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SOME IMPORTANT ASPECTS OF PHYSISORPTION EXPERIMENT TO
STUDY MESO- AND MICRO-POROUS MATERIALS
BY
D.K. SHARMAKUNASH INSTRUMENTS
GENERAL PURPOSE OF
PHYSISORPTION STUDIES
• # SURFACE AREA
• # PORE STRUCTURE
• # ADSORPTION CAPACITIES
GENERALISED SCHEME OF ADSROPTION
1. EVACUATE SAMPLE
2. DOSE N2 GAS
3. MEASURE EQUILIBRIUM PRESSURE
4. CALCULATE VOLUME ADSORBED
5. REPEAT STEP 2 TO 4
N2
GasVacuum
XCR
Sample tube
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- -- - -- - - - - - - - - - - - - --- - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - -- - - - - -- - - - - - - - - - -- - - - -- -- - - - -- - - - - - -- - - - - -- - -- - - -- - - -- - -- - - -- - -- -- - - -- - - -- - - - - -- - - - - - - -- - - - - - -- - -- - - - - -- - - - - -- - - - - -
- - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - -- - - - - - - - --- - - - - - -- - -- - - - - - - - - - - - - - -- - - - - - - - - - - - - - -- - - - - - - - -
REQUIREMENTS
• SAMPLE PRESSURE MEASUREMENT• SATURATION PRESSURE
MEASUREMENT• VOLUME- ADSORBED----- CALCULATED FROM
1. SAMPLE PRESSURE2. FREE SPACE ( COLD AS WELL AS WARM )3. MANIFOLD PRESSURE AND VOLUME -------------- USING IDEAL GAS LAWS
• ALL ABOVE MEASUREMENT AT EQUILIBRIUM CONDITIONS
• LONG DURATION OF EXPERIMENTS
NH3 Adsorption on MFI• Ammonia
chemisorption50 – 350 °C
• Automated sequence to collect the isotherms
Pressure (mmHg)00 50 100 150 200 250 300 350 400 450 500 550
Qua
ntity
Ads
orbe
d (m
mol
/g)
0.00.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
C:\...\DATA\50.SMP C:\...\DATA\75.SMP C:\...\DATA\100.SMP C:\...\DATA\150.SMP C:\...\DATA\250.SMPC:\...\DATA\300.SMP C:\...\DATA\350.SMP
Isotherm Plot
LOW SURFACE AREA SAMPLES
“Free-space”
Physical volume
that the gas occupies
in the sample cell.
Vtube-Vsample
Free space
High ads. Vs. low ads.
NN
N
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N
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N
N
N
N
N
N
N
NNN
NN
NN
N
N
N
N
N
N
N
N
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N
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N
High ads. Low ads.
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
Note:
7.3843 23.3026 23.58007.1000
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
Note:
7.0499 22.1646 22.36466.8499
Adsorption measurements are less 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 15.5229 15.42295.4433
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 22.1050 22.00507.1592
MEASURED FREE SPACE VALUES
WARM COLD
ENTERED FREE SPACE VALUES
WARM COLD
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 23.8792 23.9792 7.4184
Note: Adsorption measurements are highly sensitive to Free-Space errors
SURFACE AREA CALCULATION
REWRITING BET EQUATION WITH ADSORBED AMOUNT EXPRESSED IN TERMS OF VOLUME (STP)
p / p0 = 1 + C – 1
V [ 1 – p/p0 ] VmC VmC
WHERE,V = VOLUME AT STP ADSORBED AT PRESSURE pp0 = SATURATION PRESSURE ( VAPOUR PRESSURE OF LIQUID GAS AT ADSORPTION TEMP)Vm = VOLUME OF GAS (STP) REQUIRED TO FORM ONE MONOLAYERC = CONSTANT RELATED TO ENERGY OF ADSORPTION
p/ p0
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
RESULTS PRESENTATION1. ISOTHERM STUDIES:
a) Sample source identification
b) Pre-treatment conditions
c) Nature of Adsorptive used
d) Temperature of Adsorption
e) Mass of the sample used
f) Value of Saturation pressure
g) Type of Isotherm
h) Graphical output
Contd………..
2. SURFACE AREA CALCULATIONS – BET METHOD
a) Sample source b) Pre-treatment conditionsc) Nature of Adsorptive and Temperature of Adsorptiond) Applicable pressure rangee) ‘C’ valuef) 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
1- Sufficient number of points in the region of pore activity (between 0.4 – 0.995 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
PORE SIZE DISTRIBUTION CALCULATIONS
In addition to all the requirements under “Isotherm studies”,
the following should be reported:
a) Method of pore size calculation
b) T-plot equation used
c) 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)
PORE SIZE CALCULATION
• 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.
WHY WE SHOULD BE CAREFUL WHILE MEASURING LOW PRESSURE NITROGEN ISOTHERM
• 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:
