Adsorption On Solid Surface

1

Adsorption Adsorption is a process in which molecules from gas (or liquid) phase land

on, interact with and attach to solid surfaces. The reverse process of adsorption, i.e. the process in which adsorbed

molecules escape from solid surfaces, is called Desorption. Molecules can attach to surfaces in two different ways because of the

different forces involved. These are Physisorption (Physical adsorption) & Chemisorption (Chemical adsorption)

Physisorption Chemisorption

force van de Waal chemcal bondnumber of adsorbed layers multi only one layer

adsorption heat low (10-40 kJ/mol) high ( > 40 kJ/mol)selectivity low high

temperature to occur low high

Adsorption On Solid SurfaceCatalysis & Catalysts

2

Catalyst composition Active phase

Where the reaction occurs (mostly metal/metal oxide)

Promoter Textual promoter (e.g. Al - Fe for NH3 production) Electric or Structural modifier Poison resistant promoters

Support / carrier Increase mechanical strength Increase surface area (98% surface area is supplied within the porous structure) may or may not be catalytically active

Solid CatalystsCatalysis & Catalysts

CatalystActiv

e ph

ase

Support

Promoter

3

Some common solid support / carrier materials

Alumina Inexpensive Surface area: 1 ~ 700 m2/g Acidic

Silica Inexpensive Surface area: 100 ~ 800 m2/g Acidic

Zeolite mixture of alumina and silica, often exchanged metal ion present shape selective acidic

Solid CatalystsCatalysis & Catalysts

Other supports Active carbon (S.A. up to 1000 m2/g) Titania (S.A. 10 ~ 50 m2/g) Zirconia (S.A. 10 ~ 100 m2/g) Magnesia (S.A. 10 m2/g) Lanthana (S.A. 10 m2/g)

poreporous solid

Active site

4

Pore sizes micro pores dp <20-50 nm

meso-pores 20nm <dp<200nm

macro pores dp >200 nm Pores can be uniform (e.g. polymers) or non-uniform (most metal oxides)

Pore size distribution Typical curves to characterise pore size:

Cumulative curve Frequency curve

Uniform size distribution (a) & non-uniform size distribution (b)

Pores of Porous Solids

b

d

a

dwdd

d

wt

b awt

d

Cumulative curve Frequency curve

5

Adsorption processAdsorbent and adsorbate Adsorbent (also called substrate) - The solid that provides surface for adsorption

high surface area with proper pore structure and size distribution is essential good mechanical strength and thermal stability are necessary

Adsorbate - The gas or liquid substances which are to be adsorbed on solid

Surface coverage, The solid surface may be completely or partially covered by adsorbed molecules

Adsorption heat Adsorption is usually exothermic (in special cases dissociated adsorption can be

endothermic) The heat of chemisorption is in the same order of magnitude of reaction heat;

the heat of physisorption is in the same order of magnitude of condensation heat.


define = = 0~1number of adsorption sites occupiednumber of adsorption sites available

6

Applications of adsorption process Adsorption is a very important step in solid catalysed reaction processes

Adsorption in itself is a common process used in industry for various purposes Purification (removing impurities from a gas / liquid stream) De-pollution, de-colour, de-odour Solvent recovery, trace compound enrichment etc…

Usually adsorption is only applied for a process dealing with small capacity The operation is usually batch type and required regeneration of saturated adsorbent

Common adsorbents: molecular sieve, active carbon, silica gel, activated alumina.

Physisorption is an useful technique for determining the surface area, the pore shape, pore sizes and size distribution of porous solid materials (BET surface area)


7

Adsorption On Solid Surface Characterisation of adsorption system

Adsorption isotherm - most commonly used, especially to catalytic reaction system, T=const.The amount of adsorption as a function of pressure at set temperature

Adsorption isobar - (usage related to industrial applications)The amount of adsorption as a function of temperature at set pressure

Adsorption Isostere - (usage related to industrial applications)Adsorption pressure as a function of temperature at set volume

Pressure

Vol

. ads

orbe

d T1T2 >T1

T3 >T2

T4 >T3

T5 >T4

Vol

. ads

orbe

d

Temperature

P1

P2>P1

P3>P2P4>P3

Pre

ssur

e

Temperature

V2>V1

V1

V3>V2

V4>V3

Adsorption Isotherm Adsorption Isobar Adsorption Isostere

8

The Langmuir adsorption isotherm Basic assumptions

surface uniform (Hads does not vary with coverage) monolayer adsorption, and no interaction between adsorbed molecules and adsorbed molecules immobile

