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§8.5 adsorption at gas / solid interface

§8.5 adsorption at gas / solid interface. Levine: pp. 397– 402 section 13.5 adsorption of gases on solids. 8.3.1 Specific area. 1) Surface atoms. Si. Surface layer. Suspending bond. Matter that is at or near an interface is not in the same state as matter in the interior of a phase.

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§8.5 adsorption at gas / solid interface

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  1. §8.5 adsorption at gas / solid interface Levine: pp. 397– 402 section 13.5 adsorption of gases on solids

  2. 8.3.1 Specific area 1) Surface atoms Si Surface layer Suspending bond Matter that is at or near an interface is not in the same state as matter in the interior of a phase. Owing to the suspending bond, material is apt to react with other substances.

  3. 2) Degree of subdivision 3) Specific area: Metal catalyst: Pt, Pd, Ni, Fe Oxide: Al2O3, SiO2 Activated carbon: charcoal. Catalysts or catalyst support The surface-to-volume / mass ratio

  4. Owing to the strong interatomic action, the surface tension of solid is usually much larger than that of liquids Powered solids have very high surface energy!

  5. 4) Surface energy and micropowder explosion 400 mJm-2 6  103 m2 = 2400 J Micropowder explosion: micropowder is thermo-dynamically unstable.

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  7. gas Adsorbed molecules solid 2. The way for solids to lower their surface energy Solid can not lower surface energy by shrinking its surface. How can solid lower its surface energy? Adsorption of gas molecules

  8. 8.3.2 Adsorption Sorption: movement of a material from one phase to another. only on the surface: adsorption distributing throughout: absorption Adsorbate: the material being adsorbed. Adsorbent: the material doing the adsorbing.

  9. Adsorption on a solid surface: a : under HCOOH vapor; b: under evacuation; c: heated under evacuation Infrared spectrum of HCOOH adsorbed on Al2O3 suggested that some part of HCOOH dissociate into HCOO + H+ on the absorbent surface. chemical adsorption, physical adsorption chemisorption, physisorption

  10. Chemisorption involves the formation of chemical bonds between adsorbed molecule and solid surface, and often the breaking of preexisting bonds in the adsorbed molecule. In some cases the chemisorption step requires an activation energy. b.p. physisorption involves forces similar to the van der Waals’ forces that lead to condensation of vapors to liquid.

  11. 2Ni + 2H 2Ni + H2 HH H H NiNi NiNi Transition from physisorption to chemisorption Transition state Chemical adsorption Physical adsorption The potential curve of adsorption By following the transition from physical absorption to chemical adsorption, catalysts can activate reactants molecules at relatively mild conditions. This is the basic principal for heterogeneous catalysis.

  12. Comparison between chemical and physical adsorption Van der Vaals’ force Chemical bond Condensation heat < 40 kJ mol-1; -4~-40 Chemical reaction heat > 80 kJ mol-1. -40 ~ -800 Around b.p. Relatively higher multilayer monolayer Reversible. equilibrium established rapidly. Less reversible good poor

  13. Conditions for chemisorption 2W + N2 = 2 W-Nad adsHm = D(N2) – 2X (W-N) < 0 Thermodynamic discussion of adsorption Adsorption is spontaneous, Entropy decreases during adsorption Therefore: adsorption is a exothermic process

  14. 8.3.3 description of adsorption The adsorption of gas depends on the nature of the solid and the gas being adsorbed, the state of subdivision of the solid, the temperature and the pressure of the gases. Therefore, the amount of adsorption (a) of a certain kind of solid for certain gases is a function of T and p. constant pressure : isobar constant temperature : isotherm constant adsorption : isochore

  15. p a/mlg-1 53.3 150 13.3 100 5.3 isochore 50 a 53.3 p/kPa 80 200 300 350 150 250 T/K 13.3 60 40 20 200 300 400 T/K isobar As temperature increases, adsorption decrease. Adsorption heat can be determined from isochore on the base of Clausius-Clappeyron equation equation.

  16. a/mlg-1 T -23.5 150 0 100 30 50 80 40 80 60 20 T/K isotherm Adsorption increases with increase of pressure under low pressure. While at high pressure, adsorption attains maximum value. adsorption isotherm: an expression that gives the fraction  of a surface that is covered by adsorbed molecules in equilibrium at constant temperature as a function of pressure or concentration.

  17. a a I II Vm pc pc p p 1) Types of adsorption isotherm: Brunauer found that the isotherms of substances can be subdivided into five types. 78 K, N2 / active carbon 78 K N2 / silica gel or iron catalyst

  18. a III a IV a V pc p pc p p 323 K benzene / Fe2O3 352 K Br2 / silica gel 373 K water / active carbon

  19. 8.3.4 Langmuir adsorption isotherm Irving Langmuir 1) The basic assumptions (1) The solid surface is uniform (2) monolayer adsorption (3) no intermolecular interaction The uniform surface contains a fixed number of adsorption sites. Each site holds only one adsorbed molecule. The heat of adsorption is the same for all sites and does not depends on the fraction coverage . 1932 Noble Prize USA, 1881/01/31 – 1957/08/16 for his discoveries and investigations in surface chemistry.

  20. 2) Degree of coverage S: the total adsorption sites S1: sites occupied by molecules

  21. 3)Adsorption and desorption equilibrium b (adsorption coefficient) : the kinetic equilibrium constant of the adsorption and desorption

  22. p at low pressure, bp << 1, at high pressure, bp >> 1,

  23. For monolayer adsorption, the specific area can be thus estimated according to For Langmuir adsorption, when plotting p/V against p, straight line can be obtained.

  24. When dissociation occur during adsorption: Dissociatvie / nondissociatvie (molecular) Adsorption of mixed gases For gas A and B: A, B, PA, PB

  25. Condensation heat 4) Problems with Langmuir isotherm Adsorption of hydrogen on different metal surface. Adsorption of nitrogen on graphitized carbon black. Adsorption heat depends on the coverage, which suggests that the surface of adsorbent is not uniform. Some sites are more active than the other.

  26. 8.3.5 BET adsorption isotherm 1938, Brunauer-Emmett-Teller 1) Basic assumptions (1) uniform surface; (2) multilayer adsorption; (3) the heat of the layer other than the first layer is the condensation heat; (4) desorption only occur at the layer exposed to the gas Multilayer adsorption = condensation ? Brunauer, S., Emmett, P. H. & Teller, E. (1938) Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60, pp. 309-319

  27. V/Vm 2 c = 1000 c = 100 I 1 c = 10 II c = 1 III 0.2 0.4 0.8 0 0.6 p/p0 BET absorption isotherm is valid for type I through type III.

  28. Measurement of specific area of solid with BET

  29. 8.3.6 Other isotherms Both Langmuir and BET isotherms are based on the assumption that the surface of the solid is uniform and the adsorption heat is independent of coverage. 1) Freundilich isotherm Freundilich adsorption isotherm can be derived on the assumption that:

  30. Valid for adsorption in solution 2) Temkin adsorption isotherm Temkin adsorption isotherm can be derived on the assumption that:

  31. Conversion of CO to CO2 on nanoporous copper catalysts

  32. Discussion: 1. Give explanation to the difference of the figures presented in Levine, p. 399. Figure 13.17 2. Discuss about the behavior of the adsorption of CO on Pd as shown in Figure 13.20.

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