Template free synthesis of lithium doped three-dimensional macroporous graphitic carbon nitride for photocatalytic N2 fixation: the effect of Li–N active sites

Abstract

In this work, three-dimensional macroporous lithium doped graphitic carbon nitride was synthesized. XRD, N₂ adsorption, SEM, XPS, UV-Vis spectroscopy, N₂-TPD and photoluminescence were used to characterize the prepared catalysts. The result shows that lithium exists as a coordinative Li–N bond, which can chemisorb and activate N₂ molecules, and promote the electron transfer from the catalyst to the nitrogen molecules. The as-prepared lithium doped g-C₃N₄ shows a NH₄⁺ production rate of 4.8 mg L⁻¹ h⁻¹ g_cat⁻¹, which is 20 times that of bulk g-C₃N₄. Density functional theory (DFT) simulations show that lithium doping can not only increase the NN bond length, which activates N₂ molecules, but also promote the electron–hole separation efficiency of g-C₃N₄.