Kinetic investigation of NH3 decomposition over Ru-based catalysts: The limiting role of H* surface coverage and its impact on reactor engineering

Abstract

H₂ production via green NH₃ decomposition is a key process in the value chain of renewable energy storage and distribution, but reactor engineering studies are at an early stage due to the still open research on catalyst formulation and reaction kinetics. In this work, the kinetics of NH₃ decomposition are studied over Ru/MgAl₂O₄, Ru/γ-Al₂O₃ and Ru/MgO catalysts, that combine the best-known active metal, Ru, and supports suitable for industrial applications. For all the formulations, NH₃ concentration and H₂-cofeed are found to negatively affect the conversion, in line with the literature; however, original experiments at varying NH₃ concentration with large H₂-cofeed unambiguously show that, in the presence of H₂-rich streams representative of the catalyst application, the intrinsic kinetics have a linear dependence on NH₃ partial pressure and a negative order (−1.5) with respect to H₂ partial pressure, consistent with the hypothesis that ammonia dehydrogenation is rate determining and H* is the most abundant surface intermediate. The same kinetic dependences were obtained over a commercial Ru-based catalyst. The kinetic relevance of hydrogen coverage and the intrinsic first order dependence on NH₃ have two important implications: on a methodological plane, the performance and kinetics of the catalyst under industrially relevant conditions (e.g. pure ammonia) can be captured even under highly diluted NH₃ feeds (which guarantee the rigorous isothermal conditions) provided that the large H₂ contents are experienced; on a more applicative plane, both kinetic and thermodynamic factors play negatively at increasing concentration and pressure, thus large sizing (with GHSV values as low as 2500 Nl/kg/h) is needed to obtain complete ammonia conversion below 500 °C.