3 +ch fundamentals of ch heterogeneous catalysis · heterogeneous catalysis a molecular view of...
TRANSCRIPT
1
CH
2+C
H3
*Sasol Technology R&D
Fundamentals of Heterogeneous Catalysisa molecular view of catalysis by metals
Hans NiemantsverdrietSchuit Institute of Catalysis
Contents • Introduction
• Catalysis in terms of kinetics; coverage is the key
• Bonding on surfaces: Molecular Orbital View
• Catalysts break bonds!….How? Dissociative Adsorption
• potential energy description
• molecular orbital picture
• thermodynamics
• Elementary reaction steps and kinetics
• Adsorbate interactions and the effect on kinetics
• Concluding Remarks
What is Catalysis?
bonding
reaction
separation
AB
catalyst
catalyst
catalyst
P
P
A B
• Catalysis is a cycle of elementary steps (at least three) • Catalytic sites are regenerated
What is a catalyst?
Catalysts• increase the rate of a reaction• without being consumed in the process
offer alternative, energetically favorable pathways for reactions
enable reactions to occur under industrially achievable conditions
allow selective production routes without or with less undesirable byproducts
are the work horses of the chemical industry
are the key enablers for sustainable (green)production
supported catalyst
catalystpellets and extrudates
CourtesyHaldor Topsoe
catalyticsurface
catalytically active particles on a support
shaped catalyst particles
catalyst bed in a reactor
1 nm
10 mm
1 µm
1 m
microscopic mesoscopic macroscopic
What is an ‘energetically favourable’ reaction path?How to understand a catalytic reaction in terms of potential energies?
non-catalytic energy barrier
energy barrier catalytic route much lower!
The Sabatier Effect
metal - adsorbate bond strength
cata
lytic
act
ivity optimum interaction
catalyst - adsorbate:• not too strong• not too weak
optimum coverageat the rightsurface
Example: CO Oxidation
+
adsorption reaction desorption
CO
O2
CO2
catalyst
What is the most essential thing that the catalyst does ?
a catalyst breaks bonds……...
…...and lets other bonds form
Contents • Introduction
• Catalysis in terms of kinetics; coverage is the key
• Bonding on surfaces: Molecular Orbital View
• Catalysts break bonds!….How? Dissociative Adsorption
• potential energy description
• molecular orbital picture
• thermodynamics
• Elementary reaction steps and kinetics
• Adsorbate interactions and the effect on kinetics
• Concluding Remarks
Langmuir - Hinshelwood Kinetics
Irving Langmuir1881 - 1957
Nobel Prize 1932
Cyril NormanHinshelwood
1897 - 1967Nobel Prize 1956
1915 Langmuir: Adsorption Isotherm
1927 Hinshelwood:Kinetics of Catalytic Reactions
• Consistent with Sabatier’s Principle
• Coverage dependence: Volcano plot
• Temperature dependence: Volcano plot
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 pA
1 + KA pA
Pressure (bar)0 1 2 3 4 5 6 7 8 9 10
θ co
vera
ge
0.0
0.2
0.4
0.6
0.8
1.0
K=0.1 bar-1
K=1 bar-1
K=10 bar-1
The Langmuir Adsorption Isotherm
0A = KA pA
1 + KA pA
θA = KA pAθ*
Reaction Mechanism:
A + * ⇔ Aads equilibrium; KA
B + * ⇔ Bads equilibrium; KB
Aads + Bads → ABads + * r.d.s; k
ABads → AB + * fast
Coverages:
θA = KA pAθ*
θB = KB pB θ*
θ*
= 1
1+KApA+KBpB
Reaction rate: r=N* k KAKB pApB
(1 + KApA + KBpB)2
θA
θB
rate
θ, norm
aliz
ed
rate
pA/pB
Eads(A) = Eads(B) = 125 kJ/mol
s(B) = s(A) ; Eact = 50 kJ/mol
T = 600 K; pB is fixed
θ*
Rate of a Catalytic Reaction:Pressure Dependence
reaction orderpositive in pAnegative in pB
1.0
0.8
0.6
0.4
0.2
0.0
θAθB
rate
reaction ordernegative in pApositive in pB
0.1 1.0 10
0,0
0,2
0,4
0,6
0,8
1,0
100 300 500 700 900
θ, norm
aliz
ed
rate
T (K)
Eads (A) = 135 kJ/molEads (B) = 125 kJ/mols(B) pB = 10 s(A) pAEact = 50 kJ/mol
θA
θB
θ*rate
Rate of a Catalytic Reaction:Temperature Dependence
reaction ordernegative in pApositive in pB
reaction orderpositive in pA and pB
The Sabatier Effect
metal - adsorbate bond strength
cata
lytic
act
ivity optimum interaction
catalyst - adsorbate:• not too strong• not too weak
optimum coverageat rightsurface
r=N
* k KAKB pApB
(1 + KApA + KBpB)2
Rate of reaction, Activation energy, Order of reaction:
AA
AA
A prp
prn θ21....ln
lnln
−==∂∂
=∂∂
=
( )BBAA
rdsa
appa
HHET
rRTTrRE
∆−+∆−+=
==∂
∂=
∂∂
−=
)21()21(
......ln/1
ln 2
θθ
Kinetics of catalytic reactions:
Sabatier’s Principle: Volcano Plot Methanation
Metal oxide formationper oxygen atom
-0.8 -0.4 0.0 0.4 0.8[∆E-∆E(Ru)](eV/N2)
10-5
10-4
10-3
10-2
10-1
100
101
TOF(
s-1)
Fe
Mo
Ru
Co
Ni
Os
Calculated ammonia synthesis rates400 C, 50 bar, H2:N2=3:1, 5% NH3
Logatottir, Rod, Nørskov, Hammer, Dahl, Jacobsen, J. Catal. 197, 229 (2001)
Sabatier’s Principle: Volcano Plot Ammonia Synthesis
Courtesy Jens Nørskov
Catalysis by Metals: Trends in Reactivity
stable againstoxide, carbide, nitride formation
stable oxides, carbides, nitridesstrong, dissociative adsorption
Weak, molecular adsorption
Cr Mn Fe Co Ni Cu
Mo Tc Ru Rh Pd Ag
W Re Os Ir Pt Au
Tren
ds
in c
hem
isorp
tion
Contents • Introduction
• Catalysis in terms of kinetics; coverage is the key
• Bonding on surfaces: Molecular Orbital View
• Catalysts break bonds!….How? Dissociative Adsorption
• potential energy description
• molecular orbital picture
• thermodynamics
• Elementary reaction steps and kinetics
• Adsorbate interactions and the effect on kinetics
• Concluding Remarks
atom molecule atom
atomic molecular atomicorbital orbitals orbital
antibonding
bonding
much / little overlap
strong weak no bond
The minimum you need to know about . . . . . .
