Modeling the Ammonia Decomposition Reaction for Hydrogen ProductionDanielle A. Hansgen, Jingguang G. Chen, and Dionisios G. Vlachos
Center for Catalytic Science and Technology (CCST)Department of Chemical Engineering, University of Delaware, Newark, DE 19716 -2570
Motivation Interaction ParametersThe Vienna Ab-initio Simulation Package (VASP) was used to calculate heats of chemisorption as a function of adsorbate coverages from 1/9 to 1 monolayer (ML)
• The slope of the heat of chemisorption as a function of coverage is called the interaction parameter (Ixy)
Ammonia as a hydrogen carrier in a “hydrogen economy”• Ammonia is widely produced (100 million metric tonnes per year)• Liquid at low pressures (9.2 atmospheres at 298 K) • Compatible with the current infrastructure• Can be catalytically decomposed on-board to create CO free H2 for a fuel cell
1/9 ML 2/9 ML
1/3ML 2/3ML
Model Results
10
100
onve
rsio
n (%
)
Rh,
Co
Cu
FeIr Mo
Ru,
Ni
Pd
Pt Rh,
Co
Ru,
Ni
W
Plot of conversions at the end of the reactor at 950 K•When N-N and NHx-N interactions are included, the peak of the volcano curve shifts to metals with higher heats of chemisorption
Microkinetic Modeling
p ( xy)
Rational Catalyst Design• Microkinetic models are an inexpensive and efficient way to screen catalysts• Insights gained from models will help develop better catalysts
Interaction parameters were calculated for different adsorbates on ruthenium
• Strength of interactions are a function of atomic radius
1.0
Cu
Pt
Ir Pd Rh,
Co
Rh,
Co
Ru,
Ni
Ru,
Ni
Fe W Mo
1
Co
150140130120
Heat of nitrogen chemisorption (kcal/mol)
No Interactions N-N, NHx-N interactions
1.0
Cu
Pt Ir Pd Rh,
Co
Rh,
Co
Ru,
Ni
Ru,
Ni
Fe W Mo
2N* -> N2 +2* 950K 2N* -> N2 +2* 673KNH * > NH* +H* 950K
higher heats of chemisorption•Looking at surface coverages and sensitivity coefficients show why this shift occurs
Surface Coverages and Sensitivity Coefficients (SC) without Interactions
Qx(Өy) =Qxo-IxyӨy
The ammonia decomposition reaction consists of 12 elementary reaction steps
1: * *R NH NH+ → 7 * * * *R NH N H
Microkinetic Analysis-Modeling overall reaction in terms of elementary reactions
-No assumptions concerning the kinetically significant elementary steps, or most abundant reaction intermediate
3 2 21 32 2
N H N H→ +
a function of atomic radius• N-N interactions are much stronger than H-H interactions
N-N and H-H interaction parameters were calculated for a
f
MetalInteraction Parameters
INN IHH
Co -34 - 1.6
0.8
0.6
0.4
0.2
0.0
Cov
erag
e (M
L)
150140130120QN (kcal/mol)
N 950K N 673K H 950K H 673K NH3 950K NH3 673K
0.8
0.6
0.4
0.2
0.0
Sen
sitiv
ity C
oeffi
cien
t
150140130120
QN (kcal/mol)
NH2 -> NH +H 950K NH2* -> NH* +H* 673K NH3* -> NH2* +H* 950K NH3* -> NH2* +H* 673K
•For metals with heats of chemisorption higher than 125 kcal/mol, the surface is poisoned by nitrogen•For these metals the rate determining step is the removal of nitrogen based on the3 3
3 3
3 2
2 3
2
2
1: * *2 : * *3: * * * *4 : * * * *5 : * * * *6 : * * * *
R NH NHR NH NHR NH NH HR NH H NHR NH NH HR NH H NH
+ →→ ++ → ++ → ++ → ++ → +
2
2
2
2
7 : * * * *8 : * * * *9 : 2 * 2*10 : 2* 2 *11: 2 * 2*12 : 2* 2 *
R NH N HR N H NHR N NR N NR H HR H H
+ → ++ → +→ ++ →→ +
+ →
12 activation energies 12 pre-exponentialsOperating conditions
ConversionsSurface coverages
Reaction rates
PFRDifferential - algebraic
equation solver
number of metals• The averages and standard deviations are are shown in the figure to the right• Similar interaction parameters for all metals studied
IrMoNiPdPtRhRu
- 27- 43- 42- 32- 39- 37
- 2.7
+ 0.1- 1.6- 2.3- 0.5- 1.4
Average -36 -1.2
The binding energies of the NHx species were calculated
1.0
0.8
0.6
04vera
ge (M
L)
Cu
Pt
Ir Pd Rh,
Co
Rh,
Co
Ru,
Ni
Ru,
Ni
Fe W Mo
N 950K N 673K H 950K H 673K NH3 950K NH3 673K
