In Situ Structure Study of a TiO2 Doped MnOX-Na2WO4/SiO2 Catalyst Under Na2WO4 Melting Conditions

Abstract

MnO_X-Na₂WO₄/SiO₂ catalyst exhibited notable C₂ selectivity/yield in the oxidative coupling of methane (OCM), a promised green chemistry reaction. Nevertheless, the reaction mechanism of this catalyst remains a subject of contention, particularly regarding the role of Na₂WO₄ in the activation. In this study, in situ characterizations of a TiO₂-modified MnO_X-Na₂WO₄/SiO₂ catalyst are conducted by XRD and XPS correlating to the OCM reaction condition, focusing on the simultaneous phase transition of catalyst components within its activation temperature zone. The online MS along with XPS/XRD coupled activity study confirm that transition from Mn³⁺ to Mn²⁺ stands as a pivotal factor influencing the reactivity. In situ XRD further revealed that in this narrow temperature window there is a particular three-step Na₂WO₄ phase change, ending as molten salt, right before the substantial Mn³⁺ to Mn²⁺ transfer initiated. In addition, the rarely observed Na₂WO₄ behavior as molten salt is observed by in situ XPS with rapid spectra collected during an on-stage heating process. These comprehensive in situ catalyst characterizations, covering the extensive structure–activity relationship from solid state to partial molten salt condition, supply new important evidence of the active oxygen transfer pathway from Na₂WO₄ to Mn species which provides a key to understand the activation mechanism of MnO_X-Na₂WO₄/SiO₂ catalyst in OCM.

Graphical Abstract