Kinetic Analysis of Decomposition of Ammonia over Nickel and Ruthenium Catalysts

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.