Molecular OrbitalsOverlap:
Filling:
Formation of an electron band by addition of atoms and their orbital.
Note that the splitting between the bonding and anti bonding level increases by increasing the overlap.
Eventually when a high number of orbitals are added a continuum band is formed.
…...
and about . . . . .
Bonding in Metals
the minimum you need to know about . . . . .
Bonding in Metals
sp - band
d-banden
ergy
density of statesatom metal
4p
4s
3dFermi level
d-metal adsorbed freeatom atom
Evac
EF
a) b)
d-metal adsorbed freeatom atom
antibonding
bonding
antibonding
bonding
d-band
s-band
Atom on d-metal:Evac
EF
Cr Mn Fe Co Ni Cu
Mo Tc Ru Rh Pd Ag
W Re Os Ir Pt Au
Strong atomic adsorption
Weaker adsorption
Tren
ds
in c
hem
isorp
tion
d-band < half filledstrong bond
d-band > half filledweaker bond
585 564
543
531 531 C/Metal, eV
σ*
σ
1s 1s
Evac
EF
d-metal free molecule
antibonding
bonding
antibonding
bonding
σ-orbitals σ*-orbitals
Molecular Adsorption on a d-metal
adsorbed molecule
“relieved repulsion”favors on-top adsorptionoften called “donation”
“back donation”binds molecule to surfaceweakens internal CO bond!favors multiple coordination
This
pic
ture
is t
he k
ey t
o un
ders
tand
ing
cata
lysi
s in
ter
ms
of o
rbit
al t
heor
y
2300 2200 2100 2000 1900wave number (cm-1)
abso
rban
ce
CO gas
2143 cm-1
CO/ Ir/SiO2
SiO2
FTIR of CO 1) The infrared spectrum of gas phase CO shows rotational fine structure, which isabsent in the spectrum of CO adsorbed on an Ir/SiO2catalyst
2) The IR absorption frequency of adsorbed CO is lowered, mainly due to electron back donation into its 2π* orbital
IR of Gas Phase and Adsorbed CO
IR spectra by Leo van Gruijthuijsen, TU Eindhoven
a catalyst breaks bonds……...
…...and lets other bonds form
dissociativechemisorption physisorption
freemolecule
E=0
distance from the surface
energ
y
Heat ofphysisorption
Heat of chemisorption2 H atoms
σ*
σ
1s 1s
Evac
EF
d-metal free molecule
antibonding
bonding
antibonding
bonding
σ-orbitals σ*-orbitals
Molecular orbital picture of dissociation:
adsorbed molecule
J.W. Niemantsverdriet, Spectroscopy in Catalysis, Wiley-VCH, Weinheim, 1993 & 2000
causes H-H bond to break
Potential energy and orbitals of CO dissociation
∆Hads (AB)–150 kJ/mol Eact
75 kJ/mol
∆Hads (A+B)–600 kJ/mol
Energetics of Dissociationon a transition metal such as Fe, Ru
DrivingForce
Cr Mn Fe Co Ni Cu
Mo Tc Ru Rh Pd Ag
W Re Os Ir Pt Au
easy dissociation
no dissociation
Tren
ds
in c
hem
isorp
tion
∆Hads (AB) δEact
δ ∆Hads (A+B)
Dissociation on Different Metalse.g. Rh and Fe
δEact ≈ ½δ ∆Hads (A+B)Bronstedt-Polanyi Relation
Rh
Fe
Dissociation of CO on Fe(100)
adsorbed CO
C
Eact = 1.14 eVExp: 110 kJ/mol
Dissociation of CO on Fe(100)
transition state
∆H = - 0.34 eV
Dissociation of CO on Fe(100)
dissociated CO
∆H = - 0.34 –0.82 eV
Dissociation of CO on Fe(100)
dissociated COrepulsion relieved
Dissociation of CO on Fe(100)
adsorbed CO
Dissociation of CO on Fe(100)
-0.34 eV
1.14 eV(exp 110 kJ/mol)
-1.16 eV
2.30 eV
0.82 eV
dissociated COrepulsion relieved
dissociated CO
transition state
adsorbed CO
T.C. Bromfield, D. Curulla Ferre, J.W. Niemantsverdriet, ChemPhysChem, 6 (2005) 254
Compare CO dissociation on different metals
Energy Scaling Relations
Freek Scheijen, Dani Curulla, Hans Niemantsverdriet, J. Phys. Chem. C 113 (2009) 11041
Catalysis by Metals: Trends in Reactivity
Cr Mn Fe Co Ni Cu
Mo Tc Ru Rh Pd Ag
W Re Os Ir Pt Au
stable againstoxide, carbide, nitride formation
stable oxides, carbides, nitrides
strong, dissociative adsorption
Weak, molecular adsorption
Contents • Introduction
• Catalysis in terms of kinetics; coverage is the key
• Bonding on surfaces: Molecular Orbital View
• Catalysts break bonds!….How? Dissociative Adsorption
• potential energy description
• molecular orbital picture
• thermodynamics
• Elementary reaction steps and kinetics
• Adsorbate interactions and the effect on kinetics
• Concluding Remarks
Catalytic Reaction:
a cycle of elementary reaction steps
CO + NO Reaction Mechanism
CO + * → COads
NO + * → NOads
NOads + * → Nads + Oads
CO + Oads → CO2 + 2 *
2 Nads → N2 + 2 *
Can we determine the kinetics of each step?
Catalytic Reaction:
a cycle of elementary reaction steps
CO + NO Reaction on Rh(100) & (111)
only 30 % CO → CO2
CO2 formation slowN2 formation fast
200 300 400 500 600 700 800 900
0.15 ML 13
CO + 0.20 ML NO on Rh(111)
N2O
N2
13CO
NO
13CO2
Des
orp
tion r
ate
(a.u
.)
Temperature (K)
80 % CO → CO2
CO2 formation fastN2 formation slow
200 300 400 500 600 700 800 900
0.20 ML CO + 0.26 ML NO/Rh(100)
Temperature (K)
N2
CO
NO
CO2
Des
orp
tion r
ate
(a.u
.)
