More electrons on Fe and FeN2 promote nitrogen fixation efficiency

Abstract

Fe has been doped into many systems to adsorb and activate nitrogen, and low-valence Fe was proposed to surpass the high-valence Fe in serving as an active site for the nitrogen reduction reaction. However, it is still unclear which properties and how the properties are affected by the charge on Fe. In order to unravel the mechanism and further provide support for experimental modifications, we obtain Fe with different charges on a base of Fe/g-C₃N₄, and found that the charge is not only determined by the surrounding atoms. With the aid of first-principles calculations, Fe with a low valence state is determined to be advantageous in increasing N₂ adsorption stability, reducing the onset potential of the NRR and suppressing the competitive HER, which absorbs N₂ in an end-on configuration and reduces it easily through distal mechanisms. Notably, it is not Fe, but FeN₂ composed of Fe and two close N, which acts as a conduit to transfer electrons from the substrate to the intermediate product N_xH_y. In addition, the charge on FeN₂ is a better descriptor than that on Fe to explain the interaction between Fe and N₂ on it. Therefore, increasing the charge on Fe and FeN₂ is a promising method to enhance nitrogen fixation activity.