies”,
the following should be reported:
Method of pore size calculation
T-plot equation used
Graphical output: dV / dV vs D
dV / DlogD vs D
d) Presence of Micro-porosity, if any, as percent of Total surface area or Total pore volume (From T-plot analysis)

28. WHY WE SHOULD BE CAREFUL WHILE MEASURING LOW PRESSURE NITROGEN ISOTHERM
Nitrogen is more intensely adsorbed at 77k by microporous materials as compared with, say, Argon.
Hydrogen, Neon and Helium do not usually adsorb on microporous materials at 77k.
It is very difficult to obtain, and even more difficult to maintain, nitrogen gas purity better than a few parts per million relative to unwanted H2, He, Ne, Ar, O2, and CO.
Rubber- like polymers used as seals permits (although only slightly) gases to dissolve into them or to permeate completely through them.
Low equilibrium pressures of N2 over microporous materials lead to long equilibrium times. This allows accumulation of significant levels of less adsorbed gases. As a percentage of total pressure reading the damage is very large below 10 millitorr where the non-adsorbed gases may contribute several times as much pressure as the nitrogen.
Use of helium for free space measurement.

29. Do not allow Helium or any other five gases to be connected to instrument. Instead valve off the gas source at the regulator and leave a vacuum in the inlet lines.
Use “entered” free space value or try measuring it subsequent to the analysis.
If the manifold has been exposed to helium or any other gases mentioned earlier try an extended pumping down.
If Sample and/or sample tube has been exposed to Helium, trying baking out under vacuum for few hours prior to analysis.
THEREFORE: