8.4 adsorption at gas / solid interfacecourse.sdu.edu.cn/Download2/20201214225159883.pdfBET isotherm: Brunauer-Emmett-Teller (1938) 1) Basic assumptions (1) uniform surface; (2) multilayer

§8.4 adsorption at gas / solid interface

Levine:

pp. 397– 402 section 13.5 adsorption of gases on

solids

8.4.1 solid surface and specific surface area

Si

Suspending bond

Surface layer: enlarged interlayer space

Matter that is at or near an interface is not in the same state as those in the interior of

a phase. Owing to the suspending bond, solid surface is apt to react with other substances.

How de we stabilize the solid surface?

(1) Surface of solid: unbalanced force and suspending bond


Single atom catalyst, is it real?

检索结果: 139(来自所有数据库)您的检索: 标题: (catalyst) AND 标题: (single atom) ...

(1) Surface of solid: unbalanced force and suspending bond


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

400 mJm-2 6 103 m2 = 2400 J


Powder explosion in Baxian, Xinbei, Taiwan

http://www.iqiyi.com/w_19rt68ph15.html


Micropowder explosion: micropowder

is thermodynamically unstable.

(3) Danger of suspending powder

8.4.2 adsorption on solid surface

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

(1) The way to stabilize solid surface


Suspending bonds?

High surface energy?

Sorption: movement of a material from

one phase to another. Adsorption vs.

absorption

(2) Characteristics of adsorbents


Material

Specific

surface area

(m2 g−1)

MnFe2O4/activated carbon

magnetic composites

553

Activated Carbons from

Agricultural Residues

821

Activated carbon prepared from

lignin

931.53

Powdered activated carbon 1100---1230

Activated Carbon Fiber 1153.25

Commercial activated carbon 1200

Activated carbons from Iris

tectorum doped with ferric nitrate

1371

NaOH-activated carbon produced

from macadamia nut shells

1524

Adsorbate: the material being adsorbed.

Adsorbent: the material doing the

adsorbing.

chemical adsorption,

physical adsorption

chemisorption,

physisorption

(3) Kind of adsorption


Chemisorption

involves the formation of chemical bonds

between adsorbed molecule and solid surface,

and often involving the breaking of preexisting

bonds in the adsorbed molecule. In some cases

the chemisorption step requires an activation

energy.

physi-sorption

involves forces similar to the intermolecular

forces / weak interaction 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 reactant molecules at relatively

mild conditions. This is the basic principal

for heterogeneous catalysis.Physical adsorption

Transition state

Chemical adsorption

2Ni + H2

2Ni + 2H

−Ni−Ni−

H H −Ni−Ni−

H−H



sorption physisorption chemisorption

interaction

Thermal effect

temperature

Adsorbed layer

reversibility

selectivity

Comparison between chemical and physical adsorption



STHG −=

Adsorption is spontaneous, 0G

Entropy decreases during

adsorption 0S

0H

adsorption is a exothermic process

Therefore:

(4) Thermodynamic aspects of adsorption

Conditions for chemisorption

2W + N2 = 2 W-Nad

adsHm = D(N2) – 2X (W-N) < 0

2N

W-N2

DX


8.4.3 expressions of adsorption

The amount of adsorption (a) of a certain kind of solid

for certain gases is a function of T and p.( , )a a T p=

Ta ~ ~T p(1) For constant pressure: isobar (2) For constant adsorption : isochore

150 200 250 300 350 T/K

150

100

50

a/m

lg

-1

5.3

13.3

53.3 p

200 300 400T/K

20

p/k

Pa

13.3

53.3 a

40

60

80


ads m

2

ln

a

Hp

T RT

= −


(2) For constant adsorption : isochore

Clausius-Clappeyron equation.

Adsorption increases with increase of

pressure under low pressure. While at high

pressure, adsorption attains maximum value.

(3) adsorption isotherm:

20 40 60 80 p/atm

150

100

50

a/m

lg

-1

T-23.5

0

30

80


Types of adsorption isotherm: Brunauer: five types of isotherms(3) adsorption isotherm:


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(3) adsorption isotherm:


http://www.cnpowder.com.cn/news/33458.html

8.4.4 important adsorption isotherms

(1) Langmuir isotherm

1932 Noble Prize

USA,

1881/01/31 – 1957/08/16

for his discoveries and investigations

in surface chemistry.

Irving Langmuir

S: the total adsorption sites

S1: sites occupied by

adsorbed moleculesS

S1=

(1) Degree of coverage

(2) 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 .


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 rate and equilibrium

ads desr r=

Langmuir isotherm



1

bp

bp =

+

at low pressure, bp > 1,

kam =

mV

V=

ap =

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.

LRC=loading ratio correction



Adsorption of hydrogen on different metal

surface.

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



Condensation heat

Multilayer adsorption = condensation ?

Problems with Langmuir isotherm(1) Langmuir isotherm


BET isotherm: Brunauer-Emmett-Teller (1938) 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. Adsorption of gases in

multimolecular layers. J. Am. Chem. Soc. 1938, 60: 309-319

(2) adsorption BET isotherm


0

0

( ) 1 ( 1)m

V Cp

V pp p C

p

=

− + −

0

0

( )

1 ( 1)

mp p V CV

ppC

p

−=

+ −

0

0

1 1

( ) m m

p C p

p p V V C V C p

−= +

−

22400

mag

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

Fig. 1. Nitrogen-

adsorption

isotherms at -

195°C

Fig. 2. Linear B.E.T. plots of the

isotherms of Figure 1:



Metal catalyst: Pt, Pd, Ni, Fe

Oxide: Al2O3, SiO2Activated carbon: charcoal.