200 400 600 800
Temperature (K)
1000
Nads
NOads
TPD
SIMS
TPD+SIMS: NO Dissociation on Rh(100)
Rh(100)
NOads + * → Nads + Oads
37 ± 3 kJ/mol
N2,gas
NOgas
Nads + Nads → N2,gas + 2*
215 ± 10 kJ/mol
CO + NO Reaction Mechanism
CO + * → COads
NO + * → NOads
NOads + * → Nads + Oads
CO + Oads → CO2 + 2 *
2 Nads → N2 + 2 *
Can we determine the kinetics of each step?
2.1 2.4 2.7 3.0-6
-4
-2
0
2
1000 / T (K-1)
300 400 500 600
CO 2
form
atio
n ra
te (a
.u.)
Temperature (K)
COads+ Oads
Rh (100)θo = 0.16 MLθco= 0.07 ML
Rate equation:
r = k θO θCO = ν θOθCO e-Eact /RT
Plot:
ln (r/θO θCO) vs 1/T
Eact = 103 ± 5 kJ/mol
ν = 1012.7±0.7 s-1
M.J.P. Hopstaken, W.E. van Gennipand J.W. Niemantsverdriet,
Surface Sci., 433-435 (1999) 69
Kinetics of COads + Oads = CO2ln
r/θ
o θ
co
(s
-1)
dissociation of NOads
desorption of NOads
desorption of COads
reaction COads+Oads = CO2
reaction Nads + Nads = N2
37± 3
106± 10*
139± 3
103± 5
215± 10
1011± 1
1013.5± 1
1014± 0.3
1012.7± 0.7
1015.1± 0.5
65± 6
113± 10*
155± 5
67± 3
118± 10
1011± 1
1013.5± 1
1015± 1
107.3± .2
1010± 1
Eact ν Eact νkJ/mol s-1 kJ/mol s-1
Rh(100) Rh(111)
Kinetic ParametersCO + NO
H.J. Borg, J. Reijerse, R.A. van Santen, and J.W. Niemantsverdriet, J. Chem. Phys. 101 (1994) 10052
M.J.P. Hopstaken and J.W. Niemantsverdriet, J.Phys.Chem. B104 (2000) 3058 & J.Chem.Phys. 113 (2000) 5457
Contents • Introduction
• Catalysis in terms of kinetics; coverage is the key
• Bonding on surfaces: Molecular Orbital View
• Catalysts break bonds!….How? Dissociative Adsorption
• potential energy description
• molecular orbital picture
• thermodynamics
• Elementary reaction steps and kinetics
• Adsorbate interactions and the effect on kinetics
• Concluding Remarks
Adsorbate –adsorbate interactions
Li Be B C N O F-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5In
tera
ctio
n en
ergy
C
O –
X (e
V)
CO - Xattraction
CO - Xrepulsion
COXX
XX
COXX
XX
COXX
XX
CO
XX
X X
CO
XX
X X
CO
XX
X X
0.50 ML X0.25 ML CO
0.25 ML X0.25 ML CO
Li Be B C N O F-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
CO - Xattraction
CO - Xrepulsion
Li
B
C
N
O
F
LiBe B C
N
OF
Be
Atom–CO interactions on Rh(100):>0.5 ML
Nearest neighbour interactions >> next nearest neighbour interactionsD.L.S. Nieskens, D. Curulla, J.W. Niemantsverdriet, Chem Phys Chem 7 (2006) 1075
-0.34 eV
1.14 eV(exp 110 kJ/mol)
-1.16 eV
2.30 eV
0.82 eV
T.C. Bromfield, D. Curulla Ferre, J.W. Niemantsverdriet, ChemPhysChem, 6 (2005) 254
dissociated COrepulsion relieved
dissociated CO
transition state
adsorbed CO
“Ensemble Requirement” CO Dissociation
DFT CalculationCO on Fe(100)
Ensemble neededfor dissociation
“Ensemble Size” determined by repulsion between the dissociated atoms
1.14 eV(exp 110 kJ/mol)
0.82 eV
Dani Curulla, Ashriti Governder, Tracy Bromfield, Hans Niemantsverdriet, J. Phys. Chem. B 110 (2006) 13897
dissociated COrepulsion relieved
dissociated CO
adsorbed CO
transition state
1.29 eV
• sulfur retards CO dissociation• mainly by S-C and S-O repulsion• effect on CO is 0.26 eV• S blocks sites for C and O atoms
Sulfur Poisoning of CO Dissociation DFT CalculationFe(100) – (2x2)S
Contents • Introduction
• Catalysis in terms of kinetics; coverage is the key
• Bonding on surfaces: Molecular Orbital View
• Catalysts break bonds!….How? Dissociative Adsorption
• potential energy description
• molecular orbital picture
• thermodynamics
• Elementary reaction steps and kinetics
• Adsorbate interactions and the effect on kinetics
• Concluding Remarks
AcknowledgementsThe Fischer-Tropsch Synthesistechnology – mechanisms – catalysts
Hans Niemantsverdriet
Schuit Institute of Catalysis
Eindhoven University of Technology
Crude Oil
Gas
Coal
Biomass
Sun; H2O
Catalysis for Energy
Fuelsgasoline kerosine
diesel
CH3OHdimethyl ether
H2NH3
Energy Sources Catalytic Processes Energy Carriers
The Fischer-Tropsch Synthesis
• Fischer-Tropsch reactions & technology
• Mechanisms: iron and iron carbides
• GTL: cobalt catalysts and their stability
• Nano particle model systems
• Conclusions and outlook
FTSGas
Coal
Biomass
CxHyFuels
gasoline kerosine
Diesel
OygenatesOlefins
CO +H2
synthesisgas
Energy Source Catalytic Conversion Processes Energy Carriers
gasification
The Fischer-Tropsch Synthesis
GTL = Gas to Liquids (Sasol, Shell)CTL = Coal to Liquids (Sasol)
BTL = Biomass to Liquids
Photos: courtesy of Haldor Topsoe A/S
steam reforming of natural gas
CH4 + H2O = CO + 3H2 Ni on Al2O3 or MgAl2O4
How is synthesis gas produced?
water gas shift reaction
CO + H2O = CO2 + H2 Fe3O4 or Cu/ZnO on Al2O3
Alternatively: coal, oil fractions or biomass
more H2:
Cobalt catalystCO + 2H2 → - CH2- + H2O ΔH = -165 kJ/mol
H2/CO ratio at least 2 (CnH2n+2)
Iron catalystWater-Gas Shift (WGS) reaction:
CO + H2O → CO2 + H2 ΔH = -42 kJ/mol
FTS Catalysts with WGS activity:2CO + H2 → - CH2- + CO2 ΔH = -204 kJ/mol
H2/CO ratio > 0.5
Operating conditionsTemperatures: 200-350°C Pressures: 15-40 bar
Fischer-Tropsch Synthesis:SynFuels from Natural Gas, Coal or Biomass via Syn Gas
Reaction Mechanism of FTS: PolymerizationInitiation and building blocks
COads + * → Cads + Oads
H2 + 2 * → 2 Hads
Oads + 2 Hads → H2O + 3 *
Cads + 2 Hads → CH2ads + 2 *
CH2,ads + Hads → CH3,ads + *
Chain growth and terminationa simple mechanism
CH3,ads + CH2,ads → CH3CH2,ads + *
CH3,ads + Hads → CH4 + 2 *
CH3CH2,ads + CH2,ads → CH3CH2CH2,ads + *
CH3CH2,ads ± Hads → C2H5 ± 1 + 2 *
etc.
