Kinetic enhancement of ammonia decomposition as a chemical hydrogen carrier in palladium membrane reactor

• Published in: Catalysis today 2014-11, Vol.236, p.70-76
• Main Authors: Itoh, Naotsugu, Oshima, Atsushi, Suga, Eita, Sato, Takafumi
• Format: Article
• Language: English
• Subjects: Ammonia; Catalysis; Chemistry; Colloidal state and disperse state; Decomposition; Exact sciences and technology; General and physical chemistry; Hydrogen; Hydrogen carrier; Membranes; Palladium; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0920-5861; 1873-4308
• DOI: 10.1016/j.cattod.2014.02.054

Different effects of hydrogen separation on the conversion increment can be seen in different reaction models. The model E is limited by the surface reaction, while the model F by the desorption of nitrogen. •Ru/SiO2 catalyst showed ammonia decomposition activity even at lower temperatures below 450°C.•The decomposition of NH3 was enhanced in a palladium membrane reactor (PMR).•PMR model realized that the kinetic enhancement significantly differed among the reaction models. Decomposition of ammonia as a promising chemical hydrogen carrier is carried out at low temperatures using a ruthenium supported catalyst, where a highly efficient hydrogen recovery from ammonia is being desired. The equilibrium conversion is almost 100% in the range of 623–723K but the decomposition remains in a low conversion, so that insufficient activity of present catalysts should be improved. This study attempts to prove that this kinetically limited decomposition can be enhanced in the palladium membrane reactor, while in most of membrane reactor applications the equilibrium limited reactions can be shifted to the product side by selective hydrogen separation. The Langmuir-Hinshelwood types of rate equations, seven reaction models, for ammonia decomposition using a Ru/SiO2 catalyst were derived and compared with experimental results. As the result, the combinative desorption of nitrogen atom was found to be the rate determining step in the range of 623–723K. One dimensional model for a palladium membrane reactor could show that the kinetic enhancement differed considerably according to the reaction model (rate expression). In the ammonia decomposition employing a membrane reactor with a 200μm-thick palladium tube, 15% increase in conversion compared with the conventional packed reactor and 60% of the hydrogen recovery at 723K could be obtained. Simulation showed that further enhancement would be achievable by using thinner palladium membrane.