N-Induced Electron Transfer Effect on Low-Temperature Activation of Nitrogen for Ammonia Synthesis over Co-Based Catalysts
Xuanbei Peng, Congying Wang, Zhenni Tan, Jun Ni, Bingyu Lin, Jianxin Lin, Xiuyun Wang, Lirong Zheng, Chak‐Tong Au, Lilong Jiang
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.