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Surfaces chemistry

Surfaces chemistry. Adsorption at solids Solid: Adsorbent Gas/Solute: Adsorbate. Outline. Adsorption Comparison of physisorption and chemisorption The Langmuir treatment of adsorption Adsorption Kinetics Analytical Aspects Of Adsorption Other isotherms (BET, Freundlich)

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Surfaces chemistry

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  1. Surfaces chemistry Adsorption at solids Solid: Adsorbent Gas/Solute: Adsorbate

  2. Outline • Adsorption • Comparison of physisorption and chemisorption • The Langmuir treatment of adsorption • Adsorption Kinetics • Analytical Aspects Of Adsorption • Other isotherms (BET, Freundlich) • Texts: Introduction to colloids and Surface Chemistry – D.J. Shaw • Physical Chemistry - Atkins

  3. Applications • Central importance to many areas of pure and applied research: • Electronic device manufacture - Heterogeneous catalysis (e.g., Hydrogenation of alkenes, cracking of crude oil over silica- alumina : zeolites)

  4. Applications • Wastewater treatment  - Environmental chemistry (e.g. Leaching of pesticides in soil, chelation of metal ions in humic acids) - Chromatography

  5. Adsorption • Is the withdrawal of a substance from a bulk phase (aqueous or solution) and its accumulation at an interface. • Strictly a surface phenomenon • It is sometimes accompanied by deeper penetration of the adsorbed substance into the body (bulk) of a solid adsorbent, akin to the formation of a solid solution – absorption • The term Sorption covers both phenomena.

  6. Non-Dissociative adsorption is said to occur when a molecule adsorbs on to the surface from the gas phase without fragmentation. • When fragmentation does occur, the adsorption process is termed dissociative. • The free gas and the adsorbed gas are in dyanamic equilibrium. • Fractional coverage (ϴ )or extent of adsorption depends on : T, P (gas) or conc. (solute) and on effective surface area. • The variation of ϴ with pressure at a chosen temperature is called the adsorption isotherm.

  7. Finely divided solids possess a very high SPECIFIC SURFACE AREA (SSA) / m2g-1 • (activated C : ~ 1000 ; Si gel : ~ 500) • Adsorption is spontaneous process, therefore • (adsorption equilibrium if ) • On the other hand, the adsorbed state is more “ordered” (2D vs 3D), hence : • (non-dissociative adsorption)(translational freedom reduced) • non-dissociative adsorption exothermic

  8. Exception: Dissociative adsorption (e.g., H2 on glass 2 H(ads) ) ,,(endothermic adsorption), such that

  9. Adsorption Physical adsorption (physisorption) • The bonding interaction between adsorbate and adsorbent is long range but weak and is associated with van der Waals-type interactions. • The small DHads is insufficient to lead to bond breaking so the physiosorbed molecule retains its identity, though it might be distorted by the surface. • Chemical adsorption (chemisorption) • chemical bonds are formed between the molecules (atoms) and the surface. Note : both types of adsorption are exothermic

  10. Comparison of physisorption and chemisorption

  11. Comparison of physisorption and chemisorption

  12. Adsorption Physisorption Chemisorption Activated - temperature sensitive - varies according to a finite activation energy Non-activated rapid adsorption and near zero activation energy

  13. ENERGETICS OF ADSORPTION Adsorbate is diatom X2 X - X X - X d

  14. Physisorption • Pure physisorption (e.g. Ar / metals ): • the only attraction between the adsorbing species and the surface arises from weak, van der Waals forces. • these forces give rise to a shallow minimum in the PE curve at a relatively large distance from the surface (typically d > 0.3 nm) before the strong repulsive forces arising from electron density overlap cause a rapid increase in the total energy. • there is no barrier to prevent the atom or molecule which is approaching the surface from entering this physisorption well, i.e. the process is not activated and the kinetics of physisorption are invariably fast. P.E. d E p d- distance from surface

  15. Dissociative (chemical) adsorption - D(X2) X2 X X EP D(X2) – EC EC– EP X X X - X Metal

  16. Kinetics of Desorption/ Adsorption kd • Xads Xdes • kd / s-1 : desorption rate constant • Arrhenius: kd = A exp(- Ed/RT) • A ~ vibrational frequency • Residence time ~ half-life ; For Ed / kJ mol-1 = 25 t1/2 ~ 10-8 s (physisorption) Ed / kJ mol-1 = 100 ~ 1 hr (chemisorption) : average time between two successive attempts to escape from surface:

  17. Analytical Aspects Of Adsorption • Quantitative measures of adsorption # moles of adsorbate per gram of adsorbent : X / m (in mol g-1) or, in the case of adsorption from the gas phase, • adsorption volume (V) per gram of adsorbent, where : V = (nadsRT/P)/m evaluated at STP (25oC, 1 atm), i.e. V = (nads x 22.4 dm3)/m • V = V(T,P) gas solid • X/m = X/m(T,c) solution solid • X = Vsoln(cini – cfin) where; (cfin = c = equilibrium conc.) • T constant: V(P) or X/m(c) : Recall: relationship between the amount adsorbed (X) and the concentration (c) is known as adsorption isotherm.

