Chap. 22. Processes on Solid Surfaces
Catalyst
: A substance thataccelerates a reaction
, butundergoes no net chemical changes
.Homogeneous Catalyst: A catalyst in the same phase as the reaction mixture, e.g., the decomposition of
hydrogen peroxide in aqueous solution catalyzed by iodide ion.
Heterogeneous Catalyst: A catalyst in a different phase from the reaction mixture, e.g., hydrogenation of
ethene (C2H4) to ethane (C2H6), a gas phase
Thin-Film Deposition
- - - -
Figure 22C.1
A catalyst provides a different path with a lower activation
enthalpy. The result is an increase in the rate of formation of products.
G
Surface Characteristics of Silicon (001)
~Fig. 22A.1
~Fig. 22A.2 Terrace
~Fig. 22A.4 STM
~Fig. 22A.5 STM
~Fig. 22A.6 AFM
Seoul National University 4 Joonhyeon Kang et al.
Strongly Adsorbed Weakly Adsorbed
Catalyst: Altered Activation Barrier
U. Stimming, J. Electrochem. Soc. (2005).
T = 300 K
Pt
Au
Hydrogen chemisorption energy
Free energy for hydrogen adsorption
http://bp.snu.ac.kr
~Fig. 22C.2
Log (Exchange
Current)
Log (Exchange
Current)
Volcano Curve
22A.2. Physisorption and Chemisorption
Physisorption:
- van der Waals interactions between the adsorbate and the substrate- the enthalpy of physisorption
Chemisorption:
- the molecules (or atoms) stick to the surface by forming a chemical bond- the enthalpy of chemisorption
- usually,
∆H <0, ∆S<0
(due to the reduction of translational freedom of adsorbate)20 kJ mol
≈ −
200 kJ mol
≈ −
Assumptions
1. Adsorption cannot proceed beyond monolayer coverage.
______
_______
- - - - (skip)
Types of Adsorption Modes
Physical Adsorption or
Physisorption
Chemical Adsorption or
Chemisorption
Bonding between
molecules and surface is by
weak van der Waals forces.
Chemical bond is formed between molecules and surface.
Dr. K. L. Yeung (Hong Kong Univ.) http://teaching.ust.hk/~ceng511/notes/
6
Heterogeneous Catalysis
22B. Adsorption & Desorption
The fractional coverage θ : the extent of surface coverage
adsorption of
rate the
:
coverage monolayer
complete
to ing correspond adsorbate
of volume the
:
adsorbed adsorbate
of volume the
:
available sites
adsorption of
number
occupied sites
adsorption of
number
d V V
V V
θ θ θ
∞
∞
=
=
(22B.1a)
22B. The Dynamic Equilibrium
The rate of adsorption
The rate of desorption
At equilibrium, there is no net change.
Langmuir isotherm
) AM(surface
)
surface (
M )
(
A g + ←→ka
kd
sites vacant
of number the
: ) - 1 (
sites adsorption
of number total
the :
A of pressure partial
the :
) 1
(
θ N
N p pN dt k
d
a
θ
θ
= −θ θ
N dt k
d
=
d−
d a d
a
k K k
p K
p K
N k pN
k
+ =
=
∴
=
−
1
) 1
(
θ
θ θ
(22B.1a)
(22B.1b)
(22.B2)
______
_____
Θ : Fractional Coverage
(skip---)
Figure 22B.2
The Langmuir isotherm for
non-dissociative adsorption for different values of K.
(skip---)
Example 1
The data given below are the adsorption of CO on charcoal at 273 K. Confirm that they fit the Langmuir isotherm, and find the
constant K and the volume corresponding to complete coverage. In each case V has been corrected to 1.00 atm (101.325 kPa).
p/kPa 13.3 26.7 40.0 53.3 66.7 80.0 93.3 V/cm3 10.2 18.6 25.5 31.5 36.9 41.6 46.1 Solution
with 1
Kp Kp V V
p p
V V KV
θ θ θ ∞
∞ ∞
+ = =
= +
Hence, a plot of p/V against p should give a straight line of slope and intercept
1 V∞ 1 KV∞.
p/kPa 13.3 26.7 40.0 53.3 66.7 80.0 93.3 (p/kPa)/(V/cm3) 1.30 1.44 1.57 1.69 1.81 1.92 2.02 The slope is 0.00900 and so The intercept at p=0 is 1.20, so
111 cm .3
V∞ =
3 1
3 3
1 7.51 10 kPa
(111 cm ) (1.20 kPa cm )
K = − = × − −
×
(skip---)
Figure 22B.1
The plot of the data in
Example. As illustrated here, the Langmuir isotherm
predicts that a straight line should be obtained when p/V is plotted against p.
