Ball Milling Synthesized MnO_x as Highly Active Catalyst for Gaseous POPs Removal: Significance of Mechanochemically Induced Oxygen Vacancies

A rapid (1.5 h) one-step ball milling (BM) method was developed not only to modify commercial MnO₂ via top-down approaches (BM0), but also to bottom-up synthesize MnO_x by cogrinding of KMnO₄ and MnC₄H₆O₄ (BM1) or KMnO₄ and MnSO₄ (BM2). Catalysts activity on gaseous POPs removal was tested using hexachlorobenzene (HCBz) as surrogate. Catalytic performance decreases in the order of BM2 ≈ BM1 (T_90% = 180–200 °C) > BM0 (260 °C) > CMO ≈ cryptomelane MnO₂ (>300 °C). Both adsorption and destruction contribute to HCBz removal at 180 °C while destruction prevails at 200–300 °C. Mechanism studies show that destruction activity is lineally correlated with the amount of surface reactive oxygen species (O_ads); stability is determined by the removal of surface chloride, which is associated with the mobility of bulk lattice oxygen (O_lat); adsorption capacities are linearly correlated with surface area and pore structure. With the aid of extensive characterizations the excellent performance of BM prepared samples can be explained as (1) abundant surface vacancies enhance the generation of O_ads; (2) massive bulk vacancies promote the mobility of bulk O_lat; (3) large surface area and uniform pore size distribution facilitate the physisorption of HCBz.