High Temperature Water Gas Shift Reaction over Iron Oxide Catalysts Rainee VanNatter, Carl RF Lund
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Transcript of High Temperature Water Gas Shift Reaction over Iron Oxide Catalysts Rainee VanNatter, Carl RF Lund
High Temperature Water Gas Shift Reaction over Iron Oxide CatalystsRainee VanNatter, Carl RF Lund
Introduction
Kinetic Modeling
Computational Chemistry
?
Experimental Data
Reaction Mechanism
Model Catalyst Surface
Estimated Adsorption
& Activation Energies
Estimated Adsorption
& Activation Energies
CO + H2O ↔ H2 + CO2
Uses: Adjust synthesis gas to a H2:CO ratio needed for another process (such as ammonia synthesis); Hydrogen production; Reduction of carbon monoxide.
Commercial Process:Water gas shift reaction is reversible and exothermic. • Maximum conversion favored by low temperature • High reaction rate favored by high temperature.
Thus, reaction typically performed in 2 stages:• High temperature stage with an iron oxide catalyst containing 5-10% chromia; followed by• Low temperature stage with Copper/ZnO/alumina catalyst.
Quantum chemical calculations and microkinetic modeling are being carried out to study the water gas shift reaction over iron oxide catalysts. Cluster models corresponding to potential active sites on (100), (110), and (111) surfaces of an Fe3O4 crystal have been generated. The mechanistic thermochemistry of water-gas shift is being studied by computational chemistry using these model surfaces. At the same time, microkinetic modelling is being used to study various proposed redox and formate water gas shift mechanisms. These 2 sets of information can then be compared to discriminate between potential reaction mechanisms.
Selected Mechanisms being Studied
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)5()4()3()2()1(
22
2
2
2
22
COCOCOOCO
OHHHOHHOOH
OHOH
Forming the Model Catalyst Surfaces: 2-Unit (100) Example
The Kinetic Model:2-Step Redox ‘Mechanism’ Example
Magnetite (Fe3O4) Unit Cell
Magnetite is ferrimagnetic. The spins of the octahedral and tetrahedral iron cations have opposite ‘directions’.
Electron ‘hopping’ between octahedral iron cations results in an effective charge of +2.5
Final Cluster Models
Selected Cluster Models with an Adsorbed Oxygen Adatom
The Computational Chemistry
Cleaving of (100) Surface from Magnetite unit cell
Selection of Cluster Atoms
Selected Results of the Kinetic Modeling & Computational Chemistry: A Comparison
2rxn1rxnO*ads
exo‡2rxn
exo‡1rxn
O*ads
SSS
HHH
Adjustable
Fixed
Parameters
(100)(110)
(111)
Half the Octahedral iron cations were assigned a charge of +2. The other half were assigned a charge of +3.
Tetrahedral iron cations assigned a spin of -5/2.
Magnetite Electronic Properties
Settings for the Initial Wavefunction Guess
Geometry Optimizations
Jaguar SettingsUnrestricted DFT, tzv** basis set, ultrafine DFT grids
Transition State Searches & Geometry Scans
exo‡jrxnHi*
adsH
Assumptions• Steady-State, Isothermal, Isobaric Plug-Flow Reactor• Ideal Gas Behavior• Uniform Catalyst Sites• Heats of Adsorption independent of Surface Coverage & Temperature within Experimental Range
• Adsorption Entropy ≈ -ideal gas translational entropy of the gas phase species at experimental Tavg.
