Upload
rodger-long
View
217
Download
0
Embed Size (px)
Citation preview
ChE 553 Lecture 23 Catalysis By Surfaces
1
Objective For Today
• Ask How Surfaces Can Catalyze Reactions
2
3
Catalysis Definition
Ostwald defined a catalyst as a substance which changed the rate of reaction without itself being consumed in the process
Not being consumed catalyst does change
Freshman Chemistry View Of Catalysts
• “Catalysts lower barriers to chemical reactions”
• Catalysts often raise barriers to elementary steps in a reaction mechanism!
4
Key Mechanism Of Catalyst Action
• Stabilizing intermediates by bonding to them
• The Increase in intermediate concentration leads to a substantial increase in rate.
5
6
Catalytic Reaction Occurs Via A Catalytic Cycle:
reactants + catalyst complex
complex products + catalyst
7
Example: Rhodium Catalyzed CH3OH+COCH3COOH
HI
H O2
CH OH3
CH I3
CH COOH3
CH COI3[Rh(CO) I ]2 2
-
RhI
H C3C I
I
O
RhI
CH3
CI
I
O
CO
CO
CO
Figure 12.1 A schematic of the catalytic cycle for Acetic acid production via the Monsanto process.
Methyl radicals are not stable in the gas phase or solution but they are stable when bound to rhodium. The higher concentration leads to a higher rate
Example To Illustrate How Increasing Barriers Can Increase Rates
consider AAadB with the formation of B rate determining
The rate of reaction, r, is given by
r= KadkBPA/(1+KadPA)
Where Kad is the equilibrium constant for adsorption, kB is the rate constant for the production of B and PA is the pressure of A
8
Note the correct rate equation is
r= KadkBPA/(1+KadPA)
I am assuming that the last term is small
9
Model That I will derive In Lecture 22
• Kad = K0* exp((ΔGad+X)/RT)
Where ΔGad is free energy of adsorption on some reference surface, and X is the change in free energy in moving to some other surface
kB=ko*exp(-(Ea+0.5 X)/RT
So as you increase X, you increase the activation energy for the formation of B.
10
Now Combine The Equations
r = K0* exp((ΔGad)/RT)* ko*exp(-(Ea)/RT)* exp((0.5*X)/RT)
When X=0, r=r0 the rate on the reference surface, Combining
r = r0*exp((0.5*X)/RT)
11
Now Consider How Increases In X Changes The Rate
From before
kB=ko*exp(-(Ea+0.5 X)/RT
r = r0*exp((0.5*X)/RT)
Notice that when we increase X, we increase the activation barrier for B formation, yet the rate of B formation goes up!
12
Physical Interpretation
• The rate is given by
• r= kB θB
• When we increase X, we increase θB and decrease kB. As long as the increases in θB are larger than the decrease in kB the net rate will increase
• Most catalysts work by increasing θB. The rate increases even though the activation barrier for the rate determining step goes up. 13
Limit To The Analysis
• θB cannot be bigger than 1
• r= kB θB = KadkBPA/(1+KadPA)
• At low coverages increases in θB dominate so the net rate will increase
• Once θB gets close to 1, θB cannot increase any more. The rate decreases with increasing X.
14
Plot of actual case from lect 24
15
Heat Of Formation Of Intermediate, Kcal/mole
2R
ate
, Mol
ecul
es/
Cm
S
ec
-40 -20 0 20 40 60 801E+0
1E+2
1E+4
1E+6
1E+8
1E+10
1E+12
1E+14
1E+16
Net Result Is Volcano Behavior
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
16
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.
17
What Types Of Reactions Will Be Increased By Metal Surfaces?• Bulk metals have many free electrons
• Free electrons rapidly exchange with radicals – radical intermediates stabilized
• Free electrons neutralize carbocations– Carbocation reactions slowed down
18
19
A Selection Of The Reactions Catalyzed By Supported Metals
Reaction Catalyst Reaction Catalyst Hydrocarbon
Hydrogenation, Dehydrogenation
Pt, Pd, Ni CO + H2 Hydrocarbons
(Fischer-Tropsch)
Fe, Rh
CO oxidation, total oxidation of
hydrocarbons
Pt, Pd, Cu, Ni, Fe, Rh, Ru
Steam reforming for
production of hydrogen
Ni plus additives
CO + 2H2 CH3OH
Cu/ZnO Reforming (Isomerization of
oil)
Pt/Re/Al2O3
Key intermediates, hydrogen atoms, oxygen atoms, methyls ethyls …
Insulating Oxides Stabilize Ionic Intermediates
• Insulating oxides have no free electrons– radical intermediates not stabilized
• Insulating oxides are ionic– Ionic intermediates, i.e. carbocations can
be stabilized
20
Table 12.2-Some Reactions Commonly Catalyzed By Solid Acids And Bases
21
Reaction Example Typical Application
Isomerization(Rearranging the
structure of a molecule)
CH2=CHCH2CH3
CH3CH=CHCH3
Octane EnhancementMonomer Production
Paraxylene Production
Alkylation(Making too little molecules into a
bigger one)
CH3CH=CHCH3 + CH3CH2CH2CH3
(CH3CH2)CH(CH3)(C4H9)
Pharmaceutical Production
Monomer ProductionFine Chemicals
Butane + olefin octane
Cracking(Taking a big molecule and making it into two
littler ones).
C12H24 C7H14 + C5H10 Crude Oil ConversionDigestion
Semiconductors Are In Between
• Semiconductors have fewer free electrons, and also be ionic
• Usually use semiconductors when you want to stabilize some radicals e.g. oxygen atoms or sulfur atoms but not other radicals, such as hydrogen atoms.
22
23
A Selection Of The Reactions Catalyzed By Transition Metal Oxides, Nitrides, And Sulfides
(Semiconductors)
Reaction Catalyst Reaction Catalyst
2 SO2 + O2 2 SO3 V2O5 CO + H2O CO2+
H2 (Water Gas Shift)
FeO, CuO, ZnO
Hydrodesulfurization CoS, MoS, WS 2(CH3)3COH Þ(CH3)3COC(CH
3)3 + H2O
TiO2
CH3CH=CH2 + O2 (Bi2O3)x(MoO3)y 2 CH3CH=CH2 + 3
O2 + 2NH3
CH2=CHCHO + (Bismuth molybate)
2CH2=CHCN +
H2O Uranium Antimonate
6 H2O
(aminoxidation)
(FeO)x(Sb2O3)y
24
A Selection Of The Reactions Catalyzed By Transition Metal Oxides, Nitrides, And Sulfides
Reaction Catalyst Reaction Catalyst
4 NH3 + 4 NO +O2
4N2 + 6 H2O
benzene+O2
maleic anhydride +
water(Selective catalytic reduction)
naphthylene+O2 phthalic anhydride +
waterCH3CH2(C6H5) +O2
CH2=CH(C6H5) + H2O
NiO, Fe2O3,
V2O5, TiO2
(styrene production) CuO, Co3, O4,
MnO2
Aromatiztione.g. Heptane Tolvene
H2 or H2O
Cr2O3/Al2O3 Hydrodenitrogenation
NiS,MoS
V2O5, TiO2 (V2O5)x(PO4)y
FeO Selective oxidation of hydrocarbons
Summary
• Most catalysts work by stabilizing intermediates
• Intermediate concentration goes up
• Activation energy for product formation goes up (not down)!– Rate constant for product formation goes down
• Net effect is an increase in rate
• Metals, semiconductors, insulators stabilize different intermediates – gives different chemistry 25