Igniting the catalytic activity of Mn-Fe spinel for Fenton reactions: The doped nitrogen atoms in the interstitial sites and substitutional sites

Nitrogen doping plays a crucial role in activating the Fenton catalytic activity of transition metal oxides. However, the mechanism by which nitrogen doping sites influence the properties of these oxides remains unclear. We successfully incorporated nitrogen atoms into the internal lattice of MnFe₂O₄ spinel through thermal activation and identified two major doping sites: interstitial nitrogen (N_int) and substitutional nitrogen (N_sub). Through in-situ DIRFTS experiments and DFT calculations, we proposed a unique mechanism. Both sites can effectively activate the catalytic activity of the oxides, but N_sub is particularly effective in altering the electron distribution of Mn elements within the lattice. This alteration promotes the transformation of superoxide (•O₂^–/•OOH) to H₂O₂, thereby further enhancing catalytic activity. Compared to un-doped MnFe₂O₄, experimental results obtained using MnFe₂O₄N_0.4 as the catalyst demonstrated that the degradation rate constant of tetracycline in simulated wastewater increased from 0.0072 min⁻¹ to 0.2745 min⁻¹. Furthermore, the degradation rate of tetracycline reached 98.7 %. For real wastewater experiments, the COD removal rate was 77.4 %, and the mineralization rate was 62.5 %. This study offers new insights into the design of nitrogen-doped transition metal oxide catalysts.