The Concepts of Heterogeneous Catalysis
What is catalysis?
J.W. Niemantsverdriet
Schuit Institute of Catalysis
Eindhoven University of Technology
Lecture presented at
Mumbai University Institute of Chemical Technology
February 2003
What is catalysis
•The potential energy picture
•The kinetic picture
•The chemical bonding picture
A
B
P
separation
catalyst
P
bonding
catalyst
A
B
catalyst
reaction
potential energy
A
A
+
B
P
B
cataly st
P
A
B
P
cataly st
cataly st
cataly st
bonding
reaction
separation
reaction coordinate
What is catalysis
•The potential energy picture
•The kinetic picture
•The chemical bonding picture
Langmuir - Hinshelwood Kinetics
1915 Langmuir:
Adsorption Isotherm
1927 Hinshelwood:
Kinetics of Catalytic
Reactions
Cyril Norman
Hinshelwood
Irving Langmuir
1897 - 1967
Nobel Prize 1956
1881 - 1957
Nobel Prize 1932
• Consistent with Sabatier’s Principle
• Coverage dependence: Volcano plot
• Temperature dependence: Volcano plot
Reaction Mechanism:
A
+
*
B
+
*
Aads +
Bads
ABads
⇔
⇔
→
→
Aads
equilibrium; KA
Bads
equilibrium; KB
ABads + *
r.d.s; k
AB + *
fast
Coverages:
θ A = K A pA θ *
θB = KB pB θ*
Reaction rate:
r=
1
θ* = 1 + K p + K p
A A
B B
N* k KAKB pA pB
(1 + KA pA + KB pB )2
Irving Langmuir
(1881 - 1957)
• worked at General Electrics
• oxygen adsorption on tungsten
filaments of light bulbs
• 1932: Nobel Prize in Chemistry
• Langmuir Adsorption Isotherm:
0A =
KA [A]
1 + KA [A]
Rate of reaction, Activation energy, Order of reaction:
r=
E
app
a
(1 + KA pA + KB pB )2
∂ ln r
2 ∂ ln r
= −R
= RT
∂(1 / T )
∂T
= E
nA
N* k KAKB pA pB
rds
a
+ (1 − 2θ A ) ∆H A + (1 − 2θ B ) ∆H B
∂ ln r
=
∂ ln p A
=
∂ ln r
pA
∂ pA
= 1 − 2θ A
Rate of a Catalytic Reaction:
Pressure Dependence
Eads(A) = Eads(B) = 125 kJ/mol
s(B) = s(A) ; Eact = 50 kJ/mol
T = 600 K; pB is fixed
1,2
normalized rate
1,0
1.0
θ,
rate
0,2
0.2
rate
0.8
0,8
θA
θB
0,6
0.6
θA
0,4
0.4
θ*
0.0
0,0
-1,0
0.1
-0,8
-0,6
reaction order
positive in pA
negative in pB
-0,4
-0,2
θB
0,0
1.0
0,2
pA/pB
0,4
0,6
0,8
10
reaction order
negative in pA
positive in pB
Rate of a Catalytic Reaction:
Temperature Dependence
Eads (A)
Eads (B)
s(B) pB
Eact
=
=
=
=
135 kJ/mol
125 kJ/mol
10 s(A) pA
50 kJ/mol
θ,
normalized rate
1,0
θA
0,8
θ*
rate
0,6
0,4
0,2
θB
0,0
100
300
reaction order
negative in pA
positive in pB
500
T (K)
700
reaction order
900
positive in pA and pB
catalytic activity
The Sabatier Effect
optimum interaction
catalyst - adsorbate:
• not too strong
• not too weak
metal - adsorbate bond strength
optimum
coverage
Catalysis by Metals:
Trends in Reactivity
strong, dissociative adsorption
stable oxides, carbides, nitrides
Cr
Mn
Fe
Co
Ni
Cu
Mo
Tc
Ru
Rh
Pd
Ag
W
Re
Os
Ir
Pt
Au
Weak, molecular adsorption
stable against
oxide, carbide, nitride formation
What is catalysis
•The potential energy picture
•The kinetic picture
•The chemical bonding picture
Example: CO Oxidation
O2
CO
CO2
+
catalyst
adsorption
reaction
desorption
What is the most essential thing that the catalyst does ?
a catalyst breaks bonds……...
…...and lets other bonds form
The minimum you need to know about . . . . . .
Molecular Orbitals
Overlap:
atom
molecule
atom
anti
bonding
much
/
little overlap
Filling:
bonding
atomic
orbital
molecular
orbitals
atomic
orbital
strong
weak
no bond
and about . . . . .
