Unraveling the origin of enhanced activity of Mn species in Nb2MnO6 mixed oxide during mineralization of tetracycline via catalytic ozonation

In this study, mixed manganese-niobium oxides were synthesized using a Pechini method. The materials were fully characterized and tested in catalytic ozonation of tetracycline hydrochloride (TC) as a model antibiotic pollutant. It was revealed that the Pechini method allowed the obtainment of mixed Mn-Nb oxides containing a unique Nb₂MnO₆ phase. The most efficient formation of Nb₂MnO₆ was observed at a low Mn/Nb molar ratio, while bulk Mn₂O₃ was preferentially obtained at a higher manganese loadings. The results of the catalytic tests revealed that manganese species in Mn-Nb mixed oxides exhibit higher activity in catalytic ozonation than Mn species in bulk Mn₂O₃. The strongly enhanced activity of manganese in the Nb₂MnO₆ phase was associated with a higher electron density in the vicinity of Mn²⁺ species and the presence of strongly nucleophilic surface oxygen species, which most probably enabled more efficient electron transfer from the catalyst to adsorbed ozone molecules, leading to more efficient formation of hydroxyl radicals and mineralization of TC. Significant enhancement of TC mineralization in the presence of mixed Mn-Nb oxides was observed even at low catalyst loading (0.05 g/L). The most active mixed Mn-Nb oxides could mineralize tetracycline in a broad pH range in complex water matrices. The studies also included a detailed analysis of the possible TC degradation pathways and evaluation of the toxicity of degradation products. Results obtained in this work can have a significant impact on the development of more active catalysts based on mixed metal oxides for the removal of antibiotic pollutants from water.