Mechanism of catalytic ozonation in different surface acid sites of oxide aqueous suspensions

Abstract

The behaviour of ozone on the surface of different oxides, including MCM-41, γ-Al₂O₃, α-Fe₂O₃, α-FeOOH, Fe₃O₄, α-MnO₂ and β-MnO₂, was investigated for the degradation of ibuprofen, acyclovir and humic acid in water. According to the temperature-programmed desorption of NH₃, Fourier transform infrared (FTIR) spectroscopy of adsorbed pyridine and in situ attenuated total reflection FTIR (ATR-FTIR) spectroscopy measurements, Brønsted acid sites from the isolated hydroxyl and hydrogen-bonded hydroxyl groups were predominant on MCM-41 and α-Fe₂O₃, both Brønsted and Lewis acid sites existed on Fe₃O₄ and β-MnO₂ and Lewis acid sites were predominant on γ-Al₂O₃, α-FeOOH and α-MnO₂. In situ ATR-FTIR, in situ Raman and electron spin resonance spectroscopy using isotopic ozone revealed that ozone was electrostatically adsorbed on the Brønsted acid sites and hardly decomposed; thus, ozone served as a direct oxidant for pollutant degradation over MCM-41 and α-Fe₂O₃ suspensions. However, ozone was adsorbed on the surface Lewis acid sites, competing with water, and decomposed into surface atomic oxygen species and ˙OH and O₂˙⁻ radicals in the γ-Al₂O₃ suspension. Thus, more ˙OH and O₂˙⁻ radicals were generated on the surface of multivalent oxides (α-FeOOH, Fe₃O₄, α-MnO₂ and β-MnO₂) due to their redox activity, which was enhanced by the formation of complexes of pollutants and by-products with multivalent metals. On the basis of the experimental results, a mechanism for the transformation of ozone on the different surface acid sites is proposed.