... allowing for pore measurements. ... An Adsorption Isotherm is obtained by measuring the amount of gas .... BJH method adsorption pore Radius (Mode Dv(r)).
presented by Neal Leddy CMA Analytical Workshop 2012
SURFACE AREA AND POROSITY
Adsorption
When a gas or vapour phase is brought into contact with a solid, part of it is taken up and remains on the outside attached to the surface. In physisorption (physical adsorption), there is a weak Van der Waals attraction between the adsorbate and the solid surface. Useful to characterise porous materials allowing for the determination of specific surface area, pore size distribution and pore volume.
Characteristics of Physical Adsorption
1. Low heats of adsorption, no violent or disruptive structural changes. 2. Can involve multiple layers of adsorbate, thus allowing for pore measurements. 3. High temperatures tend to inhibit physical adsorption. 4. Adsorption equilibrium is achieved quickly since no activation energy is generally required. 5. Physical adsorption is fully reversible, allowing adsorbate to fully adsorb and desorb.
Adsorption Isotherms
An Adsorption Isotherm is obtained by measuring the amount of gas adsorbed across a wide range of relative pressures at a constant temperature (typically liquid N2, 77K). Conversely desorption Isotherms are achieved by measuring gas removed as pressure is reduced
Iostherm types
described by Brunauer, Deming, Deming and Teller.
Type I Pores are typically microporus with the exposed surface residing almost exclusively inside the microspores, which once filled with adsorbate, leave little or no external surface for further adsorption.
Type II Most frequently found when adsorption occurs on nonporous powders or powders with diameters exceeding micropores. Inflection point occurs near the completion of the first adsorbed monolayer
Type III Characterised by heats of adsorption less than the adsorbate heat of liquification, adsorption proceeds as the adsorbate interaction with an adsorbed layer is greater than the interaction with the adsorbent surface
Type IV Occur on porous adsorbents with pores in the range of 1.5 – 100nm. At higher pressures the slope shows increased uptake of adsorbate as pores become filled, inflection point typically occurs near completion of the first monolayer
Type V Are observed where there is small adsorbateabsorbent interaction potentials (similar to type III), and are also associated with pores in the 1.5 – 100nm range
Adsorbate
The most common adsorbate used is Nitrogen, however, other adsorbates are used in some circumstances.
BET
Brunauer, Emmett and Teller (BET), most common method used to describe specific surface area: The BET equation –
W= weight of gas adsorbed P/P0 =relative pressure Wm = weight of adsorbate as monolayer C = BET constant
BET equation requires a linear plot of 1/[W(P/P0)-1] against P/P0 Slope
Wm
(s) Intercept (i)
(weight of monolayer)
Total Surface area (St) can then be derived
N = Avogadro's number (6.023x1023) M = Molecular weight of Adsorbate Acs = Adsorbate cross sectional area (16.2Å2 for Nitrogen)
Specific Surface Area (S) is then determined by total Surface area by sample weight
Single point BET: Involves determining specific surface area using a single point on the isotherm Multipoint BET: Minimum of three data points.
Multipoint BET Plot
Relative Pressure P/Po
Volume @ STP 1/[W((Po/P)-1] cc/g
1.10536e-01
7.5355
1.3195e+1
1.53021e-01
8.1192
1.7804e+1
1.99422e-01
8.7403
2.2803e+1
2.48028e-01
9.4102
2.8045e+1
2.97227e-01
10.1099
3.3472e+1
Summary Slope = 108.451 Intercept = 1.195e+00 Correlation coefficient, r = 0.99999 C constant= 91.759 Surface Area = 31.762 m²/g
C Constant
Relative error between single and multipoint BET, (typically measured at P/P0 of 0.3)
Single/Multi point Comparison Constant
Relative error
1
0.70
10
0.19
50
0.04
100
0.02
1000
0.0002
Infinity
0
Langmuir
The Langmuir equation describes Microporus material exhibiting Type I Isotherms.
