Selective electrochemical oxidative coupling of methane mediated by Sr2Fe1.5Mo0.5O6-δ and its chemical stability

Abstract

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.

Highly Stable Doped Barium Niobate Based Electrocatalysts for Effective Electrochemical Coupling of Methane to Ethylene

“…This is in complete contrast to the SFMO powders that formed SrCO 3 , SrMO x , and MoO x C y upon exposure to methane under similar operating conditions. [ 18 ] Similar to SFMO, the expected reactions for the constituents of BMNF perovskites such as Ba and Mg upon exposure to methane was the formation of carbonates such as BaCO 3 , and to some extent MgCO 3 as shown in Figure S3 of the Supporting Information along with agglomeration of carbon (coking) on the surface that would result in significant weight gain. This was seen in our work on Sr 2 Fe 1.5 Mo 0.5 O 6−d (SFMO) where up to 60% weight gain was observed in TGA under methane environment along with a complete disintegration of its crystal structure.…”

“…SFMO powders under similar operating conditions showed a weight gain of about 40% to 60% that was associated with significant coke formation and crystal structure collapse. [ 18 ] To investigate any possible change to crystal structure, we analyzed the CH 4 exposed powders in PXRD. As shown in Figure 1b, the cubic perovskite structure is retained in all three samples and do not show any new peak formation or change in the peak positions associated with the cubic perovskite structure.…”

“…As previously shown, the Gibbs free energy of reaction for CO 2 is Δ G R ≈ −800 kJ mol −1 throughout the temperature range being tested (800–900 °C) and would be the dominant product if the reaction is not controlled specifically to produce partial oxidation product such as ethylene by oxide ion flux and applied potentials. [ 18 ] To understand this, we carried out temperature programmed reaction measurements where first we exposed the BMNF25 powder to a gas mixture containing 95% CH 4 and 5% O 2 and analyzed the exposed powder for BaCO 3 and coke formation while the outlet stream was analyzed by mass spectroscopy ( Figure ).…”

“…SFMO perovskites showed coke formation to start at 800 °C along with other products such as CO, CO 2 , H 2 , and small quantities of ethylene. [ 18 ] Mass spectra analysis on the outlet stream of TPR measurements obtained with BMNF33 is given in Figure 2, where the onset of ethylene production starts at temperatures as low as 600 °C and reaches the maximum in the temperature range of 750–800 °C. One of the most studied catalyst for OCM reaction, Mn–Na 2 WO 4 , is reported to show methane conversions at 800 °C at a much higher CH 4 to O 2 ratio.…”

“…[14][15][16] A novel method attempting to circumvent the overoxidation of methane to CO 2 is the electrochemical oxidative coupling of methane (E-OCM). [17][18][19] The fine-tuning of potential within an electrochemical cell allows for the regulation of oxide ion flux from cathode to anode across the electrolyte material. Further, the extent of oxidation can also be manipulated by the applied bias.…”