1- α
α
α
1- α
α probability of growth1- α termination to
paraffin or olefin
CH3
CH2 CH2
Chaingrowth byinsertion of CH2
into metal-C bond
leads to Flory-Schulzpolymerization kinetics
important implications for selectivity!
Selectivity of FTS Chain growth and termination
CH3,ads + CH2,ads → CH3CH2,ads + *
CH3,ads + Hads → CH4 + 2 *
CH3CH2,ads + CH2,ads → CH3CH2CH2,ads + *
CH3CH2,ads ± Hads → C2H5 ± 1 + 2 *
etc.
1- α
α
α
1- α
CH3
CH2 CH2
Chaingrowth byinsertion of CH2
into metal-C bond
Anderson-Schulz-Flory Distribution
wn = n (1-α)2 αn-1
C7-C11
≥ C20
≤ C2
C12-C19
C3-C4
Fischer-Tropsch: Polymerization to –(CH2)n–
Cr Mn Fe Co Ni Cu
Mo Tc Ru Rh Pd Ag
W Re Os Ir Pt Au
CO dissociation = hydrocarbons
no CO dissociation = methanol
Metal Catalysts in CO Hydrogenation
Fe gasoline range, oxygenatesCo diesel and waxes
Ru too expensive and difficult too handleNi methanation
Rh ethanol, C2 oxygenatesPd methanolCu methanol
Franz Fischer; 1918
Courtesy of Prof. Calvin H Bartholomew, Brigham Young University, Utah, USA
Catalysts:
Fischer-Tropsch Synthesis
Coal based
Since oil crises of the 80s:
Natural Gas to Liquidscobalt catalysts
40%
Cobalt catalystCO + 2H2 → - CH2- + H2O ΔH = -165 kJ/mol
H2/CO ratio at least 2 (CnH2n+2)
Iron catalystWater-Gas Shift (WGS) reaction:
CO + H2O → CO2 + H2 ΔH = -42 kJ/mol
FTS Catalysts with WGS activity:2CO + H2 → - CH2- + CO2 ΔH = -204 kJ/mol
H2/CO ratio > 0.5
Operating conditionsTemperatures: 200-350°C Pressures: 15-40 bar
Fischer-Tropsch Synthesis:SynFuels from Natural Gas, Coal or Biomass via Syn Gas
Removal of heatis a key issue in
FTS process design
lurry Bubble ColumnProf. Krishna - UvA
Fischer-Tropsch Reactor TechnologyFixed Bed versus Slurry Bubble Column
Fixed bed SlurryTemperature/partial pressure gradients
Isothermal, gradient less, well-mixed
High pressure drop Low pressure drop <2 bar
Extended shut down for catalyst removal
On-line catalyst removal/addition
Limited for scaling up Significant scope for scaling up (20 000 bbl/day)
Shell Sasol
Gas-To-Liquids process (GTL)
80% Sasol SPD™ DieselMost important productHigh performance fuel
Low emissionsEnvironmentally friendly
20% Sasol SPD™ NaphthaLow octane number
Mostly alkanesExcellent feedstock for
chemicals
Copyright reserved 2007, Sasol Technology R&D
oxygen
natural gassteam
Synthesis gas
Sasol slurry phase distillate process™
hydrocarbons
Sasol-Qatar Petroleum Oryx GTL plant
Copyright reserved 2007, Sasol Technology R&D
O2
Gasheater
ATR
FTS
Sasol-Qatar Petroleum Oryx GTL plant
Copyright reserved 2007, Sasol Technology R&D
34000 bbl/day
The Fischer-Tropsch Synthesis
• Fischer-Tropsch reactions & technology
• Mechanisms: iron and iron carbides
• GTL: cobalt catalysts and their stability
• Nano particle model systems
• Conclusions and outlook
www.fischer-tropsch.org
Bulletin 580Physical Chemistry of the Fischer-Tropsch Synthesis
R.B. AndersonJ.F. SchultzL.J.E. HoferH.H. Storch
1959
www.fischer-tropsch.org
Mechanism based on oxygenates
etc.
Mechanistic developments FTS
FTS via dissociated CO1980s: Ponec, Biloen-Sachtler, Rabo,
>2005: H-assisted CO dissociation (King, sev
Several detailed mechanisms proposals involving Biloen-Sachtler, Gaube, Schulz, Maitliss, Brad
Most popular mechanism: but also o intermediates considered
(Pichler-Sc
CH3
CH2 CH2
Chaingrowth byinsertion of CH2
into metal-C bond
CH3
CH2 CO
Chaingrowth byinsertion of CO
into metal-C bond
MossbauerSpectra:
mostlyFe5C2
Iron Converts into Carbides during FTS !Is FTS activity related to carbide?
G.B. Raupp and W.N. Delgass
J.B. Butt and L.H. Schwartz
P. Bussiere, G. leCaer, et al.
J.W. Niemantsverdriet,A.M. van der Kraan
W.L. van DijkH.S. van der Baan
J.Phys.Chem. 84 (1980) 3363
Behavior iron in FTS:FT
S A
ctiv
ity
time on stream
fast consumption of C from dissociated CO
by iron interior, (slows down rapidly)
slow build up ofsurface carbon
causes deacivation
J.W. Niemantsverdriet & A.M. van der Kraan, J. Catal. 72 (1981) 385
Questions: Are carbides essential for FTS on Fe catalysts?What is the FTS mechanism on iron carbide?
Aim: FTS Mechanism on Iron Carbide Surface
Questions:
How does CO adsorb and dissociate on a carbon containing surface?
How do hydrocarbons form on iron carbide surfaces?
