Electrochemical Oxidation of Ethylene Glycol on Palladium-Based Catalysts: Influences of Substrate and Selectivity for Glycolic Acid Production

Ethylene glycol (EG) is a crucial platform molecule derived from biomass, fossil fuels, and waste plastics like polyethylene terephthalate (PET). Nearly a hundred million metric tons of PET are produced globally, with roughly only 30% being recycled. Unrecycled PET can be depolymerized into EG, which can then be converted into glycolic acid (GA), a compound seven times more valuable than EG. However, current GA production suffers from poor selectivity and high energy demand. Electrooxidation of EG to GA presents a promising solution but requires energy-efficient, selective catalysts with a clear mechanistic understanding.

Through the testing of various Pd-M bimetallics for EG oxidation, it was found that PdBi @ Nickel foam (NF) showed high activity and selectivity towards GA. However, two questions arose from its high activity: what effects does NF have on the reaction, since it is an active substrate, and why does Bi boost the performance while being inactive on its own? To test the effects of the NF substrate, by electrodepositing PdBi @ Carbon Cloth (CC), an inert substrate, a direct comparison can be made. The PdBi @ CC maintained better Faradaic efficiencies, demonstrating that the NF is likely active and causing worse efficiencies. Using in situ Raman, it was found that at higher potentials, the PdBi was forming complex Ni-Pd-Bi oxides that were shifting the selectivity of the reaction from GA to formic acid (FA), which is supported by experimental data. As for Bi’s role in the catalyst, a series of electrochemical control tests were completed that showed Pd @ CC did not perform well and Bi @ CC had no activity. Currently, in situ Raman and density functional theory (DFT) are being used to explain Bi's effects in the catalyst.

In addition to analyzing the catalyst's activity, the effects of temperature, potential, and other reaction parameters on selectivity and Faradaic efficiency for GA production were studied. Understanding the catalyst mechanism and reaction conditions will support future work to convert crude EG from recycled PET into GA, advancing a more sustainable plastic life cycle.