Bonding in Metals
energy
4p
4s
3d
sp - band
d-band
atom
metal
density of states
Work Function: Ionization Potential of a Metal
The Surface Contribution to the Work Function
metal
Evac
ϕ
density
EF
surface
+
+
+
+
+
+
-
vacuum
+
electrons
ionic cores
distance
ϕ
: energy barrier for electrons leaving the surface
Strength of metals:
Filling of bands
EF
EF
10
Cohesive Energy (eV)
5d-series
8
EF
6
4d-series
4
2
3d-series
0
Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn
Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd
Ba La Hf Ta W Re Os Ir Pt Au Hg
Atomic Adsorption in the ‘Resonant Level Model’
(the simplest description)
EVac
Vacuum level
Ionization
energy
Work function
EF
Fermi level
ea
sp-band
Core level 1
Core level 2
Metal
Jellium has no d-band
(e.g. Al)
Adsorbed atom
Free atom
Atom on free-electron metals (no d-band):
Evac
Evac
Evac
EF
EF
EF
jellium adsorbed free
metal
atom
atom
jellium adsorbed free
metal
atom
atom
jellium adsorbed free
metal
atom
atom
Molecules on free-electron metals (no d-band):
a)
b)
Evac
Evac
σ*
1s
EF
1s
σ*
EF
1s
σ
jellium
metal
adsorbed
molecule
free
molecule
1s
σ
jellium
metal
adsorbed
molecule
free
molecule
Atom on d-metal:
a)
b)
Evac
Evac
antibonding
EF
antibonding
EF
d-band
bonding
bonding
s-band
d-metal
adsorbed
atom
free
atom
d-metal
adsorbed
atom
free
atom
Molecular Adsorption on a d-metal
σ-orbitals
Evac
σ*-orbitals
antibonding
σ*
antibonding
EF
1s
1s
bonding
σ
bonding
d-metal
adsorbed molecule
free molecule
d-5σ
Evac
d-2π*
antibonding
2π*
antibonding
EF
bonding
5σ
bonding
d-metal
adsorbed molecule
“relieved repulsion”
favors on-top adsorption
often called “donation”
free molecule
“back donation”
binds molecule to surface
weakens internal CO bond!
favors multiple coordination
This picture is the key to understanding
catalysis in terms of orbital theory
CO adsorption on a d-metal
a catalyst breaks bonds……...
How?
Atoms prefer high coordination sites
hcp
fcc
top
bridge
Carbon
on
Rh(111)
1.99 eV
0.00 eV
hcp
0.67 eV
0.24 eV
fcc
top
Nitrogen
on
Rh(111)
bridge
2.13 eV
0.00 eV
0.14 eV
0.72 eV
Paco Ample, Dani Curulla, Josep Ricart, Hans Niemantsverdriet, 2002
Dissociation:
The CO molecule dissociates in the transition state:
optimal overlap between d- and 2π*-orbitals
De Koster and Van Santen
Energetics of Dissociation
on a transition metal such as Fe, Ru
∆Hads (AB)
–150 kJ/mol
Eact
75 kJ/mol
Driving
Force
∆Hads (A+B)
–600 kJ/mol
Trends in chemisorption
Strong atomic adsorption
Fe
Co
Ni
Cu
Tc
Ru
Rh
Pd
Ag
Re
Os
Ir
Pt
Au
Cr
Mn
Mo
W
585 564
531
543
531
N/Metal, kJ/mo
Weaker adsorption
Evac
Evac
antibonding
EF
antibonding
EF
d-band
bonding
bonding
s-band
d-metal
adsorbed
atom
free
atom
d-band < half filled
strong bond
d-metal
adsorbed
atom
free
atom
d-band > half filled
weaker bond
Dissociation on Different Metals
e.g. Rh and Fe
∆Hads (AB)
δEact
Rh
δEact ≈ ½ δ ∆Hads (A+B)
Bronstedt-Polanyi Relation
δ ∆Hads (A+B)
Fe
Trends in chemisorption
easy dissociation
Cr
Mn
Fe
Co
Ni
Cu
Mo
Tc
Ru
Rh
Pd
Ag
W
Re
Os
Ir
Pt
Au
no dissociation
Dissociation on Different Metals
e.g. Rh and Fe
∆Hads (AB)
δEact
Rh
δEact ≈ ½ δ ∆Hads (A+B)
Bronstedt-Polanyi Relation
δ ∆Hads (A+B)
Fe
Coordinative unsaturation: higher reactivity
less dense surface
or defects
fcc (111)
9 neighbours per atom
Fermi
level
sp - band
energy
energy
<9 neighbours per atom
d-band
density of states
density of states
Coordinative unsaturation: higher reactivity
less dense surface
or defects
fcc (111)
9 neighbours per atom
sp - band
Fermi
level
energy
energy
<9 neighbours per atom
d-band
density of states
density of states
Evac
antibonding
antibonding
EF
bonding
bonding
er
g
n
o
r
st
!
bond
What is catalysis?
•The potential energy picture
•The kinetic picture
•The chemical bonding picture
If you can explain catalysis along
these three lines you have
a pretty good understanding of
what catalysis on metals means.