Elsevier

Journal of Catalysis

Volume 408, April 2022, Pages 423-435
Journal of Catalysis

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

https://doi.org/10.1016/j.jcat.2021.06.021Get rights and content
Under a Creative Commons license
open access

Highlights

  • W sites structure changes during the Na2WO4 and WO3 phases transitions.

  • The oxidation state of the W sites remains mainly 6+ at heating pretreatment.

  • Td-W6+ sites are less distorted and more active in the presence of Oh-Mn3+ sites.

Abstract

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.

Keywords

Methane
OCM
Active sites
XANES
XAS
Raman
In situ

Cited by (0)