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 (OLat/OAds), 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.

Keywords

• Sol-gel
• Manganese oxide
• Catalytic oxidation
• Methane
• Langmuir−Hinshelwood mechanism
• Mars−van Krevelen route