COx-Free Hydrogen by Catalytic Decomposition of Ammonia on Commercial Fe and Ru Catalysts: An Experimental and Theoretical Study
Caitlin CallaghanBarry GraceOrest SkoplyakIlie FishtikRavindra Datta
Fuel Cell CenterChemical Engineering DepartmentWorcester Polytechnic InstituteWorcester, MA 01609
Motivation
Prospect of PEM Fuel Cells Environmental benefit Limited oil reserves
Need for Suitable Hydrogen Source Hydrogen content/ energy density Fuel processing Storage / transportation
Comparison of H2 Sources
H2 Source H2 Energy Specific Cost Density Density Energy
kg
L
kW-hr
L
kW-hr
kg
$
kW hr
NH3 (liquid) 0.108 -3.43 -5.63 0.15 H2 (104 psi) 0.0380 -1.26 -33.2 23.53A
CH3OH + H2O
0.108 -2.96 -3.30 0.13 C2H5OH + 3H2O 0.115 -2.90 -3.06 0.26 CH4 (3500 psi) + 2H2O 0.113 -2.76 -3.81 0.080 C3H8 (liquid) + 6H2O 0.123 -2.99 -3.22 0.060 C4H10 (liquid) + 8H2O 0.124 -2.99 -3.14 0.062 C8H18(liquid) + 16H2O 0.123 -2.20 -2.24 0.21 NaH + H2O 0.0461 -2.06 -2.14 3.14 CaH2
+ 2H2O 0.0622 -3.02 -2.45 155.96A
A Taken from Bloomfield, Analytic Power Corporation
Objectives
Study the Decomposition of Ammonia on an Fe Synthesis Catalyst and a Supported Ruthenium Catalyst
Develop a Predictive Microkinetic Model
Design a Reactor to Produce Hydrogen for a PEM Fuel Cell Vehicle
Kinetics Rate Limiting Step
Rate Expression Derived using L-H Analysis [Chellappa et al., App. Catal. A: Gen. 227 (2002)]
Temkin-Pyzhev [Temkin, Adv. Cat. 26 (1979)]
3 2
3 2
2 3
12 3NH H2
NH 1 1 N3 2H NH
P Pr k K P
P P
0.5, 0.674, 0.75
3
3
3 2
2 22 1 NH
NH 23/ 21 NH H
k K Pr
K P P
1 *
3 2
3NH * N H
2K
* *22 N N *
Experimental
Catalysts Triply-Promoted Fe (AS-4F), (40-60 mesh) Sud-Chemie 0.5 wt% Ru on 1/8” Al2O3 pellets, Engelhard
Reduction/Stabilization Procedure 3:1 H2/N2 Diluted to 50% in Ar, 500 ºC for 4 hours 20% NH3 in Ar at 350 ºC 18 hours
Experimental Conditions Fe: W/F (1.84 - 4.91 g hr/mol), T (325 – 550 ºC)
Ru: W/F (0.0928-0.186 g hr/mol), T (225 – 500 ºC)
UBI-QEP Method Predicts Surface Energetics Di and Qi – Only Experimental Inputs
Atomic, weak, and strong binding chemisorption energies
r p b fr p b f
H Q Q D D
1
2AB C
AB C
Q QE H
Q Q
JJJJJJJJJJJJJJ
0
12A AQ Q
n
20
,0
AAB n
AAB
Q Dn
2
,A
AB nA AB
Q D
Microkinetic Model
Reaction jEJJJJJJJJJJJJJJ
jEL
jH jAJJJJJJJJJJJJJJ
jAL
[kcal/mol] [kcal/mol] [kcal/mol] [s-1] [s-1] s1:
*3 3NH ( ) * NHg 0.0 19.0 -19.0 8.83x109 a 1013
s2: * * *3 2NH * H NH 16.9 14.9 2.0 1013
1013
s3: * * *2NH * H NH 20.2 17.2 3.0 1013 1013
s4: * * *NH * H N 2.9 40.9 -38.0 1013 1013
s5: * * * *3 2NH N NH NH 40.0 0.0 40.0 1013 1013
s6: * * *3 2NH NH 2 NH 15.3 16.3 -1.0 1013 1013
s7: * * *2NH N 2 NH 43.0 2.0 41.0 1013 1013
s8: * * * *
2NH N N H 21.5 33.1 -11.7 1013 1013
s9: * *
22 N N * 47.9 21.6 26.3 1013 1013
10: *2 2N N ( ) *g 25.7 0.0 25.7 1013 106 a
s11: * *22H H * 22.5 9.5 -13.1 1013 1013
s12: *2 2H H ( ) *g 11.9 0.0 11.9 1013 3.6x107 a
a units of atm-1 s-1
Dominant Reaction Routes
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
300 350 400 450 500 550 600 650 700 750
T (ºC)
NH
3 C
on
ve
rsio
n
Equilibrium
Detailed Model
RR5
RR9
RR14
RR22
RR28
RR33
Reaction Route 5 (Dominant)
