§8.4 adsorption at gas / solid interface

Levine:

pp. 397– 402 section 13.5 adsorption of gases on solids

8.4.1 Specific area

Si

Suspending bond

Surface layer

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.

hydrogen-terminated or fluorine-terminated surface

1) Surface atoms

Nano sheet

Nano rod

Nano dot

arris Surface area Surface atom %

1 cm 6 cm2 3 10-8

1 m 6 104 cm2 30%

1 nm 6 107 cm2 ~ 100 %

2) Degree of subdivision

3) Specific area:

V

SSv

W

SSw

The surface-to-volume / mass ratio

Metal catalyst: Pt, Pd, Ni, Fe

Oxide: Al2O3, SiO2

Activated carbon: charcoal.

Catalysts or catalyst support

Owing to the strong interatomic action, the surface

tension of solid is usually much larger than that of

liquids

Liquid Hg(l) NaCl(s) MgO(s) Mica

< 80 mNm-1 486.5 400 1880 2400-5400

Powered solids have very high surface energy!

Micropowder explosion:

micropowder is thermo-

dynamically unstable.

400 mJm-2 6 103 m2 = 2400 J

4) Surface energy and micropowder explosion

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Solid can not lower surface energy by shrinking its

surface. How can solid lower its surface energy?

gas

solid

Adsorbed molecules

Adsorption of gas molecules

2. The way for solids to lower their surface energy

8.4.2 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.

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

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.

physisorption

involves forces similar to

the van der Waals’ forces

that lead to condensation

of vapors to liquid.

b.p.

Transition from physisorption to chemisorption

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.

Physical adsorption

Transition state

Chemical adsorption

2Ni + H2

2Ni + 2H

NiNiH H NiNi

HH

sorption physisorption chemisorptioninteraction

Thermal effect

temperature

Adsorbed layer

reversibility

selectivity

Comparison between chemical and physical adsorption

Van der Vaals’ forceVan der Vaals’ force Chemical bondChemical bond

Condensation heat < 40 kJ Condensation heat < 40 kJ molmol-1; -1; -4~-40-4~-40

Chemical reaction heat > 80 Chemical reaction heat > 80 kJ molkJ mol-1-1. -40 ~ -800. -40 ~ -800

Around b.p.Around b.p. Relatively higherRelatively higher

multilayermultilayer monolayermonolayer

Reversible. equilibrium Reversible. equilibrium established rapidly.established rapidly.

Less reversibleLess reversible

poorpoor goodgood

Conditions for chemisorption

2W + N2 = 2 W-Nad

adsHm = D(N2) – 2X (W-N) < 0

2N

W-N 2

DX

Thermodynamic discussion of adsorption

STHG

Adsorption is spontaneous, 0G

Entropy decreases during adsorption 0S0H

adsorption is a exothermic process

Therefore:

8.4.3 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.

),( PTaa

Ta ~

PT ~

Pa ~

constant pressure : isobar

constant temperature : isotherm

constant adsorption : isochore

isobar

As temperature increases, adsorption decrease.

Adsorption heat can be determined

from isochore on the base of Clausius-

Clappeyron equation equation.

ads m2

ln

a

Hp

T RT

150 200 250 300 350 T/K

150

100

50

a/m

lg-1

5.3

13.3

53.3 p

isochore

200 300 400 T/K

20

p/k

Pa

13.3

53.3 a

40

60

80

Adsorption increases

with increase of

pressure under low

pressure. While at high

pressure, adsorption

attains maximum

value.

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.

adsorption isotherm:

20 40 60 80 T/K

150

100

50

a/m

lg-1

T-23.5

0

30

80

1) Types of adsorption isotherm:

Brunauer found that the isotherms of substances can be subdivided into five types.

78 K, N2 / active carbon78 K N2 / silica gel or iron catalyst

I

Vm

a

ppc

IIa

ppc

352 K Br2 / silica gel

323 K benzene / Fe2O3

373 K water / active carbon

IIIa

ppc

IVa

ppc

Va

p

8.4.4 Langmuir adsorption isotherm

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 PrizeUSA, 1881/01/31 – 1957/08/16for his discoveries and investigations in surface chemistry.

Irving Langmuir

S: the total adsorption sites

S1: sites occupied by molecules

2) Degree of coverage

S

S1

1

2 1 1

k p bp

k k p bp

2

1

k

kb

b (adsorption coefficient) :

the kinetic equilibrium constant of the adsorption and desorption

2krdes

1(1 )adsr k p

3) Adsorption and desorption equilibrium

ads desr r

1

bp

bp

at low pressure, bp << 1,

a kbp

1

kbpa k

bp

at high pressure, bp >> 1,

kam

mV

Vap

1

ap

ap

1

p

For monolayer adsorption, the specific area can be thus estimated according to

W

LV

S

m

W

022400

1m

V bp

V bp

1

m m

p p

V bV V

For Langmuir adsorption, when plotting p/V against p, straight line can be obtained.

mVS

1

mbVI

1

When dissociation occur during adsorption:

1/ 2 1/ 2A A

1/ 2 1/ 2A A1

b p

b p

Adsorption of mixed gases

For gas A and B: A, B, PA, PB

A AA

A A B B1

b p

b p b p

B B

BA A B B1

b p

b p b p

A AA

1

1n

i ii

b p

b p

Dissociatvie / nondissociatvie (molecular)

Adsorption of hydrogen on different metal surface.

4) Problems with Langmuir isotherm

Adsorption heat depends on the coverage, which suggests

that the surface of adsorbent is not uniform. Some sites are

more active than the other.

Adsorption of nitrogen on graphitized carbon black.

Condensation heat

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

00

( ) 1 ( 1)m

V cp

V pp p C

p

0

0

( )

1 ( 1)

mp p V cVpp Cp

00

1 1

( ) m m

p c p

p p V V c V c p

cV

cS

m

1

cVI

m

1

mVIS

1

22400m

ag

VLS

0 0.2 0.4 0.6 0.8

p/p0

V/V

m

1

2

c = 1

c = 10

c = 100

c = 1000

III

III

BET absorption isotherm is valid for type I through type III.

Measurement of specific area of solid with BET

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:

0ads m ads m lnH H

1

na kp1

ln ln lna k pn

2) Temkin adsorption isotherm

Temkin adsorption isotherm can be derived on the assumption that:

0ads m ads mΔ Δ (1 )H H

ln( )a k bp

1ln ln lna k c

n

Valid for adsorption in solution