Greatly enhanced oxidative activity of δ-MnO2 to degrade organic pollutants driven by dominantly exposed {−111} facets

The reactivity of oxidizing materials is highly related to the exposed crystal facets. Herein, δ-MnO2 with different exposure facets were synthesized and the oxidative activities of the as-prepared materials were evaluated by degrading phenol in water without light. The degradation rate of phenol by δ-MnO2-{−111} was significantly higher than that by δ-MnO2-{001}. δ-MnO2-{−111} also displayed high degradation efficiency to a variety of other organic pollutants, such as ciprofloxacin, bisphenol A, 3-chlorophenol and sulfadiazine. Comprehensive characterization and theoretical calculation verified that the {−111} facet had high density of Mn3+, thus displaying enhanced direct oxidative capacity to degrade organic pollutants. In addition, the dominant {−111} facet promoted adsorption/activation of O2, thus favored the generation of superoxide radical (O2•−), which actively participated in the degradation of pollutants. The phenol degradation kinetics could be divided into two distinct phases: the rapid phase (k1obs = 0.468 min−1) induced by Mn3+ and the slower phase (k2obs = 0.048 min−1) dominated by O2•−. The synergistically promoted non-radical and radical based reactions resulted in greatly enhanced the oxidative activity of the δ-MnO2-{−111}. These findings deepen the understanding of facet-dependent oxidative performance of materials and provided valuable insights into the possible practical application of δ-MnO2 for water purification.