Method: Computational Chemistry, DFT and UBI-QEPwith Dani Curulla and Jose Gracia
DFT = Density Functional Theory
UBI-QEP = unity bond index – quadratic exponential potentialE. Shustorovich, H. Sellers, Surf. Sci. Rep. 31 (1998) 1
DFT CO Dissociation on Fe(100)
-0.34 eV
1.14 eV(exp 110 kJ/mol)
-1.16 eV
2.30 eV
0.82 eV
Tracy Bromfield, Dani Curulla, Hans Niemantsverdriet, ChemPhysChem, 6 (2005) 254
dissociated COrepulsion relieved
dissociated CO
transition state
adsorbed CO
size of the catalytic ensemble
for dissociationdetermined by
the atoms!!
Ener
gy
(eV)
-8,28 eV
-6,82 eVtransition state
-5,34 eVtransition state-6,54 eV
-6,02 eV
-5,22 eV
-2,59 eV -2,55 eV-2,03 eV
transition stateH
O
C
C, O, H - Atoms on Fe(100)
Atoms prefer high coordination; Mobility: H > O >> CNote that diffusion into the lattice has been excluded
activation energydiffusion ≈ 1.5 eV
Water formation on Fe(100)(with reference to water in the gas phase and atomic oxygen adsorbed on the slab)
Two reaction pathways:
OH + OH more likely than OH + HAshriti Govender, Dani Curulla, Hans Niemantsverdriet, 2007
-1.0
0.0
1.0
2.0
CCH
CH2
CH3 CH4
Ener
gy
(eV)
Top
Top
Top
Top
Bridge BridgeBridge
Bridge
HollowHollow
Hollow
Non specific
H addition: stabilizes top/bridge, destabilizes hollowmobility increases
CH3 is a highly mobile species
CHx intermediates on Fe(100)
Surface Reaction C + H = CH
Reaction Coordinate
Ener
gy
(eV)
1) Endothermic reaction
2) Activation barrier corresponds
roughly to a reaction temperature
of ~300 K0.74 eV
0.44 eV
fill emptysite with H
0.42 eV
or exothermic reaction!
-1.5
-1.0
-0.5
0.0
0.5
1.0
C+4H
(C+H)+3H
Transition state
CH+3H
(CH+H)+2H
Transition state
CH2+2H
(CH2+H)+H
Transition state
CH3+H
(CH3+H) Transition state
CH4(a) CH4(g)
Ener
gy (e
V)
∆Er = +0.31 eV
0.71 eV
∆Er = +0.50 eV
0.81 eV
∆Er = +0.19 eV
0.77 eV
∆Er = -0.23 eV
1.5
(C+H) +3H
(CH+H) +2H
(CH2+H) +H(CH3+H)
0.75 eV
0.44 eV
0.21 eV
0.62 eV1.00 eV
Transition state I
Transition state III
Transition state II
CH4(a)
Transition state IV
Methane formation on Fe(100) from C + 4 H(with reference to methane in the gas phase and the clean slab)
In agreement with DC Sorescu, Phys Rev B 73 (2006)155420
-2.0
-1.5
-1.0
-0.5
0.0
0.5
C+4H
(C+H)+3H
CH+4H
(CH+H)+3H
CH2+4H
(CH2+H)+3H
CH3+4H
(CH3+H)+3H
CH4(a)+4H
CH4(g)+4H
Ener
gy (e
V)
∆H = -0.11 eV
0.71 eV
∆H = +0.08 eV
0.81 eV
∆H = -0.23 eV
0.77 eV
∆H = -0.65 eV
(C+H) +3H (CH+H) +2H (CH2+H) +H (CH3+H)
0.75 eV0.86 eV 0.63 eV 1.04 eV
1.42 eV
Transition state I Transition state IIITransition state II Transition state IV
Methane formation on Fe(100) from C + H(with reference to methane in the gas phase and the clean slab)
Ashriti Govender, Dani Curulla, Hans Niemantsverdriet, 2008
Example of a C-C coupling reaction stepCH3+CH2 CH3CH2
CH2+CH3
TS[CH2+CH3]
CH2CH3
CH2CH3+H
TS[CH2CH3+H]
CH3CH3
TS[CH2CH2+H]
CH2CH2
-1.00
-0.50
0.00
0.50
1.00
1.50
Ene
rgy
(eV
)
0.72 0.75
0.67
0.87 1.17
0.59
0.10
… several other mechanisms with similar barriersare conceivable
Ethylene formation pathways on Fe(100)
CHCHCCH2
CHCH+H
[CHCH+H]
CHCH2CHCH2+H
[CHCH2+H]
CHCH3
CCH2+H
[CCH2+H]
CCH3 CCH3+H
[CCH3+H][CH+CH3]
[H+CCH2]CH2+CH2
[CH2+CH2]
CH2CH2
CH+CH3CH+CH2
[CH+CH2][H+CHCH2]
C+CH3
[C+CH3]
-2.00
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
Ener
gy (e
V)
CCH2
CHCH
CHCH+H
CH+CH2
CCH2+H
C+CH3
TS[CHCH+H]
TS[CH+CH2]
TS[CCH2+H]
TS[H+CCH2]
CCH3
CHCH2
CH2CH2(g)
CHCH2+H
CH+CH3
CCH3+H
TS[H+CHCH2]
TS[CHCH2+H]
TS[CCH3+H]
TS[CH+CH3]
CH2CH2
CHCH3
0.76
0.69
0.43
1.27
0.48
1.55
0.57
0.55
0.72
0.78
0.10
-0.11
-0.08
Transition states
Transition states
TS[C+CH3]
CH2+CH2
TS[CH2+CH2]
DFT activation energies (eV) on Fe(100)
• FTS on Fe(100): several pathways possible• reactions involving CH3 are almost always favored
• but…. all surface reactions to hydrocarbons are endothermic!• and… iron is a carbide in FTS ………
C CH CH2 CH3 H
C 1.97 1.37 0.68 0.48 0.75
CH 1.37 1.14 1.27 0.57 0.71
CH2 0.68 1.27 1.55 0.72 0.81
CH3 0.48 0.57 0.72 high 0.77
in good agreement with DC Sorescu, Phys Rev B 73 (2006)155420JMH Lo, T Ziegler, J Phys. Chem. C 111 (2007) 13149
J Cheng, P Hu, P Ellis, S French, G Kelly, CM Lok, J Phys Chem C 112 (2008) 6082
FTS mechanism on Fe5C2(100)Jose Gracia, Frans Prinsloo, Hans Niemantsverdriet, TU/e + Sasol, 2009
