Catalytic Activity of Unsupported Pd-Pt Nanoalloys Towards Formic Acid and Ethanol Electrooxidation
Authors: Gralec Barbara , Lewera Adam 
Published: February 2020

Formic acid and ethanol oxidation reactions on noble metals were extensively studied in the past. However it is still unknown how the catalytic activity of Pd-Pt alloys towards formic acid and ethanol scale with composition. In case of formic acid oxidation, palladium is active at low electrode potentials, where platinum is quickly poisoned by strongly adsorbed CO. On the contrary, for ethanol oxidation, only platinum is active. Thus the activity of the alloys should scale with composition. However multitude of effects, such as surface morphology, segregation and changes in electronic properties make the relationship between composition of Pt-Pd alloys and catalytic activity a complicated one.

Oxidation of formic acid on Pt, Pd and Pd-Pt alloys follows a so-called “dual pathway” mechanism, namely formic acid can undergo dehydrogenation leading directly to CO2 (not involving strongly adsorbed CO, COads, as an intermediate) or dehydration leading to formation of COads and its possible further oxidation to CO2. Direct path (dehydrogenation) is operative at relatively low potentials but, in case of Pt, strongly adsorbed COads quickly blocks the surface sites, causing overall low activity of Pt in formic acid oxidation at low potentials. On the contrary for Pd the poisoning by COads at low potentials is much slower and the direct path (dehydrogenation) is operative to a much large extend. As a result Pd is significantly more active than Pt towards formic acid oxidation at low potential. In case of Pt-Pd alloys additional factors, related to surface morphology, must be considered: the number of adjacent Pd atoms required to adsorb formic acid molecule and number of adjacent Pt atoms required to form CO from formic acid. In particular is known that COads cannot be formed from formic acid on isolated Pt atoms, because at least two adjacent Pt atoms are required to dehydrate formic acid molecule and form COads, a phenomenon known as the “ensemble effect”. As a result such isolated Pt atoms can be much more active towards formic acid oxidation directly to CO2, and low Pt concentration Pd-Pt nanoalloys favor formation of such sites, a so called “third body effect”. Also changes in electronic properties, due to Pt-Pd interactions, cannot be excluded. In particular we observed changes in onset potential of formic acid and in the catalytic current density. The changes in surface morphology can to a large extend explain the changes in catalytic currents, and the only observation tentatively suggesting the changes in electronic properties are the difference between Pd and Pd-Pt nanoalloys catalytic activity observed in the cathodic scan. However the changes in formic acid oxidation potential cannot be fully explained by surface morphology. As a result we explain the changes in formic acid oxidation potential based on changes in the electronic properties of the Pd-Pt surface due to interactions between Pt and Pd.

In case of ethanol electrooxidation on Pt-Pd alloys similar factors, as mentioned above, must be considered. Additionally we used Differential Electrochemical Mass Spectrometry to distinguish between the possible ethanol oxidation products at different electrode potentials. Most common products of ethanol oxidation on Pt and Pt-Pd is carbon dioxide, acetic acid and acetaldehyde. Although it is known that all of these products are produced during the process of ethanol oxidation, the details of this reaction are still unknown, and how the ethanol oxidation mechanism change with Pt-Pd alloy composition. The use of DEMS allowed us to address those issues and to elucidate the changes in reaction mechanism when Pt is diluted by Pd. Better understanding of the mechanism of ethanol electrooxidation can help to develop new, more active catalysts for low temperature direct ethanol fuel cells.

This project was funded from Polish National Science Centre budget based on decision number DEC-2013/09/B/ST4/00099