• Simple Transition State Theory rate expressions
T, P, mcat 0 rates flow gasInlet
in expn %Conversio COy
0 rates flow gasInlet in simCOy Conversion
SimulatedSimulation Routine (Single
Data Point)GuessesParameter
Calculate Rates
vacCOOCO
OHvacOH
PKkPkrate
PKkPkrate
2
22
2
222
1
111
Material Balance Equations(ODE’s)
1
2
1
22
2
2
2 0
ratedmnd
ratedmnd
ratedmnd
ratedmnd
dmnd
H
CO
OH
CO
N
Surface Coverage Equations(NLAE’s)
1
0 21
Ovac
O rateratedm
d
Load Initial Parameter Guesses
Load Experiment
cati mnPT 0
Calculate Rate Constants
RTH
RSk
RTH
RSk
‡2
‡2
2
‡1
‡1
1
expexp
expexp
RTH
RSK
RTH
RSK
222
111
expexp
expexp
adsO
fO
fCO
fCO
adsO
fO
fH
fOH
HTHTHTHTH
HTHTHTHTH
)()()()(
)()()()(
2
22
2
1
adsO
fO
fCO
fCO
adsO
fO
fH
fOH
STSTSTSTS
STSTSTSTS
)()()()(
)()()()(
2
22
2
1
Repeat for entire data set of 189 Experiments
Calculate simulated conversion
0
,0
,CO
finalsimCOCO
n
nnsimkCOy
Integrate through the simulated reactor
Calculate Objective Function
k
kCOsim
kCO s,Experiment All
2exp,, yy
Adjust Parameters to minimize Objective
Function
(Nelder Mead Simplex Method)
Continue .
until Objective Function
Converges to a minimum value
Results of the 2-step Redox ‘Mechanism’ Fit
Θ(O)
Θ(vac)
Simulated Catalyst Coverage
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.900.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
R² = 0.909498965529318
CO Conversion
Measured (Experiment)
Sim
ulat
ed (S
hort
Red
ox 'M
echa
nism
')
-5% -4% -3% -2% -1% 0% 1% 2% 3% 4% 5%0%
500%
1000%
1500%
2000%
2500%
3000%
3500%
4000%Sensitivity of Fit to Parameters
ΔH_ads(O)ΔH_act(1)ΔH_act(2)ΔS_ads(O)ΔS_act(1)ΔS_act(2)
%Change in Parameter
%Ch
ange
in O
bjec
tive
Func
tion
• The 2-step redox model provided a good fit to the experimental results, with an R2 value of 0.91
• The model predicted a catalyst surface fully covered with adsorbed oxygen
• The model was very sensitive to the oxygen atom adsorption energy (predicted to be -605 kJ/mol), and insensitive to the other parameters.
2-step Redox ‘Mechanism’
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2
22
COOCOOHOH
(100) (110) (111)
Estimation of Parameters: Examples
molkJE estimateadsO /8.933,
H2O*
H2O-Fe Distance
Results of Geometry Scan
0,‡n/desorptioadsorption 2 estimateexo
OHE
ads*CO
adsO*
adsHHO*
adsO*H
exo‡5rxn
exo‡4rxn
exo‡3rxn
exo‡2rxn
exo‡1rxn
exo‡5rxn
exo‡4rxn
exo‡3rxn
exo‡2rxn
exo‡1rxn
ads*CO
adsO*
adsHHO*
adsO*H
22
22
SSSS
SSSSS
HHHHH
HHHH
Adjustable
Fixed
Parameters
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****
**
22
22
2
22
COCOCOHHHCOO
HHCOOHCHOOHCHOOOHCO
OHOHFormate Mechanism 1
OCOOCOOCOOOCO
OOHOHHOOHHOOOHOOHOOH
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******
22
2
2
2
22
Redox Mechanism 2
0% 10% 20% 30% 40% 50% 60% 70% 80% 90%0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
R² = 0.920095537288159
CO Conversion, Measured
CO C
onve
rsio
n, S
imul
ated
Θ(H2O)Θ(HO*H)Θ(O)
Θ(CO2)
Θ(vac)
Average Simulated Site Coverage
-5% -4% -3% -2% -1% 0% 1% 2% 3% 4% 5%
Sensitivity to Parametershloc_H2O
hloc_HO*H
hloc_O
hloc_CO2
hact(1)
hact(2)
hact(3)
hact(4)
hact(5)
%Change in Parameter
• The redox model provided a good fit to the experimental results, with an R2 value of 0.92
• The model predicted a catalyst surface largely covered with adsorbed oxygen
• The model was most sensitive to the adsorption energies of dissociated water (-565 kJ/mol) and oxygen (-558 kJ/mol), and the activation energy of the hydrogen formation step (123.0 kJ/mol).
