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8.5 adsorption at gas / solid interface
Levine pp. 397 402 section 13.5 adsorption of
gases on solids
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8.3.1 Specific area
1) Surface atoms
Si
Surface layer
Suspending bond
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.
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2) Degree of subdivision
arris Surface area Surface atom
1 cm 6 cm2 3 ? 10-8
1 ?m 6 ? 104 cm2 30
1 nm 6 ? 107 cm2 100
3) Specific area
Metal catalyst Pt, Pd, Ni, Fe Oxide Al2O3,
SiO2 Activated carbon charcoal. Catalysts or
catalyst support
The surface-to-volume / mass ratio
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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
lt 80 mNm-1 486.5 400 1880 2400-5400
Powered solids have very high surface energy!
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4) Surface energy and micropowder explosion
400 mJ?m-2 ? 6 ? 103 m2 2400 J
Micropowder explosion micropowder
is thermo-dynamically unstable.
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2. The way for solids to lower their surface
energy
Solid can not lower surface energy by
shrinking its surface. How can solid lower its
surface energy?
Adsorption of gas molecules
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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.
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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
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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.
b.p.
physisorption involves forces similar to the
van der Waals forces that lead to condensation
of vapors to liquid.
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Transition from physisorption to chemisorption
Transition state
Chemical adsorption
Physical adsorption
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.
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Comparison between chemical and physical
adsorption
sorption physisorption chemisorption
interaction
Thermal effect
temperature
Adsorbed layer
reversibility
selectivity
Van der Vaals force
Chemical bond
Condensation heat lt 40 kJ mol-1 -4-40
Chemical reaction heat gt 80 kJ mol-1. -40 -800
Around b.p.
Relatively higher
multilayer
monolayer
Reversible. equilibrium established rapidly.
Less reversible
good
poor
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Conditions for chemisorption
2W N2 2 W-Nad
?adsHm D(N2) 2X (W-N) lt 0
Thermodynamic discussion of adsorption
Adsorption is spontaneous,
Entropy decreases during adsorption
Therefore
adsorption is a exothermic process
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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.
constant pressure isobar
constant temperature isotherm
constant adsorption isochore
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isobar
As temperature increases, adsorption decrease.
Adsorption heat can be determined from isochore
on the base of Clausius-Clappeyron equation
equation.
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isotherm
Adsorption increases with increase of pressure
under low pressure. While at high pressure,
adsorption attains maximum value.
adsorption 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.
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1) Types of adsorption isotherm
Brunauer found that the isotherms of
substances can be subdivided into five types.
78 K, N2 / active carbon
78 K N2 / silica gel or iron catalyst
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323 K benzene / Fe2O3
352 K Br2 / silica gel
373 K water / active carbon
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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 Prize USA, 1881/01/31
1957/08/16 for his discoveries and investigations
in surface chemistry.
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2) Degree of coverage
S the total adsorption sites S1 sites occupied
by molecules
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3)Adsorption and desorption equilibrium
b (adsorption coefficient) the kinetic
equilibrium constant of the adsorption and
desorption
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at low pressure, bp ltlt 1,
at high pressure, bp gtgt 1,
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For monolayer adsorption, the specific area can
be thus estimated according to
For Langmuir adsorption, when plotting p/V
against p, straight line can be obtained.
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When dissociation occur during adsorption
Dissociatvie / nondissociatvie (molecular)
Adsorption of mixed gases
For gas A and B ?A, ?B, PA, PB
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4) Problems with Langmuir isotherm
Adsorption of hydrogen on different metal
surface.
Adsorption of nitrogen on graphitized carbon
black.
Adsorption heat depends on the coverage,
which suggests that the surface of adsorbent is
not uniform. Some sites are more active than the
other.
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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
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BET absorption isotherm is valid for type I
through type III.
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Measurement of specific area of solid with BET
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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
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Valid for adsorption in solution
2) Temkin adsorption isotherm
Temkin adsorption isotherm can be derived on
the assumption that
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Conversion of CO to CO2 on nanoporous copper
catalysts
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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.