Hydrothermal Synthesis of Manganese Oxide Nanomaterials and Their Catalytic and Electrochemical Properties

Manganese oxides were fabricated by hydrothermal reactions of KMnO4 and MnSO4 solutions. Crystal structures and morphologies of samples were characterized by X-ray diffraction, scanning electron microscopy, Brunauer–Emmett–Teller (BET) measurements, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The combined effects of temperature, additives of H2SO4, and tetrabutylammonium bromide (TBAB) on manganese oxide crystal structures were investigated. The key factors affecting catalytic activity and electrochemical performance of manganese oxides were studied using oxidation of benzyl alcohol and cyclic voltammetry, respectively. The results indicated that pyrolusite (OMS-7) was formed in 0.1 mol·L–1 MnSO4 solution with KMnO4/MnSO4 molar ratios of 7:18 and 8:17 at 120 °C, respectively. A mixture of OMS-7 and cryptomelane (OMS-2) was prepared when the molar ratio increased. Potassium ion plays an important role in the formation of OMS-2. Steric effects arose from complexation reactions of TBAB and MnSO4 solutions and facilitated the formation of OMS-2. K-OMS-2 and H-K-OMS-2 were formed by adding 0.1 mol·L–1 TBAB and 0.2 mol·L–1 H2SO4 to 0.1 mol·L–1 MnSO4 solution with a KMnO4/MnSO4 molar ratio of 11:14 at 120 °C, respectively. The catalytic oxidation activity was found to follow this trend: H-K-OMS-2 > K-OMS-2 > OMS-7 likely because of the larger amount of acid sites in H-K-OMS-2 . However, the exchangeable active oxygen and specific surface area had a greater impact on the electrochemical performance of manganese oxides. The electrocatalytic activity of synthesized manganese oxides for oxygen reduction increased in the order: OMS-7 < H-K-OMS-2 < K-OMS-2. Supercapacitor performance was compared, and the capacitance also increased in the sequence: OMS-7 < H-K-OMS-2 < K-OMS-2.