P. J. Steynberg, J. A. van den Berg, W. J. van Rensburg, J. Phys.: Condens. Matter. 20 (2008) 064238.
top view side view‘4-fold hollow sites’ for Cc-atoms
carbide lattice
‘cartoon’:
CO adsorbs on Fe5C2(100)…Ccarbide =C=O Ccarbide + C=O (TOP)
a) C carbide =C + O (BRIDGE) b) C carbide =C + O (BRIDGE)
Eads = -0.51 eV Eads = -1.29 eV
or
but dissociation is not feasible
ΔH = +2.4 eV endothermic
in agreement with D.-B. Cao, F.-Q. Zhang, Y.-W. Li, J. Wang, H. Jiao, J. Phys. Chem. B 109 (2005) 10922
Hydrogenation Ccarb - exchange with CO
However, • formation of CH2 is endothermic (+0.33 eV)• reaction on to CH3 is favorable (-0.53 eV)
• hence: CH2 coverage low, coupling unlikely
C carbide H2 (4 - fold) + CO (top) CO (4 - fold) + C carbide H2 (top)
Hydrogenated Ccarb exchanges with CO
• formation CH3 is exothermic (-0.53 eV):→ CH3 abundantly present on the surface
• exchange CH3 - CO also exothermic (-0.63 eV)with 0.2 eV activation energy only
• places CO in a 4-fold site, where it may dissociate
C carbide H3 (bridge- ) + CcarbCO CO (4 - fold) + C carbide H3 (bridge)
Catalytic Cycle Methanation on Carbide
Similar to ‘Mars - van Krevelen’ mechanism
+CH4
in a simplified cartoon
H-assisted CO dissociation
Catalytic Cycle Methanation on Carbide
Similar to ‘Mars - van Krevelen’ mechanism
+CH4
in a simplified cartoon
H-assisted CO dissociation
Catalytic Cycle Methanation on Carbide
Similar to ‘Mars - van Krevelen’ mechanism
+CH4
in a simplified cartoon
H-assisted CO dissociation
Catalytic Cycle Methanation on Carbide
Similar to ‘Mars - van Krevelen’ mechanism
+CH4
in a simplified cartoon
H-assisted CO dissociation
Catalytic Cycle Methanation on Carbide
Similar to ‘Mars - van Krevelen’ mechanism
+CH4
in a simplified cartoon
H-assisted CO dissociation
Jose Gracia, Frans Prinsloo, Hans Niemantsverdriet, 2009
DFT – UBI-QEP Methanation on Fe5C2
ENERGY PROFILE ON FE5C2 (100)– 0.05 FOR THE REACTION:
CO + 3H2 CH4 + H2O
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
CC-H
CC-H3
H2O*
1H2
CC-H3 + CO(Fe-4- fold)
H2 O formation
C-OH bondactivation
eV
CO + 3 H2 CH4 + H2O
direct C-O bond activation
CH4
CO4-fold
COH
Cc +OH
½H2
½H2
½H2
½H2
H2O
all barriers below 0.7 eV
Jose Gracia, Frans Prinsloo, Hans Niemantsverdriet, 2009
0.5 eV
0.65 eV
DFT – UBI-QEP: H-assisted CO dissociation
Direct CO dissociation: Eact = 1.4 eV
Towards a FTS mechanism on iron carbides Fe5C2(100)
Conclusion
Feasible and plausible mechanism for methane formation on iron carbide
• Direct dissociation of CO on carbide difficult
• Ccarb from hollow site becomes CH3
• Empty hollow site available for CO
• H-assisted dissociation of CO = Ccarb + OH
Mars –van Krevelen likereaction cycle
Energy Profile Methanation Reaction on Group VIII and IB metals
G Jones, T Bligaard, F Abild-Pedersen, JK Norskov, J.Phys: Condens. Matter 20 (2008) 064239
Ag
Au
Cu
Pd, Pt
RuFe
W
Prediction:Ni, Rh, Co
can domethanation
as clean metal
Ni, RhCo
Ni, RhCo
Energy Profile Methanation Reaction on Group VIII and IB metals
G Jones, T Bligaard, F Abild-Pedersen, JK Norskov, J.Phys: Condens. Matter 20 (2008) 064239
Ag
Au
Cu
Pd, Pt
RuFe
W
Prediction:Ni, Rh, Co
can domethanation
as clean metal
but Iron worksonly as carbide
FTS Mechanism over Iron Carbide: longer hydrocarbons
Jose Gracia, Frans Prinsloo, Hans Niemantsverdriet, 2009
The Fischer-Tropsch Synthesis
• Fischer-Tropsch reactions & technology
• Mechanisms: iron and iron carbides
• GTL: cobalt catalysts and their stability
• Nano particle model systems
• Conclusions and outlook
Co/Al2O3 catalyst
20wt.% Co;
0.04 Pt
prepared by
slurry impregnation,
calcined at 250 oC
reduced at 425 oC
Copyright reserved 2006, Sasol Technology R&D
0
0.05
0.1
0.15
0.2
0.25
0.3
0 5 10 15 20Co size (nm)
Nor
mal
ized
pop
ulat
ion
TEM size distribution
TEM, XRD, Hydrogen Chemisorption: Co ∼ 6nm
G.L. Bezemer, J.H. Bitter, H.P.C.E. Kuipers, H. Oosterbeek, J.E. Holewijn,X. Xu, F. Kapteijn, A.J. van Dillen and K.P. de Jong,
J. Am. Chem. Soc. 128 (2006) 3956
Particle Size Dependence FTS Cobalt on Carbon Nano Fibres - 35 bar, 210 °C
C5+ selectivityTOF
Optimum: Co particles of 6-8 nm
Dissociation easier on steps
K. Honkala, J. Norskov et al. Science 307, 555 - 558 (2005)
Diameter approximately 6 nm:Smallest size that supports steps
needed to dissociate CO
Long term catalyst performance testingunder realistic Fischer-Tropsch synthesis
100 bbl/day slurry bubble column reactor, 230 °C, 20 bar, (H2+CO) conversion: 50-70 %,
feed gas: 50 vol. % H2, 25 vol. % CO, PH2O = 4-6 bar)
0.0
0.2
0.4
0.6
0.8
1.0
0 10 20 30 40 50 60Time on line (days)
RIAF
rela
tive
activ
ity
0 10 20 30 40 50 60 Time on line (days)
Cobalt is expensive; need to maximize catalyst life
0.0
0.2
0.4
0.6
0.8
1.0
0 10 20 30 40 50 60Time on line (days)
RIA
F
Co/Al2O3
Deactivation mechanisms:
(postulated)
• Oxidation
• Poisoning (S, HCN,NH3)
• Sintering
• Carbon deposition
Long term catalyst performance testing
Time on stream (days)
Nor
mal
ized
act
ivity
What causes the deactivation of Co FTS catalysts?
