Surfaces chemistry

Adsorption at solidsSolid: Adsorbent

Gas/Solute: Adsorbate

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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

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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)

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Applications

- Wastewater treatment

 - Environmental chemistry(e.g. Leaching of pesticides in soil, chelation of metal ions in humic acids)

- Chromatography

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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.5

• 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.

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• 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

0)( , PTadsG

0)( , PTadsG

0 adsS

0 adsadsads STHG

0 adsads STH

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• Exception: Dissociative adsorption (e.g., H2 on glass 2 H(ads) )

, ,(endothermic adsorption), such that 0 adsS 0 adsH

0 adsG

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AdsorptionPhysical 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 Hads 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 exothermic9

Comparison of physisorption and chemisorption

Physisorption Chemisorption

Cause non-specific, long range (dispersion, forces) Van der Waals forcesNo electron Transfer (redistribution of e- density)

Covalent/electrostatistic forces, electron transfer

Adsorbents All solid Some solids

Adsorbates All gases below the critical point, intact molecules

Some chemically reactive gases, dissociation into atoms, ions, radicals

Temperature range Low temperatures Generally high temperatures

Heat of adsorption Low,~heat of condensation (typical ads ≈ - 20kJmol-

1), always exothermic

High, ~heat of reaction (ads ≈ - 200kJmol-

1)Exothermic

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Comparison of physisorption and chemisorptionPhysisorption Chemisorption

Rate Very fast Strongly temp. dependent

Activation energy no barrier activation barrier; Low

Generally high (unactivated: low)

Surface Coverage multilayer monolayer

reversibility Highly reversible (adsorbate layer is always in eqm with molecules of gas phase)

Often irrreversible (C + O2(ads) CO, CO2 at high T)

Applications Determination of surface area and pore size

Determination of surface concentrations and kinetics, rates of adsorption and desorption, determination of active centres

Example N2 on C C6H6 on Pd11

Adsorption

Chemisorption Physisorption

Activated

Non-activated

- temperature sensitive- varies according to a finite activation energy

rapid adsorption and near zero activation energy

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ENERGETICS OF ADSORPTION 

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dX - X

X - X

Adsorbate is diatom X2

Physisorption

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E p

d

P.E.

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.

d- distance from surface

Dissociative (chemical) adsorption  

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- D(X2)

D(X2) – EC

X2

X - X X X

X X

EC – EP EP

Metal

Kinetics of Desorption/ Adsorption• 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)

kd

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: average time between two successive attempts to escape from surface:

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)/mevaluated 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.17

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TYPE I ISOTHERMS: THE LANGMUIR MODEL

• Monolayer adsorption (Chemi/Physisorption)V/cm3g-1

p/atm

Vm

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 site4. Molar heat of adsorption is the same for all sites and independent of fractional coverage θ5. No dissociation 

adsH

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• Fractional coverage θ = Ns/N

= # sites occupied by adsorbate per cm2

total number of available adsorption sitesCan also be defined in terms of relative volumes and relative masses

θ = V = X Vm Xm

(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) = kd θ • (Adsorption from solution: replace p by c)

K p = θ / (1 – θ)  adsorption constant (in atm-1 or M-1) K(T) = ka / kd

kd

ka

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As p 0; θ = 0when Kp << 1(low p) ; θ ~ Kpwhen p ∞ : θ 1 • In terms of adsorption volume: 

KpKpVV m

1 pKVVV mm

1111

1/V

1/p

1/VmK

1/Vm

Vm = 1 / intercept K = intercept / slope

Langmuir isotherm (T const.)  Kc

KccT

1

,Kp

KppT

1

),(

p = gaseous partial pressurec = aqueous concentrationK= Langmuir equilibrium constant

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Alternatively:

- Graph of p/V vs p has slope = 1/Vm, intercept = 1/VmK

mm Vp

KVVp 1

• Vm and mm : total number of sites corresponding to a

monolayer

pKmmm

henceKp

KpVV

mmpT

mm

mm

1111

1),(

corresponding mass

mm = 1 / intercept K = intercept / slope

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Dissociative adsorption

S SS SS

YY Y

S SS SS

YYY

 

Y2 + 2 S 2 Y – S  p 1 – θ θ

 Adsorption equilibriumka p (1 – θ)2 = kd θ2

Thus:

from which we obtain: (K = K(T))

kd

ka

2

1

Kp

KpKp

pT

1

),(24

• with θ = V/Vm (V = adsorption volume of Y2 at STP) this can be reorganised to:

OR

where:

pKVVV mm

1111

1/V

1/√p

1/Vm√K

1/Vm

pVKVV

p

mm

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Vm = 1 / intercept;

K = (intercept / slope)2

Vm = 1 / slope;

K = (slope / intercept)225

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.

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MULTILAYER ADSORPTION – BET Model

• Model assumptions:(1) smooth, uniform surface(2)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(6)non-dissociative adsorption

vapadsadsads HHHH ...3,2,1,

12345

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• 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

pkk

TKd

a

)1()(

1

21)1(

)1(

1

)(/)()( 21 TKTKTc

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• 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)

)/1(ln

)/1(ln

)/1(ln 21

TdKd

TdKd

Tdcd

RH

RH vapads

1,

RTHH

Tc vapads 1,exp)(

01, vapads HH

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Brunauer-Emmett-Teller (BET) isotherm

• contains 2 parameters : Vm and c(T)•  Linearised form (multiply both sides by and invert):

 

 

))1(1)(1( zczcz

VV

m

zcVc

cVVzz

mm

11

)1(

Vzz

)1(

mcV1

mcVc 1

z

T

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• 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 dm3 m)

• 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

1,adsH

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