Ozonation has been widely undertaken in industrial wastewater treatment because ozone poses the outstanding oxidation potential on a large number of organic pollutants (Chen et al., 2017; Wang and Chen, 2020). However, single ozonation commonly presented a low removal rate on organic compounds degradation (Hu and Xia, 2018). To overcome the limitation of this process, ozonation coupled with catalyst have been attracted much attention. Heterogeneous catalytic ozonation can enhance the oxidation of ozone by selecting appropriate solid catalysts. This method can promote ozone decomposition, which will generate more reactive oxygen species (ROS) and further enhance the degradation of organic compounds (Peng et al., 2018; Zhang et al., 2020).
Metal oxides (e.g., MnO2, Fe oxides, CoFe2O4, TiO2) have been widely used as the catalysts in heterogeneous catalytic ozonation (Du et al., 2020; Liu et al., 2019; Oliveira et al., 2019; Yang et al., 2014). Among these metal oxides, MnO2 showed the excellent catalytic performance on benzene series degradation due to its strong redox coples of Mn2+/Mn3+ and Mn3+/Mn4+ on the surface of catalysts, diverse and crystallographic structure (Niu et al., 2019; Zhang et al., 2020). MnO2 has great structural flexibility and crystallographic polymorphs (e.g., α-, β-, γ-, and δ-MnO2) because the basic structural MnO6 units can be linked in different manners forming tunnels (Wang et al., 2016). α-MnO2 has been extensively studied due to its structural characteristics and excellent activity for ozone decomposition and catalytic ozonation (Wang et al., 2015; Zhu et al., 2017). Moreover, recent studies have showed that metal doping can improve the catalytic performance by modifying α-MnO2 structure. Wang et al. synthesized Zr4+ doped α-MnO2 nanowires, and observed that Zr4+ ions originally occupied the positions belonging to elemental manganese in the crystal structure and resulted in a mutual action between Zr4+ ions and Mn3+ ions, thus improving the catalytic performance of α-MnO2 (Wang et al., 2018). Uematsu et al. reported that the morphology of α-MnO2 was changed by doping with Mo6+, leading to an increase in its specific surface area and the number of catalytically active surface positions, which in turn improved the catalytic performance of the catalyst (Uematsu et al., 2016).
As a kind of transition metal, Co has a strong Co2+/Co3+ redox cycle which can also promote electron transfer and thus improve catalytic oxidation efficiency (Anfar et al., 2021). Lv et al. reported that Co doping on Fe3O4 increased the catalytic activity and stability of Fe3O4 (Lv et al., 2012). Li et al. observed that an interface synergistic effect between the doped metal Co and cerium oxide on catalyst Co-Ce-MCM-48 improved the interface electronic behavior and promoted the production of ROS (Li et al., 2019). Faleh et al. developed a new heterogeneous cobalt (Co) catalyst supported on activated carbon (Co/AC) and found the doping of Co improved the surface adsorption capacity of activated carbon which enhanced the degradation efficiency of oxalic acid in catalytic ozonation process (Faleh et al., 2019). Additionally, some studies have showed that the synergistic role can be well presented when the radius of doped metal ions is close to that of metal ions in the catalyst (Kang et al., 2013). Therefore, due to the close ionic radius between Co2+ and Mn2+, the catalyst with Co doping on the α-MnO2 might have a better catalytic activity than that of the sole α-MnO2 catalyst. However, it is not clear how influence cobalt doping on its structure and physical and chemical properties.
In this article, α-MnO2 catalyst doped with Co2+ was prepared by hydrothermal method, and then investigate the catalytic ozonation activities on phenol removal. BET, XRD, XPS and FTIR were used to analyze the phase, morphology and structural properties of the synthesized catalysts. The catalytic ozonation mechanism of Co-doped α-MnO2 catalyst on phenol removal was explored in depth by the masking experiment of free radicals combining with catalyst structure characteristics.