<italic>In-situ</italic> characterization technique in electrocatalytic nitrogen reduction to ammonia

Abstract

<p indent="0mm">As a basic chemical raw material, ammonia is one of the most important chemicals and noncarbon-based energy carriers. Industrial ammonia synthesis mainly relies on the Haber-Bosch process, which requires Fe (or Ru)-based catalysts, high temperature, and high pressure. High-purity H₂ gas is produced by steam reforming of methane from natural gas or coal, resulting in 3%–5% of global natural gas consumption and 1%–2% of global CO₂ emissions. In recent years, under the background of energy strategy and “double carbon”, using renewable energy to develop low energy consumption, low carbon emission, and environmentally friendly technology for ammonia synthesis has become important. Electrochemical nitrogen reduction technology has received extensive attention in the field of synthetic ammonia. This method can potentially provide a new pathway to an alternate Haber-Bosch process, using nitrogen gas and water as raw materials to realize the conversion of nitrogen to ammonia under ambient temperature and pressure. In this process, the nitrogen gas is adsorbed on the surface of a catalyst and then activated by electrons, and the activated nitrogen species are further hydrogenated with the protons supplied by water. Although this electrochemical nitrogen reduction technology has rapidly developed in recent years, the efficiency of ammonia synthesis is still very low and many problems are to be solved: (1) N₂ is a stable, nonpolar molecule with a high bond energy of <sc>941 kJ mol⁻¹,</sc> and hence, N ≡ N is difficult to activate; (2) most catalysts have slow kinetics for nitrogen adsorption and decomposition; (3) most metal atoms are more easily bonded to hydrogen atoms, leading to a competitive hydrogen evolution reaction; (4) the nitrogen reduction reaction is a multielectron, multiproton reaction process, which involves complex proton-coupled electron transfer processes and multiple reaction intermediates, so the relevant reaction mechanisms are very unclear. Importantly, ammonia synthesis catalysts with high activity and stability have not yet been developed. Therefore, to improve the selectivity and efficiency of ammonia synthesis, <italic>in-situ</italic> characterization techniques must be developed to monitor the changes in the catalyst structure and the dynamic evolution of the reaction species under the real conditions of catalytic reactions. This paper summarizes the application of various <italic>in-situ</italic> characterization techniques, such as X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy, particularly in the study of catalysts and catalytic reactions. More importantly, it summarizes the existing problems and challenges in combining multiple <italic>in-situ</italic> technologies and enhancing the spatial and temporal resolution of<italic> in-situ</italic> characterization technologies for the nitrogen-water system.