Temporal post-discharge reactions effect on the oxidative catalytic properties of plasma-synthesized α-MnO2 nanorods

Abstract

The evolution of the morphology, crystalline structure, and textural properties of α-MnO2 nanorods, which were obtained after reduction of a KMnO4 solution via gliding arc plasma process, have been followed along their ageing under temporal post-discharge conditions. When aged at 25 °C a partial transformation of α-MnO2 (cryptomelane) to γ-MnO2 (nsutite) occurred with an increase of specific surface area from 98 to 141 m2/g. In parallel, the starting nanorods were converted to nanoneedles after 24 and 48 h, and into nanosheets after 72 h. These transformations are ascribed to the evolution of maturation process of crystallites, thanks to post-discharge species (H2O2, HOONO,…). At the opposite, ageing at 100 °C induced a remarkable transformation of α-MnO2 to γ-MnO2 with a decrease of specific surface area from 98 to 35 m2/g after 3 h, which was in line with the observed agglomeration of nanorods. Finally, whereas a longer ageing at 100 °C provoked an activity diminution of the material, an augmentation of activity of plasma-synthesized α-MnO2 was recorded when implementing ageing at 25 °C.