Novel Catalysts for Methane Combustion Based on Cobalt-Doped Lanthanum Silicates Having an Apatite-type Structure

Novel PdO/La₁₀Si_6-xCo_xO_27-δ/γ-Al₂O₃ catalysts with applications to methane combustion were developed. These materials were based on the use of La₁₀Si_6-xCo_xO_27-δ as a promoter because this compound has an oxide-ion conducting apatite-type structure that allows the smooth migration of active oxygen to the PdO activator. Co^3+/2+ ions were also introduced into the original La₁₀Si₆O₂₇ lattice to enhance its redox properties. Temperature-programmed reduction measurements revealed that the oxygen supply was facilitated by introducing Co^3+/2+, where the reduction was observed at 290 °C for La₁₀Si_6-xCo_xO_27-δ (x = 1.0), whereas no reduction was observed below 460 °C for La₁₀Si₆O₂₇. Among the samples synthesized in this work, PdO/La₁₀Si_6-xCo_xO_27-δ/γ-Al₂O₃ (x = 1.0) exhibited the highest catalytic activity, allowing the complete oxidation of methane at 310 °C, a temperature 80 °C lower than the 390 °C required when employing PdO/La₁₀Si₆O₂₇/γ-Al₂O₃.