(skip---)
The rate of adsorption for adsorption with dissociation is proportional to the pressure and to the probability that both atoms will find sites, which is proportional to the square of the number of vacant sites,
The rate of desorption
At equilibrium
Langmuir isotherm for
adsorption with dissociation
{
(1θ
)}
2θ
= −N p dt k
d
a
( ) θ
2θ
k Ndt d
= d
−
{ } ( )
Kp Kp
N k N
p
ka d
= +
∴
=
−
1
) 1
( 2 2
θ
θ
θ
Figure 22B.3
The Langmuir isotherm for dissociative adsorption, for different values of K.
) surface (
2X )
(
X2 g →
The isosteric standard enthalpy of adsorption
: thestandard enthalpy of adsorption at a fixed surface coverage the van’t Hoff equation
Example 2:
The data below show the pressures needed for the volume of adsorption (corrected to 1.00 atm and 273 K) to be 10.0 cm3 using the same sample as in Example 1. Calculate the
adsorption enthalpy at this surface coverage.
T/K 200 210 220 230 240 250
p/kPa 4.00 4.95 6.03 7.20 8.47 9.85
2 0
ln ads
RT H T
K ∆
∂ =
∂
Solution:
The Langmuir isotherm can be rearranged to
Therefore, a plot of ln p against 1/T should be a straight line R
H T
p
T dT
T d
RT H T
K T
p p K
Kp
0 ads
2
2 0 ads
) / 1 ( ln
, / 1 /
) / 1 ( with
ln ln
constant ln
ln
constant is
when 1
= ∆
∂
∂
−
=
− ∆
=
∂
− ∂
=
∂
∴ ∂
= +
= −
θ
θ θ
θ θ
θ
The slope is −0.904, so
Figure 22B.4
The isoteric enthalpy of
adsorption can be obtained
from the slope of the plot of lnp against 1/T, where p is the
pressure needed to achieve the specified coverage.
( )
1 - 3 0
ads
mol kJ
52 . 7
10 904
. 0
−
=
×
×
−
=
∆H K R
Isotherm with multilayer adsorption
BET isothem:
Stephen Brunauer, Paul Emmett, Edward Teller{ }
( H H ) RT e
c c
V p
p z p
z c z
cz V
V
/ mon
mon
0 vap 0
des
constant :
coverage monolayer
to ing correspond volume
the :
liquid bulk
pure a
resembling
adsorbate, layer
a above pressure
vapor the
:
*
*
) 1
( 1 ) 1
(
∆
−
=
∆=
−
−
= −
Figure 22B.5
Plot of the BET isotherm for different values of c . The value of V /Vmon rises indefinitely
because the adsorbate may condense on the covered substrate surface.
Example 3:
The data below relate the adsorption of N2 on rutile (TiO2) at 75 K.
Confirm that they fit a BET isotherm in the range of pressures reported, and determine Vmon and c.
p/kPa 0.160 1.87 6.11 11.67 17.02 21.92 27.29
V/mm3 601 720 822 935 1046 1146 1254
At 75 K, p*=76.0 kPa. The volumes have been corrected to 1.00 atm and 273 K and refer to 1.00 g of substrate.
Solution:
The BET equation can be reorganized into
(c−1)/cV can be obtained from the slope of a plot of the expression
mon mon
) 1 (
1 )
1
(
cVz c
cV V
z
z
−
+
− =
p/kPa 0.160 1.87 6.11 11.67 17.02 21.92 27.29
103z 2.11 24.6 80.4 154 224 288 359
104z/(1−z)(V/mm3) 0.035 0.350 1.06 1.95 2.76 3.53 4.47
When 104z/(1−z) is plotted against 103z, the intercept is 0.0398, so
The slope of the line is
3 - 6
mon
mm 10
98 .
1 3 −
× cV =
( )
3 mon
3 - 3
3 - 4
3 2
mon
mm 811
and 310
mm 10
23 . 1 mm
10 10
10 23
. 1 1
=
=
∴
×
=
×
×
×
− = − − −
V c
cV c
so , 10 23 .
1 × −2
Figure 22B.6
The BET isotherm can be tested, and the parameters determined, by plotting z/(1−z)V against z=p/p*.
