Total oxidation of methane over rare earth cation-containing mixed oxides derived from LDH precursors

Abstract

Ln(x)MgAlO (Ln=Ce, Sm, Dy and Yb) (x=5% and also 7, 10 and 20% in the case of Ce) mixed oxide catalysts obtained from layered double hydroxide (LDH) precursors calcined at 750°C were used in the complete oxidation of methane. The catalysts were characterized by XRD, N2 adsorption, EDX, XPS and H2-TPR experiments. Their catalytic activity in the complete oxidation of methane indicated by the temperature for 50% conversion (T50) followed the order MgAlO<Dy(5)MgAlO<Sm(5)MgAlO<Yb(5)MgAlO<Ce(5)MgAlO for Ln(5)MgAlO catalysts and CeO2<Ce(20)MgAlO<Ce(5)MgAlO<Ce(7)MgAlO<Ce(10)MgAlO for Ce(x)MgAlO catalysts. Total conversion was achieved at ca. 700°C with Ce(10)MgAlO, the most active and highly stable catalyst. Nevertheless, the intrinsic reaction rates of the Ce(x)MgAlO catalysts followed the order: Ce(5)MgAlO<Ce(7)MgAlO<Ce(10)MgAlO<Ce(20)MgAlO and were correlated to the reducibility of the cerium-containing species below 600°C showing that their oxido-reduction ability is involved in the catalytic combustion of methane. This was confirmed by the existence of Ce(IV)/Ce(III) redox couple on the catalyst surface. The oxido-reduction of the catalyst was also involved in the reaction over Sm(5)MgAlO, while for MgAlO support as well as for Yb(5)MgAlO and Dy(5)MgAlO catalysts the surface adsorbed oxygen species were mostly involved. The apparent activation energy of methane oxidation calculated based on a pseudo first order kinetics was in the range from 72.0kJmol−1 to 100.9kJmol−1 for Ce-based catalysts and much higher for the other catalysts.