Case I - single molecule adsorptionwhen adsorption is in a dynamic equilibrium A(g) + M(surface site) AMthe rate of adsorptionrads = kads (1-) P

the rate of desorptionrdes = kdes

at equilibrium rads = rdes kads (1-) P = kdes

rearrange it for

let B0 is adsorption coefficient

Adsorption On Solid Surface

CC

B PB P

s 0

01des

ads

kkB 0

PBk/kPk/k

desads

desads

0)(1)(

case I

A

9

Adsorption On Solid Surface The Langmuir adsorption isotherm (cont’d)

Case II - single molecule adsorbed dissociatively on one site

A-B(g) + M(surface site) A-M-B

the rate of A-B adsorption rads=kads (1))PAB=kads (1)2PAB

the rate of A-B desorption rdes=kdes=kdes2

at equilibrium rads = rdes kads (1)2PAB= kdes2

rearrange it for

Let.

case II

A BBA

==

1/20

1/20

)(1)(

AB

ABs

PBPB

CC

des

ads

kkB 0

)(1

)(

ABdesads

ABdesads

Pk/kPk/k

10

The Langmuir adsorption isotherm (cont’d) Case III - two molecules adsorbed on two sites

A(g) + B(g) + 2M(surface site) A-M + B-M

the rate of A adsorptionrads,A = kads,A (1) PA

the rate of B adsorptionrads,B = kads,B (1) PB

the rate of A desorptionrdes,A = kdes,A

the rate of B desorptionrdes,B = kdes,B

at equilibrium rads ,A = rdes ,A and rads ,B = rdes ,B

kads,A(1)PA=kdes,A and kads,B(1)PB=kdes,B

rearrange it for

where are adsorption coefficients of A & B.


B,des

B,adsB,

A,des

A,adsA, k

kB

kk

B 00 and

BB,AA,

BB,B,sB

BB,AA,

AA,A,sA PBPB

PBCC

PBPBPB

CC

00

0

00

0

1

1

case III

A B

11

The Langmuir adsorption isotherm (cont’d)


B,des

B,adsB,

A,des

A,adsA, k

kB

kk

B 00 and

BB,AA,

BB,B,sB

BB,AA,

AA,A,sA

PBPBPB

CC

PBPBPB

CC

00

0

00

0

1

1

AdsorptionStrong kads>> kdes kads>> kdes

B0>>1 B0>>1

Weak kads<< kdes kads<< kdes

B0<<1 B0<<1

1/20

1/20

)(1)(

AB

ABs

PBPB

CC

des

ads

kkB 0

case II

A B

CC

B PB P

s 0

01

des

ads

kkB 0

case I

A

1CCs 1

CCs

PBCCs

0

1/20 )( PB

CCs

AdsorptionA, B both strong

A strong, B weak

A weak, B weak

BB,AA,

BB,B,sB

BB,AA,

AA,A,sA

PBPBPB

CC

PBPBPB

CC

00

0

00

0

BB,B,sB

AA,A,sA

PBC/CPBC/C

0

0

A

BA,B,B,sB

A,sA

PPB/BC/C

C/C

)(

1

00

case III

A B

12

Langmuir adsorption isothermcase I

case II

Case III


Langmuir adsorption isotherm established a logic picture of adsorption process It fits many adsorption systems but not at all The assumptions made by Langmuir do not hold in all situation, that causing error

Solid surface is heterogeneous thus the heat of adsorption is not a constant at different Physisorption of gas molecules on a solid surface can be more than one layer

BB,AA,

BB,B,sB

BB,AA,

AA,A,sA

PBPBPB

CC

PBPBPB

CC

00

0

00

0

1

1

1/20

1/20

)(1)(

AB

ABs

PBPB

CC

CC

B PB P

s 0

01

large B0 (strong adsorp.)

small B0 (weak adsorp.)moderate B0

Pressure

Am

ount

ads

orbe

d

mono-layer

1CCs

PBCCs

0

Strong adsorption kads>> kdes

Weak adsorption kads<< kdes

13

Five types of physisorption isotherms are found over all solids

Type I is found for porous materials with small pores e.g. charcoal. It is clearly Langmuir monolayer type, but the other 4 are not

Type II for non-porous materials

Type III porous materials with cohesive force between adsorbate molecules greater than the adhesive force between adsorbate molecules and adsorbent

Type IV staged adsorption (first monolayer then build up of additional layers)

Type V porous materials with cohesive force between adsorbate molecules and adsorbent being greater than that between adsorbate molecules


I

II

III

IV

V

relative pres. P/P0

1.0

amou

nt a

dsor

bed

14

Other adsorption isothermsMany other isotherms are proposed in order to explain the observations

The Temkin (or Slygin-Frumkin) isotherm Assuming the adsorption enthalpy H decreases linearly with surface coverage

From ads-des equilibrium, ads. rate des. rate

rads=kads(1-)P rdes=kdes

where Qs is the heat of adsorption. When Qs is a linear function of i. Qs=Q0-iS (Q0 is a constant, i is the number and S represents the surface site),

the overall coverage

When b1P >>1 and b1Pexp(-i/RT) <<1, we have =c1ln(c2P), where c1 & c2 are constants

Valid for some adsorption systems.


