Effect of KOH on the continuous synthesis of cobalt oxide and manganese oxide nanoparticles in supercritical water

• Cobalt oxide and manganese oxide synthesized continuously in supercritical water.
• KOH concentrations have effect on oxidation state of metal oxides.
• OH− of KOH may act as a reducing agent as well as a supersaturation enhancing agent.

The effects of KOH on the supercritical hydrothermal synthesis of cobalt oxide and manganese oxide particles are investigated using a continuous-flow reactor. Significant changes in morphology, particle size, and oxidation state are observed by adding KOH. The spinel Co3O4 phase is transformed to a rocksalt CoO phase and the pyrolusite MnO2 phase is transformed to a hausmannite Mn3O4 phase in the presence of 0.5 M KOH. The average particle size of the metal oxides decreased with an addition of KOH. The OH− ions of KOH may act as a reducing agent as well as a supersaturation enhancing agent under supercritical water conditions.