Low-Temperature Catalytic Methane Deep Oxidation over Sol-gel derived Mesoporous Hausmannite (Mn3O4) Spherical Particles

Herein, Mn3O4 spherical particles (SPs) were synthesized by the sol-gel process, followed by thermal annealing at 400 °C. The sample was analyzed by XRD, FTIR, SEM, SBET, XPS, H2-TPR and UV-vis spectroscopy. XRD and FTIR analysis show that Mn3O4 exhibits a tetragonal spinel structure. SEM and SBET analysis display a porous homogeneous surface made of strongly agglomerated spherical-like grains size with an estimated average particle size of ~35 nm, corresponding to a large specific surface area (SBET) of ~81.5 m2/g. XPS analysis indicated that, Mn3O4 were composed of metallic cations (Mn4+, Mn3+, Mn2+) and oxygen species (O2-, OH- and CO32-). The optical energy band gap energy was ~2.55eV. Moreover, Mn3O4 SPs were successfully tested as catalyst with almost 100% conversion of CH4 to CO2 and H2O at a gas hourly space velocity (GHSV) of 72 000 mL3.g-1h-1. The observed performance can be assigned to the cooperated effects of the smallest spherical-like grain size with mesoporous structure as responsible for the larger SBET, and the available surface-active oxygenated species. The cooperative effect of the good reducibility, the higher ratio of active species (OLat/OAds), as well as Density Functional Theory (DFT) calculations suggested that, CH4 total oxidation over the mesoporous Mn3O4 SPs might follow a two-term process in which both Langmuir-Hinshelwood and Mars-van Krevelen mechanisms are cooperatively involved.