Transcript Catalysis - University of Notre Dame

CBE 40445
Lecture 15
Introduction to Catalysis
William F. Schneider
Department of Chemical and Biomolecular Engineering
Department of Chemistry and Biochemistry
University of Notre Dame
[email protected]
Fall Semester 2005
W. F. Schneider
CBE 40445
What is a “Catalyst”
 A catalyst (Greek: καταλύτης, catalytēs) is a substance that
accelerates the rate of a chemical reaction without itself being
transformed or consumed by the reaction. (thank you Wikipedia)
k(T) = k0e-Ea/RT
Ea′ < Ea
k0′ > k0
k′ > k
Ea
Ea′
ΔG = ΔG
A+B
A+B+
catalyst
ΔG
C
ΔG
C + catalyst
uncatalyzed
catalyzed
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CBE 40445
Catalysts Open Up New Reaction Pathways
‡
O
H
O
H2C
C
OH
CH3
C
CH3
C
CH3
CH2
‡
CH3
propenol
propanone
propenol
propanone
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W. F. Schneider
CBE 40445
Catalysts Open Up New Reaction Pathways
O−
C
CH2
OH−
+ H2O
CH3
−OH−
Base catalyzed
O
OH
rate = k[OH−][acetone]
C
CH3
C
CH2
CH3
propanone
‡
CH3
propenol
‡
propenol
intermediate
propanone
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W. F. Schneider
CBE 40445
Catalysts Open Up New Reaction Pathways
‡
‡
propenol
different
intermediate
propanone
propenol
O
propanone
C
CH3
OH
rate = k[H3O+][acetone]
CH3
C
Acid catalyzed
H3O+
CH3
CH3
−H3O+
OH
C
+
CH2
CH3
+ H2O
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CBE 40445
Types of Catalysts - Enzymes
 The “Gold Standard” of
catalysts
 Highly specific
 Highly selective
 Highly efficient
 Catalyze very difficult
reactions
 N2  NH3
 CO2 + H2O  C6H12O6
Triosephosphateisomerase
“TIM”
Cytochrome C Oxidase
Highly tailored “active sites”
Often contain metal atoms
 Works better in a cell
than in a 100000 l
reactor
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CBE 40445
Types of Catalysts – Organometallic Complexes
 Perhaps closest man has
come to mimicking
nature’s success
 2005 Noble Prize in
Chemistry
 Well-defined, metal-based
active sites
 Selective, efficient
manipulation of organic
functional groups
 Various forms, especially
for polymerization
catalysis
Polymerization:
 Difficult to generalize
beyond organic
transformations
Termination:
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CBE 40445
Types of Catalysts – Homogeneous vs.
Heterogeneous
Zeolite catalyst
Catalyst powders
Homogeneous catalysis
Heterogeneous catalysis
Single phase
(Typically liquid)
Low temperature
Separations are tricky
Multiphase
(Mostly solid-liquid and solid-gas)
High temperature
Design and optimization tricky
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CBE 40445
Types of Catalysts: Crystalline Microporous
Catalysts
 Regular crystalline structure
 Porous on the scale of molecular dimensions
 10 – 100 Å
 Up to 1000’s m2/g surface area
 Catalysis through
 shape selection
 acidity/basicity
 incorporation of metal particles
10 Å
100 Å
Zeolite (silica-aluminate)
Silico-titanate
MCM-41 (mesoporous silica)
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CBE 40445
Types of Catalysts: Amorphous Heterogeneous
Catalysts
 Amorphous, high surface area supports
 Alumina, silica, activated carbon, …
 Up to 100’s of m2/g of surface area
 Impregnated with catalytic transition metals
 Pt, Pd, Ni, Fe, Ru, Cu, Ru, …
 Typically pelletized or on monoliths
 Cheap, high stability, catalyze many types of reactions
 Most used, least well understood of all classes
SEM micrographs of alumina and Pt/alumina
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W. F. Schneider
CBE 40445
Important Heterogeneous Catalytic Processes
 Haber-Bosch process
 N2 + 3 H2 → 2 NH3
 Fe/Ru catalysts, high pressure and temperature
 Critical for fertilizer and nitric acid production
 Fischer-Tropsch chemistry
 n CO + 2n H2 → (CH2)n + n H2O , syn gas to liquid fuels
 Fe/Co catalysts
 Source of fuel for Axis in WWII
 Fluidized catalytic cracking
 High MW petroleum → low MW fuels, like gasoline
 Zeolite catalysts, high temperature combustor
 In your fuel tank!
 Automotive three-way catalysis
 NOx/CO/HC → H2O/CO2/H2O
 Pt/Rh/Pd supported on ceria/alumina
 Makes exhaust 99% cleaner
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W. F. Schneider
CBE 40445
Heterogeneous Catalytic Reactors
 Design goals
 rapid and intimate contact
between catalyst and
reactants
 ease of separation of
products from catalyst
Packed Bed
(single or multi-tube)
Fluidized
Bed
Slurry
Reactor
Catalyst
Recycle
Reactor
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CBE 40445
Automotive Emissions Control System
“Three-way” Catalyst
CO  CO2
HC  CO2 + H2O
NOx  N2
Monolith reactor
Most widely deployed
heterogeneous catalyst in
the world – you probably
own one!
Pt, Rh, Pd
Alumina, ceria, lanthana, …
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W. F. Schneider
CBE 40445
Length Scales in Heterogeneous Catalysis
Mass transport/diffusion
Chemical adsorption
and reaction
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W. F. Schneider
CBE 40445
Characteristics of Heterogeneous Supported
Catalysts
 Surface area:
 Amount of internal support surface accessible to a fluid
 Measured by gas adsorption isotherms
 Loading:
 Mass of transition metal per mass of support
 Dispersion:
 Percent of metal atoms accessible to a fluid
M
M
M
support
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W. F. Schneider
CBE 40445
Rates of Catalytic Reactions
 Pseudo-homogeneous reaction rate
 r = moles / volume · time
 Mass-based rate
 r′ = moles / masscat · time
 r′ = r / ρcat
 Heterogeneous reactions happen at surfaces
 Area-based rate
 r′′ = moles / areacat · time
 r′′ = r′ / SA,
SA = area / mass
 Heterogeneous reactions happen at active sites
 Active site-based rate
TOF (s−1)
Hetero. cats. ~101
Enzymes ~106
 Turn-over frequency TOF = moles / site · time
 TOF = r′′ / ρsite
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W. F. Schneider
CBE 40445
Adsorption and Reaction at Solid Surfaces
 Physisorption: weak van der Waals attraction of a fluid
(like N2 gas) for any surface
 Eads ~10 – 40 kJ/mol
 Low temperature phenomenon
 Exploited in measuring gross surface area
 Chemisorption: chemical bond formation between a fluid
molecule (like CO or ethylene) and a surface site
 Eads ~ 100 – 500 kJ/mol
 Essential element of catalytic activity
 Exploited in measuring catalytically active sites
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W. F. Schneider
CBE 40445
Comparing Physi- and Chemisorption on MgO(001)
1.25
Calculated from first-principles DFT
O
1.48
O
Physisorbed CO2
-2 kcal mol-1 GGA
: :
CO2
C
2-
:O:surf
1.51
SO2
O
O
O
:
Mg
2.10
1.77
Chemisorbed SO2
(“sulfite”)
-25 kcal mol-1 GGA
: :
S
2-
:O:surf
2.60
1.45
SO3
1.48
1.66
2.12
Chemisorbed SO3
(“sulfate”)
-50 kcal mol-1 GGA
O
O
MgO(001) supercell
O
: :
S
2-
:O:surf
Schneider, Li, and Hass, J. Phys. Chem. B 2001, 105, 6972
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W. F. Schneider
CBE 40445
Measuring Concentrations in Heterogeneous
Reactions Kinetics
 Fluid concentrations
 Traditionally reported as pressures (torr, atm, bar)
 Ideal gas assumption: Pj = Cj RT
Rate = f(Pj,θj)
 Surface concentrations
Metal particle surface
 “Coverage” per unit area
 nj = molesj / area
 Maximum coverage called monolayer
 1 ML: nj,max = ~ 1015 molecules / cm2
 Fractional coverage
 θj = nj / nj,max
 0 ≤ θj ≤ 1
θj = 1/6
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W. F. Schneider
CBE 40445
Adsorption Isotherms
 Molecules in gas and surface are in dynamic equilibrium
A (g) + M (surface) ↔ M-A
 Isotherm describes pressure dependence of equilibrium
 Langmuir isotherm proposed by Irving Langmuir, GE, 1915




