Facile synthesis of MnO2 nanorods and ZnMn2O4 nanohexagons: a comparison of microwave-assisted catalytic activity against 4-nitrophenol degradation

The present preliminary research work reports the synthesis of MnO2 nanorods and ZnMn2O4 spinel via co-precipitation method. The synthesized nanomaterials were characterized via scanning electron microscopy coupled energy-dispersive X-ray spectroscopy (SEM-EDX) with elemental mapping, transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) techniques. The chemical composition and purity were established via XRD and XPS studies. Formation of MnO2 nanorods and hexagonal shaped ZnMn2O4 nanostructures was confirmed via SEM and TEM studies. The TGA studies confirmed high thermal stability of MnO2 as compared to ZnMn2O4. Microwave-assisted catalytic activity was assessed by degrading 50 ppm of 4-nitrophenol solution in presence of MnO2 and ZnMn2O4 under microwave irradiation for 30 min. almost 85% degradation within 30 min was achieved using ZnMn2O4 as catalyst. The degradation followed first order kinetics in both cases and the rate constant (k) values were found to be 0.02075 in presence of MnO2 min(-1) and 0.05912 min(-1) in presence of ZnMn2O4. The effect of addition of hydrogen peroxide was also investigated and the increase in catalyst dosage from 50 mg to 250 mg increased the degradation efficiency from 71% to 94% using ZnMn2O4 as catalyst. Likewise, upon increasing the microwave power from 200 W to 1000 W, the degradation efficiency increased from 71% to 94%. The radicals responsible for degradation were confirmed by the scavenging experiments using tert-butyl alcohol (TBA) and 1,4-Benzoquinone (BQ) as radical scavengers. The degradation efficiency decreased to almost 50% in presence of TBA as compared to BQ which revealed 75% degradation of 4-nitrophenol solution under microwave irradiation for 30 min. A plausible mechanism of microwave-assisted catalytic degradation was proposed. (C) 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).