Deviations from the Langmuir isotherm which assumes the independence and equivalence of the adsorption sites
The Temkin isotherm
1
( )
2 1 2ln
, : constants c c p c c
θ
=The Freundlich isotherm
2
1
1
c pc
θ
=the BET isotherm
(skip)
z V
V c
= −
≈
1 1 ,
1
mon
22C. Homogeneous Catalysis
• Features of homogeneous catalysis
[ ]
+ 3 2
+ +
3 2 2 3 2 2 +
2 2 3
+ +
3 2 2 3 2
+
H O +H O H O +H O H O
H O H O
H O +I HOI+H O H O I
HOI+H O H O +O +I (fast)
a
K
v k
− −
−
=
→ =
→
- - - -
(skip---)
2000.
of factor a
by increases constant
rate The
ion iodide of
presence in the
mol kJ
57
catalyst of
absence in the
mol kJ
76
) ( O )
( O H 2 )
( O H 2
1 -
1 -
2 2
2 2
=
=
+
→
a a
E E
g l
aq
The second step is rate-determining step, therefore we can obtain the rate law of the overall reaction by setting the overall rate
equal to the rate of the second step:
[ ]
2 r[
2 2]
3 +r a
O H O H O I
where .
d k
dt
k k K
−
=
=
In acid catalysis, the crucial step is the transfer of a proton to the substrate:
In base catalysis, a proton is transferred from the substrate to a base:
X + HA → HX+ + A− HX+ → products
XH + →B X− + BH+ X− → products
(skip---)
(ES): active combination of the enzyme and substrate
Experiments show that the rate of formation of the product depends on the concentration of the enzyme, and so although the net reaction is simply S→P, this must reflect an underlying mechanism with
steps that involve the enzyme.
The Michaelis-Menten Mechanism
Enzymes
E P
S
E + → +
E P
ES)
( S
E +
←→ka+
k 'a →kb
[ ] [ ( ) ]
[ ( ) ES ] [ ][ ] E S ' [ ( ) ES ] [ ( ) ES ]
P ES
b
k k
d k dt k d
−
−
=
=
Figure
The basis of the Michaelis-
Menten mechanism of enzyme action. Only a fragment of the large enzyme molecule E is shown.
S
E
(ES)
E P
The steady-state approximation
If [E]0 is the total concentration of the enzyme, then [E]+[(ES)]=[E]0. Since only a little enzyme is added,
[ ][ ] [ ( ) ] [ ( ) ] [ ( ) ] [ ][ ]
a b
a
b a
a
k k
k
k k
k
' S ES E
0 ES
ES '
S E
≈ +
∴
≈
−
−
[ ] [ ( ) ] [ ]
[ ( ) ] { [ ] [ ( ) ] } [ ]
[ ] [ ]
'
S ES
ES E
S ES
S
0
a b
a
k k
k
⇓
+
≈ −
∴
≈
+
The rate of enzymolysis depends linearly on the amount of enzyme added, and also on the amount of substrate present.
[ ] [ ( ) ]
[ ( ) ] [ ] [ ] [ ] [ ] [ ] [ ]
[ ] [ ] [ ] [ ]
constant Michaelis
the ' is
where
S S E S
'
S E P
S '
S ES E
P ES
0 0
0
a a b
M
M a a
a b
b a
a a
b a b
k k k k
k k k
k k
k k dt
d
k k
k k dt k
d
= +
= + +
≈ +
∴
+
≈ +
=
[ ] [ ] [ ]
[ ] [ ] S
1 1
S S
, P E
0
b M b
M b
k k k
k
k k k
dt k d
+
=
= +
=
Figure
The variation of the effective rate constant k with substrate
concentration according to the Michaelis-Menten mechanism.
0 2 8
0.2 0.4 0.6 0.8 1.0
k/k b
4 6 10
0
[ ] [ ] [ ]
[ ] [ ]
[ ] [ ] [ ]
[ ] [ ] S
1 1
/ S 1
/ S
S S
S E S E
P
M
M M 1 0
0
b b
b M
b M b
k k k
k
k k k
k k
k k
k k k dt
d
+
=
= +
= +
+ ≅
=
Figure
A Lineweaver-Burk plot for the analysis of an enzymolysis that proceeds by a Michaelis-
Menten mechanism, and the significance of the intercepts and the slope.
[ ]
M1 1
b b S k k = k + k
1/[S]
0
1/kb
−1/KM 1/k
Slope = KM/kb
(skip)