A facile synthesis of octahedral layered birnessite-type manganese oxide (OL-1) nanostructures with tremendous catalytic activity for methylene blue degradation

Abstract

Birnesitte-type octahedral layer manganese oxides were successfully synthesized via precipitation method with the different mole ratios of potassium permanganate and oxalic acid (KMnO4/H2C2O4). The precipitation reaction occurs only at certain mole ratios (i.e. 3:5 and 2:3) of KMnO4/H2C2O4 were applied. However, at lower mole ratio of KMnO4/H2C2O4 (1:4) or higher mole ratio KMnO4/H2C2O4 (3:1) the reactants remain solution even at longer the reaction times. The precipitated solid product was calcined up to 600 °C and referred to as the CC catalysts, whereas the products without calcination were denoted to as the CNC catalysts. The solid CC and CNC catalysts were then characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Absorption Spectroscopy (AAS) and the oxidation states of manganese in the manganese oxide catalysts were determined together with the zero point charge (pHzpc) as well as particle sizes. The data from XRD indicated the CNC catalysts showed wide peaks with low intensities, indicating poor crystallinity of birnessite whereas the CC catalysts generated much higher crystalline birnessite. The morphology of the CC and CNC catalysts displayed similar aggregates of particles with irregular shapes and sizes but CNC catalyst exhibit smaller particle size. Data from AOS of manganese indicated that CNC catalyst (3.12) have lower values than that of CC (4.00), indicating the presence of more defects which was consistent with XRD result, low values pHzpc (3.17-3.92) and K/Mn (0.44-0.50). The catalytic activities of birnessite catalysts were tested for the degradation of methylene blue (MB) dye with H2O2 as an oxidant. The CNC catalysts showed substantially higher catalytic activities for MB degradation compared to the CC catalysts. The maximum degradation of methylene blue was 97.73% for the CNC catalyst using mole ratio (KMnO4/H2C2O4) of 2:3, whereas the CC catalysts can degrade only 83.27% of MB using mole ratio (KMnO4/H2C2O4) of 3:5.