δ-MnO2 with an ultrahigh Mn4+ fraction is highly active and stable for catalytic wet air oxidation of phenol under mild conditions

δ-MnO2 with a very high amount of Mn4+ showed high catalytic activity and stability for the catalytic wet air oxidation (CWAO) of phenol at a very low temperature of 70 °C. The abundance of Mn4+ and reactive surface oxygen species in this catalyst, its resistance to Mn leaching, and its repeatable Mn4+/Mn3+ (Mn4+/Mn2+) redox cycles contributed greatly to its remarkable catalytic performance.