(1932 Noble Prize)
Adsorption saturates at 1 monolayer
All sites are equivalent
Adsorption is independent of coverage
rated  kd NA
ratea  ka PA N *
Site conservation
θA + θ* = 1
+
Equilibrium
rateads = ratedes
A 
KPA
, K  ka kd
1  KPA
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W. F. Schneider
CBE 40445
Using the Langmuir Isotherm
 Example: CO adsorption on 10% Ru/Al2O3 @ 100°C
PCO (torr)
100
150
200
COads (μmol/gcat)
1.28
1.63
1.77
1.94
2.06
2.21
CO adsorption on Ru/Al O at 100C
CO adsorption on Ru/Al 2O3 at 100C
Non-linear regression
250
300
2
400
3
Linearized model
2.6
200
nCO,∞ = 2.89 μmol/gcat
K = 0.0082
2.4
1.6
nCO 
1.4
nCO, KPCO
1  KPCO
P /n
1.8
n
CO
cat
(mol/g )
2
CO CO
(torr g /mol)
cat
2.2
150
100
PCO
P
1
 CO 
nCO nCO, KnCO,
1.2
1
0.8
50
200
300
400
100
200
300
400
Pressure
(torr)
Pressure
(torr)
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W. F. Schneider
100
CBE 40445
Brunauer-Emmett-Teller Isotherm (BET)
 Relaxes Langmuir restriction to single layer adsorption
 Monolayer adsorption; multilayer condensation
 Useful for total surface area measurement
 Adsorption of boiling N2 (78 K)
V
Vmono
ΔHads/ΔHcond
cz

(1  z )(1  (1  c) z )
z P
Pvap
, ce
( H ads H cond )
ΔHcond
RT
ΔHads
Solid Surface
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CBE 40445