Promotion effect of rare-earth elements on the catalytic decomposition of ammonia over Ni/Al₂O₃ catalyst

Highlights

• The Ni/Al₂O₃ catalysts modified by rare-earth elements were investigated.

• The addition of rare-earth elements promoted ammonia decomposition reaction.

• La-modified Ni/Al₂O₃ was most active in this study.

• The hydrogen inhibition phenomenon was alleviated by the modification.

Abstract

Ammonia decomposition has attracted much attention as an efficient method for the on-site generation of hydrogen. In this study, alumina-supported nickel catalysts (Ni/Al₂O₃) modified by rare-earth elements were prepared by the impregnation method and their catalytic activity for the ammonia decomposition was investigated. The addition of rare-earth elements promoted the decomposition reaction over catalysts and the La-modified catalyst achieved the highest ammonia conversion in this work. For the modified catalysts, the adsorbed hydrogen, which is known to be an inhibitive species for the ammonia decomposition, desorbed at lower temperature compared to the unmodified one. Therefore, the effective alleviation of hydrogen inhibition would be responsible for the activity enhancement for the modified catalysts. The reaction kinetics study also supported this proposed mechanism. For the La-modified catalyst, the optimal pretreatment condition was investigated to enhance the catalytic activity. The catalyst calcined at 400 °C followed by reduction at 600 °C exhibited the highest ammonia conversion of 94% at 550 °C.

Introduction

In recent years, fuel cells have attracted much attention as promising power generation devices because of high energy conversion efficiency and low emission. Hydrogen is a primary fuel source for fuel cells especially operated at low temperatures. However, the storage and transportation of hydrogen are major obstacles for the spread of fuel cell systems due to its low volumetric density and boiling point. For this reason, on-site generation of hydrogen has been accomplished by the reforming of hydrocarbons. However, CO is produced as a by-product upon the reforming process and degrades the performance of Pt electrode in polymer electrolyte fuel cells (PEFCs). Thus, the CO concentration in the fuel gas is reduced less than 10 ppm via water gas shift reaction and preferential oxidation reaction in the residential PEFC cogeneration system [1], [2]. This makes the system complicated.

Ammonia is regarded as a prospective hydrogen carrier because of its favorable properties for on-site hydrogen generation. Ammonia can be liquefied under mild conditions (–33.4 °C at atmospheric pressure or 8.46 atm at 20 °C) and possesses high hydrogen storage capacity (17.6 wt.%) compared to other hydrogen carriers. In addition, the hydrogen production via the ammonia decomposition does not emit CO. This decomposition process corresponds to a reverse reaction of ammonia synthesis. Ammonia decomposes endothermically into nitrogen and hydrogen as follows;

2NH₃ → N₂ + 3H₂ ΔH° = +46 kJ mol⁻¹

Many studies have been reported for the development of ammonia decomposition catalysts such as metals, alloys, carbides, and nitrides, and it was revealed that Ru, Ir, Ni, Rh, Pt, Pd, and Fe metals, MoN_x, and VC_x catalysts promoted this reaction [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Since Ru catalysts are the most active among them, much effort has been devoted for the catalyst development [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. In previous reports, Ru catalysts supported on carbon nanotube (CNT) showed very high activity for ammonia decomposition because the high surface area of CNT enabled high dispersion of Ru species [2], [8], [9]. Moreover, the addition of basic components such as alkali metals to the catalysts promoted the decomposition reaction. These species could facilitate the combinative desorption of nitrogen atoms, which is generally accepted as the rate-determining step in the ammonia decomposition, via the electron transfer from the additives to active species. Especially, the prominent effect of basic additives was confirmed for the CNT-supported catalysts due to high electron conductivity of CNT as compared with other metal oxide supports [8]. Recently, it was reported that the Ru catalyst supported on inorganic electride, [Ca₂₄Al₂₈O₆₄]⁴⁺(e^–)⁴, showed higher activity than K-modified Ru/graphite carbon and Cs-modified Ru/MgO catalysts. The electron injection from the electride to the antibonding orbital of Ru–N bond reduced the activation energy of decomposition reaction [14]. However, the limited availability of noble metals makes it necessary to develop catalysts with low cost. Thus, Ni catalysts have attracted much interest as alternative materials for ammonia decomposition because they attained the highest activity among non-noble metal catalysts [15], [16], [17], [18], [19], [20], [21], [22]. In previous works, La and Ce species were known as effective promoters for Ni catalysts [20], [21], [22]. However, the promotion effect of other rare-earth elements has not been investigated sufficiently for Ni catalysts. In this study, therefore, we investigated the activity of Ni/Al₂O₃ catalysts modified by various rare-earth elements for ammonia decomposition. To clarify the additive effects on the reaction behavior of reactant and products during ammonia decomposition, their desorption processes were also examined. Besides, the kinetics analysis was carried out to elucidate the influence of hydrogen, which is known to be an inhibitive species for the ammonia decomposition [2], [14], [26], [27], on the catalytic performance. Furthermore, the optimization of heat treatment condition for the synthesis of La-modified catalyst was conducted because the La element significantly enhanced the catalytic activity of Ni/Al₂O₃.