Catalytic partial oxidation of methane over oxide-ion-conductive lanthanum silicate apatites†

A lanthanum silicate La_9.33Si₆O₂₆ (LSO) crystallizes in an apatite-type structure and has been known as a promising oxide-ion conductor. Here, we report the activity of LSO for catalytic partial oxidation of methane (CPOX) to synthesis gas. The LSO catalyst demonstrated relatively high catalytic activity from 500 to 700 °C, with CH₄ conversion reaching 22.1% at 700 °C while retaining moderate CO and H₂ selectivities of 20–60%. Notably, LSO exhibited higher CPOX activity than non-apatite-type La₂SiO₅ despite their similar specific surface areas. The higher CPOX activity of LSO is likely attributed to its structural superiority involving mobile oxide ions in the crystal structure. The reaction kinetic study showed that the reaction orders for methane and oxygen in the CPOX reaction over the LSO catalyst were 0.69–0.73 and 0.08–0.21, respectively. Furthermore, the small contribution of adsorbed O species generated from gas-phase O₂ molecules indicated that the lattice oxygen may be involved in the reaction mechanism. The kinetic isotope effect (KIE) study using a CD₄ suggested that C–H bond breaking is the rate-determining step of CPOX over LSO.