100 bbl/day slurry bubble column reactor, 230°C, 20 bar, H2+CO conversion: 50-70 %, feed gas: 50 vol. % H2, 25 vol. % CO, PH2O = 4-6 bar
free atom
atoms in a lattice
Ek = hν - Eb
hν
hν
abso
rption a
bso
rption
hν
hν
Eb
Eb
EXAFS
XANES
edgepreedge
EXAFS and XANES
/Al2O3
53% Co0
80% Co0
85%
88%
89%
XANES of Cobalt Phases
LURE, ORSAY LURE, ORSAY
XANES:
• phase identification
• oxidation state
• in situ measurement
• at synchrotron
• quantitation
straightforward
/Al2O3
53% Co0
80% Co0
85%
88%
89%
XANES of wax coated/protected catalystsfrom FT demonstration reactor
A.M. Saib, A. Borgna, J. van de Loosdrecht, P.J. van Berge, J.W. Niemantsverdriet Appl. Catal. A: General 312 (2006) 12
No oxidation of Co> 6 nm, instead reduction of unreduced cobalt.
LURE, ORSAY LURE, ORSAY
AFM Co/SiO2/Si(100) Model Catalysts
Abdool Saib, Armando Borgna, Jan van de Loosdrecht, Peter van Berge, Hans Niemantsverdriet,
J. Phys. Chem. B 110 (2006) 8657
XANES of Co/SiO2/Si(100) Oxidation
Question:Can cobalt FTS catalysts
oxidize under FTS?
Conclusion:Co/SiO2
highly resistant to oxidation by water
Abdool Saib, Armando Borgna, Jan van de Loosdrecht, Peter van Berge, Hans Niemantsverdriet,
J. Phys. Chem. B 110 (2006) 8657
0.0
0.2
0.4
0.6
0.8
1.0
0 10 20 30 40 50 60Time on line (days)
RIA
F
Co/Al2O3
Deactivation mechanisms:
(postulated)
• Oxidation
• Poisoning (S, HCN,NH3)
• Sintering
• Carbon deposition
Long term catalyst performance testing
Time on stream (days)
Nor
mal
ized
act
ivity
What causes the deactivation of Co FTS catalysts?
100 bbl/day slurry bubble column reactor, 230°C, 20 bar, H2+CO conversion: 50-70 %, feed gas: 50 vol. % H2, 25 vol. % CO, PH2O = 4-6 bar
Deactivation mechanisms: poisoning
Sulphur: irreversible effect
0 10 20 30
Time (days)
Act
ivity
(a.u
.)
Sulphur in syngas
J. van de Loosdrecht, M.M. Hauman, D.J. Moodley, S.D. Nthute, A.M. Saib, B.H. Sigwebela, Sasol Technology (Pty) Ltd, South Africa, presented at ACS Philadelphia, 2008.
0.0
0.2
0.4
0.6
0.8
1.0
0 10 20 30 40 50 60Time on line (days)
RIA
F
Co/Al2O3Deactivation mechanisms:
(postulated)
• Oxidation
• Poisoning (S, HCN,NH3) V• Sintering
• Carbon deposition
Long term catalyst performance testing
Time on stream (days)
Nor
mal
ized
act
ivity
What causes the deactivation of Co FTS catalysts?
100 bbl/day slurry bubble column reactor, 230°C, 20 bar, H2+CO conversion: 50-70 %, feed gas: 50 vol. % H2, 25 vol. % CO, PH2O = 4-6 bar
fresh catalyst after 3 days after 14 days
HAADF-TEM of CoPt/Al2O3
Sintering Cobalt Catalyst in FTSM.J. Overett, B. Breedt, E. du Plessis, W. Erasmus, J. van de Loosdrecht,
Prepr. Pap.-Am. Chem. Soc., Div. Petr. Chem. 2008, 53(2), 126
Sintering Cobalt Catalyst in FTSM.J. Overett, B. Breedt, E. du Plessis, W. Erasmus, J. van de Loosdrecht,
Prepr. Pap.-Am. Chem. Soc., Div. Petr. Chem. 2008, 53(2), 126
35% surface area loss due to sinteringin the first 20 days
0.0
0.2
0.4
0.6
0.8
1.0
0 10 20 30 40 50 60Time on line (days)
RIA
F
Co/Al2O3Deactivation mechanisms:
(postulated)
• Oxidation
• Poisoning (S, HCN,NH3) V
• Sintering V• Carbon deposition
Long term catalyst performance testing
Time on stream (days)
Nor
mal
ized
act
ivity
What causes the deactivation of Co FTS catalysts?
100 bbl/day slurry bubble column reactor, 230°C, 20 bar, H2+CO conversion: 50-70 %, feed gas: 50 vol. % H2, 25 vol. % CO, PH2O = 4-6 bar
CATALYST COVERED IN WAX…
CATALYST POWDER
Denzil Moodley
1 2 3
0.0
0.2
0.4
0.6
0.8
1.0
0 10 20 30 40 50 60Time on line (days)
RIA
F
Co/Al2O3Deactivation mechanisms:
(postulated)
• Oxidation
• Poisoning (S, HCN,NH3) V
• Sintering V• Carbon deposition V
Long term catalyst performance testing
Time on stream (days)
Nor
mal
ized
act
ivity
What causes the deactivation of Co FTS catalysts?
