Low-temperature catalytic methane deep oxidation over sol-gel derived mesoporous hausmannite (Mn3O4) spherical particles

Abstract

In this study, Mn₃O₄ spherical particles (SPs) were synthesized by the sol-gel process, after which they were thermally annealed at 400 °C, and comprehensively characterized. X-ray Diffraction (XRD) revealed that Mn₃O₄ exhibited a tetragonal spinel structure, and Fourier transformed infrared (FTIR) spectroscopy identified surface-adsorbed functional groups. Scanning electron microscopy (SEM) and the specific surface area analyses by Brunauer−Emmett−Teller (BET) revealed a porous, homogeneous surface composed of strongly agglomerated spherical grains with an estimated average particle size of ∼35 nm, which corresponded to a large specific surface area of ∼81.5 m²/g. X-ray photoelectron spectroscopy (XPS) analysis indicated that Mn₃O₄ was composed of metallic cations (Mn⁴⁺, Mn³⁺, and Mn²⁺) and oxygen species (O²⁻, OH⁻ and CO₃²⁻). The optical bandgap energy is ∼2.55 eV. Assessment of the catalytic performance of the Mn₃O₄ SPs indicated T₉₀ conversion of CH₄ to CO₂ and H₂O at 398 °C for gas hourly space velocity (GHSV) of 72000 mL³ g⁻¹ h⁻¹. This observed performance can be attributed to the cooperative effects of the smallest spherical grain size with a mesoporous structure, which is responsible for the larger specific surface area and available surface-active oxygenated species. The cooperative effect of the good reducibility, higher ratio of active species (O_Lat/O_Ads), and results of density functional theory (DFT) calculations suggested that the total oxidation of CH₄ over the mesoporous Mn₃O₄ SPs might take place via a two-term process in which both the Langmuir−Hinshelwood and Mars−van Krevelen mechanisms are cooperatively involved.