(698c) Insight into the Dynamic Evolution of Co3Mo3n Surface during Ammonia Synthesis

Ammonia produced through green hydrogen is considered a promising energy carrier for the future. Ammonia synthesis is a structure-sensitive reaction and the dynamic evolution of catalyst surfaces induced by reaction atmosphere leads to significant differences between the ideal surface and the realistic surface during the reaction process. Identifying the realistic surface is crucial to gain a deeper understanding of reaction mechanisms. In this work, we focus on the formation of stable surfaces for Co₃Mo₃N catalyst in different scenarios under the atmosphere of nitrogen and hydrogen. Theoretical and experimental studies have demonstrated that Co₃Mo₃N catalyst can adjust surface states in response to various conditions. Both nitrogen and hydrogen possess two types of sites with distinct coordination environments. Based on ab initio thermodynamics phase diagram, metal-hydrogenated surfaces and subsurface defects are more likely to form under realistic gas-phase environments of nitrogen and hydrogen. There is no competitive adsorption behavior between nitrogen and hydrogen. Clean surfaces are thermodynamically less stable than hydrogenated surfaces under a wide range of reaction and activation conditions, which has been also confirmed by a series of temperature-programmed desorption experiments. The evolution of the surface has a significant impact on the electronic structure and reaction performance of Co₃Mo₃N catalyst. The presence of surface H species inhibits the nitrogen dissociation process, but has no effect on the lattice nitrogen hydrogenation process. This insight on the realistic surface is crucial for understanding of reaction mechanism and designing of high-performance catalysts.