Mechanisms of the Oxidation of Carbon Monoxide and Small Organic Molecules at Metal Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 194

currents. However, for fuel cell purposes, one has to consider the progressive carbona-
tion of the electrolyte solution due to CO 2 , which is produced as a product of the oxi-
dation reaction, leading to a decrease in the pH of the solution. A study on the electro-
oxidation of C 1 -C 4 alcohols by Tripkovi´ and co-workers on Pt(111), Pt(755) ;
Pt[6(111) (100)] and Pt(332) ; Pt[5(111) (110)] in an alkaline electrolyte has
shown that the potentials at which ethanol can be oxidized are mainly affected by
the ability of the surface to adsorb OH 2 anions [Tripkovic et al., 2001]. On all
three surfaces, the onset potential of ethanol oxidation coincides with the completion
of hydrogen desorption, and accelerates in the potential region in which the hydroxide
anions can adsorb reversibly. The peak potential in all cases was found to be coupled
to the onset of the irreversible hydroxide adsorption or oxide layer formation, which
occurs at a lower potential on the stepped surfaces than on Pt(111).
6.5.2 Ethanol Oxidation on Gold
Gold is generally considered a poor electro-catalyst for oxidation of small alcohols,
particularly in acid media. In alkaline media, however, the reactivity increases,
which is related to that fact that no poisoning CO-like species can be formed or
adsorbed on the surface [Nishimura et al., 1989; Tremiliosi-Filho et al., 1998].
Similar to Pt electrodes, the oxidation of ethanol starts at potentials corresponding
to the onset of surface oxidation, emphasizing the key role of surface oxides and
hydroxides in the oxidation process. The only product observed upon the electro-
oxidation of ethanol on Au in an alkaline electrolyte is acetate, the deprotonated
form of acetic acid. The lack of carbon dioxide as a reaction product again suggests
that adsorbed CO-like species are an essential intermediate in CO 2 formation.
In acidic media, the reactivity of ethanol on Au electrodes is much lower than in
alkaline media. The main product of the oxidation of ethanol on Au in an acidic
electrolyte was found to be acetaldehyde, with small amounts of acetic acid
[Tremiliosi-Filho et al., 1998]. The different reactivities and the product distributions
in different media were explained by considering the interactions between the active
sites on Au, ethanol, and active oxygen species absorbed on or near the electrode sur-
face. In acidic media, surface hydroxide concentrations are low, leading to relatively
slow dehydrogenation of ethanol to form acetaldehyde as the main oxidation pathway.
In contrast, in alkaline media, ethanol, adsorbed as an ethoxy species, reacts with a
surface hydroxide, forming adsorbed acetate, leading to acetate (acetic acid) as the
main reaction product.
6.5.3 Ethanol Oxidation on Rhodium
Another metal that has attracted interest for use as electrode material is rhodium,
inspired by its high activity in the catalytic oxidation of CO in automotive catalysis.
It is found that Rh is a far less active catalyst for the ethanol electro-oxidation reaction
than Pt [de Souza et al., 2002; Leung et al., 1989]. Similar to ethanol oxidation on Pt,
the main reactions products were CO 2 , acetaldehyde, and acetic acid. Rh, however, pre-
sents a significant better CO 2 yield relative to the C 2 compounds than Pt, indicating a