Catalytic Decomposition of Ammonia for Fuel Cell Applications
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Transcript Catalytic Decomposition of Ammonia for Fuel Cell Applications
COx-Free Hydrogen by Catalytic
Decomposition of Ammonia on
Commercial Fe and Ru Catalysts:
An Experimental and Theoretical Study
Caitlin Callaghan
Barry Grace
Orest Skoplyak
Ilie Fishtik
Ravindra Datta
Fuel Cell Center
Chemical Engineering
Department
Worcester Polytechnic
Institute
Worcester, 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
Energy
Density
Specific
Energy
Cost
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
H2
Density
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
N *2 *
2 N*
Rate Expression Derived using L-H Analysis
[Chellappa et al., App. Catal. A: Gen. 227 (2002)]
rNH3
2
k2 K12 PNH
3
K P
1 NH3
P
3/ 2
H2
Temkin-Pyzhev
3
K1
NH 3 *
N* H 2
2
2
[Temkin, Adv. Cat. 26 (1979)]
rNH3
P 2
NH
k1 3 3
PH2
3
P
H
2
K1 PN2 2 2
PNH
3
1
0.5, 0.674, 0.75
Experimental Setup
Experimental
Catalysts
Reduction/Stabilization Procedure
Triply-Promoted Fe (AS-4F), (40-60 mesh) Sud-Chemie
0.5 wt% Ru on 1/8” Al2O3 pellets, Engelhard
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
H Qr Q p Db D f
r
p
b
f
QAB QC
1
E H
2
QAB QC
Atomic, weak, and strong binding
chemisorption energies
1
QA Q0 A 2
n
QAB ,n
Q02A
Q0 A
n
DAB
QAB ,n
QA2
QA DAB
Microkinetic Model
E j
E j
H j
[kcal/mol] [kcal/mol] [kcal/mol]
Reaction
s1:
NH 3 ( g ) *
s2:
Aj
[s-1]
Aj
[s-1]
8.83x109 a 1013
NH*3
0.0
19.0
-19.0
NH*3 *
H* NH*2
16.9
14.9
2.0
1013
1013
s3:
NH*2 *
H* NH*
20.2
17.2
3.0
1013
1013
s4:
s5:
NH* *
NH*3 N*
H* N*
NH*2 NH*
2.9
40.0
40.9
0.0
-38.0
40.0
1013
1013
1013
1013
s6:
NH*3 NH*
2 NH*2
15.3
16.3
-1.0
1013
1013
s7:
NH*2 N*
2 NH*
43.0
2.0
41.0
1013
1013
s8:
NH* N*
N*2 H*
21.5
33.1
-11.7
1013
1013
s9:
2 N*
N*2 *
47.9
21.6
26.3
1013
1013
10:
N*2
N2 ( g) *
25.7
0.0
25.7
1013
106
s11:
2 H*
H*2 *
22.5
9.5
-13.1
1013
1013
s12:
H*2
H2 ( g) *
11.9
0.0
11.9
1013 3.6x107
a
units of atm-1 s-1
a
a
Dominant Reaction Routes
1
0.9
0.8
NH3 Conversion
0.7
0.6
Equilibrium
0.5
Detailed Model
RR5
0.4
RR9
0.3
RR14
RR22
0.2
RR28
RR33
0.1
0
300
350
400
450
500
550
T (ºC)
600
650
700
750
Reaction Route 5 (Dominant)
Quasi-Equilibrium and
Quasi-Steady State Assumptions
Step
s1:
s2:
s3:
s4:
s9:
s10:
s11:
s12:
Elementary Reaction
NH 3 ( g ) *
NH*3
NH*3 *
H* NH*2
NH*2 *
H* NH*
NH* *
H* N*
2 N*
N*2 *
N*2
N2 ( g) *
2 H*
H*2 *
H*2
H2 ( g) *
2NH3 = N2 + 3H2
σi
2
2
2
2
1
1
3
3
Reaction Rate Expression
rNH3
k9 a02
k3 Kiv PH1/2 2 k4
S
2
P P 2 a P P a P P1/ 2 a P a P
H2
1 N2 H2
2 N2 H2
3 N2
4 NH3
H2
2
k K P
a5 PNH3 PH-1/2
9
ii
N
S
2
2
2
1
1/ 2
k
K
P
P
3
v
NH
H
3
2
1 Ki PNH3 K ii PN2 Kiii PH2 K iv PH1/2 2 Kv PNH3 PH21/ 2
1/ 2
k3 K iv PH2 k4
1/ 2
a0 k4 k3 k4 Kiv PH2
2
1/ 2
-1/2
PH2 PH2 a1 PN2 PH2 a2 PN2 PH2 a3 PN2 a4 PNH3 a5 PNH3 PH2
2
1/
2
k3 Kiv PH2 k4
Surface Coverages
on Fe Catalyst
1
0.1
0.01
0.001
0.0001
1E-05
i 1E-06
1E-07
1E-08
1E-09
1E-10
1E-11
1E-12
300
350
400
450
500
550
T (ºC)
NH3*
NH2*
NH*
N*
N2*
H2*
H*
*
600
650
700
750
Surface Coverages
on Ru Catalyst
1
0.01
0.0001
1E-06
i
1E-08
1E-10
1E-12
1E-14
200
300
400
500
T (ºC)
NH3*
N2*
H2*
H*
NH2*
N*
NH*
*
600
700
Apparent Activation Energy
54
H eff E9 2 NH3S H1 NH2S 2H1 2H 2 H11 H12 2 NHS H1 H 2 H11 H12
1 NS 2H1 2H 2 3H11 3H12 HS H11 H12 2 N2S H10 2 H2S H12
Apparent Activation Energy (kcal/mol)
52
50
48
46
H eff
ln r9
RT
T
2
44
Fe
Ru
42
40
300
350
400
450
500
550
T (ºC)
600
650
700
750
Model vs. Experimental
Data on Fe Catalyst
1.00
0.90
0.80
0.70
X(NH3)
0.60
0.50
0.40
0.30
0.20
Forward (325 - 550 ºC)
Reverse (525 - 350 ºC)
Equilibrium Conversion
0.10
Microkinetic model
0.00
300
350
400
450
T [ºC]
500
550
600
Model vs. Experimental
Data on Ru Catalyst
1.00
0.90
0.80
0.70
X(NH3)
0.60
0.50
0.40
0.30
0.20
1.0g Ru 20sccm NH3
1.0g Ru 10sccm NH3
Equilibrium
Microkinetic model 20%
Microkinetic model 10%
0.10
0.00
200
250
300
350
400
T [°C]
450
500
550
600
Experimental Activation
Energy on Fe and Ru Catalyst
-1
Ea(Ru) = 21.4 kcal/mol
-1.5
-2
-2.5
ln(TOF)
-3
Ea(Fe) = 29.8 kcal/mol
-3.5
y = -10784x + 15.657
2
R = 0.9995
-4
-4.5
-5
y = -15049x + 17.417
2
R = 0.9916
-5.5
-6
0.0013
0.00135
0.0014
0.00145
0.0015
0.00155
-1
1/T [K ]
0.0016
0.00165
0.0017
0.00175
0.0018
Comparison of Iron
and Ruthenium Activity
1.0E+01
1.0E+00
TOF [s-1]
1.0E-01
1.0E-02
1.0E-03
Ru on Alumina
Fe
1.0E-04
200
250
300
350
400
T [°C]
450
500
550
600
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