NH 3 Decomposition Kinetics on Supported Ru Clusters: Morphology and Particle Size Effect
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NH3 Decomposition Kinetics on Supported Ru Clusters: Morphology and Particle Size Effect Weiqing Zheng Æ Jian Zhang Æ Hengyong Xu Æ Wenzhao Li
Received: 1 July 2007 / Accepted: 1 August 2007 / Published online: 15 August 2007 Springer Science+Business Media, LLC 2007
Abstract The supported Ru clusters with mean sizes ranging from 1.9 to 4.6 nm showed a high activity towards the NH3 decomposition reaction. The structural properties of catalysts were characterized by N2 adsorption/desorption, X-ray diffraction (XRD) and transmission electron micrograph (TEM). Steady-state reaction kinetics revealed that the apparent activation energy increased with a decrease in Ru particle size and ranged from 79 kJ mol1 to 122 kJ mol1. The decomposition rate over Ru nanoparticles showed a strong dependency on mean crystallite size and the optimum appeared at dRu = 2.2 nm. The dependencies of reaction rate on partial pressures of NH3 and H2 were also sensitive to the varying Ru particle size. Experimental data could be well fitted by the Temkin–Pyzhev equation, indicating that the recombinative desorption of surface nitrogen atom acts as the rate-determining step. A compensation effect between the pre-exponential factor (k0) and activation energy (Ea) was quantified. Keywords Ammonia decomposition Ru Kenitics Size effects Compensation effect
1 Introduction Catalytic decomposition of NH3 provides an applicable route to COx-free H2 stream for the fuel cell uses. Over the past few years, the NH3 decomposition process has been extensively studied mainly driving by either the scientific view to get insight into the mechanism of NH3 synthesis W. Zheng J. Zhang H. Xu (&) W. Li Dalian Institute of Chemical Physics, Graduate School of the Chinese Academy of Sciences, Dalian 116023, China e-mail: [email protected]
[1, 2] or the technological opinion to use liquid NH3 as a suitable H2 carrier [3–5]. It has been reported that group VIII metals (Fe, Ni, Ru, Ir, Co, Pt) catalyze the NH3 decomposition reaction and Ru is generally considered to be the most active single metal catalysts [6–9]. Ru-CeO2/Y Zeolite [2], Ru/Al2O3 [6, 8], Ru/SiO2 [6, 10], Ru/C [11–15], Ru/MgAl2O4 [9, 16], Ru/CNTs [8, 10, 15, 17], and Ru/MgO [8, 10, 17, 18] have been proved to catalyze decomposition of NH3. Although the assessments of relevant elementary steps and structure sensitivity in NH3 decomposition have been widely reported, the kinetics of NH3 decomposition over Ru is not well established and many contradictory findings still remain unresolved. Tsai and Weinberg [19] investigated the kinetics of NH3 dissociation over Ru (001) plane. The authors found that the rate determining step (RDS) changed with the temperature and established a reaction expression under the hypothesis that both the recombinative desorption of N* as the most abundant reactive intermediate (MARI) and the initial N–H bond cleavage are slow elementary steps. However, their expression could not describe the H2 inhibition phenomenon that was widely observed in the later studies. I
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