Efficient conversion of methane to value-added products such as olefins and aromatics has been in pursuit for the past few decades. The demand has increased further due to the recent discoveries of shale gas reserves. Oxidative and non-oxidative coupling of methane (OCM and NOCM) have been actively researched, although catalysts with commercially viable conversion rates are not yet available. Recently, $${{{{{{{\mathrm{Sr}}}}}}}}_2Fe_{1.5 + 0.075}Mo_{0.5}O_{6 - \delta }$$ Sr 2 F e 1.5 + 0.075 M o 0.5 O 6 − δ (SFMO-075Fe) has been reported to activate methane in an electrochemical OCM (EC-OCM) set up with a C2 selectivity of 82.2%1. However, alkaline earth metal-based materials are known to suffer chemical instability in carbon-rich environments. Hence, here we evaluated the chemical stability of SFMO in carbon-rich conditions with varying oxygen concentrations at temperatures relevant for EC-OCM. SFMO-075Fe showed good methane activation properties especially at low overpotentials but suffered poor chemical stability as observed via thermogravimetric, powder XRD, and XPS measurements where SrCO3 was observed to be a major decomposition product along with SrMoO3 and MoC. Nevertheless, our study demonstrates that electrochemical methods could be used to selectively activate methane towards partial oxidation products such as ethylene at low overpotentials while higher applied biases result in the complete oxidation of methane to carbon dioxide and water.

Methane conversion into value‐added products such as olefins and aromatics is gaining increased attention in the wake of new natural gas reserve discoveries. Electrochemical oxidative coupling of methane (E‐OCM) provides better product selectivity as the product distribution can be controlled by applied potential as well as the oxide ion flux. Here a new catalyst based on Mg and Fe codoped barium niobate perovskites is reported. The prepared perovskites show excellent chemical stability in CH4‐rich environments up to 925 °C while showing methane activation properties from 600 °C. E‐OCM measurements indicate an ethylene production rate of 277 µmol cm−2 h−1 with a faradaic efficiency of 20% at 1 V and durable operation for six continuous days. X‐ray photoelectron spectroscopic measurements indicate significant Nb valency reorganization that can be the reason for its chemical stability. Nevertheless, the surface area of the catalyst is significantly lower and requires improvements in synthesis methodologies to improve catalytic activity further. The exceptional chemical stability of this perovskite material under methane exposure at high temperatures has significant importance as this material can be used as a catalyst and/or support in a wide variety of applications relevant for efficient energy conversion and storage.

Doped perovskite metal oxide catalysts of the form A(BxM1-x)O3-δ have been instrumental in the development of solid oxide electrolyzers/fuel cells. In addition, this material class has also been demonstrated to be effective as a heterogeneous catalyst. Co-doped barium niobate perovskites have shown remarkable stability in highly acidic CO2 sensing measurements/environments (1). However, the reason for their chemical stability is not well understood. Doping with transition metal cations for B site cations often leads to exsolution under reducing conditions. Many perovskites used for the oxidative coupling of methane (OCM) or the electrochemical oxidative coupling of methane (E-OCM) either lack long term stability, or catalytic activity within these highly reducing methane environments. The Mg and Fe co-doped barium niobate BaMg0.33Nb0.67-xFexO3-δ shown activity in E-OCM reactors over long periods (2) (>100 hrs) with no iron metal exsolution observed by diffraction or STEM EDX measurements. In contrast, iron decorated BaMg0.33Nb0.67O3 showed little C2 conversion activity.