  18. TYPE I ISOTHERMS: THE LANGMUIR MODEL • Monolayer adsorption (Chemi/Physisorption) V/cm3g-1 Vm p/atm Model assumptions: 1.Uniform surface with N equivalent adsorption sites per cm2 2. No interference of adsorbed particles with an adjacently adsorbed molecule 3. One molecule per site 4. Molar heat of adsorption is the same for all sites and independent of fractional coverage θ 5. No dissociation

  19. Fractional coverage θ = Ns/N = # sites occupied by adsorbate per cm2 total number of available adsorption sites Can also be defined in terms of relative volumes and relative masses θ = V = X VmXm (gas/solid) (solution/solid) Kinetic scheme: Y(g) + S(surface site) Y - S   (associative adsorption) p 1 – θ θ • Equilibrium : rate of adsorption = rate of desorption ka p (1 – θ) = kdθ • (Adsorption from solution: replace p by c) K p = θ / (1 – θ)  adsorption constant (in atm-1or M-1)  K(T) = ka/ kd ka kd

  20. Langmuir isotherm (T const.)  p = gaseous partial pressure c = aqueous concentration K= Langmuir equilibrium constant As p 0; θ = 0 when Kp << 1(low p) ; θ ~ Kp when p ∞ : θ 1  • In terms of adsorption volume: 1/V Vm = 1 / intercept K = intercept / slope 1/VmK 1/Vm 1/p

  21. Alternatively: - Graph of p/V vs p has slope = 1/Vm, intercept = 1/VmK mm = 1 / intercept K = intercept / slope corresponding mass • Vm and mm : total number of sites corresponding to a monolayer

  22. Dissociative adsorption Y Y Y Y Y Y Y2 + 2 S 2 Y – S   p 1 – θ θ  Adsorption equilibrium ka p (1 – θ)2 = kd θ2 Thus: from which we obtain: (K = K(T)) S S S S S S S S S S ka kd

  23. with θ = V/Vm (V = adsorption volume of Y2 at STP) this can be reorganised to: OR where: 1/V Vm = 1 / intercept; K = (intercept / slope)2 1/Vm√K 1/Vm 1/√p Vm = 1 / slope; K = (slope / intercept)2

  24. Limitations to Langmuir Model • Does not explain multi-layer adsorption and limited to low pressure studies. • ∆Hads is not independent of coverage: • also on a real surface some sites are better so gas molecules search for these first and ∆Hads is greater for better sites. • as molecules of adsorbate pack closer on the surface with increasing coverage, inevitably some lateral interactions will result, which will change ∆Hads.

  25. MULTILAYER ADSORPTION – BET Model • Model assumptions: (1) smooth, uniform surface • same number of adsorbate molecules in each layer when full (3) no lateral interactions (4) (heat of adsorption is same for each layer except layer 1) (5) dynamic equilibrium between adjacent layers • non-dissociative adsorption 5 4 3 2 1

  26. Coverage : θ = V / Vm • Vm = adsorption volume (STP) occupied by molecules covering a monolayer (so θ may now become > 1!)  • 2 equilibrium constants: • K2(T) defined analogously for layers 2, 3,… • Define : • C is BET constant

  27. so that : • Chemisorption in layer # 1 (type II isotherms) c(T) decreases with increasing T • Define : z = p / p0 ( p and p0 = equilibrium and saturated vapour pressure of adsorbate at temperature T respectively)

  28. Brunauer-Emmett-Teller (BET) isotherm • contains 2 parameters : Vm and c(T) •  Linearised form (multiply both sides by and invert): T z

  29. Vm = 1 / (slope + intercept) # adsorption sites • c(T) = 1 + (slope / intercept) • Vm allows us to calculate an effective surface area of substrate. • Determination of specific surface area • SSA = adsorbent area / adsorbent mass •  = NA nmax a / m = NA Vm a /(22.4 dm3m) • NA = 6.0 x 1023 mol-1 ; a = area of one adsorbate molecule Vm = volume corresponding to one monolayer nmax total number of moles corresponding to one monolayer

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