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) mechanism of methane oxidation. A region of slightly defected surface (450 °C < T < 650 °C, 25% < XCH4 < 80%), in which oxygen vacancies produced upon CO2 and H2O release are virtually refilled by dioxygen, is characterized by coexistence of the suprafacial (Langmuir–Hinshelwood) and intrafacial (Mars–van Krevelen) mechanistic steps. In a nonstoichiometric surface region (T > 650 °C, XCH4 > 80%), the oxygen vacancies are only partially refilled, the catalyst is significantly reduced, and methane is combusted according to the Mars–van Krevelen scheme. Molecular modeling revealed that the suprafacial Co–Oads adoxygen species are more active (ΔEa = 0.83 eV) than the intrafacial Co–Osurf surface sites (ΔEa = 1.11 eV) in the CH4 oxidation. The (100) surface state diagrams for the three distinguished conversion regions were constructed to elucidate the catalyst thermodynamic behavior under those conditions. It was shown that the activity of cobalt spinel is maintained by redox autotuning of the catalyst and dynamic adjustment of uneven participation of the suprafacial and intrafacial oxygen species in methane oxidation to the actual reaction conditions. These factors have important structural and mechanistic consequences for the catalytic CH4 combustion on cobalt spinel and related systems, controlling not only the sustainable versus the stoichiometric turnovers but also for the prevalence or coexistence of the Langmuir–Hinshelwood and the Mars–van Krevelen mechanisms with the reaction progress.