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

The industrial synthesis of ammonia (NH 3 ) 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 NH 3 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 NH 3 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 N 2, leading to a decrease in activation energy for the N 2 molecule. The third is the involvement of nitrogen vacancies. The pyridine N weakly coordinated with highly dispersed Co reacts with adsorbed H 2 to form NH 3, simultaneously generating N vacancies; then, the consumed N species can be replenished via N 2 adsorption on vacancy sites. These factors contribute to the superiority of the N-doped carbon-supported Co-based catalyst, which achieves an NH 3 production rate of 1.59 mmol NH 3 ·g cat –1 ·h –1 at even 250 °C and 1 MPa.