(321c) First Principles Analysis of Ethanol Electrooxidation on Pt (111), (211), and (221) Surfaces

As concern about anthropogenic carbon emissions grows, the need for alternative sources of energy has become increasingly acute. Among many other technologies, fuel cells have the potential to alleviate such challenges in the context of long-distance and/or heavy-duty vehicles, where the current energy density of batteries is insufficient. Ethanol is an attractive fuel for use in such devices since it is agriculturally derived, non-toxic, and liquid at ambient conditions. However, current Pt-catalyzed ethanol fuel cells struggle with complete oxidation to CO₂ and instead produce the partial oxidation products acetic acid and acetaldehyde. The possible elementary reaction pathways for this chemistry are numerous in nature, experimental investigations of the mechanism are non-trivial, and existing computational studies of this system are not comprehensive. Thus, there remains a strong need to elucidate the fundamental mechanistic features of this chemistry under realistic electrochemical conditions.

In this work, DFT calculations are performed on a series of single crystal Pt surfaces ((111), (211), and (221)). For each possible reaction intermediate between ethanol and the fully oxidized product CO₂, the adsorption energy and associated transition state energies for both dehydrogenation and C-C dissociation reactions, are determined. Using this data, a free energy diagram is constructed that shows the minimum energy pathway for the reaction network. Further, Brønsted-Evans-Polanyi (BEP) relationships are identified using the transition state calculations for dehydrogenation and C-C dissociation. The results suggest that, on all surfaces, ethanol is initially dehydrogenated via proton-coupled electron transfer reactions, and once it is deeply dehydrogenated, to species such as CH₃CO and CH₂CO, C-C dissociation begins to compete with further dehydrogenation reactions. C-C scission is, in turn, promoted by steps over terrace features. We close by presenting preliminary results from a microkinetic model based on these energetic analyses, and outline strategies for extension of these analyses to Pt-based alloys.