Mechanistic Study of the Electrochemical Pathways for Methyl Surface Intermediates during Organic Oxidations
Authors: Peng Xiong , Mustain William E. 
Published: February 2020

The electrochemical activation of methane has been an area of significant discussion and growing research interest in recent years as society looks for more environmentally benign processes to synthetically create complex organic molecules – preferably under mild conditions. This requires an electrochemical cascade that starts with easy to obtain small molecules (e.g. methane, ethane, acetic acid) and preferably ends with intermediate-length (4-10 carbon) paraffins and olefins – not just the formation of single-carbon oxygenates. Forming longer-chain molecules requires the surface stabilization of the reaction intermediates that drive C-C bond formation, such as methyl (CH3) and methylene (CH2) radicals. Therefore, understanding the behavior of these surface species during methane electrochemical oxidation is extremely important to synthesize multi-carbon species. Unfortunately, for methane activation, very high electrode potentials are often required to break the C-H bond, and the resulting electrode potential is so high that methane and nearly all of its reaction intermediates are completely oxidized to CO2. Because of this, not only is the faradaic efficiency low on many existing catalysts, but very little is known about the behavior of the surface intermediates that are formed, such as methyl (CH3) and methylene (CH2) radicals.

Because of the difficulty in creating surface-stable radicals at the high potentials needed for methane activation, most of the experimental work and DFT calculations work that have been done regarding CH4 activation and upgrading has focused on understanding how the C-H bond breaks and how catalyst composition influences the location and orientation of the resulting methyl radical (CH3). Though this understanding is important and investment in this area needs to be continued for the foreseeable future, it will not answer other fundamental and practical questions about what happens after the C-H bond is broken on the catalyst surface – including understanding the variables that control C-C coupling to create multi-carbon paraffins, olefins or alcohols as well as selectivity.

In this work, we have taken the unique approach of creating these radicals from a proxy reaction instead – acetic acid electrochemical oxidation [1]. The first step in the reaction of acetic acid is breaking the C-C bond, forming CO2 and a high surface density of CH3 radicals (likely more than methane activation due to the much higher concentration in the aqueous electrolyte and lower potential needed for activation). In this work, a series of electrochemical and isotope labeling experiments were done in order to understand the reactivity and reaction pathways of the CH3 radicals. A series of hydrocarbons and oxygenates such as ethane, ethylene, methanol and ethanol were observed. A comprehensive mechanism for the reaction of CH3 radicals on Pt electrodes will be proposed. These results enable a more fundamental understanding of how the CH3 radical behaves under oxidizing conditions, which can provide insight for reactor designs that enable methane electrochemical upgrading.

Reference:

Peng, X.; Omasta, T.J.; Xinyi, Z.; Mustain, W.E. Electrochemical Pathways for Electrochemical Oxidation of Acetic Acid. ECS Trans.
2018, 85, 29–34.