New Mechanistic and Reaction Pathway Insights for Oxidative Coupling of Methane (OCM) over Supported Na WO /SiO Catalysts.

The complex structure of the catalytic active phase, and surface-gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na WO /SiO catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H -TPR, TAP and steady-state kinetics) experiments, that the long speculated crystalline Na WO active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na-WO sites. Kinetic analysis via temporal analysis of products (TAP) and steady-state OCM reaction studies demonstrate that (i) surface Na-WO sites are responsible for selectively activating CH to C H and over-oxidizing CH to CO and (ii) molten Na WO phase is mainly responsible for over-oxidation of CH to CO and also assists in oxidative dehydrogenation of C H to C H . These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na WO /SiO catalysts.