Stabilizing active N species on support for enhancing ammonia synthesis

Abstract 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 other support effect will also contribute 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 N* reservoir to form the Mg-N* interaction, as evidenced by both in situ EELS and diffuse reflectance infrared Fourier transformation spectroscopy. The density functional theory calculation confirmed that the dissociated N* over 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, 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 boost the ammonia productivity.