High-Performance Catalytic CH₄ Oxidation at Low Temperatures: Inverse Micelle Synthesis of Amorphous Mesoporous Manganese Oxides and Mild Transformation to K_2–<i>x</i>Mn₈O₁₆ and ϵ-MnO₂

Abstract

Catalytic combustion of methane at low temperature under lean conditions was investigated over mesoporous amorphous manganese oxide (Meso-Mn-A), Mn2O3 (Meso-Mn2O3), MnO2 (epsilon phase) (Meso-ϵ-MnO2), and octahedral molecular sieves MnO2 (Meso-OMS-2) synthesized using an inverse surfactant micelle method. The prepared materials are monodispersed nanoparticle aggregates, and the mesopores are formed by connected interparticle voids. All the mesoporous manganese oxides proved to be significantly active compared to nonporous, similar phase materials. However, among the tested materials Meso-Mn-A showed the lowest light-off temperature of 229 °C, but Meso-OMS-2 showed the highest conversion (90%) at the lowest temperature of 373 °C. Despite the low light-off temperatures of mesoporous materials, even nonporous K-OMS-2 (cryptomelane) showed 90% conversion at 403 °C illustrating not only the effect of mesopore size but also the oxidation state of manganese and the structure of the catalyst having effects on the activity of manganese oxides. X-ray photoelectron spectroscopy (XPS), H2-temperature-programmed reduction (H2-TPR), and N2 sorption analysis indicated that the oxidation states of catalysts, surface oxygen vacancies, and large surface areas promoted the lattice oxygen mobility of the catalysts. Thus, activities of the catalysts were correlated to the oxidation states, the lattice oxygen mobility, and the reducibility of the catalysts. The apparent activation energy of methane oxidation calculated based on a pseudo-first-order kinetics ranged from 70.5 to 107.2 kJ mol–1 for the manganese oxides, and the values are comparable with catalysts containing precious metals.