1.0
0.8
0.6
0 4vity
Coe
ffici
ent
Cu
Pt Ir Pd
Rh,
Co
Rh,
Co
Ru,
Ni
Ru,
Ni
Fe W Mo
2N* -> N2 +2* 950K 2N* -> N2 +2* 673K NH2* -> NH* +H* 950K NH2* -> NH* +H* 673K NH3* -> NH2* +H* 950K NH3* -> NH2* +H* 673K
•For these metals, the rate determining step is the removal of nitrogen based on the sensitivity analysis
Surface Coverages and SCs with N-N and NHx-N Interactions
Model Inputs
3 differential equations6 algebraic equations
Activation Energies (Ea) - Bond Order Conservation (BOC)
Atomic heats of chemisorption (QH, QN)+
Gas phase molecular bond energies Coverage effects:density
Low coverage values: temperature programmed desorption (TPD)
g g pusing VASP at a coverage of 1/9 ML while varying nitrogen coadsorbate coverages (from 0 to 2/3 ML)
NHx-N interactions were also calculated through a DFT-BOC hybrid method where the DFT N-N interaction parameter was included in the heat of chemisorption input
The DFT-BOC hybrid method does well at approximating the NHx-N interactions and is computationally inexpensive
0.4
0.2
0.0
Cov
150140130120
QN (kcal/mol)
0.4
0.2
0.0
Sen
sitiv
150140130120
QN (kcal/mol)
• N-N interaction parameters were calculated through DFT• NHx-N interactions were calculated through the DFT-BOC hybrid method.• The amount of nitrogen on the surface is significantly reduced for most metals and the amount of hydrogen increased• When interactions are included the rate determining step is the removal of the
Future WorkPre-exponentials Fit to Ruthenium Experimental Dataa
(Assumed valid for All Metals)
Gas phase molecular bond energies
Activation energies for all 12 elementary steps
Coverage effects:density functional theory (DFT)
Conclusions
• Complete microkinetic model development• Calculate nitrogen – hydrogen adsorbate interactions using VASP
• Compare microkinetic models to experimental data
• A library of thermodynamically consistent microkinetic models has been developed for the first time to describe the chemistry on various single metals
• Nitrogen-nitrogen adsorbate interactions were estimated using VASP for metals with high nitrogen coverages and these interactions were included in the microkinetic models
• A linear correlation was used to estimate the heat of nitrogen chemisorption as
ReactionPre-exponential factor (s-1) or sticking coefficient (unitless)
NH3 + * NH3 * 1.5 x 10-4
NH3* NH3 + * 8.1 x 1011
NH3* + * NH2* +H* 2.0 x 1012
NH2* +H* NH3* +* 3.4 x 109
Experimental Operating Conditions/Parameters
When interactions are included, the rate determining step is the removal of the second hydrogen for most metals
Acknowledgments• US Department of Energy, Grant FG02-03ER15468• The Chen and Vlachos research groups
• Predict activity for more complex catalytic surfaces (bimetallics, surface defects, etc.) in order to develop higher activity catalysts using microkinetic modeling
a function of nitrogen coverage• Interactions reduced the amount of nitrogen adsorbed on the surface• Increased the amount of surface hydrogen• Increased the catalytic activity
• VASP calculations were used to confirm the validity of a DFT-BOC hybrid method of approximating NHx-nitrogen interactions
• This method was used to calculate NHx-nitrogen interactions for metals in the microkinetic library
NH2 H NH3 3.4 x 10NH2* +* NH* + H* 2.0 x 1012
NH* + H* NH2* +* 1.4 x 1010
NH* + * N* + H* 1.9 x 1012
N* + H* NH* + * 7.6 x 109
2N* N2 + 2* 1.7 x 1012
N2 + 2* 2N* 2.0 x 10-1
2H* H2 + 2* 1.1 x 1011
H2 + 2* 2H* 8.7 x 10-1
aA.B. Mhadeshwar, J.R. Kitchen, M.A. Barteau, and D.G. Vlachos, Catalysis Letters 2004, 96, 13-22.bJ.C. Slater, Journal of Chemical Physics 1964, 41, 3199-3204.
Diameter = 0.32 cmLength = 1.0 cm
Pressure =1.0 atmFlow rate = 70 sccm
Surface density = 13,100 cm2/cm3
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