§8.5 adsorption at gas / solid interface levine: pp. 397– 402 section 13.5 adsorption of gases on...
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§8.5 adsorption at gas / solid interface
Levine:
pp. 397– 402 section 13.5 adsorption of gases on solids
8.3.1 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.
1) Surface atoms
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 t
ension of solid is usually much larger than that of liqui
ds
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-dyna
mically 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
8.3.2 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 t
hat 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 chemisorption
interaction
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:
8.3.3 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 fr
om isochore on the base of Clausius-Cl
appeyron 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 c
overed by adsorbed molecules in equilibrium at constant tem
perature 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
8.3.4 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 t
he assumption that the surface of the solid is uniform a
nd 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
Conversion of CO to CO2 on nanoporous copper catalysts
Discussion:
1. Give explanation to the difference of the figures presented in Levine, p. 399. Figure 13.17
2. Discuss about the behavior of the adsorption of CO on Pd as shown in Figure 13.20.
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