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Efficient conversion of methane to value added products such as olefins and aromatics has been in pursuit for the past several decades. The demand has increased further due to the recent discoveries of shale gas reserves. Electrochemical methane conversion is gaining attention due to its ability to control the oxide ion flux that will help reduce the over-oxidation of methane while also help activate methane via applied potential. High temperature electrolysis further benefits this process due to improved kinetics. Unfortunately, high temperature operation also leads to materials degradation via sintering, crystal structure disproportion to thermodynamically more stable phases, and interfacial reactions that reduces the performance. For example, lifetime requirements for energy conversion technologies often times exceed 10 years of usage with no more than 20% degradation.[1] Similarly, we demonstrated the chemical instability of Sr2Fe1.5Mo0.5O6- d (SFMO) perovskite that was reported to show good methane activation properties.[2] [3] SFMO formed carbonates and coke upon exposure to CH4. Hence, the durability measurement results are often not reported for these catalysts under the extremely reducing or oxidizing high temperature environments. We have developed an exciting class of barium niobate perovskite materials with varying levels of Mg/Ca and Fe co-doping that show good catalytic activity towards methane activation in the electrochemical and conventional heterogeneous oxidative coupling environment.[4] These catalysts further demonstrate durable electrochemical activities over five days of continuous operation. We have performed thermogravimetric, FT-IR and electrochemical linear sweep voltammetry methods to rapidly determine their stability under operationally relevant conditions and these results are compared to stability calculations. Stability determinations of our perovskite oxide electrocatalysts for EC-OCM offer an excellent example of our approach towards evaluation of materials durability under challenging temperature and reducing conditions. These perovskite materials could also serve as a support for a wide variety of catalyst materials for high temperature applications thus opening up new possibilities. References [1] A. Hauch, S. D. Ebbesen, S. H. Jensen, and M. Mogensen, “Highly efficient high temperature electrolysis,” J. Mater. Chem., vol. 18, no. 20, pp. 2331–2340, 2008, doi: 10.1039/B718822F. [2] K. P. Ramaiyan, L. H. Denoyer, A. Benavidez, and F. H. Garzon, “Selective electrochemical oxidative coupling of methane mediated by Sr2Fe1.5Mo0.5O6-δ and its chemical stability,” Commun. Chem., vol. 4, no. 1, p. 139, 2021, doi: 10.1038/s42004-021-00568-1. [3] C. Zhu, S. Hou, X. Hu, J. Lu, F. Chen, and K. Xie, “Electrochemical conversion of methane to ethylene in a solid oxide electrolyzer,” Nat. Commun., vol. 10, no. 1, p. 1173, 2019, doi: 10.1038/s41467-019-09083-3. [4] F. H. G. Kannan P. Ramaiyan, Luke H. Denoyer, Angelica Benavidez, “Highly Stable Doped Barium Niobate Based Electrocatalysts for Effective Electrochemical Coupling of Methane to Ethylene,” ACS Catal. Under Rivsion.

Identification of large shale gas reserves has reduced the price of methane significantly. Interest in converting methane to value-added commodity chemicals has increased as a result of these findings [1]. Direct conversion of methane to ethylene using heterogeneous methods such as oxidative and non-oxidative coupling of methane (OCM and NOCM) utilize metal oxide catalysts with varying success. However, a catalyst with commercial viability remains elusive. Recently, iron-doped Sr2Fe1.5+0.075Mo0.5O6-δ electrodes (SFMO-075) have been shown to convert methane to ethylene more efficiently in an electrochemical OCM set up (EC-OCM) [2]. However, one of the main concerns with perovskite electrodes is the stability and durability of these materials. We have developed two different perovskites comprising Ba, Mg, Ca, Nb and Fe and evaluated them for EC-OCM. However, the chemical and electrochemical stability of these materials is largely unexplored. Specifically, during EC-OCM, these materials are exposed to CH4, CO2 and H2O. Each metal oxide’s chemical stability needs to be evaluated under these gases at concentrations relevant to EC-OCM. Further, pure methane supplied at the anode presents a highly-reducing atmosphere, necessitating a study on the redox stability of these materials under very low oxygen partial pressures. We shall report our results on the chemical stability of three different perovskite materials (including SFMO) in EC-OCM relevant conditions. We evaluate these perovskite powders before and after gas exposure using TGA, XPS, XRD and FT-IR measurements. While SFMO was found to be unstable in these conditions with a crystal structure collapse, the other two perovskites show promising stability. Their thermodynamic stability in these conditions along with the feasibility of various reaction mechanisms (analyzed through HSC Chemistry) will be discussed. Our study demonstrates important characteristics of catalysts for EC-OCM and may lend to the design of innovative perovskites capable of converting methane to ethylene electrochemically. References: 1) P. Schwach, X. Pan, and X. Bao, “Direct Conversion of Methane to Value-Added Chemicals over Heterogeneous Catalysts: Challenges and Prospects,” Chem. Rev., 117, 8497, 2017. 2) C. Zhu, S. Hou, X. Hu, J. Lu, F. Chen, and K. Xie, “Electrochemical conversion of methane to ethylene in a solid oxide electrolyzer,” Nat. Commun., 10, 1173, 2019.

