Electrocatalytic Oxidation of Ethanol on Noble Metal Nanoalloys
Authors: Lewera Adam , Piwowar Justyna , Gorzkowski Maciej Tomasz , Gralec Barbara 
Published: February 2020

Electrooxidation of ethanol on noble metals is of particular interest due to its potential application in low temperature direct ethanol fuel cells (DEFCs). However there is no known ethanol electrooxidation reaction (EOR) catalyst, allowing for complete electrooxidation of ethanol to CO2. Even for the most active systems, incomplete and slow EOR renders the DEFCs not economically viable. As a result significant effort has been devoted to understand the reasons of low activity of known catalysts and to finding better, more active systems.

It is widely accepted that catalytic and electronic properties of catalyst‘s surface are strongly related. Electronic properties of catalyst can be purposely modified, for instance by modifying its lattice parameter. In general, changes of the lattice parameter of metals will induce changes in their electronic properties as degree of overlapping of band forming orbitals change. As a result, when metal lattice is contracted, the valence band becomes broader, and when metal lattice is expanded the valence band (d-band) shrinks. The changes in d-band width causes the shift of the d-band: the broadening of the valence band leads to shift of the d-band center away from Fermi level and when the d-band shrinks, the d-band center shifts in the opposite direction. The changes in d-band center position relative to Fermi level can be correlated to adsorption properties of the surface. We used the above mechanism to purposely induce the changes in the electronic properties and to investigate how the changed electronic properties influence the catalytic activity.

It is also known, that in ethanol oxidation reaction the activity and product selectivity varies broadly among different catalysts. In general systems consisting of platinum, and platinum alloyed with other metals are the most active towards ethanol electrooxidation in terms of highest current density. In particular, Pt-Ru and Pt-Sn systems are often reported as those with the highest current density in acidic media. However, despite the higher current, addition of Ru or Sn to Pt further decrease the already small amount of CO2, produced in EOR on pure Pt. Selectivity of a given catalyst towards CO2 in EOR is especially important, due to the facts, that CO2 is the product of complete, 12-electron oxidation of ethanol molecule, where acetaldehyde and acetic acid (other possible products) yields respectively 2- or 4-electrons. Additionally acetic acid cannot be further oxidized in a working fuel cell, thus it is effectively a dead-end of EOR in a working low temperature DEFC. As a result other catalysts, more active and selective toward ethanol electrooxidation to CO2, are needed for a commercially viable DEFC.

 In this context Rh-containing systems seem very attractive. In numerous reports higher selectivity of ethanol oxidation to CO2 has been reported for Rh containing catalysts. Also higher activity for C-C bond scission for Pt-Rh/C systems has been reported multiple times based on differential electrochemical mass spectrometry (DEMS) experiments.

This contribution will address both: the role of electronic properties and the selectivity of electrocatalysts in ethanol electrooxidation reaction for selected systems containing Pt, Pd and Rh.

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