Total Oxidation of Lean Methane over Cobalt Spinel Nanocubes Controlled by the Self-Adjusted Redox State of the Catalyst: Experimental and Theoretical Account for Interplay between the Langmuir–Hinshelwood and Mars–Van Krevelen Mechanisms

Abstract

Involvement of suprafacial and intrafacial oxygen species in catalytic combustion of methane over the (100) faceted cobalt spinel was systematically examined as a function of temperature and CH4 conversion (XCH4). The clear-cut Co3O4 nanocubes of uniform size were synthesized using a hydrothermal method and characterized with XRD, RS, HR-TEM, XRF, TPSR (CH4/16/18O2), and SSITKA (CH4/16/18O2) techniques. The experimental results were corroborated by first-principles thermodynamic and DFT+U molecular modeling, providing a rational framework for a detailed understanding of the origin of a different redox comportment of the catalyst with the varying temperature and its mechanistic implications. Three temperature/conversion stages of the methane oxidation reaction were distinguished, depending on involvement of the adsorbed or lattice oxygen and the redox state of the catalyst. A stoichiometric (100) surface region (300 °C < T < 450 °C, XCH4 < 25%) is featured by the dominant suprafacial (Langmuir–Hinshelwood)...