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

Abstract

Manganese oxide-based (MnO_x) 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 MnO_x-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 MnO_x exhibited high catalytic activity in hydrolysis of phosphate diester-based substrate bis(4-nitrophenyl)phosphate (BNPP) at 328 K. Furthermore, MnO_x 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 MnO_x in both investigated catalytic reactions. Based on the experimental data, possible mechanisms of BNPP hydrolysis and methanol decomposition were proposed and discussed in detail.