1

1 1

1

0

0

PebPeb

PBPB

RT/Q

RT/Q

s s

s

H

of a

ds

LangmuirTemkin

RTiRT/Q

RT/Q

s expPP

iRTdS

PebPebdS

s

s

1

11

01

11

0 b1b1ln

(1[

15

The Freundlich isotherm assuming logarithmic change of adsorption enthalpy H with surface coverage

From ads-des equilibrium, ads. rate des. rate

rads=kads(1-)P rdes=kdes

where Qi is the heat of adsorption which is a function of i. If there are Ni types of surface sites, each can be expressed as Ni=aexp(-Q/Q0) (a and Q0 are constants), corresponding to a fractional coverage i,

the overall coverage

the solution for this integration expression at small is:

ln=(RT/Q0)lnP+constant, or

as is the Freundlich equation normally written, where c1=constant, 1/c2=RT/Q0

Freundlich isotherm fits, not all, but many adsorption systems.


0

0 11

0

0

e

e)](1[

dQa

dQaPeb/Peb

N

N

Q/Q

Q/QRT/QRT/Q

ii

iii

1

1 1

1

0

0

PebPeb

PBPB

RT/Q

RT/Q

i i

i

H

of a

ds

Langmuir

Freundlich

211

C/pc

16

BET (Brunauer-Emmett-Teller) isotherm Many physical adsorption isotherms were found, such as the types II and III, that the

adsorption does not complete the first layer (monolayer) before it continues to stack on the subsequent layer (thus the S-shape of types II and III isotherms)

Basic assumptions the same assumptions as that of Langmuir but allow multi-layer adsorption the heat of ads. of additional layer equals to the latent heat of condensation based on the rate of adsorption=the rate of desorption for each layer of ads.

the following BET equation was derived

Where P - equilibrium pressureP0 - saturate vapour pressure of the adsorbed gas at the temperatureP/P0 is called relative pressureV - volume of adsorbed gas per kg adsorbentVm - volume of monolayer adsorbed gas per kg adsorbentc - constant associated with adsorption heat and condensation heatNote: for many adsorption systems c=exp[(H1-HL)/RT], where H1 is adsorption heat of 1st layer & HL is liquefaction heat, so that the adsorption heat can be determined from constant c.


)(111 0

0

0 P/PcVc

cV)P/P(VP/P

mm

17

Comment on the BET isotherm BET equation fits reasonably well all known adsorption isotherms observed so far

(types I to V) for various types of solid, although there is fundamental defect in the theory because of the assumptions made (no interaction between adsorbed molecules, surface homogeneity and liquefaction heat for all subsequent layers being equal).

BET isotherm, as well as all other isotherms, gives accurate account of adsorption isotherm only within restricted pressure range. At very low (P/P0<0.05) and high relative pressure (P/P0>0.35) it becomes less applicable.

The most significant contribution of BET isotherm to the surface science is that the theory provided the first applicable means of accurate determination of the surface area of a solid (since in 1945).

Many new development in relation to the theory of adsorption isotherm, most of them are accurate for a specific system under specific conditions.


18

Use of BET isotherm to determine the surface area of a solid At low relative pressure P/P0 = 0.05~0.35 it is found that

Y = a + b XThe principle of surface area determination by BET method:

A plot of against P/P0 will yield a straight line with slope of equal to (c-1)/(cVm) and

intersect 1/(cVm).

For a given adsorption system, c and Vm are constant values, the surface area of a solid material can be determined by measuring the amount of a particular gas adsorbed on the surface with known molecular cross-section area Am,

* In practice, measurement of BET surface area of a solid is carried out by N2 physisorption at liquid N2 temperature; for N2, Am = 16.2 x 10-20 m2


)( )(111 00

0

0 P/PP/PcVc

cV)P/P(VP/P

mm

P PV P P

/( / )

0

01

P/P0

P PV P P

/( / )

0

01

A A N AVVs m m mm

T P

,

.6 022 1023Vm - volume of monolayer adsorbed gas molecules calculated from the plot, L

