Gas-Phase Electrochemical Conversion of Methane on Boron-Doped Diamond Gas Diffusion Anodes
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2023-12-22 |
| Journal | ECS Meeting Abstracts |
| Authors | ĂdĂĄm Vass, Hanadi Ghanem, Stefan Rosiwal, Tanja Franken, Regina Palkovits |
| Institutions | RWTH Aachen University, Friedrich-Alexander-Universität Erlangen-Nßrnberg |
| Citations | 2 |
Abstract
Section titled âAbstractâMethane as the primary component of natural gas is used as fuel for energy supply and serves as a major feedstock for the chemical industry to produce hydrogen. In addition, renewable methane is becoming available via thermochemical, photo- or electrochemical, and biogenic routes. It remains a challenge, however, to efficiently activate the C-H bond of methane while keeping the reaction kinetics under control to form value added products (hydrocarbons or alcohols) and avoid complete oxidation to CO 2 [1]. A promising way to selectively activate the C-H bond of methane is to utilize reactive oxygen species (ROS) [2]. ROS play a major role in the partial electrochemical oxidation of water to produce hydrogen peroxide with boron-doped diamond (BDD) anodes [3]. However, little is known about the role of ROS towards CH 4 activation under electrochemical conditions. The goal of our work is to achieve control over product selectivity in the gas-phase electrochemical conversion of methane towards methanol or C 2+ products [4, 5]. We also aim to explore the role of water in the methane conversion mechanism using in-situ electrochemical FT-IR spectroscopy. Experimentally, we use a BDD coated mesh as gas-diffusion anode electrode in a zero-gap gas-phase electrolyzer. The anode side of the cell is fed with humidified or dry CH 4 (or He for control experiments) and humidified He or H 2 O are fed to the cathode side (to keep the Nafion 117 membrane humidified). The electro-oxidation of water is paired with hydrogen evolution.. The scheme of the membrane electrode assembly is shown on Fig. 1. A . In this talk, we discuss the effects of water content, gas flow rate and methane pressure on product selectivity. We observe the formation of CO 2 and CO along with O 2 when humidified CH 4 is fed to the anode side of the electrolyzer (and H 2 O is fed to the cathode). CO 2 and CO originate from the oxidation of CH 4 , as confirmed by control experiment with only He fed to the anode ( Fig. 1. B ). This suggests that ROS, generated at the BDD electrode by water partial oxidation, can oxidize methane. However, O 2 is still the major product even in the presence of CH 4 and both conversion and selectivity are low. The missing Faradaic efficiency ( Fig. 1. C ) can be related to non-detected CH 4 oxidation products - for this, we are currently deploying additional analytical techniques besides gas-chromatography, e.g., IR, NMR, and HPLC, to detect minor products in both gas and liquid phases. When running experiments with humidified CH 4 gas feed on the anode and humidified He gas feed on the cathode, we observe that the CO to CO 2 ratio increases. This indicates that the selectivity changes significantly while the cell (and the membrane) dries out. Hence, we propose that the product selectivity can be controlled by tuning the humidity level. Finally, in experiments with dry CH 4 gas fed to the anode and H 2 O recirculated to the cathode, we observe that both the increase of the CH 4 gas flow rate (from 10 to 20 cm 3 min -1 ) and pressure (atmospheric, +1 and +2 bar) lead to a consistent increase of the CO 2 and CO formation yield. Hence, controlling CH 4 gas flow rate and pressure provides further opportunity to control the conversion and selectivity. The results achieved so far motivate us to further investigate the joint effect of the above parameters by using a membrane capable to operate under dry conditions, in a system with improved H 2 O management. References [1] R. Horn et al. Cat. Lett., 145, 23-39 (2015). [2] R. F. B de Souza et al. Catalysts, 12, 217 (2022). [3] K. Wenderich et al. ACS Sustainable Chem. Eng., 9, 23, 7803-7812 (2021). [4] J. A. Arminio-Ravelo et al. Curr. Op. in Green and Sustainable Chemistry, 30, 100489 (2021). [5] A. S. Ramos et al. ChemCatChem, 12, 18, 4517-4521 (2020) Figure 1. Schematic figure of the membrane electrode assembly (A). Chronovoltammetric curve (top) and product formation rates in the anode compartment (bottom, O 2 , CO 2 and CO respectively) recorded during continuous electrolysis applying humidified He or CH 4 anodic gas feeds (B). Faradaic efficiencies for anodic processes of electrolysis with humidified He or CH 4 anodic gas feeds (C). Electrolysis conditions: T cell = 25 °C, j = 20 mA cm -2 , 10 cm 3 min -1 gas feed rate, atmospheric pressure, H 2 O recirculated in the cathode compartment. Figure 1