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Transcript of Gas-Solid Interactions Light bulbs, Three way catalysts, Cracking, Corrosion, Electronic Devices...
Gas-Solid Interactions
Light bulbs, Three way catalysts, Cracking, Corrosion, Electronic
Devices etc.
Adsorption-the key stepExtent of adsorption usually given by fractional coverage
coveragemonolayer have we1when
)(N sites surface ofnumber total
)(N occupied sites surface ofnumber s
V
V
N is often equivalent to number of surface atoms of the substrate
Associative (or non-dissociative) adsorption is when a molecule adsorbs without fragmentation
Dissociative adsorption is when fragmentation occurs during the adsorption process
Adsorption IsothermsModels describing equilibrium between the gaseous and the
adsorbed phases at a given fixed temperature
Simplest is that based on Irving Langmuir’s proposals
http://www.nobel.se/chemistry/laureates/1932/langmuir-bio.html
Born in Brooklyn January 31, 1881Graduated from Columbia University in 1903
Postgraduate under Nernst in Göttingen Post at Stevens Institute of Technology New Jersey
1909 hired by General Electric CompanyStudies embraced chemistry, physics, and engineering
Investigated properties of adsorbed films and the nature of electric discharges in high vacuum and in certain gases at
low pressures.
Langmuir Isotherm
• Adsorption proceeds to monolayer formation only
• All sites are equivalent and the surface is uniform
• Molecule adsorption is independent of occupation of neighbouring sites
Simplifying assumptions
d
a
da
d
a
surface
d
a
surfaceg
k
k
p
p
kpk
kdt
d
pkdt
d
SMk
kSM
K ,K1
K
)1(
mequilibriuAt
rate desorption
)1( rate adsorption
Langmuir Isotherms
Vm
pV
p
m
p
N
p
V
p
VV
p
m
p
mm
p
N
p
NN
p
p
p
V
V
m
m
N
N
s
s
s
, N, determine to
vs,,Plot
K
1 ,
K
1 ,
K
1
Can write
K1
K
Using the isotherm
RTnpVL
Nmn
N
Vm
m
m
massmolar
determinemay we
or From
Atkins & dePaula 8th p. 918 Attard & Barnes p. 4
SA = N x Am
Specific surface area
SA/(mass of substrate)
Dissociative Adsorption
p
p
ppk
pk
kpk
kdt
dpk
dt
d
SMk
kSM
d
a
da
da
surface
d
a
surfaceg
K1
K
K)1(
,K)1(
)1(
mequilibriuAt
rate desorption ,)1( rate adsorption
)(22
2
2
22
22
2
Typical Langmuir isotherms
http://www.oup.com/uk/orc/bin/9780199271832/01student/graphs/lg_16_17_18_20.htm
Associative adsorption isotherms Dissociative adsorption isotherms
Heats of Adsorption
Vi n
Gas adsorption to a solid is exothermic.The magnitude and variation as a function of coverage may
reveal information concerning the bonding to the surface.
Calorimetric methods determine heat, Q evolved.
qi = integral heat of adsorption
TVD n
,
qD = differential heat of
adsorption
Enthalpy of Adsorption
Heats of adsorption change as a function of surface coverage
2
00
000
0000
Kln
Kln
Kln
RT
H
T
R
S
RT
H
STHRTG
SMSM
AD
ADAD
ADADAD
surfacesurfaceg
differentiate
Van’t Hoff equation
Isosteric enthalpy of adsorption
21
0
2
1
2
0
11ln
ln
lnKln
1lnlnKln
1K
TTR
H
p
p
RT
Hp
T
pTT
p
p
AD
AD
Re-arranging Langmuir
Differentiate & re-arrange
Use van’t Hoff
Measuring isosteric enthalpies
21
0
2
1 11ln
TTR
H
p
p AD
Attard & Barnes p. 83
Isosteric HEATS of adsorption sometimes used instead of enthalpies
RTqHq DADST 0
Measuring isosteric enthalpies
R
H
T
p
TdT
Td
AD0
2
/1
ln
1/1
Atkins & de Paula, 8th p. 919-920
Note
BET IsothermWhen adsorption of a gas can occur over a previously
adsorbed monolayer of the gas
Brunauer, Emmett & Teller extends the Langmuir isotherm model to multilayer adsorption
Assumptions:
Adsorption of 1st layer takes place on a surface of uniform energy
2nd layer only adsorbs on 1st, 3rd on 2nd, etc. When p=p*, infinite layers form.
At equilibrium, rates of condensation & evapouration are same for each individual layer
For layers ≥ 2, ΔH0AD = -Δ H0
VAP
BET
RTHH
s
s
VAPADec
p
p
Nc
c
NcNpp
pp
p
p
Nc
c
NcppN
p
/
**
*
**
00
11
/1
/
11
As before, we can replace N with masses or volumes.
BET
“knee” in some isotherms represents monolayer coverage
*
00
/1
1
ppV
V
HHc VAPAD
BET underestimates adsorption at low p and overestimates adsorption at high p
Using the BET
Atkins & dePaula 8th
p. 921
Principle behind the surface area and pore size analyzers on the market. Use nitrogen at 77K as adsorbate. Knowing size of molecule, the surface area and/or pore size can be determined from the isotherm.
http://www.beckman.com/products/instrument/partChar/pc_sa3100.asp
IUPAC Classification
Other isothermsWhen adsorption sites are not equivalent, enthalpy of adsorption
changes as a function of coverage
Temkin: Assumes enthalpy changes linearly with pressure
Freundlich: Assumes enthalpy changes logarithmically with pressure
2/11
21 ln
cpc
pcc
Try example in Attard & Barnes, p.83