(621em) Kinetic Analysis of Decomposition of Ammonia over Nickel and Ruthenium Catalysts | AIChE

(621em) Kinetic Analysis of Decomposition of Ammonia over Nickel and Ruthenium Catalysts

Authors 

Fujitani, T. - Presenter, National Institute of Advanced Industrial Science and Technology (AIST)
1         
Introduction

Hydrogen (H2)
is a promising energy resource and can be utilized in high-efficiency power
generation systems such as fuel cells. However, the transport and storage of H2
are obstacles for its practical use. Ammonia (NH3) is a good candidate
as an alternative H2 carrier because of its high H2
storage capacity and facile liquefaction under mild conditions [1]. Because
high-purity H2 is required to generate electricity with a fuel cell,
a high-performance catalyst for producing H2 from decomposition of
NH3 is needed when NH3 is used as a H2
carrier.

Previous studies
indicated that Ru catalysts have the highest catalytic activity [1,2]. However,
because precious metals are very expensive, alternative inexpensive Ni-based
catalysts for decomposition of NH3 have been widely investigated
[3,4]. Nevertheless, the
catalytic activity of Ni does not reach that of the Ru catalyst. Furthermore,
it is unclear why there is a difference between the catalytic activities of the
Ru and Ni catalysts.

In this study,
the decomposition of NH3 over Ni and Ru catalysts was investigated
by using a kinetic model based on a reaction mechanism consisting of
kinetically important elementary steps. The origin of the difference between
the catalytic activities of the Ni and Ru catalysts was clarified.

  2         
Experimental

MgO-supported Ni and Ru catalysts were prepared by
an impregnation method using aqueous solutions of Ni(NO3)2
and RuCl3, respectively, followed by drying and calcination. The
loading of metals was set at 30 wt % for Ni and 5 wt % for Ru. 

The NH3 decomposition reaction was
carried out in a continuous-flow fixed-bed quartz tubular reactor at
atmospheric pressure. The reaction products were analyzed by means of an
on-line gas chromatograph equipped with a thermal conductivity detector and a
Shincarbon ST column (Shinwa Chemical Industries Ltd., Kyoto, Japan) for N2
and H2. The NH3 conversion was calculated on the basis of
the production of H2.

  3         
Results and discussion

A kinetic model
for decomposition of NH3 was constructed on the basis of the
reaction mechanism including the following elementary steps: NH3
adsorptionDdesorption (Eq. (1)), dehydrogenation of adsorbed NH3
(Eq. (2)), recombinative N2 desorption (Eq. (3)), and recombinative
H2 desorptionDadsorption of gas-phase H2 (Eq. (4)).

The 6 unknown
constants were estimated by fitting the model equations with the experimental
results. The fitted results were in good agreement with the experimental data.
Estimated values of the constants are listed in Table 1. Recombinative N2
desorption (Eq. (3)) has been suggested to be the rate-determining step in the
decomposition of NH3 over most catalysts [1], and several research
groups have reported values of the activation energy for recombinative N2
dissociation. The estimated activation energy for recombinative N2
dissociation, k3, on the Ni and Ru catalyst are in agreement
with these reported values.

 

Table 1  Estimated values of constants in the
model

Constant

Ni catalyst

Ru catalyst

Pre-exponential factor, A

Activation energy, Ea / kJ·mol-1

Pre-exponential factor, A

Activation energy,

Ea / kJ·mol-1

k1 [mol·s-1]

7.55 x 101

0

8.36 x 101

0

k2 [mol·s-1]

7.54 x 1015

144.0

1.34 x 1014

105.8

k3 [mol·s-1]

3.12 x 106

127.8

6.26 x 105

123.5

k4 [mol·s-1]

3.92 x 104

109.0

7.18 x 102

67.8

K1 [–]

2.26 x 10-14

-79.4

3.49 x 10-13

-64.0

K4 [–]

4.13 x 106

109.0

1.51 x 104

67.8

 

Furthermore,
the experimental results in the high-conversion region (20–100% conversion of
NH3) at various NH3 partial pressures and space
velocities (SVs) under high-temperature conditions were compared with simulated
results. The simulated lines were in good agreement with the experimental
results over a wide range of reaction temperatures, NH3 partial
pressures, and space velocities (SVs). Thus, the proposed model can predict the
catalytic activities of both the Ni and the Ru catalysts under a remarkably
wide range of reaction conditions.

  4         
Conclusions

Using the
minimum numbers of elementary steps necessary to understand the chemistry on
the catalyst surface, the kinetic analysis presented herein has clarified the
true reaction step governing the decomposition of NH3 on Ni and Ru
catalysts. Thus, the kinetic approach described herein should be a useful tool
for designing new high-performance catalysts.

Acknowledgements

This work was
supported by the Cross-ministerial Strategic Innovation Promotion Program (SIP)
of the Cabinet Office, Government of Japan.

References

[1] S. F. Yin, B. Q. Xu, X. P. Zhou, C. T. Au,
Appl. Catal. A 277 (2004) 1.

[2] T. V. Choudhary, , C. Sivadibarayana, D. W.
Goodman, Catal. Lett. 72 (2001) 197.

[3] J. Zhang, H. Xu, X. Jin, Q. Ge, W. Li,
Appl. Catal. A 290 (2005) 87.

[4] H. Muroyama, C. Saburi, T. Matsui and K.
Eguchi, Appl. Catal. A 443-444 (2012) 119.