Area and Porosity utilising Gas Sorption Neal Leddy CMA Postgraduate Analytical Workshop 2010 Vital for many manufacturing and research processes Common Characterisation techniques Optical and Scanning Electron Microscopy ID: 1026835
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1. Material Characterisation for Surface Area and Porosity utilising Gas SorptionNeal LeddyCMA Postgraduate Analytical Workshop 2010
2. Vital for many manufacturing and research processes. Common Characterisation techniques - Optical and Scanning Electron Microscopy - Transmission and Scanning Probe Microscopy - Dynamic Light Scattering - X-ray Diffraction - Differential Scanning Calorimertry and Thermogravimetric AnalysisMaterial Characterisation
3. Surface Area and Porosity measurement data important for many sectors:PharmaceuticalsPaint & Surface CoatingCeramicsCatalystsGas Sensors & Filters etc.
4. 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 tool to characterise porous materials allowing for the determination of specific surface area, pore size distribution and porosity.Adsorption
5. 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.Characteristics of Physical Adsorption
6. 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 reduced5 Classical Iostherm types described by Brunauer, Deming, Deming and Teller. Adsorption Isotherms
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8. Type I Pores are typically microporus with the exposed surface residing almost exclusively inside the micropores, which once filled with adsorbate, leave little or no external surface for further adsorption.
9. 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
10. 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
11. 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
12. Type V Are observed where there is small adsorbate-absorbent interaction potentials (similar to type III), and are also associated with pores in the 1.5 – 100nm range
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14. The most common adsorbate used is Nitrogen, however, other adsorbates are used in some circumstances.AdsorbateGASTemp (˚C)α factor x105 (1/mm Hg)CrossSectionalArea(Å2/mol.)Molecularweight(g/mol)Ar-195.811.414.239.948-1833.94CO2-782.7519.544.0101.75251.55CO-1833.4216.328.01N2-195.86.5816.228.0134-1833.78O2-1834.17C4H10014.246.958.12254.21
15. Surface area: Best described as the external surface area of a solid object including surface attributable to pores. Gas adsorption provides a distinct advantage as many classical models for particle measurement and characterisation fail to consider porositySurface area
16. 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 pressureWm = weight of adsorbate as monolayerC = BET constantBET
17. BET equation requires a linear plot of 1/[W(P/P0)-1] against P/P0Slope (s) Intercept (i)Wm (weight of monolayer)
18. Total Surface area (St) can then be derived N = Avagadro’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
19. Single point BET: Involves determining specific surface area using a single on the isothermMultipoint BET: Minimum of three data points.
20. Plot:Summary:Multipoint BET Plot Relative Volume@STP 1 / [ W((Po/P) - 1) ] Pressure P/Po cc/g 1.10536e-01 7.5355 1.3195e+01 1.53021e-01 8.1192 1.7804e+01 1.99422e-01 8.7403 2.2803e+01 2.48028e-01 9.4102 2.8045e+01 2.97227e-01 10.1099 3.3472e+01 BET summary Slope = 108.451, Intercept = 1.195e+00, Correlation coefficient, r = 0.99999 C constant= 91.759 Surface Area = 31.762 m²/g
21. Relative error between single and multipoint BET, (typically measured at P/P0 of 0.3)C Constant
22. The Langmuir equation describes Microporus material exhibiting Type I Isotherms. Assumes adsorption limited to one monolayer.Langmuir
23. Macroporous (>50nm)Mesoporus (2-50nm)Microporus (<2nm)IUPAC classification on pores
24. Pore Volume – Total pore volume is derived from the amount of vapour adsorbed at a relative temperature close to unity (assuming pores are filled with liquid adsorbate).Vads = volume of gas adsorbedVliq = volume of liquid N2 in poresVm = molar vol. of liquid adsorbate (N2=34.7cm3/mol)Pa = ambient pressureT = ambient temperaturePorosity
25. Pore Radius - The average pore size can be estimated from the pore volume. Assuming cylindrical pore geometry (type A hysteresis) average pore radius (rp) can be expressed as: Other pore geometry models may require further information on the isotherm hysteresis before applying appropriate model.
26. Adsorption/Desorption Isotherm
27. Pore Volume Data Total pore volume for pores with Radius less than 15.93 Å at P/Po = 0.395090 5.787e-01 cc/g BJH method cumulative adsorption pore volume 2.103e+00 cc/g BJH method cumulative desorption pore volume 2.192e+00 cc/g DH method cumulative adsorption pore volume 2.054e+00 cc/g DH method cumulative desorption pore volume 2.146e+00 cc/g HK method cumulative pore volume 4.257e-01 cc/g SF method cumulative pore volume 4.358e-01 cc/g NLDFT method cumulative pore volume 1.904e+00 cc/gPore Size Data Average pore Radius 3.505e+01 Å BJH method adsorption pore Radius (Mode Dv(r)) 1.698e+01 Å BJH method desorption pore Radius (Mode Dv(r)) 1.710e+01 Å DH method adsorption pore Radius (Mode Dv(r)) 1.698e+01 Å DH method desorption pore Radius (Mode Dv(r)) 1.710e+01 Å HK method pore Radius (Mode) 1.838e+00 Å SF method pore Radius (Mode) 2.261e+00 Å NLDFT pore Radius (Mode) 2.376e+01 Å
28. Barrett-Joyner-Halenda Method (BJH)Dollimore Heal Method (DH)Alpha S Method (αs)MP Method (MP)Dubinn-Radushkevic Method (DR)Dubinin-Astakhov Method (DA)Horvath-Kawazoe Method (HK)Saito-Foley Method (SF)Density Functional Theory Method (DFT)Frenkel-Halsey-Hill Method (FHH)Neimark-Kiselev Method (NK)Other Methods
29. Important step before measurement of surface area or pore size/volumeSurfaces are ‘cleaned’ of water/organic vapours in two ways:- 1. With heating under a vacuum 2. Under a flow of dry, inert gas.Degas
30. Adsorbate is introduced in to the manifoldThe valve to the sample cell is opened allowing the adsorbate to interact with the sample material.The pressure is repeatedly measured for the preset equilibration time, if the pressure drops dosing recurs and measurement proceeds until a stable reading is achieved.Analysis
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32. Nova Quantachrome 4200e
33. Criticism on BET
34. Brunauer [11] answers these criticisms by pointing out that lateral interaction between adsorbate molecules necessarily increases as the surface becomes more completely covered. The interaction with the surface, however, decreases with increasing adsorption up to monolayer coverage since on an energetically heterogeneous surface the high energy sites will be occupied at lower relative pressures with occupancy of the lower energy sites occurring nearer to completion of the monolayer.Brunaeur’s Answer
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36. A further criticism of the BET theory is the assumption that the heat of adsorption of the second and higher layers is equal to the heat of liquefaction.It seems reasonable to expect that polarization forces would induce a higher heat of adsorption in the second layer than in the third, and so forth. Only after several layers are adsorbed should the heat of adsorption equal the heat of liquefaction. It is, therefore, difficult to resolve a model of molecules adsorbed in stacks while postulating that all layers above the first are thermodynamically a true liquid structure.
37. The apparent validity of these criticisms contributes to the failure of the BET equation at high relative pressures (PI Po > 0.35). However, in the range of relative pressure leading to coverage near WIWm = 1, the BET C values usually give heats of adsorption that are reasonable.Thus, for the great majority of isotherms the range of relative pressures between 0.05 and 0.35, the linear BET range, apparently represents a condition in which the very high energy sites have been occupied and extensive multilayer adsorption has not yet commenced. It is within these limits that the BET theory is generally valid.