Gliding Arc Plasma Synthesis of MnO2 Nanomaterials for Catalytic Oxidation of Benzene: Effect of Plasmagenic Gas

MnO2 nanostructures were successfully synthesized via the reduction of KMnO4 solutions using the gliding arc plasma (Plasma Glidarc) approach. Here, we highlight the effect of different plasmagenic gases, such as moist air (atmospheric air), dry air, nitrogen (N2) or oxygen (O2). The obtained materials were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption and scanning electron microscopy (SEM). The crystalline structures of obtained MnO2 polymorphs are mainly γ-MnO2 and α-MnO2, regardless of the feeding gas. The main reactive species, in addition to nitrogenous species like NO· radical generated with moist air, dry air or N2 gas, other oxygenated species such as H2O2 (E°(O2/H2O2) = 0.69 V) are produced with O2 able to reduce KMnO4 solution (E°(KMnO4/MnO2) = 1.70 V). Helium gas did not allow for the plasma reduction of the KMnO4 solution, even after 60 min of exposure. Furthermore, gas humidification did not significantly affect the precipitation time or the properties of plasma-synthesized MnO2. Atmospheric humidified air appears to be the best plasmagenic gas, as it allows for a shorter synthesis time and leads to a large specific surface area. All plasma-synthesized MnO2 showed good activity during the catalytic oxidation of benzene. The use of different MnO2 polymorphs (α-, δ- and γ-MnO2) showed that, in addition to the specific surface area, the crystalline structure significantly affects the catalytic oxidation of benzene. K+ species inserted within the MnO2 structure allow for their stability during the catalytic process. This work highlights the possibility to use different plasmagenic gases to prepare MnO2 nanostructures through plasma glidarc for the catalytic oxidation of benzene.