Phosphorus Modification of Iron: Mechanistic Insights into Ammonia Synthesis on Fe2P Catalyst.

Abstract

Ammonia (NH3) is a critical chemical for fertilizer production and a potential future energy carrier within a sustainable hydrogen economy. The industrial Haber-Bosch process, though effective, operates under harsh conditions due to the high thermodynamic stability of the nitrogen molecule (N2). This motivates the search for alternative catalysts that facilitate ammonia synthesis at milder temperatures and pressures. Theoretical and experimental studies suggest that circumventing the trade-off between N-N activation and subsequent NHx hydrogenation, governed by the Brønsted-Evans-Polanyi (BEP) relationship, is key to achieving this goal. Recent studies indicate metal phosphides as promising catalyst materials. In this work, a comprehensive density functional theory (DFT) study comparing the mechanisms and potential reaction pathways for ammonia synthesis on Fe(110) and Fe2P(001) is presented. The results reveal substantial differences in the adsorption strengths of NHx intermediates, with Fe2P(001) exhibiting weaker binding compared to Fe(110). For N-N bond cleavage, multiple competing pathways become viable on Fe2P(001), including routes involving the pre-hydrogenation of adsorbed N2 (e.g., through *NNH*). Analysis of DFT-derived turnover rates as a function of hydrogen pressure (H2) highlights the increased importance of these hydrogenated intermediates on Fe2P(001) compared to Fe(110) where direct N2 dissociation dominates. These findings suggest that phosphorus incorporation modifies the ammonia synthesis mechanism, offering alternative pathways that may circumvent the limitations of traditional transition metal catalysts. This work provides theoretical insights for the rational design of Fe-based catalysts and motivates further exploration of phosphide-based materials for sustainable ammonia production.