Stabilizing active N species on support for enhancing ammonia synthesis

As an important chemical raw material, ammonia is of great significance in fertilizer production and hydrogen energy storage. Ammonia synthesis via the Haber-Bosch process has been under continuous development for over a century, with previous studies predominantly emphasizing the electron donation effect from the support. However, various phenomena hint at other support effects also contributing to this reaction, while the detailed mechanism is yet to be fully discovered. This research reveals an additional role of the supports and an alternative reaction pathway to boost the ammonia synthesis, based on in situ environmental transmission electron microscopy and electron energy loss spectroscopy (EELS) investigation of two selected model catalysts, Ru/MgO and Ru/Al₂O₃. The activation and dissociation of N₂ and H₂ molecules occurred on Ru particles to generate active N* and H* species. During the reaction, the in situ-generated oxygen defects on MgO act as an N* reservoir to form the Mg-N* interaction, as evidenced by both in situ EELS and diffuse reflectance infrared Fourier-transform spectroscopy. The density functional theory calculation confirmed that the dissociated N* over the Ru particle will automatically refill into the surface oxygen defects on MgO due to the lower refilling energy. Thus, the support not only extracts the active N* from the Ru surface by nitrogen spillover, thereby suppressing N* recombination to N₂, but also promptly frees Ru active sites for subsequent activation. This strongly enhances the reaction rate of N₂ activation and dissociation, and further boosts the ammonia productivity.