Nanostructured manganese oxides as highly active catalysts for enhanced hydrolysis of bis(4-nitrophenyl)phosphate and catalytic decomposition of methanol

Manganese oxide-based (MnOx) catalysts have received increasing attention due to their low cost, low toxicity, and the ability to degrade organic molecules under mild conditions. In this work, several nanostructured MnOx-based catalysts were prepared via redox reactions of manganese compounds in an aqueous solution and alkaline precipitation with aqueous ammonia. A wide arsenal of analytical techniques, including nitrogen physisorption (BJH and BET), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF), and Raman spectroscopy were applied for their characterization. The nanostructured MnOx exhibited high catalytic activity in hydrolysis of phosphate diester-based substrate bis(4-nitrophenyl)phosphate (BNPP) at 328 K. Furthermore, MnOx specimens were also studied in decomposition of methanol to carbon monoxide and hydrogen as a potential alternative fuel. The results show high dependency of the materials catalytic properties on the synthesis method. It was found that the varying fractions of redox-active Mn2+/Mn3+/Mn4+ surface sites and the high proportion of oxygen species (such as O2− or O−) together with the particle dispersion and morphology are important for high catalytic activity of MnOx in both investigated catalytic reactions. Based on the experimental data, possible mechanisms of BNPP hydrolysis and methanol decomposition were proposed and discussed in detail.