VT,P - molar volume of the adsorbed gas, L/mol

Am - cross-section area of a single gas molecule, m2

19

Summary of adsorption isotherms

Name Isotherm equation Application Note

Langmuir

Temkin =c1ln(c2P)

Freundlich

BET


)(111 0

0

0 P/PcVc

cV)P/P(VP/P

mm

CC

B PB P

s 0

01

211

C/pc

Chemisorption andphysisorption

Chemisorption

Chemisorption andphysisorption Multilayer physisorption

Useful in analysis of reaction mechanism

Chemisorption

Easy to fit adsorption data Useful in surface area determination

20

The BET isotherm

00a

0mm0a

ppsI

ppvp

or

pp

Cv1C

Cv1

ppvp

Theoretical development based on several assumptions: multimolecular adsorption 1st layer with fixed heat of adsorption H1

following layers with heat of adsorption constant (= latent heat of condensation)

constant surface (i.e. no capillary condensation) gives

OT fig1.3

21

The BET isotherm, cont.

00a ppsI

ppvp

Plot of left side vs. p/p0 should give straight line with slope s and intercept I

Reorganizing gives

Knowledge of S0 (specific area for a volume of gas then allows the calculation of the specific surface area Sg:

where mp is the mass of the sample

IsICand

Is1vm

OT fig1.5

p

0mg m

SvS

22

BET cont’d

BET method useful, but has limitations microporous materials: mono - multilayer adsorption cannot occur, (although BET surface

areas are reported routinely) assumption about constant packing of N2 molecules not always correct? theoretical development dubious (recent molecular simulation studies, statistical

mechanics) - value of C is indication o f the shape of the isotherm, but not necessarily related to heat of adsorption

23

Simplified method

1-point method simplefied BET assuming value of C 100 (usually the case), gives

usually choose p/p0 0,15 method underestimates the surface area by approx. 5%.

0

0a'm

0'm0mm0a

pppvv

pvp

pp

Cv1C

Cv1

ppvp

24

Adsorbates An adsorbate molecule covers an area , calculated assuming dense packing of the

molecules in the multilayer. The corresponding area per volume gas is S0:

Gas Temp.[K]

σ[Å2/molecule]

S0[m2/cm3 gas (STP)]

N2 77,5 16,2 4,36Kr 77,5 19,5 5,24Ar 77,5 14,6 3,92

H2O 298 10,8 2,90C2H6 90 22,5 6,05CO2 195 19,5 5,24

25

Porosity and pore size

The pore structure (porosity, pore diameter, pore shape) is important for the catalytic properties pore diffusion may influence rates pores may be too small for large molecules to diffuse into

Measurement techniques: Hg penetration interpretation of the adsorption - desorption isotherms electron microscopy techniques

26

Hg penetration

Based on measuring the volume of a non-wetting liquid forced into the pores by pressure (typically mercury)

Surface tension will hinder the filling of the pores, at a given pressure an equilibrium between the force due to pressure and the surface tension is established:

where P = pressure of Hg, is surface tension and is the angle of wetting Common values used: = 480 dyn/cm and = 140° give average pore radius

valid in the range 50 - 50000Å

cosr2rP 2

Å]cm/kp[P

75000r 2

27

Pore size distribution

If the Hg-volume is recorded as a function of pressure and this curve is differentiated we can find the pore size distribution function V(r)=dV/dr

OT fig 2.3.

28

The Kelvin equation

0

_

ln

2

ppRT

Vrk

If adsorbent is mesoporous we get Type IV isotherm

Deviation upwards is due to filling of mesopores by capillary condensation - curved liquid meniscus in narrow pores with radius rk:

V is molar volume of the liquid, minus sign introduced since in the actual range of measurement 0 < p/p0 <1

29

The Kelvin equation

Since capillary condensation is preceeded by multilayer adsorption on the wall the value is corrected with t, the thickness of this layer:Cylindrical pores: rp = rk + t

Parallell sided slits: dp = rk + 2t

Value of t determined from measurements without capillary condensation Practical experience, typical values give for circular pores:

Values for t have been found to be a function of rk, e.g. for rk > 20Å:

][ln

547,9

0

Å

pp

rk

Årt k61,2ln429,0

30

Adsorption-desorption hysteresis

Hysteresis is classified by IUPAC (see fig.) Traditionally desorption branch used for

calculation H1: narrow distribution of mesopores H2: complex pore structure, network

effects, analysis of desorption loop misleading H2: typical for activated carbons

H3 & 4: no plateau, hence no well-defined mesopore structure, analysis difficult H3: typical for clays

Handbookfig 2 s 431

Adsorption On Solid Surface

Documents

Transcript of Adsorption On Solid Surface