Key Takeaways & Executive Findings
- •• • At a Mg/Ba molar ratio of 1:1 and 700 °C, the co-doped La2O3 catalyst achieved a CH4 conversion of 29.5%, a C2+ selectivity of 54.5%, and a C2+ yield of 16.1%, demonstrating a balanced activity-selectivity profile that is industrially relevant for OCM process intensification. • • In situ DRIFTS confirmed a significantly more intense signal for superoxide (O2−) species on the Mg-Ba co-doped catalyst compared to single-doped counterparts, indicating a synergistic effect that enhances the density of reactive oxygen intermediates critical for C–H bond activation. • • O2/H2-TPR and CH4/O2 pulse experiments showed that the co-doped catalyst exhibits superior O2 activation and faster establishment of oxygen adsorption equilibrium, which directly correlates with improved CH4 activation and C2 product formation kinetics. • • The hydrothermal synthesis method yields a catalyst with a stable Mg-Ba co-doped La2O3 structure, offering a scalable route to tune surface oxygen species, a key lever for optimizing OCM performance beyond conventional single-dopant systems.
Abstract
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.
1. Introduction
The oxidative coupling of methane (OCM) to C2+ hydrocarbons remains a grand challenge in catalysis, as the direct conversion of methane into valuable olefins and paraffins is thermodynamically limited and kinetically plagued by over-oxidation to COx. While indirect routes via syngas are energy-intensive and capital-heavy, direct OCM offers a streamlined pathway with higher carbon atom economy. However, existing catalytic systems, such as Li/MgO and MnOx-Na2WO4/SiO2, suffer from either rapid deactivation due to lithium volatility or require high operating temperatures and complex multi-component formulations, hindering industrial deployment. Rare earth oxides, particularly La2O3, have shown promise due to their basicity and ability to stabilize active oxygen species, yet their performance is often constrained by a trade-off between activity and selectivity.
This study addresses this bottleneck by systematically investigating the co-doping of La2O3 with alkaline earth metals, specifically Mg and Ba, to modulate surface oxygen species. The authors demonstrate that Mg doping enhances O2 and CH4 activation but yields moderate C2+ selectivity, while Ba doping improves selectivity but reduces activity. The synergistic combination of Mg and Ba at a 1:1 molar ratio achieves a balanced performance, with a CH4 conversion of 29.5% and C2+ selectivity of 54.5% at 700 °C. Mechanistic studies reveal that co-doping promotes the formation of superoxide (O2−) species, which are critical for selective methane activation. This work provides a rational design strategy for tuning surface oxygen chemistry in La2O3-based catalysts, offering a path toward more efficient and selective OCM processes.
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WANG Ke, ZHANG Qi, NIU Pengyu, LIN Minggui, JIA Litao, LI Debao, ZHANG Riguang (2026). Tuning surface oxygen species via Mg-Ba co-doping on La2O3 to enhance oxidative coupling of methane performance. Journal of Fuel Chemistry and Technology. https://doi.org/10.1016/S1872-5813(26)60677-9
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Frequently Asked Questions
What is the specific role of superoxide (O2−) species in enhancing both activity and selectivity in the Mg-Ba co-doped La2O3 catalyst?
In situ DRIFTS revealed a significantly more intense signal for O2− species on the co-doped catalyst compared to single-doped analogues. These species are believed to act as selective oxidants that activate methane via hydrogen abstraction while minimizing over-oxidation to COx. The enhanced concentration of O2− correlates with the observed CH4 conversion of 29.5% and C2+ selectivity of 54.5% at 700 °C, indicating that O2− species facilitate a balanced reaction pathway.
How does the Mg-Ba co-doped catalyst compare to the benchmark MnOx-Na2WO4/SiO2 system in terms of operating temperature and C2+ yield?
The benchmark MnOx-Na2WO4/SiO2 system typically requires temperatures above 750 °C to achieve C2+ yields around 20%. In contrast, the Mg-Ba co-doped La2O3 catalyst achieves a C2+ yield of 16.1% at a lower temperature of 700 °C, suggesting a potential energy efficiency advantage. However, direct comparisons under identical conditions are needed to fully assess relative performance.
What are the long-term stability and deactivation mechanisms of the Mg-Ba co-doped La2O3 catalyst under OCM conditions?
The study does not report long-term stability tests. However, the hydrothermal synthesis method typically yields catalysts with good thermal stability. Potential deactivation mechanisms could include sintering of La2O3 at high temperatures or loss of dopant homogeneity. Further studies are required to evaluate stability over extended time-on-stream and under industrially relevant pressures.
Can the hydrothermal synthesis method be scaled up for industrial production, and what are the cost implications compared to conventional impregnation methods?
Hydrothermal synthesis is scalable and offers better control over dopant distribution and crystallinity. While it may require autoclave equipment, the process is relatively simple and cost-effective for producing homogeneous mixed oxides. The use of abundant alkaline earth metals (Mg, Ba) and lanthanum precursors suggests moderate raw material costs, but a detailed techno-economic analysis is necessary to compare with conventional methods.
What is the significance of the Mg/Ba molar ratio of 1:1, and how does varying this ratio affect catalytic performance?
The study specifically optimized the Mg/Ba ratio to 1:1, achieving the best balance between activity and selectivity. At this ratio, the synergistic effect between Mg (which enhances O2 activation) and Ba (which promotes C2+ selectivity) is maximized. Deviating from this ratio likely leads to either reduced activity (if Ba dominates) or reduced selectivity (if Mg dominates), as observed in single-doped catalysts.
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