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 Setup

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

QQ

Q Dn

2

,A

AB nA AB

QQ

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