Structure-dependent catalysis of Co3O4 crystals in persulfate activation via nonradical pathway

Co3O4 crystals with different shapes and surface structures were successfully synthesized. The shapes obtained included flower-like nanoplates (FN), nanocubes (NC), nanosheets (NS) and flake-stacked microsphere (FM). The shapes and surface structures of the Co3O4 catalysts significantly affected their catalytic activity. Among the as-synthesized Co3O4 catalysts, the NC sample displayed the highest catalytic activity in persulfate (PS) activation, achieving almost complete removal of 30 mg L-1 orange G (OG) within 10 min at the reaction conditions of 0.5 g L-1 catalyst and 8.0 mM PS. The excellent catalytic performance of NC sample could be ascribed to dominant exposed crystal plane and abundant surface hydroxyl groups. The electron paramagnetic resonance (EPR) and radical quenching tests demonstrated that reactive oxygen species (ROS) including sulfate radical (.SO4-), hydroxyl radical (.OH), superoxide radical (.O-2 ) and singlet oxygen (O-1(2)) were generated in the NC/PS system, while O-1(2) was dominantly attributed to the OG degradation. The exposed plane of (1 1 1) may be responsible for the generation of 1O2. The study dedicates a novel mechanism of PS heterogeneous activation by Co3O4 crystals and provides an ideal catalyst for efficient removal of organic pollutants in wastewater.