SOME IMPORTANT ASPECTS OF
PHYSISORPTION EXPERIMENT TO
STUDY MESO- AND MICRO-POROUS
MATERIALS
BY
D.K. SHARMA
KUNASH INSTRUMENTS
GENERAL PURPOSE
OF
PHYSISORPTION STUDIES
• # SURFACE AREA
• # PORE STRUCTURE
• # ADSORPTION CAPACITIES
GENERALISED SCHEME OF ADSROPTION
Vacuum
N2
Gas
-------------- - ------------- -------- - - - -- - - - ---- -------- ------- ----------------- - - - - - -------- - -- - - - - - - - -- -- --------- - - - - - - - -- - - - - - - - - - - --- - - - - - - - --- - - - - - - ---------------------------------- ------ -- ---- - - - - - - - - - - -- - - - - - -- - - - - -- - - - - - - - - - -- - - - -- - - - - -- - - - - - -- - - - - -- - -- - - -- - - -- - -- - - -- - -- -- - - -- - -- - - - - -- - - - - - - -- - - - - - -- - -- - - - - -- - - - - -- - - - - -
XCR
1.
2.
3.
Sample
tube
4.
5.
EVACUATE SAMPLE
DOSE N2 GAS
MEASURE
EQUILIBRIUM
PRESSURE
CALCULATE
VOLUME ADSORBED
REPEAT STEP 2 TO 4
REQUIREMENTS
• SAMPLE PRESSURE MEASUREMENT
• SATURATION PRESSURE
MEASUREMENT
• VOLUME- ADSORBED----- CALCULATED FROM
1. SAMPLE PRESSURE
2. 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
Isotherm Plot
3.0
• Ammonia
chemisorption
2.5
50 – 350 °C
C:\...\DATA\50.SMP C:\...\DATA\75.SMP C:\...\DATA\100.SMP C:\...\DATA\150.SMP C:\...\DATA\250.SMP
C:\...\DATA\300.SMP C:\...\DATA\350.SMP
Quantity Adsorbed (mmol/g)
3.5
2.0
1.5
1.0
0.5
0.0
0
50
100
150
200
250
300
Pressure (mmHg)
350
400
450
500
550
• Automated
sequence to
collect the
isotherms
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.
N N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
NN
N
NN
N
N
N
N
N
N
N
N
N
N
N
N
High ads.
N
N
N N
N
N
N
N
N
NNN
N
NN
Low ads.
EFFECT OF FREE SPACE MEASUREMENT
----- ON MESOPOROUS MATERIAL CHARACTERISATION:
MEASURED FREE SPACE VALUES
WARM
COLD
7.3843
BET SURFACE AREA
(sqm / g)
23.3026
ENTERED FREE SPACE VALUES
WARM
COLD
7.1000
23.5800
928
925
TOTAL PORE VOLUME
(single point)
2.0294
2.025
AVERAGE PORE
DIAMETER (B.J.H)
77.326
76.106
Note:
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
7.0499
22.1646
ENTERED FREE SPACE VALUES
WARM
COLD
6.8499
22.3646
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:
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
5.3433
BET SURFACE AREA (sqm/ g)
15.5229
ENTERED FREE SPACE VALUES
WARM
COLD
5.4433
15.4229
0.1465
0.2127
TOTAL PORE VOLUME (single point)
-
-
AVERAGE PORE DIAMETER (B.J.H)
-
-
MEASURED FREE SPACE VALUES
WARM
COLD
7.0592
BET SURFACE AREA (sqm/ g)
22.1050
ENTERED FREE SPACE VALUES
WARM
COLD
7.1592
22.0050
5.12
6.69
TOTAL PORE VOLUME (single point)
0.0977
0.1000
AVERAGE PORE DIAMETER (B.J.H)
689
510
MEASURED FREE SPACE VALUES
WARM
COLD
7.5184
BET SURFACE AREA (sqm/ g)
23.8792
ENTERED FREE SPACE VALUES
WARM
COLD
7.4184
23.9792
6.64
5.22
TOTAL PORE VOLUME (single point)
0.0986
0.0964
AVERAGE PORE DIAMETER (B.J.H)
573
879
Note:
Adsorption measurements are highly sensitive to FreeSpace errors
SURFACE AREA CALCULATION
REWRITING BET EQUATION WITH ADSORBED AMOUNT
EXPRESSED IN TERMS OF VOLUME (STP)
p / p0
V [ 1 – p/p0 ]
=
1
+
VmC
C – 1 p/ p0
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
BET SURFACE AREA
CALCULATION
WHAT TO LOOK FOR……….
a) BET Transform --------
plot linearity
b) Y-Intercept
---------
+ve
c) ‘C’ value
---------
(Correlation coefficient > 99.99)
> 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 PRESENTATION
1.
ISOTHERM STUDIES:
a)
b)
c)
d)
e)
f)
g)
h)
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
Contd………..
2.
SURFACE AREA CALCULATIONS – BET METHOD
a)
b)
c)
d)
e)
f)
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
PORE SIZE DISTRIBUTION CALCULATIONS
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 CALCULATION
In addition to all the requirements under “Isotherm studies”,
the following should be reported:
a)
b)
c)
d)
Method of pore size calculation
T-plot equation used
Graphical output: dV / dV vs D
dV / DlogD vs D
Presence of Micro-porosity, if any, as percent of Total surface
area or Total pore volume (From T-plot analysis)
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.
THEREFORE:
•
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.