Catalysts or catalyst support



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.

(2) Freundlich isotherm

Freundilich adsorption isotherm can be

derived on the assumption that:

0

ads m ads m lnH H = −

1

na kp=1

ln ln lna k pn

= +

1ln ln lna k c

n= +

Valid for adsorption in

solution


1

2 1 1

k p bp

k k p bp = =

+ + 2

1

k

kb =

ads0 exp

mHb ART

= −


Fig. 8. Comparison between experimental and computed

equilibrium data derived from the modified Freundlich

model in the case of adsorption of mixtures



(3) Temkin adsorption isotherm

Temkin adsorption isotherm can be derived

on the assumption that:

0

ads m ads mΔ Δ (1 )H H = −

ln( )a k bp=

(4) Temkin isotherm


ln( )a k bp=

ln( )a k bc=

Other isotherms:

Frenkel–Halsey–Hill Adsorption Isotherm Model

Hygroscopic Growth Theory

Frumkin adsorption model

(4) Temkin isotherm


8.4.5 Theoretical consideration of heterogeneous catalysis

（1） Mechanism of heterogeneous catalysis

A surface reaction can usually be divided into five elementary steps:

diffusion

adsorption

reaction

desorption

diffusion

1) diffusion of reactants to surface;

2) adsorption of reactants at surface;

3) reaction on the surface;

4) desorption of product from surface;

5) diffusion of product from surface.

Which is r.d.s.?


Many surface reactions can be treated successfully on the basis of the following assumptions:

For unimolecular reaction over catalyst

Catalyzed isomerization or decomposition

A (g) +

A

B +

B

1) the r.d.s. is a reaction of adsorbed molecules;

2) the reaction rate per unit surface area is proportional to .



For bimolecular reaction over catalyst

Langmuir-Hinshelwood mechanism (L-H mechanism)

Langmuir-Rideal mechanism (L-R mechanism)

A (g)

B (g)

A B

Transition state

A-B

+

A (g) +

A-B +

A

+ B (g)

A B



Synthesis of ammonia

Langmuir-Hinshelwood mechanism (L-H mechanism)

Hydrogenation of ethylene

Langmuir-Rideal mechanism (L-R mechanism)



In middle 1970s, Ertl began to studied the

mechanism of Haber-Bosch process. Ertl used

LEED, AES, UPS, TDS to study this process and

measured the work function of N adsorption on

Fe(111), Fe(100) and Fe(110). He found the

dissociation of N2 on Fe surface, and the Haber-

Bosch process obeyed L-H mechanism.

He won 2017 Noble Prize.



For unimolecular reaction

Ar Akr =

According to Langmuir isotherm

1

bp

bp =

+ 1

A A

A A

kb pr

b p=

+

（2） kinetics for heterogeneous catalysis

Under low pressure, when bAPA > 1

kr =

1

A A

A A

kb pr

b p=

+

A Ar kb p=

pA

r

rmax


When bApA > 1

1

A A

B B

kb pr

b p=

+' A

B

pr k

p=

For example

Decomposition of ammonia over Pt3

2

[NH ]

[H ]r k=

1

A A

A

A A B B

b p

b p b p =

+ +

when competing adsorption

exists:

(3) Some phenomena for heterogeneous catalysis


The situation of the L-R mechanism is the same as that of unimolecular reaction over catalyst.

For L-H mechanism, small modification should be made.

A Br k =

A A B B

2

A A B B(1 )

kb p b pr

b p b p=

+ +

Rate~ partial pressure relation of L-H mechanism

pA

pB = constant

r

(3) Some phenomena for heterogeneous catalysis


Ununiformity of solid surface and catalysis

10-9 PH3, which is insufficient for formation

of monolayer, can destroy completely the

activity of Pt catalyst toward oxidation of

ammonia.

In 1926, Talyor proposed the active site model

(4) structure of catalyst--Active sites

1) Only the molecules adsorbed on the active

sites can lead to reaction.

2) The fraction of active sites on the catalyst

surface is very low.


Fe(100)Fe(111) Fe(211)

Fe(110)

Fe(210)

Active sites in iron catalyst

for ammonia synthesis

C7: active sites

The active site is in fact atom cluster

comprising of several metal atoms.

Increase of the degree of subdivision will

increase the ununiformity of catalyst surface

and increase the number of active sites.

(4) structure of catalyst--Active sites


The potential curve of adsorption Interaction between molecule and catalyst on

catalytic activity

(5) Activity of of catalyst


If bB is very large, even at low pB, A will be

very small. The reaction of A will be greatly

retarded. The impurities with high b is catalyst

poison.

(6) Poison and renewal of catalyst

1

A A

A A B B

kb pr

b p b p=

+ +


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8.4 adsorption at gas / solid interfacecourse.sdu.edu.cn/Download2/20201214225159883.pdfBET isotherm: Brunauer-Emmett-Teller (1938) 1) Basic assumptions (1) uniform surface; (2) multilayer