Quasi-Equilibrium and Quasi-Steady State
Assumptions
Step Elementary Reaction σi s1: *
3 3NH ( ) * NHg 2 s2: * * *
3 2NH * H NH 2 s3: * * *
2N * NH H H 2 s4: * * *N * NHH 2 s9: * *
22 N *N 1 s10: *
2 2N N ( ) *g 1 s11: * *
22H H * 3 s12: *
2 2H H ( ) *g 3
2NH3 = N2 + 3H2
Reaction Rate Expression
3 2 2 2 2 2 2 2 3 3 2 2
2
2 22 1/ 2 -1/2 29 0
NH H H 1 N H 2 N H 3 N 4 NH 5 NH H 9 N S21/ 2
3 H 4
ii
iv
k ar P P a P P a P P a P a P a P P k K P
k K P k
L
LJ
3 2
3 2 2 2 3 2
2
2
2 2 2 2 2 2 2 3 3 2
2
S 1/ 23 NH H1/ 2 1/ 2
NH N H H NH H 1/ 23 H 4
1/ 20 4 3 4 H 2 1/ 2 -1/2
H H 1 N H 2 N H 3 N 4 NH 5 NH H21/ 2
3 H 4
1
1 vi ii iii iv v
iv
iv
iv
k K P PK P K P K P K P K P P
k K P k
a k k k K PP P a P P a P P a P a P a P P
k K P k
LJ
JLL
LJ
Surface Coverages on Fe Catalyst
1E-12
1E-11
1E-10
1E-09
1E-08
1E-07
1E-06
1E-05
0.0001
0.001
0.01
0.1
1
300 350 400 450 500 550 600 650 700 750
T (ºC)
i
NH3* NH2* NH* N*
N2* H2* H* *
Surface Coverages on Ru Catalyst
1E-14
1E-12
1E-10
1E-08
1E-06
0.0001
0.01
1
200 300 400 500 600 700
T (ºC)
i
NH3* N2* H2*
H* NH2* N*
NH* *
Apparent Activation Energy
T
rRTH eff
92 ln
40
42
44
46
48
50
52
54
300 350 400 450 500 550 600 650 700 750
T (ºC)
Ap
pa
ren
t A
cti
va
tio
n E
ne
rgy
(k
ca
l/m
ol)
Fe
Ru
3 2
2 2
9 NH S 1 NH S 1 2 11 12 NHS 1 2 11 12
NS 1 2 11 12 HS 11 12 N S 10 H S 12
2 2 2 2
1 2 2 3 3 2 2
effH E H H H H H H H H H
H H H H H H H H
Model vs. Experimental Data on Fe Catalyst
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
300 350 400 450 500 550 600
T [ºC]
X(N
H3)
Forward (325 - 550 ºC)
Reverse (525 - 350 ºC)
Equilibrium Conversion
Microkinetic model
Model vs. Experimental Data on Ru Catalyst
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
200 250 300 350 400 450 500 550 600
T [°C]
X(N
H3)
1.0g Ru 20sccm NH31.0g Ru 10sccm NH3EquilibriumMicrokinetic model 20%Microkinetic model 10%
Experimental Activation Energy on Fe and Ru Catalyst
y = -10784x + 15.657
R2 = 0.9995
y = -15049x + 17.417
R2 = 0.9916
-6
-5.5
-5
-4.5
-4
-3.5
-3
-2.5
-2
-1.5
-1
0.0013 0.00135 0.0014 0.00145 0.0015 0.00155 0.0016 0.00165 0.0017 0.00175 0.0018
1/T [K-1]
ln(T
OF
)
Ea(Ru) = 21.4 kcal/mol
Ea(Fe) = 29.8 kcal/mol
Comparison of Iron and Ruthenium Activity
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
200 250 300 350 400 450 500 550 600
T [°C]
TO
F [
s-1
]
Ru on Alumina
Fe
Reactor Design for a PEM Operated Automobile 10.5% of H2 is consumed to provide
heat of reaction 5.40 kg/hr of NH3 required to operate
at 55 mph Capable of traveling 434 miles at 55
mph, compared to 592 miles for gasoline powered vehicle
150 g of Fe catalyst required to obtain 600 ppm NH3 effluent at 600 C
Conclusions It is possible to predict activity of transition
metal catalysts for ammonia decomposition Experimental activation energies for Fe and Ru
are 29.8 kcal/mol and 21.4 kcal/mol, respectively, compared to predicted values of 47.9 kcal/mol and 43.0 kcal/mol
Ru catalyst is 10 times more active than Fe catalyst
A fuel cell operated automobile requires 5.40 kg/hr of NH3
An absorber is required to remove trace levels (600 ppm) of NH3 from H2 stream
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