chbe 452 lecture 28 mechanism of catalyst action 1
TRANSCRIPT
CHBE 452 Lecture 28Mechanism of Catalyst Action
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Last Time We Introduced Catalysis
Homogeneous catalysts
Acids and bases Metal salts Enzymes Radical initiators Solvents
Heterogeneous catalysts
Supported metals Metal oxides,
nitrides, sulfides Solid acids and
bases Polymer bound
species
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Increase rates by 1020 or more
Today: Mechanisms Of Catalyst Action
Catalysts can be designed to help initiate reactions.
Catalysts can be designed to stabilize the intermediates of a reaction.
Catalysts can be designed to hold the reactants in close proximity.
Catalysts can be designed to hold the reactants in the right configuration to react.
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Mechanism Of Catalysts, Continued
Catalysts can be designed to block side reactions.
Catalysts can be designed to sequentially stretch bonds and otherwise make bonds easier to break.
Catalysts can be designed to donate and accept electrons.
Catalysts can be designed to act as efficient means for energy transfer.
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Mechanism Of Catalysis Continued
Is is also important to realize that:
One needs a catalytic cycle to get reactions to happen.
Mass transfer limitations are more important when a catalyst is present.
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Today: The Role Of Catalysts In Initiating Reactions, Stabilizing
Intermediates Catalysts initiate reactions by help
creating active centers (i.e. a radical or ion). Active center could be catalyst itself Could be a radical R-O.
Catalysts stabilize intermediates. Catalyst binds to intermediate, lowering the
free energy of the reactive intermediates. Raises intermediate concentration. Intermediates less reactive.
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How Could Catalysts Change H2+Br22HBr
MechanismBr2→2Br
Br+H2→HBr+H
H+Br2→HBr+Br
2Br→Br2
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Fre
e en
ergy
, kca
l/mol
e of
bro
min
e at
oms
-50
0
50
1/2Br2
Br
H+HBr
Br+2HBr
1/2 Br2+2HBr
Reaction Progress
Modify Intrinsic Barriers
+H2
+Br2
-50
0
50
1/2Br2
Br
H+HBr
Br+2HBr
1/2 Br2+2HBr
Reaction Progress
Initiate Reaction
+H2
+Br2
-50
0
50
1/2Br2
Br
H+HBr
Br+2HBr
1/2 Br2+2HBr
Reaction Progress
Stabilize Intermediates
+H2
+Br2
-50
0
50
1/2Br2
Br
H+HBr
Br+2HBr
1/2 Br2+2HBr
Reaction Progress
Gas Phase
+H2
+Br2
B
D
A
C
Key Principles Of Catalytic Mechanisms
Catalysts bind intermediates at distinct sites.
Mechanism often the same in gas phase & on catalyst.
Initiation much faster – in effect do not need initiation reacion.
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Example Of Catalysts Initiating Reactions
C2H6C2H4+H2
(12.41)
C2H6 2CH3
(12.42)
CH3+C2H6C2H5+CH4
(12.43)
C2H5C2H4+H
(12.44)
H+C2H6C2H5+H2
(12.45)
2CH3C2H6
(12.46)
2c2H5C4H10
(12.47)
CH3C2H5 C3H8
(12.48)
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Gas phase mechanism
Catalysis By NO2
Consider adding NO2
(12.50) Catalysts can initiate reactions. The mechanisms
are similar to the mechanisms without a catalyst, but the initiation process is much faster with the catalyst.
