Catalytic Ammonia Decomposition Over Fe/Fe 4 N
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Catalytic Ammonia Decomposition Over Fe/Fe4N R. Pelka Æ I. Moszyn´ska Æ W. Arabczyk
Received: 2 July 2008 / Accepted: 31 October 2008 / Published online: 15 November 2008 Ó Springer Science+Business Media, LLC 2008
Abstract The catalytic ammonia decomposition over iron and iron nitride, Fe4N, under the atmosphere of ammonia–hydrogen mixtures of different amounts of ammonia in the temperature range of 400–550 °C by means of thermogravimetry has been studied. A differential tubular reactor with mixing has been used. The ammonia concentration in the gas phase during all the process was analysed. The balance between the inlet and outlet ammonia quantity has been used to determine a degree of ammonia conversion and the values of decomposition reaction rate. The activation energy of ammonia decomposition reaction over Fe and Fe4N was found to be 68 and 143 kJ/mol, respectively. Keywords Ammonia decomposition Nitriding process Iron catalyst Nitrides
1 Introduction In ammonia synthesis process, apart from a synthesis reaction, catalytic ammonia decomposition reaction over iron also occurs [1, 2]. Ammonia decomposition reaction also takes place in the first stage of iron nitriding process under ammonia atmosphere [3, 4], while the formation of nitrides is favoured from the thermodynamical point of view. Grabke [5, 6] showed, that in the initial stage of the nitriding process, the surface catalytic ammonia decomposition reaction over iron is a rate determining step.
R. Pelka (&) I. Moszyn´ska W. Arabczyk Institute of Chemical and Environment Engineering, Szczecin University of Technology, Pułaskiego 10, 70-322 Szczecin, Poland e-mail: [email protected]
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Nitriding process was studied over bulk iron [7], iron catalyst for ammonia synthesis [8] and iron thin films [9]. The decomposition of ammonia on nitrided iron can be studied when the equilibrium between the formation and decomposition of iron nitrides is achieved during the nitriding process [10]. The experimentally measured catalytic ammonia decomposition on nanocrystalline iron surface occurs at temperatures higher than 400 °C [11, 12]. Nitrogen content in the obtained nitrides depends on the temperature and ammonia concentration in a gas phase [11, 12]. Catalytic ammonia decomposition over iron depends on many factors such as, for example, chemical composition of a gas phase or a surface, on which the reaction proceeds. The influence of sulphur [13, 14] or water vapour [15] present in the gas phase on the rate of ammonia decomposition process over iron was investigated. Catalytic ammonia decomposition reaction at a stable temperature occurs with various rates depending on the solid state phase (Fe or FexN) present in the reaction system [16]. Therefore, during the analysis of ammonia decomposition reaction over iron catalyst one is to take into account the fact, that in parallel to this reaction a nitriding reaction of iron, resulting in iron nitrides formation, also occurs. It was previously found [11, 12, 17, 18], that the rates of reactions taking place in
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