Study of Electrochemical Methane Oxidation to Oxygenates
Creators: name: Al-Attas, Tareq Ali Salem
Publisher: name: Graduate Studies
Year: 2024
Subjects: subject: Electrochemical process
subject: CH4 oxidation
subject: Iron (IV)-oxo species
subject: Oxygenates
subject: Renewables
subject: Oxygen species
subject: ATR-SEIRAS
subject: Electrolyzer
subject: PTR-TOF-MS
subject: Mechanistic pathway
subject: Engineering--Chemical

description: Methane (CH4), a primary component of natural gas, plays a vital role in energy and chemical production. However, conventional methods of chemical production tend to be resource-intensive and often result in the release of CH4 through venting or flaring in oil and gas operations. Decentralized technologies that leverage renewable energy can mitigate CH4 emissions while generating revenue. Electrochemical oxidation of CH4 (eCH4OR) into valuable fuels and materials offers a flexible solution that can operate in varied conditions. Nevertheless, achieving desirable outcomes at scale is challenging due to the high energy requirements for breaking the C–H bonds of CH4. This thesis focuses on developing catalysts for high-rate and selective electrooxidation of CH4 while also aiming to deepen our understanding of the reaction mechanism. Drawing inspiration from iron (IV)-oxo (FeIVO) metalloenzymes that activate C–H bonds, a copper-iron-nickel (CuFeNi) catalyst for selectively oxidizing CH4 into formate using a peroxide-assisted pathway is presented. The synergistic effect of the metals to selectively oxidize CH4 is explored by in situ spectroelectrochemical studies (i.e. XANES and UV-Vis) and density functional theory (DFT) calculations. Specifically, the analyses revealed the presence of high valent FeIV as the active site for CH4 oxidation, attained by the reactive oxygen species generated during the partial oxidation of H2O2 at low overpotentials compared to water oxidation reaction (OER) on nickel. Furthermore, the critical role of copper in preventing the overoxidation of valuable oxygenates to CO2 was revealed. We achieved a formate faradaic efficiency of ~42% at a current density of 32 mA cm−2 (i.e., partial current of ~13 mA cm−2) and a low applied potential of 0.9 VRHE. Additionally, the thesis examines the reaction pathways of the eCH4OR using hematite (α-Fe2O3) as an electrocatalyst. Different electrochemical and in situ spectroelectrochemical techniques, including ATR-SEIRAS, EIS, and PTR-TOF-MS, were employed to comprehend the mechanism of this reaction. The electrochemical oxidation current density uncovered non-faradaic adsorption of CH4 molecules at lower applied potentials. The non-faradaic adsorption of CH4 molecules was further confirmed through ATR-SEIRAS. In situ ATR-SEIRAS also revealed the presence of the FeIVO species and CH4 oxidation intermediates, correlating them with the applied potential. Thisanalysis unveiled the formation of oxygenated products, including CH3OH, HCOOH, CH3COOH, and their corresponding intermediate adsorbed species, such as •OCH3, •OCOH, and •OCOCH3. Notably, C–C coupling between –COCH3 and –CH3 via ketonization resulted in CH3COCH3 formation. PTR-TOF-MS supported our findings by confirming that acetone is the primary liquid product generated, achieving a faradaic efficiency of 6.3% at 2.3 VRHE. This result is attributed to its formation by coupling acetate and formate intermediates. Consequently, we developed proposed reaction pathways for the selective electrooxidation of CH4 to C1-C3 products. The thesis extends to present techno-economic and life-cycle assessments of electrification options for CH4 utilization. Initially, the study highlights the economic viability of electrifying reformers and boilers in traditional technology. A futuristic scenario discusses one-step methanol synthesis via the direct eCH4OR and illustrates the impact of cell voltage and electricity prices on the calculated minimum selling price of methanol. For instance, methanol production can be profitable at electricity prices below ¢4 per kWh at a total operating cell voltage of 2.0 V. The analysis establishes electricity emission goals to maintain net CO2 emissions within the acceptable range for current methanol synthesis. It is shown that the emissions intensity of the electricity source must be under 181 kgCO2 per MWh for the electrochemical route.