Tailoring the Active Oxygen Species in the Electrochemical Oxidative Coupling of Methane to Suppress Deep Oxidation

Ethylene (C₂H₄) is a crucial industrial product with a broad range of applications such as the manufacturing of plastics. Commercial C₂H₄ production involves the steam cracking of ethane/naphtha, resulting in ~1.5-3 tonnes of CO₂ released for every tonne of C₂H₄: this makes C₂H₄ production the second-biggest contributor to industrial CO₂ emissions globally. A promising route to low carbon C₂H₄ production is the oxidative coupling of methane (OCM)¹ which can directly convert CH₄ into C₂H₄. Unfortunately, this OCM approach is severely limited by significant “deep oxidation” where CH₄ is instead combusted to CO/CO₂ and H₂O. To improve the C₂ (C₂H₄ and C₂H₆) selectivity and yield of OCM, recent works have integrated OCM activity into solid oxide electrolyzers. In this electrochemical OCM (EOCM) approach, O^2- ions from the cathode are transported to the anode where they oxidise CH₄ to yield the desired C₂ products. While the initial results are somewhat promising,² it is unclear what active oxygen species are involved in the activation of CH₄ and how these active oxygen species then affect the overall reaction selectivity.

In this work, we first adopt the model titanate-based mixed ionic-electronic conductor La_0.3Sr_0.7TiO₃ (LST30) as an anode material for EOCM. We demonstrate that by coupling the EOCM reactions to oxygen evolution at the anode surface, we can increase the C₂ selectivity of the system by >3x relative to the baseline performance of LST30. We rationalize this observation by hypothesizing that the oxygen intermediates produced as part of the oxygen evolution reaction are also OCM-active and therefore enable the selective oxidation of CH₄ into C₂ products. We can thus utilize this insight into the nature of the OCM-active oxygen species to further tune the reaction selectivity. Since it is known that the OCM selectivity is limited by deleterious reactions of C₂H₄ with the surface oxygen species,³ herein we attempt to suppress these reactions by changing the surface binding strength of these oxygen species. We use the well-established O 2p-band model for oxides⁴ to tune the oxygen binding strength: doping SrTiO₃ with Ta to form SrTi_1-xTa_xO₃ can therefore allow us to monotonically shift the O 2p-band down and thus increase the oxygen binding strength. By exploring this family of materials, we demonstrate that the C₂H₄/C₂H₆ ratio can be significantly manipulated along this series, highlighting that the oxygen binding strength changes the relative rates of the C₂ oxidation reactions. To complement this, we utilize ab initio density functional theory (DFT) calculations and we confirm that the binding strengths of the active oxygen species can be significantly increased using Ta-doping, providing a materials-based approach towards altering the selectivity of the EOCM process. Overall, our general materials design strategy enables us to target electrode surfaces with optimal oxygen binding strengths to favor CH₄ activation while also suppressing the deep oxidation reactions, providing a pathway towards EOCM systems with enhanced C₂ selectivity.

(1) Keller, G. E.; Bhasin, M. M. Synthesis of ethylene via oxidative coupling of methane: I. Determination of active catalysts. Journal of Catalysis 1982, 73 (1), 9-19. DOI: https://doi.org/10.1016/0021-9517(82)90075-6.
(2) Zhu, C.; Hou, S.; Hu, X.; Lu, J.; Chen, F.; Xie, K. Electrochemical conversion of methane to ethylene in a solid oxide electrolyzer. Nature Communications 2019, 10 (1), 1173. DOI: 10.1038/s41467-019-09083-3.
(3) Su, Y. S.; Ying, J. Y.; Green, W. H. Upper bound on the yield for oxidative coupling of methane. Journal of Catalysis 2003, 218 (2), 321-333. DOI: https://doi.org/10.1016/S0021-9517(03)00043-5.
(4) Giordano, L.; Akkiraju, K.; Jacobs, R.; Vivona, D.; Morgan, D.; Shao-Horn, Y. Electronic Structure-Based Descriptors for Oxide Properties and Functions. Accounts of Chemical Research 2022, 55 (3), 298-308. DOI: 10.1021/acs.accounts.1c00509.