Elucidating the structure of the W and Mn sites on the Mn-Na2WO4/SiO2 catalyst for the oxidative coupling of methane (OCM) at real reaction temperatures
2022
Ortiz-Bravo, Carlos A. | Figueroa, Santiago J.A. | Portela, Raquel | Chagas, Carlos A. | Bañares, Miguel A. | Toniolo, Fabio Souza
The performance of the Mn-Na₂WO₄/SiO₂ catalyst for oxidative coupling of methane (OCM) has been ascribed to crystalline phases that are not present at reaction temperatures (>700 °C). The evolution of W and Mn sites structure was monitored via in situ TPO-XRD, -Raman, and -XANES spectroscopies. TPO-XRD shows that the crystalline phases identified on the Mn-Na₂WO₄/SiO₂, Na₂WO₄/SiO₂, and WO₃/SiO₂ catalysts at room temperature do not exist at relevant OCM temperatures. The γ → β → α-WO₃, α → β-cristobalite, and cubic → orthorhombic → molten-Na₂WO₄ phase transitions occur upon heating. TPO-Raman spectra show that the bond order of W sites with octahedral (Oₕ) and tetrahedral (Td) symmetry changes during the δ → γ → β → α-WO₃ and cubic → orthorhombic → molten-Na₂WO₄ transitions, respectively. TPO-XANES spectra show that bond order changes are due to distortion degree variations because all samples preserve essentially W⁶⁺ valence and Oₕ-Mn³⁺ sites are always present on Mn-Na₂WO₄/SiO₂ catalyst. Steady-state OCM tests show that Oₕ-W⁶⁺ sites are inactive and Td-W⁶⁺ sites are less distorted and more active towards methane activation in the presence of Oₕ-Mn³⁺ sites.
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