Nitrogen-Doped MgO as an Efficient Photocatalyst Under Visible Light for the Degradation of Methylene Blue in Wastewater Treatment

In this study, two nitrogen-doping strategies for magnesium oxide—an in situ method and a post-synthesis modification—were developed, and their visible-light photocatalytic activity was evaluated using methylene blue (MB) as a model organic pollutant. The materials were characterized using a combination of structural (SEM–EDX), spectroscopic (WAXRD, FTIR, Raman), optical (UV–DRS, PL), and thermal (TGA–MS) analyses. Both nitrogen-doped MgO samples exhibited significantly enhanced MB degradation compared to commercial MgO. Additional photocatalytic tests using phenol, a colorless contaminant, as a probe molecule suggested the occurrence of two distinct degradation pathways: direct photocatalysis for the in situ nitrogen-doped MgO, and a sensitization-mediated degradation process for the post-synthesis nitrogen-doped MgO. Based on the experimental results, a reaction mechanism is proposed.