Highly porous α-MnO2 nanorods with enhanced defect accessibility for efficient catalytic ozonation of refractory pollutants

Herein we reported the first example of preparing α-MnO2 by selective acid etching from Mn-containing spinel. The defects, facet, and surface area of α-MnO2 were cooperatively engineered by an all-in-one acid etching method to enhance the defect accessibility to the reactants. The obtained highly porous α-MnO2 nanorods have rich defects of Mn3+ (24.9%) and oxygen vacancies (31.4%), mainly active crystal facets of (110), and an ultrahigh surface area of 271.1 m2/g. With α-MnO2 nanorods as the catalysts, more than 90.9% of 4-chlorophenol can be degraded within 12 min by catalytic ozonation in a wide work pH of 4.5-10.5. The experiments and DFT theory calculations reveal that α-MnO2 with (110) facet promotes the adsorption and activation of ozone directly over the defects or indirectly over H2O adsorbed on the defects. Thus, more reactive oxygen species (e.g., •OH, •O2-, 1O2, surface *O) are generated and get involved in pollutant degradation. This work provides a facile method to maximize the defect accessibility, and a deeper mechanistic study to understand the roles of the defects.