Deciphering the Mechanistic Role of Individual Oxide Phases and Their Combinations in Supported Mn–Na₂WO₄ Catalysts for Oxidative Coupling of Methane

Oxidative coupling of methane (OCM) is an attractive direct route for upgrading methane to valuable chemicals. In this study, temporal analysis of products (TAP) and steady-state experiments are conducted to understand the role of individual oxide phases and their combinations in supported Mn–Na₂WO₄/SiO₂ catalysts for OCM. The results from TAP transient kinetic studies indicate that Mn plays an important role in promoting gas-phase oxygen activation, while NaO_x/SiO₂ and WO_x/SiO₂ are relatively inert toward gas-phase oxygen and methane activation. However, the supported catalyst combining Na and W in the form of Na₂WO₄ shows enhanced gas-phase oxygen activation, exhibiting a much lower oxygen activation energy (148 kJ/mol) and enhanced activity toward methane activation as compared to the individual supported oxide catalysts. The addition of Mn to Na₂WO₄/SiO₂ further decreases the oxygen activation energy by 40 kJ/mol. Moreover, methane activation is also enhanced with CH₃ as the main intermediate, but with increasing Mn content, more CH₂ intermediates are observed. Different forms of oxygen (both dioxygen and atomic) are detected on the catalyst surface using isotopic pump/probe pulsing and their distribution is found to depend on the catalyst composition. An optimal Mn content in the Na₂WO₄/SiO₂ catalyst system is needed to enhance the amount of dioxide surface species (e.g., superoxide ¹⁶O₂^– or peroxide ¹⁶O₂^2–) associated with Na₂WO₄, leading to high C₂ selectivity for OCM. When the Mn content is too high, the larger MnO_x domains are shown to contribute to the formation of higher concentrations of monoxide surface species that lead to nonselective OCM pathways. This insight from transient kinetic characterization using TAP combined with conventional steady-state studies provides a deeper understanding of the role of individual oxide phases and their combination on supported catalysts toward the formation of intermediate surface species and their impact on the OCM reaction mechanism. This knowledge is critical for designing superior catalyst formulations for OCM.