Efficient conversion of methane to value added products such as olefins and aromatics has been in pursuit for the past several decades. The demand has increased further due to the recent discoveries of shale gas reserves (1). Oxidative and non-oxidative coupling of methane (OCM and NOCM) has been actively researched although commercially viable conversion rates have not been achieved with any catalyst yet. Electrochemical OCM is gaining attention due to its ability to control the oxide ion flux that will help reduce the over-oxidation of methane while also helping to activate methane. Recently, (SFMO-075Fe) has been reported to activate methane with a C2 product selectivity of 82.2% (2). However, alkaline earth metal-based materials are known to suffer chemical instability in carbon rich environments at elevated operating temperatures. Hence, we evaluated the chemical stability of SFMO-075Fe and SFMO in carbon rich conditions at temperatures relevance for EC-OCM. SFMO-075 showed wonderful methane activation properties but suffered poor chemical stability as observed in thermogravimetric and powder XRD measurements. XPS and XRD measurements revealed that SrCO3 as the major product along with MoC when SFMO is exposed to methane at 900°C. We further found that methane could be selectively activated towards partial oxidation products such as ethylene at low overpotentials while higher applied biases results in complete oxidation of methane to carbon dioxide and water. We report in this presentation our findings with SFMO along with potential benefits of EC-OCM for methane activation. References Schwach, X. Pan, and X. Bao, “Direct Conversion of Methane to Value-Added Chemicals over Heterogeneous Catalysts: Challenges and Prospects,” Chem. Rev., 117, 8497, 2017. Zhu, S. Hou, X. Hu, J. Lu, F. Chen, and K. Xie, “Electrochemical conversion of methane to ethylene in a solid oxide electrolyzer,” Nat. Commun., 10, 1173, 2019.

Methane is the major component of many gas sources such as natural gas and shale gas and more than 500 million tons of methane is produced every year. Unfortunately, most of it is burnt as an energy source due to difficulties in converting it into easily transportable fuels. Oxidative and non-oxidative coupling of methane (OCM and NOCM) has been actively researched to produce ethylene and higher olefins from methane although catalysts with commercially viable conversion rates have not been developed yet.[1] Electrochemical OCM (EC-OCM) is gaining attention due to its ability to regulate the oxide ion flux that will help reduce the over-oxidation of methane while also helping to activate methane.[2] Fe based catalysts have been shown to activate methane in both OCM and NOCM although suffer from coking related durability challenges. We have developed two novel perovskites comprising Ba, Ca, Mg, Nb and Ta where incorporation of Fe in the crystal lattice has resulted in mixed ionic electronic conductivity. Their physical and chemical properties are evaluated using TGA, XRD, XPS and FT-IR measurements. We further examined these catalysts for EC-OCM along with structural and chemical stability characterizations. Both these perovskites show very good chemical stability in methane under EC-OCM conditions. TGA under methane environment further reveal adsorption of methane on our perovskite material at about 600°C. Ethylene production was observed to increase with increasing Fe content in the perovskite structure. In this presentation we report our findings on the application of both perovskites towards EC-OCM and discuss potential future directions. References Schwach, X. Pan, and X. Bao, “Direct Conversion of Methane to Value-Added Chemicals over Heterogeneous Catalysts: Challenges and Prospects,” Chem. Rev., 117, 8497, 2017. Zhu, S. Hou, X. Hu, J. Lu, F. Chen, and K. Xie, “Electrochemical conversion of methane to ethylene in a solid oxide electrolyzer,” Nat. Commun., 10, 1173, 2019.