Crystal phase-driven performance of MnO₂ in aqueous phase low-temperature thermal catalysis: Synergistic interactions...

Catalytic oxidation at mild conditions is crucial for mitigating the high pressure and high temperature challenges associated with current catalytic wet air oxidation (CWAO) technologies in wastewater treatment. Among potential materials for catalytic oxidation reactions, polycrystalline MnO₂ existed in natural minerals holds considerable promise. However, the relationships between different crystal phases of MnO₂ and their catalytic activity sources in aqueous phase remain uncertain and subject to debate. In this research, we synthesized various MnO₂ crystal phases, comprising α-, β-, δ-, γ-, ε-, and λ-MnO₂, and assessed their catalytic oxidation efficiency during low-temperature heating for treatment of organic pollutants. Our findings demonstrate that λ-MnO₂ exhibits the highest catalytic activity, followed by δ-MnO₂, γ-MnO₂, α-MnO₂, ε-MnO₂, and β-MnO₂. The variations in catalytic activity among different MnO₂ are attributed to variances in their oxygen vacancy abundance and redox activity. Furthermore, we identified the primary active species, which include Mn³⁺ and superoxide radicals (•O₂^-) generated by surface lattice oxygen of MnO₂. This research highlights the critical role of crystal phases in influencing oxygen vacancy content, redox activity, and overall catalytic performance, providing valuable insights for the rational design of MnO₂ catalysts tailored for effective organic pollutant degradation in CWAO applications.