Influence of solvothermal post-treatment on photochemical nitrogen conversion to ammonia with g-C3N4 catalyst

Abstract

In this work, graphitic carbon nitride (g-C3N4) with large surface area and many nitrogen vacancies was synthesized by introducing ionic liquid [Bmim]Br as a solvent into the solvothermal post-treatment. X-ray diffraction (XRD), N2 adsorption, scanning electron microscopy (SEM), UV-Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), temperature-programmed desorption of N2 (N2-TPD), and photoluminescence (PL) spectroscopy were used to characterize the prepared catalysts. The morphology of the as- prepared g- C3N4 was markedly changed from an orderless layered structure to nanoparticles with a uniform size distribution of around 30-40 nm after the introduction of [Bmim]Br, leading an increase in surface area from 8.6 to 37.9 m2∙g-1. N2-TPD, photoluminescence spectra, and density functional theory (DFT) simulations indicated that the nitrogen vacancies not only trapped the photogenerated electrons to enhance their separation rate, but also served as active sites for the adsorption and activation of N2 molecules. The increased surface area of the as-prepared g-C3N4 meant that more nitrogen vacancies were exposed on the surface, leading to a markedly promoted nitrogen photofixation ability. The possible reaction mechanism is proposed.