Influence of the interaction between alkaline earth and rare earth metal oxides on OCM reaction: elucidating the impact of different loading steps

Abstract

This study systematically investigated the effects of Ca/La₂O₃- and La/CaO-supported catalysts with different loading steps on the active sites and catalytic OCM performance. Ca/La₂O₃ and La/CaO exhibited good catalytic OCM performance in the low-temperature (500–550 °C) and high-temperature (≥700 °C) ranges, respectively, given that loading CaO onto the La₂O₃ surface leads to a greater abundance of moderate and strong basic sites and low-temperature easily desorbable reactive oxygen species, although the basicity of La/CaO is stronger than that of Ca/La₂O₃. The strong interaction between CaO and La₂O₃ and the conductivity of their respective oxides are key factors determining their low-temperature catalytic OCM performance, while their high-temperature catalytic performance is influenced by the mobility of oxygen ions. Multiple characterization methods combined with DFT theoretical calculations demonstrated that O₂²⁻ is the active site for the low-temperature catalytic performance on Ca/La₂O₃. This is attributed to the relatively low reaction energy barriers for activating oxygen molecules to O₂²⁻ and activating methane with O₂²⁻ on the Ca/La₂O₃ (001) surface at low temperatures. On the other hand, O₂⁻ constitutes the active site for the high-temperature OCM on La/CaO, given that the La/CaO (100) surface is more prone to activating oxygen molecules to O₂⁻ and since the energy barrier for activating methane with O₂⁻ is minimal at high temperatures. The surface lattice oxygen species (O²⁻) leads to the deep oxidation of methane. This work provides theoretical guidance for the development of low-temperature or high-temperature OCM catalysts.