(453f) Electrochemical Oxidation of Methane to Methanol on Electrodeposited Transition Metal Oxides Under Ambient Temperature

Methane is the primary constituent of natural gas and broadly utilized as an essential feedstock and fuel in chemical manufacturing units and energy transformations. From the current energy outlook, natural gas is the prominent option for fuel, energy, and feedstock in the chemical and petroleum industries. Methane in associated petroleum gas cannot be transported economically from remote oil fields and is thus usually flared resulting in significant greenhouse gas (GHG) emissions and energy losses leading to adverse influences on the climate, environment and energy markets. In this regard, electrochemical oxidation of methane to value-added chemicals such as methanol under ambient temperatures via modular systems is of great interest.

Research in the electrochemical oxidation of methane under ambient temperature using transition metal oxides as electrocatalysts has shown significant advances over the last decade. Binary transition metal oxides of NiO/ZrO₂ in a carbonate electrolyte have been shown to catalyze the electrochemical oxidation of methane. An analogous example utilizing chemical precipitation of Co₃O₄/ZrO₂ nanocomposite and Co₃O₄ powder/ZrO₂ nanotubes has been shown to produce higher alcohols such as 1-propanol and 2-propanol. Introducing zirconia by using co-precipitation to unary transition metal oxide as catalyst has shown to promote the methane oxidation in the presence of carbonate ions enabling the system to operate at room temperature. Intriguingly, the faradaic efficiencies (FEs) in some of these systems exceed 100% implying that chemical reactions between methane and stoichiometric oxidants could be responsible for the production of a large fraction of the oxygenates. Oxidation products could be also the result of the degradation of carbon conductive materials and binders added in the preparation of catalyst inks.

Methane oxidation reaction via electrochemistry is more favorable than water oxidation as it requires a less positive potential (E₀=0.58V vs SHE). However, because methane activation and the regeneration of oxidative species on the surface of the electrode are kinetically sluggish, this transformation may require higher overpotentials to drive the rate-limiting step at appreciable rates. At high overpotentials, the oxygen evolution reaction (OER, E₀=1.23V vs SHE) competes with the methane oxidation reaction and the high oxidation potential result in the over-oxidation of methane derived products to carbon dioxide and carbon monoxide which are more thermodynamically favorable. Accordingly, the major challenge is to develop electrocatalysts that can not only activate the C-H bond of methane at low overpotentials but also prevent the overoxidation of desirable products such as methanol. Many studies within the existing literature show inconsistent results due to the use of catalysts of varying morphology and composition as well as the use of different electrolyte compositions. This highlights the difficulties to determine product distribution accurately for methane partial oxidation and results in a lack of fundamental understandings of the kinetics associated with these electrochemical systems. The discussion of reaction mechanisms, theory-driven catalyst discovery, and optimization of reaction environments must be built upon reliable experimental evidence, which are still in short availability. In this study, a one-step electrodeposition method has been developed for the preparation of thin film transition metal oxides as model electrocatalysts without binders or carbon conductive materials. A systematic understanding of the electrochemistry in methane oxidation towards methanol is gained through kinetic studies for CH₄ partial oxidation in a customized gas-tight cell with a rotating cylinder electrode (RCE). Product analysis using NMR shows methanol and acetate are the major products during the oxidation. Additionally, by collecting and analyzing the liquid products with a 20-min interval, fluctuations in concentrations of products over time are observed thus highlighting the dynamic nature of partial oxidation studies. This is the case particularly for experiments where many of the liquid products such as methanol accumulate in the electrolyte and can be further transported back to the electrode surface to be oxidized. To further understand these phenomena, different factors including catalyst porosity, the applied potentials, the operating temperature, and the rotational speed of the electrode were systematically studied. Different unary transition metal oxides (CoO_x, NiO_x, CuO_x and MnO_x) were discovered to be active for methane partial oxidation with this simpler and cleaner fabrication method. Acetate is detected at low overpotentials near the thermodynamic minimum and methanol is the favored product at intermediate overpotentials, while high overpotentials result in the over-oxidation of the produced methanol and the production of oxygen. The temperature also plays an important role in both the activation of methane and the over-oxidation of the produced methanol. Furthermore, thicker catalyst films lead to more significant over-oxidation of methanol due to the highly porous structures and thus the longer paths for methanol to exit the catalyst layer. Higher electrode rotation speed of the RCE increase the cycling time of the concentration profiles in the electrochemical cell by affecting convection in the cell. These two last factors (catalyst porosity and electrolyte convection) are both correlated with the effect of mass transfer, implying the importance of understanding mass transport of the relevant species in partial oxidation studies of methane. To better illustrate the whole process, a numerical model describing the local concentrations of all species is built. From the evidence shown above, we propose that methane needs to go through a thermal activation step during the electrochemical oxidation and that the transport of methanol away from the electrode will be sufficient to obtain high selectivity to partial oxidation products. The experimental results as well as the numerical model in this study demonstrate a new approach tor catalyst fabrication and benchmarking and may inspire further studies of the electrochemical partial oxidation of methane under well defined conditions of mass, heat and charge transport.