Effect 109 (small for catalysis)
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NO2 C2H6 C2H5 HNO2
Some Examples Of Reactions Initiated By Catalysts
Reaction Catalyst Mechanism of Initiation
CH3 CH3 C2H4 +H2 NO2 NO2 + CH3CH3 HNO2 + CH3CH2
X+I2 2I+X
I + CH3COH HI + CH3CO
ROOR 2RO
RO+CH2=CH2ROCH2CH2
H2 + Br2 2HBr metalic platinum
Br2 + 2S 2Brad
Propylene
PolypropyleneTi+ Tl
+ + propylene CH3CHTiCH2+
C2H5OH + H+ [C2H5OH2]+
[C2H5OH3]+ [C2H5]
+ + H2O
[C2H5]+ C2H4 + H+
2O3 O2 Cl O3 + Cl O2 + ClO
C2H5OH C2H4 + H2O H+
CH3COH CH4 + CO I2
Ethylene polyethylene ROOR
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Initiation Often Not Enough. Also Need To Stabilize Intermediates
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-50
0
50
1/2Br2
Ent
halp
y, k
cal/m
ole
of b
rom
ine
atom
s
Br
H+HBr
Br+2HBr
1/2 Br2+2HBr
Reaction Progress
Gas Phase
+H2
+Br2
-50
0
50
+Br2
Ent
halp
y, k
cal/m
ole
of b
rom
ine
atom
s
Br(ad)
H(ad)+HBr
Br(ad)+2HBr
1/2 Br2+2HBr
Reaction Progress
1/2Br2
Surface
+H2
Termination
Figure 12.7 The enthalpy changes during the gas phase reaction H2 + Br2 2 HBr assuming that the reaction terminates after one cycle
Figure 12.8 The enthalpy changes during the Rideal-Eley surface reaction H2 + Br2 2 HBr on Pt(111) assuming that the reaction terminates after one cycle
Stabilization Of Ionic Intermediates
R R RRHC C H C CHH
(12.63)
Possible gas phase mechanism
X + HC C H HC CH XR R R R (12.64)
R R R R R HC C H X 2HCC H X
(12.65)
R R2HCCH X 2C CH H X
(12.66)
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Note big barrier to first step
Acid Catalyzed Reaction
R R R RHC C H H HC C HH (12.67)
R R RRHC C HH HC CHH (12.68)
RR RRHC CHH C CHH H (12.69)
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Stabilization Of Intermediates. Can We Have Too Much of a Good Thing?
When we stabilize intermediates we increase the intermediate concentration. We also decrease the reactivity of the intermediates.
Which wins?
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Experimental Evidence HCOOHH2+CO2
Heat Of Formation Of Formate
Tem
pera
ture
For
50
% C
onve
rsio
n
Au
Ag
Pt
Pd
Ir
Rh
Ru
Cu
Co
Ni Fe
W
50 60 70 80 90 100 110 120
350
400
450
500
550
600
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HCOOH HCOO(ad) Had
H(ad) HCOOad CO2 H2
(12.75)
Sabatier’s Principle
The best catalysts are substances which bind the reactants strongly, but not too strongly.
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Consider H2+Br2HBr
Rideal Eley mechanism Complicated derivation (see text)
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Br2 2S 2BradBrad H2 HBr
Had Br2 HBr Br
Had
(12.76)
rHBr 2 kBrexp p,2 Had / BT [H2][S][Br2]12 ( )1
kBr 2k20 Ea,2
0p,2 HO T Sad BT
exp /
(12.85)
(12.86)
For Unlimited Sites
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-40 -20 0 20 40 60 801E+6
1E+11
1E+16
1E+21
1E+26
1E+31
Heat Of Formation Of Intermediate, Kcal/mole
2R
ate,
Mol
ecul
es/C
m
Sec
Figure 12.10 The rate of HBr formation as calculated from Equation (12.85), with [S] = 1e14/cm2 and p= 0.5, T = 500K, .P P atmH Br2 2
1
For Finite Number Of Sites
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[S]So
1 KBr2 PBr2 KH2 PH2
Heat Of Formation Of Intermediate, Kcal/mole
2R
ate,
Mol
ecul
es/C
m
Sec
-40 -20 0 20 40 60 801E+0
1E+2
1E+4
1E+6
1E+8
1E+10
1E+12
1E+14
1E+16
Figure 12.11 The rate of HBr formation calculated from Equation (12.85), with [S] from Equation (12.87) and p= 0.5, T = 500K, .P P atmH Br2 2
1
(12.87)
Common Plots In Literature
Sachtler-Frahenfort plots:
Use heat of oxidation per mole of oxygen as surrogate for heat of formation of product.
Tanaka-Tamaru plots:
Use heat of oxidation per mole of metal as surrogate for heat of formation of product.
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Comparison Of Sachtler-Frahrenfort And Tanaka-Tamaru
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Pt
Pd Ir Rh
Ru
Cu
Co
Ni
Fe
W Ta
-300-250
-200-150
-100-50
01E+10
1E+12
1E+14
1E+16
1E+18
1E+20
Heat of formation of oxideper mole of metal, Kcal/mol
Pt Ir Rh
Ru
Cu
Co
Ni
Fe
W Ta
-110-100
-90-80
-70-60
-50-40
-30-20
-100
1E+10
1E+12
1E+14
1E+16
1E+18
1E+20
Heat of formation of oxideper mole of oxygen, Kcal/mol
Tanaka-TamaruSachtler-Frahrenfort
Pd
Rat
e, M
olec
ules
/cm
/se
c2
Rat
e, M
olec
ules
/cm
/se
c2
Figure 12.13 A Sachtler-Frahrenfort and Tanaka-Tamaru plot for the hydrogenation of ethylene.
