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.

Keywords

Sol-gel / Manganese oxide / Catalytic oxidation / Methane / Langmuir-Hinshelwood mechanism / Mars-van Krevelen route / Two-term process

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Patrick Ndouka Ndouka: Writing - review & editing, Writing - original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Stephane Kenmoe: Writing - review & editing, Writing - original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Formal analysis, Conceptualization. Jacques Richard Mache: Methodology, Investigation, Formal analysis. Elie Acayanka: Validation, Methodology, Investigation, Formal analysis. Dick Hartmann Douma: Writing - original draft, Visualization, Validation, Formal analysis. Ralph Gebauer: Writing - review & editing, Validation, Methodology, Investigation, Formal analysis. Patrick Mountapmbeme Kouotou: Writing - review & editing, Writing - original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Formal analysis, Conceptualization.

Acknowledgments

S.K. acknowledges computing time granted by the Center for Computational Sciences and Simulation (CCSS) at the Universität DuisburgEssen and provided on the supercomputer magnitude (DFG grants INST 20876/209-1 FUGG, INST 20876/243-1 FUGG) at the Zentrum für Informations- und Mediendienste (ZIM). S.K. gratefully acknowledges the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) for funding 388390466—TRR 247.

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