Tailoring Lewis acidic metals and SO42- functionalities on bimetallic Mn-Fe oxo-spinels to exploit supported SO4radical dot- in aqueous pollutant fragmentation

Abstract

Generation of SO4?? anchored on metal oxides via radical transfer from ?OH to surface SO42? functionality (?OH ? SO4?? ) is singular, unraveled recently, and promising to decompose aqueous refractory contaminants. The core in furthering supported SO4?? production is to reduce the energy required to accelerate the rate-determining step of the ?OH ? SO4?? (?OH desorption), while increasing the collision frequency between the ?OH precursors (H2O2) and H2O2 activators (Lewis acidic metals) or between SO42? -attacking radicals (?OH) and supported SO4?? precursors (SO42? ). Herein, Mn-substituted Fe3O4 oxo-spinels (MnXFe3-XO4; MnX) served as reservoirs to accommodate the Lewis acidic Fe/Mn and SO42?, whose properties were tailored by altering the metal compositions (X). The production of supported SO4?? via the ?OH ? SO4?? was of high tangibility, as confirmed by their electron paramagnetic resonance spectra coupled with those simulated. A concave trend was observed in the plot of the Lewis acidic strength of Fe/Mn versus X of MnX with the minimum at X - 1.5. Hence, Mn1.5 could expedite ?OH liberation from the surface most proficiently and therefore exhibited the greatest initial H2O2 scission rate, as corroborated by its lowest energy barrier needed for activating the ?OH ? SO4?? . Meanwhile, a volcano-shaped trend was found in the plot of SO42? concentration versus X of MnX (other than Mn3). This could tentatively increase the collision frequency between ?OH and SO42? on the surface of Mn1.5, as partially substantiated by its second largest pre-factor among the catalysts. Therefore, Mn1.5 exhibited the highest phenol consumption rate (-rPHENOL, 0) among the catalysts, which was - 20-fold larger than those for SO42? -modified Fe2O3 and NiO, which we reported previously. Mn1.5 also outperformed other catalysts in recycling phenol degradation, fragmenting another pollutant (aniline), and mineralizing phenol/aniline.