Novel Manganese-Based Catalysts For Catalytic Oxidation Of Bisphenol A With Peroxymonosulfate

Sulfate radical(SO₄^·-)-based advanced oxidation processes（SRAOPs）have been regarded as a powerful wastewater treatment technology for the elimination of recalcitrant organic pollutants.Compared with the·OH generated from traditional Fenton technology,the SO₄^·-generated in SRAOPs has advantages of longer lifetime and wider working pH range.SO₄^·-can be effectively generated from the activation of peroxymonosulfate（PMS）by heterogeneous cobalt（Co）based catalysts.However,due to the high price of Co and the leaching of toxic Co ions,an alternative metal material with low toxicity and environmental benignity for PMS activation is highly desired.Recently,manganese（Mn）is one of the research hotspots in the field of catalysts because of its abundant existence in soil and low toxicity to the environment as well as multiple valence states.Nevertheless,the performance of Mn-based catalysts in PMS activation is not ideal yet.How to improve the catalytic activity of these materials has become a noticed project.In this article,two novel Mn-based PMS activators,MnO_x/N-HPCS and OD-MnO_2-x-RM,were prepared by morphology control,carrier process and defect engineering.The performance of these two catalysts was evaluated by degradation of bisphenol A（BPA）which is one of the typical endocrine disrupting chemicals（EDCs）.The main conclusions are as follow:（1）A unique hybrid hollow structure with ultrasmall size MnO_x nanoparticle homogeneously dispersed on porous N-doped carbon（NC）shells was designed as the novel MnO_x-based catalyst（denoted as MnO_x/N-HPCS）.The characterization results showed that the hydrothermal nucleation process well removed the SiO₂ template,resulting in a unique hollow and porous structure of MnO_x/N-HPCS.This special structure increased the specific surface area（227.91 m²/g）,pore volume（0.616 cm³/g）and pore diameter（⁶.2 nm on average）of the material.Based on the result of XRD and Raman analyses,the main phases of MnO_x areα-MnO₂ andα-Mn₂O₃,and that of NC shell is amorphous N-doped graphitized carbon.The NC shell not only effectively avoid the agglomeration of MnO_x,but also serve as metal-free active sites for PMS activation.（2）OD-MnO_2-x-RM,another MnO_x-based catalyst,was prepared by morphology controlling and defect engineering.This well-designed material has rattle-type structure with plenty of oxygen vacancies（OVs）modified MnO₂ nanoflakes vertically aligned on the surface.TEM and BET results show that the etching process caused the material to form a 52-nm-thick cavity layer between core and shell,increasing the specific surface area（178.9 m²/g）,pore volume（0.23 cm³/g）and pore diameter（⁷.6 nm on average）of this material.The results of XRD and XPS analyses indicate that the main phase of the MnO₂ nanoflakes isδ-MnO₂.The H₂ reduction treatment reduced theδ-MnO₂ intoγ-Mn₃O₄ and decreased the ratio of oxygen atoms in OD-MnO_2-x-RM,accompanied by the formation of OVs and an increase of adsorbed oxygen ratio on material surface.This phenomenon indicates that the reduction treatment can effectively enhance the adsorption capacity of OD-MnO_2-x-RM to small oxygen-containing molecules（such as O₂ and PMS）.（3）The performance of MnO_x/N-HPCS was evaluated by activation of PMS for BPA removal.The result shows that not only the design of hollow porous structure but also the introduction of NC carrier improve the catalytic activity of this material.When the concentration of PMS and catalyst were 0.2 g/L and 0.06 g/L respectively,10 mg/L of BPA could be completely removed within 30 min,and the reaction kinetics constant was 0.29 min^-1.For the mechanism of PMS activation,the mixed valence states of Mn,the hydroxyl species induced complexation of≡Mn(^3+/4+)-PMS and the metal-free N-doped sites synergistically enhancing the activation of PMS.The main reactive oxygen species（ROSs）generated during the reaction were·OH and SO₄^·-,among which SO₄^·-played a leading role in the degradation of BPA.Furthermore,the MnO_x/N-HPCS catalyst displayed good reusability（successive 4 runs）,negligible Mn leaching（less than 2μg/L）,excellent stability in actual water（tap water and effluent from sewage plants）.（4）The performance of OD-MnO_2-x-RM was also evaluated by activation of PMS for BPA removal.The result shows that not only the design of rattle type structure but also the introduction of OVs improve the catalytic activity of this material.Specially,OVs increased the catalytic activity of OD-MnO_2-x-RM by 8 times.When the concentration of PMS and catalyst were 0.1 g/L and 0.05 g/L respectively,10 mg/L of BPA could be completely removed within 45 min,and the reaction kinetics constant was 65.86 g^-1 min^-1.Besides,the turnover frequency（TOF）of optimized OD-MnO_2-x-RM catalyst reaches up to 66.7 min^-1,making it one of the best-performing Mn-based catalysts ever reported for BPA removal via PMS activation.Various experimental and density functional theory（DFT）findings indicate OVs enhances the interaction betweenδ-MnO₂ and PMS（from repulsion to attraction）,inducing the dissociation of PMS molecule on OVs site with formation of HSO₄^-and ¹O₂.Chemical probe experiment and quenching tests confirmed that ¹O₂ was the main ROSs responsible for BPA oxidation by OD-MnO_2-x-RM+PMS system.Additionally,the OD-MnO_2-x-RM catalyst displayed good reusability after 4 runs（more than 86.1%of BPA was removed）,and outstanding practical application potential（98.1%and 90.8%of BPA were removed in tap water and effluent from sewage plant,respectively）.