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.<br/><br/>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 &gt;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 <i>ab initio</i> 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.<br/><br/>(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.<br/>(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.<br/>(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.<br/>(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.