Formate Mechanism 2
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22
22
2
22
COCOCOHHHCOO
HHCOOHHOCOHHOOH
OHOH
adsO*H
K.M.: 2-step Redox
K.M.: Redox1, 9-Param
K.M.: Redox1, 14-Param
(111) pre-capped, O + * → O*
(111) pre-capped, O + *O → O*O
(111), O + * → O*
(111), O + *O → O*O
(110) 3x, O + * → O*
(100) 2x, O + * → O*
(100) 2x, O + *O → O*O
(100) 4x pre-capped, O + * → O*
(100) 4x, O + * → O*
K.M.: Redox1, 9-Param
K.M.: Redox1, 14-Param
(111) pre-capped, H + OH + *O → HO*OH †
(110) 3x, H + OH + * → HO*H
(100) 2x, H + OH + * → HO*H
K.M.: Redox1, 9-Param ‡
K.M.: Redox1, 14-Param ‡
(111) 4x, H2O + * → H2O*
(111) 4x, Pre-capped, H2O + *O → H2O*O
(110) 3x, H2O + * → H2O*
(100) 2x, H2O + * → H2O*
K.M.: Redox1, 9-Param ‡
K.M.: Redox1, 14-Param ‡
(111) 4x, CO2 + * → CO2*
(111) 4x, Pre-capped, CO2 + *O → CO2*O
(110) 3x, CO2 + * → CO2*
(100) 2x, CO2 + * → CO2*
-605.0 kJ/mol
-557.5 kJ/mol
-581.6 kJ/mol
-849.9 kJ/mol
-668.8 kJ/mol
-680.9 kJ/mol
-701.5 kJ/mol
-404.5 kJ/mol
-163.0 kJ/mol
-22.4 kJ/mol
-193.2 kJ/mol
-130.7 kJ/mol
-565.0 kJ/mol
-604.5 kJ/mol
-810.4 kJ/mol
-446.5 kJ/mol
-505.6 kJ/mol
-108.7 kJ/mol
-135.8 kJ/mol
-62.2 kJ/mol
-31.0 kJ/mol
-158.6 kJ/mol
-54.9 kJ/mol
-123.0 kJ/mol
-148.1 kJ/mol
-103.1 kJ/mol
-37.5 kJ/mol
-69.1 kJ/mol
-51.2 kJ/mol
adsHHO*H
adsO*H2
H
ads*CO2
H
† HO*H not possible on this surface. ‡ This kinetic model is fairly insensitive to this parameter
Preliminary Results
• The fitted 2-step redox model’s sensitive parameter (hloc(O)) best agrees with the computational chemistry results for the (111) surfaces. The computational chemistry results for the (100) and (110) surfaces predict too weak of a bond with the surface for any reaction to happen.
• The adsorption energy of O estimated using the cluster models varies largely with coverage. The Kinetic modeling results are thus best compared with surfaces corresponding to the coverage (largely O-covered) predicted by the kinetic modeling results.
Future Work
• Larger clusters are being studied to examine the effect of cluster size on the results, and whether our current clusters are too small to obtain accurate estimates.
• Kinetic Modeling needs to be carried out for other mechanisms. This includes a redox mechanism utilizing neighboring Fe-sites (for the (111) surfaces) and various formate and carbonate mechanisms.
• Quantum chemistry computations need to be carried out for cluster models having occupied neighboring sites as predicted by the kinetic modeling, where not already performed.
• Transition state searches and geometry scans need to be performed for the majority of clusters to estimate activation energies.
Kinetic Modeling Results of Redox Mechanism 1Redox Mechanism 1