Revealing the reaction mechanism of ammonia synthesis over bimetallic nitride catalyst from a kinetic perspective

Abstract

Ammonia is one of the most important feedstocks for both fertilizer production and energy carriers. Identifying the appropriate reaction mechanism from the multiple pathways of ammonia synthesis is critical to the rational design of efficient catalysts. However, the low adhesion of nitrogen molecules hinders the observation of the behavior of reaction intermediates and the understanding of the reaction mechanisms. Kinetic analysis is a powerful tool to recognize reaction mechanisms, but it is difficult to expand case-by-case research to target systems. Herein, we establish a framework for the investigation of reaction mechanisms based on kinetic analysis and apply it to ammonia synthesis over a Co₃Mo₃N bimetallic nitride catalyst. The energetics of elementary reactions calculated by density functional theory (DFT) is used for a microkinetic model to obtain information about reaction mechanisms. Theoretical calculations indicate that the reaction rate via the surface reaction mechanism is much higher than that via the MvK mechanism. Nitrogen-vacancy-generation-induced charge redispersion is the major hindrance to the subsequent hydrogenation of NH_x species for the MvK mechanism. This information guides the design and analysis of kinetic experiments. A series of steady-state and transient kinetic experiments demonstrate the dominant role of the dissociation mechanism over associative and MvK mechanisms. The low coverage of surface N species derived from both DFT-based microkinetic simulations and transient kinetic experiments originates from the high energy barrier to N₂ dissociation. This work reveals the reaction mechanism of ammonia synthesis over bimetallic nitrides based on both theoretical calculations and experimental results and proposes a new paradigm for elucidating reaction mechanisms in heterogeneous catalysis from a kinetic perspective.