N-Induced Electron Transfer Effect on Low-Temperature Activation of Nitrogen for Ammonia Synthesis over Co-Based Catalysts

Abstract

The industrial synthesis of ammonia (NH₃) using Fe-based derived catalysts requires harsh reaction conditions (400–600 °C, 20–40 MPa). It is desirable to develop catalysts that perform well at low temperature and pressure (<400 °C, <2 MPa). The main challenge of low-temperature NH₃ synthesis is the dissociation of the extremely stable N≡N triple bond (945 kJ/mol). Herein, the N-doped C-supported Co catalyst was demonstrated to be active and efficient for NH₃ synthesis under mild conditions, resulting from the hybridization of the d orbitals of Co with p orbitals of nitrogen for Co–N coordination. The doping of N has three relevant effects. The first is the size decrease in Co nanoparticles. It is proven by in situ X-ray photoelectron spectroscopy and synchrotron-based X-ray absorption near-edge structure and extended X-ray absorption fine structure analyses coupled with density functional theory calculation that there is strong electron transfer from the doped N to Co. Hence, the second effect of N doping is the increase in electronic state of Co d orbitals which promotes the donation of 3d electrons from Co to the π* orbital of N₂, leading to a decrease in activation energy for the N₂ molecule. The third is the involvement of nitrogen vacancies. The pyridine N weakly coordinated with highly dispersed Co reacts with adsorbed H₂ to form NH₃, simultaneously generating N vacancies; then, the consumed N species can be replenished via N₂ adsorption on vacancy sites. These factors contribute to the superiority of the N-doped carbon-supported Co-based catalyst, which achieves an NH₃ production rate of 1.59 mmolNH₃·gcₐₜ–¹·h–¹ at even 250 °C and 1 MPa.