Role of apatite-type lanthanoid silicate promoter in PdO/Ln₁₀Si₆O₂₇/γ-Al₂O₃ catalysts for catalytic combustion-type methane sensor: Contrasting trends between oxide-ion conductivity and sensor performance

Abstract

Apatite-type lanthanoid silicates (Ln₁₀Si₆O₂₇; Ln = La, Pr, Sm, Eu, Gd) were investigated as promoters for PdO/γ-Al₂O₃ catalysts to enhance the performance of catalytic combustion-type methane sensors. X-ray diffraction and fluorescence analyses confirmed successful synthesis of phase-pure Ln₁₀Si₆O₂₇ and PdO/Ln₁₀Si₆O₂₇/γ-Al₂O₃ catalysts with compositions close to the intended stoichiometry. All sensors exhibited fast responses (T₅₀ ≈ 10 s) and excellent linearity (R² > 0.990) in the range of 0–1000 ppm CH₄. Remarkably, the PdO/Gd₁₀Si₆O₂₇/γ-Al₂O₃ sensor enabled quantitative CH₄ detection at 320 °C, the lowest operating temperature among the tested catalysts. Interestingly, sensor sensitivity increased systematically with increasing lanthanoid atomic number, opposite to the trend expected from the oxide-ion conductivity of Ln₁₀Si₆O₂₇, which decreases from La to Gd. Detailed analyses revealed that this discrepancy originated from catalytic activity and oxygen release properties rather than bulk conductivity. H₂-TPR results demonstrated that PdO/Gd₁₀Si₆O₂₇/γ-Al₂O₃ released active oxygen species more readily at low temperatures than the La-based catalyst. XPS further showed that the Pd²⁺/Pd⁰ ratio decreased from La to Gd, with the Gd-based catalyst achieving a balanced distribution that facilitated continuous redox cycling and sustained oxygen supply. This optimized Pd redox environment enhanced the proportion of complete methane oxidation, resulting in higher combustion heat and improved sensor output. These findings highlight that the performance of catalytic combustion-type methane sensors is governed not by intrinsic oxide-ion conductivity of the promoter, but by promoter–PdO interactions controlling Pd redox states and oxygen release dynamics. The results provide new insights into the design of advanced methane sensors operating at lower temperatures with higher sensitivity.