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BET.pptx
- 1. Brunauer–Emmett–Teller (BET) surface areaanalysis: brief theory and instrumentation
- 2. OUTLINE 1. Definition-discovery &what technique does 2. Definition-theory of BET , adsorption vs. desorption and adsorbates 3. Types of pores 4. Equipment 5. Sample preparation 6. Results obtained from instrument/software
- 3. DEFINITION • Discovered byStephen Brunauer, Paul Hugh Emmett and Edward Teller in 1938 • Studies physical adsorption of gas molecules on a solid surface • Can therefore be used to study surface area & pore size distribution in given material • Depends on T , P , surface energy distribution and surface area
- 5. DEFINITION • Gases thatadsorb onto material surface are called adsorbates react • These gases should not chemically with material surfaces • Most common adsorbate is N2 • Ar, CO, CO2, and O2 may also be used • Volume of gas adsorbed is measured at boiling point of nitrogen (-196 ºC)
- 6. DEFINITION • Adsorption vs.desorption: • Adsorption is the sticking of gas molecules onto the surface of a solid • Desorption is the removal of gas molecules from the surface of a solid • These include surfaces inside open pores
- 7. DEFINITION • When theinteraction between a surface and an adsorbate is relatively weak only physisorption takes place When electrons available for formation of Chemical bond on surface atoms, Chemisorption occurs. • Activation energy-adsorbate molecules must overcome an energy barrier
- 8. BASIC PRINCIPLE 3 Ideal PV= nRT P/Po Q Ads No gas molecules Q Ads P/Po Nitrogen gas molecules Saturated Nitrogen gas molecules P/Po Q Ads Actual Never start from no gas molecules Monolayer: Gas molecules clump together Multilayer: Gas molecules clump together Some pores are not filled
- 9. BET isan extension of Langmuir model BASIC PRINCIPLE CONT. Kinetic behavior of the adsorption process Rate of arrival of adsorption is equal to the rate of desorption Heat of adsorption was taken to be constant and unchanging with the degree of coverage, θ 9
- 10. BASIC PRINCIPLE Assumptions Gas moleculesbehave ideally Only 1 monolayer forms All sites on the surface are equal No adsorbate-adsorbate interaction Adsorbate molecule is immobile 5
- 11. BASIC PRINCIPLE Homogeneoussurface No lateral interactions between molecules Uppermost layer is in equilibrium with vapour phase First and Higher layer: Heat adsorption All surface sites have same adsorption energy for adsorbate Adsorption on the adsorbent occurs in infinite layers The theory can be applied to each layer BET Theory
- 12. PORE CLASSIFICATION Pore Size: •Microporus <2nm • Mesoporus 2-50nm • Macroporous >50nm
- 13. DEFINITION • across theentire surface with no preference for one or more sites • Each site is either unoccupied or occupied by a single adsorbate molecule • Once adsorbed, molecule can act as sorption site for another gas molecule • Assumptions: 1. Homogeneous surface- adsorption Occurs
- 14. DEFINITION • Assumptions: 2. Allsites on the surface are equal 3. No lateral interactions between molecules 4. Adsorbate molecule is immobile 5.The rate of adsorption is equal to the rate of desorption
- 15. BASIC PRINCIPLE Adsorption Mechanism MonolayerMultilayer Capillary condensation
- 16. INSTRUMENTATION: • Basic Construction –Removablesample cell –Dosing manifold –Pressure transducers –Vacuum system –Analysis gas –Valves to move gas in and out of manifold and sample cell –Sample thermostat (dewar, furnace, cryostat) *** Before performing gas sorption expts, solid surfaces must be freed from contaminants like water and oils Sample cell Analysisgas to vacuum Manifold Pressure transducer(s)
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- 20. SAMPLE PREPARATION • Weighsolid sample (200 ≈ 300 mg) • Clean, pure & dry sample • Measurement of sample mass • Degassing the sample using heat & vacuum • Heating releases water vapor • Measuring adsorption • Analyzing measurement
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- 22. ISOTHERMS • 6 isotherms& BET applicable to: type II (disperse, nonporous or macroporous solids) type IV (mesoporous solids; pore diameter 2-50 nm)
- 23. THE BET LINEAREQUATION
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- 27. Limitations •BET theory ignoresinhomogeneities of the surface and lateral adsorbate-adsorbate interactions High energy sites will be occupied at lower relative pressures Reason for the nonlinearity of BET plots at p/p0 <0.05 •Polarization forces would induce a higher heat of adsorption in the second layer than in the third and so forth Reason for the failure of the BET equation at p/p0 >0.3 •The BET equation is applicable for surface area analysis of nonporous materials Difficulties to separate mono-multilayer adsorption from pore filling Filling of micropores is completed at p/p0 <0.1, however, linear BET plot are found at even lower relative pressures → measured surface area reflects not a real internal surface, but a „characteristic“ BET surface area For mesoporous materials exhibiting pore diameters between 2 and 4 nm (MCM-41, MCM-48) pore filling is observed at pressures close to mono-multilayer formation → overestimation of monolayer capacity
- 28. CONCLUSIONS • BET canbe used for: 1. Fibers 2. Nanocatalysts (chemisorption) 3. Nanotubes 4. Polymer membranes
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