Th0.8Ca0.2O2-δ and Na/K dispersed Th0.8Ca0.2O2-δ catalysts, synthesized by the citric acid gel method, have been tested concerning their efficiency in the oxidative coupling of methane. The structural properties of the catalysts were characterized by BET, XRD, Raman, TEM, TPD, and EPR techniques. The presence of oxide ion vacancies due to Ca ion incorporation into ThO2 lattice was evidenced by Raman spectroscopy. EPR revealed the formation of radical oxygen in the Ca-doped ThO2 sample after oxygen treatment at 500 °C. Th0.8Ca0.2O2-δ showed high catalytic activity reaching about 24% methane conversion (rate: 2.3 mol g-1 h-1) at 56% selectivity toward Cn+ products at 600°C, whose performance is comparable to that of the well known Sr doped La 2O3 catalyst. Other reference catalysts like 1.9%Mn-4%Na2WO4/SiO2 and 5%Li/MgO showed considerably lower activity under these conditions. The high catalytic activity and selectivity of the Ca-doped sample is likely due to formation of stable active Ȯ- species in the oxide ion vacancy site in the lattice.
- Ca and alkali metal doping
- Oxidative coupling of methane
- Oxide ion radical
- Oxide ion vacancy