(712d) Transient Kinetic Characterization Surface Oxygen Species to Understand Steady-State Performance of Oxidative Coupling of Methane

The Mn/Na₂WO₄/SiO₂ catalyst is one of the most advanced catalysts for the OCM process but still shows comparatively low C₂ yield that is below the threshold for industrial implementation. The adsorbed oxygen species on the catalyst surface are thought to play a major role in OCM performance [1]. The literature contains few reports regarding the correlation of the OCM reactivity with kinetic processes of surface oxygen [2]. An experimental methodology using isotopic O₂ pump/probe transient kinetic experiments was described to characterize the surface oxygen transformation process. In addition, steady state flow experiments were applied to study OCM performance.

We performed pump/probe experiments over Mn/Na₂WO₄/SiO₂ catalyst system between ¹⁶O₂ and ¹⁸O₂ at 700 ℃. Based on the kinetic analysis, monoxo species were suggested as the main oxygen species on Mn site. 2%Mn/SiO₂ showed the highest oxygen activation ability while C₂₊ yield was less than 8%, which suggested that monoxo species was not active for OCM performance. The poor OCM performance of Na₂WO₄/SiO₂ was due to the low gas-phase oxygen activation.

Dioxo species were indicated as the main oxygen species in the Mn and W complex system based on the kinetic analysis. Both 1.5%Mn and 4%Mn over Na₂WO₄/SiO₂ showed best OCM performance with the highest ratio of Y(¹⁶O₂)/Y(¹⁸O¹⁶O). The mean desorption time of dioxo species was longer than the monoxo species. These experiments implied that the dioxo species played determining roles in enhancing OCM performance and the monoxo species may act as unselective oxygen. An optimum Mn loading for C₂ yield is correlated to an optimal surface residence time and concentration of dioxo and monoxo species. These transient kinetic experiments provide a new understanding for OCM catalyst operation at industrial conditions.

[1] Kim, I., et al., Molecular Catalysis, 2017. 435: 13-23.

[2] Kondratenko, E.V. and J. Pérez-Ramírez, Catalysis today, 2007. 119(1-4):243-246.