Unprecedented promotion of NH 3 decomposition over Ru via H 2scavenging: a chemical reaction engineering analysis

Abstract

Ammonia offers great potential as hydrogen carrier, facilitating the storage, transport, and distribution of renewable energy. Nevertheless, the decomposition of ammonia, a key process for releasing hydrogen to distributed end users, represents a critical gap in the developing NH3-based energy value chain. Two scientific issues are fully open and slow-down the process development: the catalyst formulation and the reactor design. Ruthenium is recognized as the most active metal for ammonia decomposition; however, its reactivity is strongly limited by kinetic constraints. Although the mechanistic understanding remains under debate, substantial evidence from the literature and further analysed herein through kinetic modeling demonstrates that ammonia decomposition on Ru-based catalysts is severely hindered by hydrogen inhibition. Building on this evidence, novel diagnostic tests (where the “restoring” of Ru sites was obtained by O2 cofeeding and H2 oxidation) and modelling (simulating the ideal condition of zero H* coverage) reveal that a huge potential is disclosed if the reactor is designed to obtain full H2-scavenging, thus solving both the issues above mentioned. The turnover rate is shown herein to increase by a factor of 14 at 250 °C under diluted feed conditions and is projected to rise by several orders of magnitude under pure ammonia feeds at atmospheric pressure. Even greater enhancements are expected under pressurized conditions of industrial relevance.