(712f) Roles of Different Oxide Phases Present in Na- and Mn- Promoted W/SiO2 Catalysts during Oxidative Coupling of Methane (OCM)

The oxidative coupling of methane (OCM) over Mn₂O₃-Na₂WO₄/SiO₂ catalyst (prepared by incipient wetness impregnation method) was studied using various in-situ spectroscopic techniques along with reaction performance to identify the catalyst structure, roles of various oxide phases and catalytic active sites. In-situ Raman spectra of this catalyst under dehydrated conditions (400°C, 10%O₂/Ar) revealed, to our knowledge for the first time, the presence of surface Na-WO_x and MnO_x species along with the presence of Na₂WO₄ and Mn₂O₃ (corroborated by in situ XRD) crystalline nano-particles (NPs). Stability analysis of above metal oxide phases under OCM reaction conditions (900°C, CH₄+O₂+N₂ 3.3:1:4), through in-situ Raman spectroscopy, established that only surface Na-WO_x and MnO_x species, anchored to the SiO₂ support, are thermally stable in the OCM environment; whereas Na₂WO₄ and Mn₂O₃ NPs are unstable due to melting and reduction of these oxide phases, respectively.

Further steady-state performance investigation of the thermally stable, SiO₂ supported, surface WO_x species with and without Na and Mn promoters, confirmed that surface WO_x species are catalytically active as they can perform OCM without Na and Mn promoters. Addition of Na to this catalyst (producing Na-WO_x surface sites) was found to improve the selectivity significantly. Subsequently, an OCM performance comparison of Na-WO_x surface sites vs Na₂WO₄ NPs, through CH₄+O₂ temperature programmed surface reaction (CH₄+O₂-TPSR) (substantiated by H₂-temperature programmed reduction (H₂-TPR) and Temporal Analysis of Products (TAP) experiments), found that the Na₂WO₄ crystals are more active (activate CH₄ at a lower temperature), but exhibit ~20% less C₂ selectivity . TAP and H₂-TPR analysis revealed that surface MnO_x species significantly enhance reducibility and redox ability of supported Mn₂O₃-Na₂WO₄/SiO₂ catalyst. The additional role of Na- and Mn- promoters, on catalyst structure and OCM performance, will be discussed from in-situ spectroscopy (Raman, UV-Vis, XPS, and XAS), HS-LEIS, SSITKA, TAP and steady-state OCM studies.