Ball Milling Synthesized MnOx 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 MnOx 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 approximate to BM1 (T-90% = 180-200 degrees C) > BM0 (260 degrees C) > CMO approximate to cryptomelane MnO2 (>300 degrees C). Both adsorption and destruction contribute to HCBz removal at 180 degrees C while destruction prevails at 200-300 degrees 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.