First-Principles and Microkinetic Study on the Mechanism for Ammonia Synthesis Using Ru-Loaded Hydride Catalyst

Ru-loaded hydride is an efficient catalyst for ammonia (NH3) synthesis under mild conditions. Metal hydrides such as Ca2NH with surface anionic electrons at hydrogen vacancies (Ca2NH1–x e x –) function well as active catalytic support materials for Ru. The resultant catalysts exhibit good performance for NH3 synthesis with a large reduction of the apparent activation energy and the suppression of hydrogen poisoning of Ru. However, the reaction mechanism and the rate-determining step (RDS) have not yet been clarified from a microscopic viewpoint. Here, we have successfully reproduced the experimental results of NH3 synthesis by microkinetic modeling using density functional theory (DFT) calculations. Three essential mechanisms were identified: (i) the promotion of nitrogen cleavage with electron injection from Ca2NH1–x e x – to Ru, (ii) the formation of NH x species promoted at the Ru/Ca2NH1–x e x – interface, and (iii) hydrogen poisoning suppression of Ru by fast hydrogen migration at the Ru/Ca2NH1–x e x – interface. Microkinetic modeling also revealed that NH3 formation (NH2 + H → NH3) at the Ru/Ca2NH1–x e x – interface is the RDS. These findings are consistent with the experimental results and validate the reaction mechanism dealt with in this research.