Influence of water molecule on active sites of manganese oxide-based catalysts for ozone decomposition

Abstract

Developing an efficient approach to decompose ground-level O₃ in humidity is crucial for preventing O₃ pollution in practical application scenes. In this study, MnO_x, CuO, and Cu/MnO_x were synthesized to investigate the influence of H₂O on the variation of active sites during O₃ decomposition. The structural characterizations of the as-synthetic catalysts were measured by N2 physisorption, XRD, SEM, O₂-TPD, H₂-TPR, TG, and FT-IR analyses. In dry conditions, the elimination rate of O₃ followed the sequence of MnO_x > Cu/MnO_x > CuO. The introduction of Cu to MnO_x enhanced the surface area and pore volume of Cu/MnO_x, accordingly diminishing the amounts of surface defects and the participation of sub-surface lattice oxygen for catalytic cycle, indicating that surface defects and oxygen vacancies (V_O) determined the catalytic activity for O₃ decomposition. In humid conditions, the elimination rate of O₃ changed to the sequence of Cu/MnO_x > MnO_x > CuO, with a variation rate compared to dry conditions of -62.9% for MnO_x, 14.2% for CuO, and 27.7% for Cu/MnO_x. The decrease of participant sub-surface lattice oxygen and the accumulation of intermediates in humidity diminished the decomposition of O₃ on MnO_x, while the active species such as superoxide radicals generating from the reaction of H₂O and Cu/MnO_x facilitated the participation of V_O and the desorption of O₂ from the occupied active sites, accelerating the catalytic cycle on Cu/MnO_x. This work developed a deeper understanding of the influence of H₂O on catalytic activity, promoting the performance of MnO_x-based catalysts for practical O₃ decomposition.

Keywords: Humidity influence; MnO(x)-based catalysts; Oxygen vacancies; Ozone decomposition; Sub-surface lattice oxygen.