Mechanocatalytic Synthesis of Ammonia: State of the Catalyst During Reaction and Deactivation Pathway

Ammonia synthesis holds significant importance for both agricultural fertilizer production and emerging green energy applications. Here, we present a comprehensive characterization of a catalyst for mechanochemical ammonia synthesis, based on Cs-promoted Fe. The study sheds light on the catalyst's dynamic evolution under reaction conditions and the origin of deactivation. Initially, elemental Cs converts to CsH, followed by partial CsOH formation due to trace oxygen impurities on the surface of the Fe metal and the equipment. Concurrently, the mechanical milling process comminutes Fe, exposing fresh metallic Fe surfaces. This comminution correlates with an induction period observed during ammonia formation. Critical to the study, degradation of active Cs promoter species (CsH and CsNH2) into inactive CsOH emerged as the primary deactivation mechanism. By increasing the Cs content from 2.2 mol % to 4.2 mol %, we achieved stable, continuous ammonia synthesis for nearly 90 hours, showcasing one of the longest-running mechanocatalytic gas phase reactions. Studies of the temperature dependence of the reaction revealed negligible bulk temperature influence in the range of -10 degrees C to 100 degrees C, highlighting the dominance of mechanical action over bulk thermal effects. This study offers insights into the complex interplay between mechanical processing, reactive species, and deactivation mechanisms in mechanocatalytic ammonia synthesis. The synthesis of ammonia over a Cs-promoted Fe catalyst via the novel approach of mechanocatalysis is carefully studied by ex situ characterizations of the catalyst. Physicochemical changes are related to the different stages of ammonia formation. Optimizations based on these results resulted in the intrinsically complicated low-temperature ammonia synthesis being one of the longest stable mechanocatalytic gas-phase reactions to date. image