100 bbl/day slurry bubble column reactor, 230°C, 20 bar, H2+CO conversion: 50-70 %, feed gas: 50 vol. % H2, 25 vol. % CO, PH2O = 4-6 bar
Co/Al2O3 FTS catalyst
• Cobalt metal is active phase
• Contains reduction promotor (Pt)
• Particles in the 6-10 nm size range
• Catalyst deactivates due to a number of factors
• Sintering
• Poisons
• Inactive carbon formation (at a very slow rate)
• Oxidation is not a factor in deactivation
The Fischer-Tropsch Synthesis
• Fischer-Tropsch reactions & technology
• Mechanisms: iron and iron carbides
• GTL: cobalt catalysts and their stability
• Nano particle model systems
• Conclusions and outlook
Synthesis of FeO (wustite) particles
0
10
20
30
40
50
60
31 32 34 36 39
Diameter (nm)
Part
icle
s (
%)
Synthesis from thermal decomposition of iron carboxylate salts (22nm)
0
5
10
15
20
25
30
18 20 21 22 23 24 26-29
Diameter (nm)
Par
ticle
s (%
)
Synthesis of 12nm FeOx - T Hyeon
05
1015202530354045
13-14 15-16 17-18 19-20Diameter (nm)
% P
artic
les
Synthesis of 5nm particles -T.Hyeon
0
510
15
20
2530
35
40
6 7 8 9 10 11Diameter (nm)
% P
artic
les
Seed mediated growth after 110 min. reaction time
0
10
20
30
40
50
60
6 7 8
Diameter (nm)
Par
ticle
s (%
)
Particle growth after 30 minutes
0
10
20
30
40
50
2 3 4 5 6 7
Diameter (nm)
Part
icle
s (
%)
Particle growth after 10 minutes
0
10
20
30
40
50
60
2 3 4 5
Diameter (nm)
Par
ticle
s(%
)
4 nm
± 15%
5 nm
± 17%
7 nm
± 8%
9 nm
± 11%
iron oxide nano particlesseveral diameters - with narrow size distribution
Prabashini Moodley, Freek Scheijen, Peter Thüne
16 nm± 9%
22 nm± 7%
35 nm± 6%
Variety of preparations
iron oxide; cobalt oxide
silicon wafer
SiO2 / SiNx / SiO2membrane15 nm thick
Electronbeam-
Silicon wafer
9 TEM grids
TEM Grid: Support for Model Catalyst
Particles deposited by spin coating
Spin Coater
TEMgrid
Procedure:1. Add 600 µl iron particle suspension 2. Rotate at 2800 RPM for ca. 5 sec3. Remove disk; place in oven 450°C
4. Surfactant burns off5. Iron oxide binds to silica surface
Thermal stability iron oxide particles (oxidation 500˚C)
A
B C
E
DA
BC
E
D
9.5 nm particles
Inter-particle distance (spincoated)A1-A2 = 39.2nmB1-B2 = 14.4nmC1-C2 = 12.7nmE1-E2 = 12.8nmD1-D2 = 13.0nm
Inter particle distance (calcined)A1-A2 = 39.2nmB1-B2 = 12.3nmC1-C2 = 12.3nmE1-E2 = 11.7nmD1-D2 = 11.1nm
spin coated calcined
Super-imposed: spin coated and calcined particles
spin coated calcined
Part. SC (nm) Calc (nm)A1 10.3 10.5A2 11.6 12.5B1 10.0 10.3B2 13.2 14.2C1 9.0 9.8C2 10.3 11.2D1 10.6 11.4D2 9.0 10.3E1 10.6 11.4E2 10.0 10.6
A1 A2D1
D2
B1
B2
C1C2
E1 E2spin coatedcalcined
Super-imposed: spin coated and calcined particles
Diameter increases by about 10% upon calcination
Particle pair after spin coatingCenter 1 - Center 2 = 13nm
Particle pair A after calcinationCenter 1 – Center 2 = 11nm
• particles make contact after calcination• no major rearrangements during calcination
• no sintering
Thermal Stability of Iron Oxide Particles
1
2
1
2
During heating to 500 ˚Cthe particles flatten out
(diameter increases by ~10%)
silica support silica support
If the particles are close enough they will make contact
Tentative explanation how particles come together
(a) (b)20 nm
Reduced in H2 at 700 ºC for 45 min
Rearrangements upon reduction Calcined at 500 ºC
9.5 nm iron oxide particles core shell morphologycore sizes from 4 - 20 nm
Prabashini Moodley, TU/e
Reduction in H2 – passivation in air320 °C 500 °C 700 °C 800 °C
No reduction Appearance of hollow particles
Some particles unaffected
Partial reduction
Complete reduction
Sintering
Appearance of small particles
Appearance of core shell particles
Reduction incomplete
reduction temperature
Freek Scheijen, TU/e
TEM on the same particles after treatments
28 nm iron oxide particles on SiO2/Si(100)impregnated calcined & reduced after syngas 270ºC
20 nm
20 nm10 nm
Rearrangements: Tentative Explanation
key ingredient: iron diffusion during oxidation
fresh calcined reduced and reoxidized
core and shell hollow donut like
silicon
SiO2
We acknowledge valuable discussions with Prof Abhaya Datye on particle rearrangements
The Fischer-Tropsch Synthesis
• Fischer-Tropsch reactions & technology
• Mechanisms: iron and iron carbides
• GTL: cobalt catalysts and their stability
• Nano particle model systems
• Conclusions and outlook
Crude Oil
Gas
Coal
Biomass
Sun; H2O
Catalysis for Energy
Fuelsgasoline kerosine
diesel
CH3OHdimethyl ether
H2NH3
Energy Sources Catalytic Processes Energy Carriers
The Fischer-Tropsch Synthesis
• old technology – many new opportunities (GTL, CTL, BTL, SNG)
• mechanistically: metals are too reactive
• many endothermic surface reactions
• iron carbide: energetically favorable MvK mechanism
• Fe FTS catalyst: self assembling system
• other metals…?
• cobalt catalyst:
• stability is the major challenge
• sintering, poisoning, carbon deposition (?) contribute to deactivation
• oxidation is not a factor in deactivation
• nano particle models reveal massive rearrangement in FTS
ammonia
synthesis
Langmuir
HinshelwoodIR
Surface
ScienceComputational
chemistryBerzelius
Equilibrium
Thermodynamics
1800 1900 2000
TST
Know
ledge
Fundamental & Applied CatalysisHow much do we know?
prac
tical cat
alys
is
unde
rsta
nding
Heterogeneous Catalysis:
• having the right species
• with the right coverages
• at the right temperature
• on the right surface
Acknowledgements
Cobalt• Abdool Saib (TU/e - Sasol)
• Denzil Moodley (TU/e – Sasol)• Kees-Jan Weststrate (TU/e – Sasol))• Armando Borgna (ICES – Singapore)
• Jan van de Loosdrecht (Sasol)• Peter van Berge (Sasol)
• Abhaya Datye (Univ New Mexico)• Tiny Verhoeven (TU/e)
Iron and Iron CarbidesAshriti Govender (TU/e – Sasol)
Prabashini Moodley (TU/e – Sasol)Dani Curulla (TU/e)
Tracy Bromfield (Sasol)Freek Scheijen (TU/e)
Peter Thüne (TU/e)Frans Prinsloo (Sasol)
Jose Gracia (TU/e – Sasol)
National Computer FacilityThe Netherlands