Nitrogen-Doped TiO₂/Nitrogen-Containing Biochar Composite Catalyst as a Photocatalytic Material for the Decontamination of Aqueous Organic Pollutants

In this study, a waste walnut shell-derived biochar enriched with nitrogen (N-biochar) is mixed with nitrogen-doped TiO2 (N-TiO2) to fulfill an affordable composite material for the degradation of methyl orange (MO). Results showed that porous structure and oxygen-containing functional groups of biochar facilitate contact with MO during the reaction process. Meanwhile, doped nitrogen has a positive effect on improving the reaction activity due to the existence of a substituted state and a gap state in the catalyst. It was revealed that the N-TiO2/N-biochar (NCNT0.2/1) exhibited better photocatalytic degradation efficiency (97.6%) and mineralization rate (85.4%) of MO than that of TiO2, N-TiO2, and TiO2/N-biochar due to its stronger synergistic effect of N, TiO2, and biochar, in accordance with its high charge separation by photoluminescence (PL) analysis. Trapping experiments showed that ·OH is the predominant active species during the decolorization and mineralization process of MO. After five repeated use, the loss of activity of the catalyst was negligible. In addition, the catalytic degradation process was consistent with the pseudo-first-order kinetic model with the rate constant of 4.02 × 10–2 min–1