Tuning surface oxygen species via Mg-Ba co-doping on La2O3 to enhance oxidative coupling of methane performance
Mg-, Ca-, Sr-, and Ba-single-doped La2O3 as well as Mg-Ba co-doped La2O3 catalysts were synthesized via a hydrothermal method and evaluated for the oxidative coupling of methane (OCM). The experimental results revealed that the Mg-modified La2O3 catalyst activates O2 and CH4 effectively, yet achieves only moderate C2+ selectivity. Conversely, the Ba-modified analogue affords high C2+ selectivity, albeit at the expense of lower reaction activity. Notably, the Mg-Ba co-doped La2O3 catalyst strikes an effective balance between activity and selectivity, enhancing catalytic performance while maintaining a high C2+ selectivity. Specifically, at a Mg/Ba molar ratio of 1:1 and 700 °C, it achieved a CH4 conversion of 29.5%, a C2+ selectivity of 54.5% and a corresponding C2+ yield of 16.1%. The characterization results indicate that Mg and Ba co-doped La2O3 catalysts promote the formation of more superoxide (O2−) species on the catalyst surface, which in turn significantly enhances both the activity and selectivity of La2O3 catalysts. In situ DRIFTS revealed the presence of superoxide species on the surface of both Mg- and Ba-doped catalysts, with the co-doped system exhibiting a significantly more intense signal for the superoxide species. O2/H2-TPR studies revealed that Mg and Ba co-doped La2O3 catalysts exhibit superior O2 activation capabilities compared to those doped with Mg or Ba alone. CH4/O2 pulse experiments revealed that the co-doped catalysts facilitate faster establishment of oxygen adsorption equilibrium, thereby enhancing CH4 activation and the subsequent formation of C2 products. This work establishes that co-doping La2O3 with Mg and Ba represents an effective strategy for improving catalytic performance in OCM, primarily by modulating the generation and stabilization of key active oxygen species.