Summary So Far
Catalysts work by initiating reactions, stabilizing intermediates.
Leads to 1020 increase in rates – need other effects to get to 1040.
Can stabilize too much.
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Example: Constructing Sachtler-Frauhenfort And Tanaka-Tamaru
Plots
Table 12.E.1 some data for the rate of ethylene hydrogenation on a number of metals 0 C. Next lets construct a Sachtler-Fahrenfort plot of the data.
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Table 12.E.1 The spreadsheet for example 12.E
A B C D E F G 4 Metal rate Hf Of
Oxide Oxygens in oxide
Metal atoms
in oxide
Hf per mole of oxide
Hf per mole of metal
5 Pt 1.0E+16 -9.7 1 1 =$C5/D5 =$C5/E5 6 Pd 3.0E+16 -20.4 1 1 =$C6/D6 =$C6/E6 7 Ir 3.0E+16 -40.1 2 1 =$C7/D7 =$C7/E7 8 Rh 3.0E+17 -21.7 1 1 =$C8/D8 =$C8/E8 9 Ru 9.0E+18 -52.5 2 1 =$C9/D9 =$C9/E9
10 Cu 8.3E+10 -39.8 1 2 =$C10/D10 11 Cu 8.3E+10 -37.1 1 1 =$C11/E11 12 Co 3.0E+16 -57.2 1 1 =$C12/D12 =$C12/E12 13 Ni 1.0E+14 -58.4 1 1 =$C13/D13 =$C13/E13 14 Fe 1.0E+16 -197.5 3 2 =$C14/D14 =$C14/E14 15 Fe 1.0E+16 -63.7 1 1 16 W 3.0E+13 -136 2 1 =$C16/D16 =$C16/E16 17 Ta 3.0E+13 -499.9 5 2 =$C17/D17 =$C17/E17
Solution
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A B C D E F G 4 Metal rate Hf Of
Oxide Oxygens in oxide
Metal atoms
in oxide
Hf per mole of oxide
Hf per mole of metal
5 Pt 1.0E+16 -9.7 1 1 =$C5/D5 =$C5/E5 6 Pd 3.0E+16 -20.4 1 1 =$C6/D6 =$C6/E6 7 Ir 3.0E+16 -40.1 2 1 =$C7/D7 =$C7/E7 8 Rh 3.0E+17 -21.7 1 1 =$C8/D8 =$C8/E8 9 Ru 9.0E+18 -52.5 2 1 =$C9/D9 =$C9/E9
10 Cu 8.3E+10 -39.8 1 2 =$C10/D10 11 Cu 8.3E+10 -37.1 1 1 =$C11/E11 12 Co 3.0E+16 -57.2 1 1 =$C12/D12 =$C12/E12 13 Ni 1.0E+14 -58.4 1 1 =$C13/D13 =$C13/E13 14 Fe 1.0E+16 -197.5 3 2 =$C14/D14 =$C14/E14 15 Fe 1.0E+16 -63.7 1 1 16 W 3.0E+13 -136 2 1 =$C16/D16 =$C16/E16 17 Ta 3.0E+13 -499.9 5 2 =$C17/D17 =$C17/E17
Comparison Of Sachtler-Frahrenfort And Tanaka-Tamaru
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Pt
Pd Ir Rh
Ru
Cu
Co
Ni
Fe
W Ta
-300-250
-200-150
-100-50
01E+10
1E+12
1E+14
1E+16
1E+18
1E+20
Heat of formation of oxideper mole of metal, Kcal/mol
Pt Ir Rh
Ru
Cu
Co
Ni
Fe
W Ta
-110-100
-90-80
-70-60
-50-40
-30-20
-100
1E+10
1E+12
1E+14
1E+16
1E+18
1E+20
Heat of formation of oxideper mole of oxygen, Kcal/mol
Tanaka-TamaruSachtler-Frahrenfort
Pd
Rat
e, M
olec
ules
/cm
/se
c2
Rat
e, M
olec
ules
/cm
/se
c2
Figure 12.13 A Sachtler-Frahrenfort and Tanaka-Tamaru plot for the hydrogenation of ethylene.
Stabilizing Intermediates Not Entire Effect
Leads to 1020 increases in rates – -need other effects to get to 1040
Does not lead to selectivity
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Summary
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Catalysts work by initiating reactions, stabilizing intermediates.
Leads to 1020 increase in rates – need other effects to get to 1040.
Can stabilize too much.
Other effects connect selectivity.
Query
What did you learn new in this lecture?
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