Electro Oxidation of Methane Under Ambient Conditions

Abstract

Methane is one of the main components of natural gas, which is generally used as a worldwide energy source for daily and industrial activities. However, methane is also a kind of greenhouse gas, which has 30 times more global warming potential than that of CO2.[1] The severe methane leak problem has been underestimated for a long time. It was not gaining increasing attention until recently. Environmentally friendly converting methane to other value-added chemicals is attractive. The large-scale conventional methane reforming is under practical application that converts methane to methanol, but the two-steps endothermic reaction requires high energy input.[2] Effective decomposition of methane still remains challenging, due to the methane intrinsic chemical feature of high bond strength and high ionization potential. The unique symmetric structure makes it hard to break the C-H chemical bond and overcome the activation energy.[3] Several strategies have been built to achieve a lower energy consumption without CO2 emission, for example, photocatalyst technology, plasma technology, methanotrophs technology, electrosynthesis technology, etc. Among them, the electrosynthesis method stands out as the ideal one to oxidize methane to valuable oxygenates in a wide range of PH values and temperatures with an easy scale-up modular design.[4]In this report, we experimentally and theoretically explored the methane partial oxidation on FeNi(OH)x electrocatalysts under ambient conditions. The mixed transition metal hydroxide nanoplate compositions (Ni(OH)2, Fe10%Ni90%(OH)x, Fe20%Ni80%(OH)x, Fe30%Ni70%(OH)x, Fe50% Ni50%(OH)x) were synthesized by the facial hydrothermal method and characterized by SEM, TEM, HRTEM, XPS, and XRD techniques. The effect of incorporating Fe in the nickel hydroxide lamella structure was discussed. Evidently, Fe doping content modified the electrochemical property. We could manipulate the methane conversion process and promote the reaction selectivity by controlling the ratio of Fe in the prepared electrocatalysts. Of all the catalyst materials, Fe30%Ni70%(OH)x exhibited the best methane oxidation activity that greatly enhanced the catalytic performance. Electrochemical behavior studies were then developed in a three electrodes system to investigate methane activation regarding reaction time, applied potential, and methane partial pressure. The optimum electrode potential was determined, involving a key intermediate formation of methane activation. Under this potential, peroxide (NiOOH) formed, but oxygen evolution reaction (OER) onsite potential yet reached, thus the electrochemical performance could attain a relatively high methane oxidation current density and low competition with water oxidation. From cyclic voltammogram (CV) cycling evidence, a slight increase in the NiOOH anodic current and decrease in the OER oxidation peak current was observed in the presence of methane. The oxygenate products from methane electrooxidation were collected and analyzed by NMR and gas chromatograph (GC) instruments. Ethanol turned out to be the main product. The integrated design with ultrathin Fe30%Ni70%(OH)x nanosheet catalyst resulted in a maximum ethanol formation rate of 9.09 mmol/gcatalyst·h under 1.46 V vs. RHE with 87 % faradaic efficiency (FE) and 0.26 s-1 turnover frequency (TOF). In order to further understand the reaction mechanism, in situ attenuated total reflectance fourier transform infrared (ATR-FTIR) experiments and density functional theory (DFT) calculation were provided. We detected the HCO and CH3OH intermediates generated at the catalyst surface. DFT calculation with an ab initio modeling strongly supported the experimental results and rationalized the room temperature selective methane upgrading reaction pathways on Fe30%Ni70%(OH)x. We believe this fundamental work will provide insight and guidance in economically efficient methane conversion.[1] S. M. Miller et al., “Anthropogenic emissions of methane in the United States,” Proc. Natl. Acad. Sci., vol. 110, no. 50, pp. 20018–20022, Dec. 2013, doi: 10.1073/PNAS.1314392110.[2] X. D. Peng, “Analysis of the thermal efficiency limit of the steam methane reforming process,” Ind. Eng. Chem. Res., vol. 51, no. 50, pp. 16385–16392, 2012.[3] A. A. Latimer et al., “Understanding trends in C–H bond activation in heterogeneous catalysis,” Nat. Mater., vol. 16, no. 2, pp. 225–229, 2017.[4] A. H. B. Mostaghimi, T. A. Al-Attas, M. G. Kibria, and S. Siahrostami, “A review on electrocatalytic oxidation of methane to oxygenates,” J. Mater. Chem. A, vol. 8, no. 31, pp. 15575–15590, 2020. Figure 1