The Effect of Strontium Content on the Activity and Selectivity of Sr-Doped La2O3 Catalysts in Oxidative Coupling of Methane

Thirty-five doped and undoped rare earth oxide catalysts were synthesized and tested under identical conditions for the oxidative coupling of methane. While La2O3 was found to be the most active, La2O3 doped with Sr was found to be the most selective. High resolution low energy ion scattering (HR-LEIS) showed that doping with Sr above 2 mole % led to migration of Sr to the surface of the catalyst producing surface (Sr/La) ratios of 2.7 - 4.4 from bulk (Sr/La) ratios of 0.053 and 0.13, respectively. Operando Raman analysis run under OCM conditions identified La2O3 and SrCO3 as the predominant lanthanum and strontium phases during reaction. A bilayer structure comprising a thermally stable Sr-doped La2O3 base covered with a mostly amorphous Sr phase containing SrCO3, SrO and SrO2 was proposed. Selectivity to C-2 + for this structure increased with surface Sr content; and SrO2 is believed to be responsible for this effect.