72
portable electronics, and stationary use. However, the kinetics of ethanol oxidation
reaction is slow even on the best available catalysts, which is the major obstacle that
has hampered commercialization of direct ethanol fuel cells (DEFCs). The major
problem in ethanol oxidation is particularly the difficult breaking of the C–C bond,
which is a necessary step for a complete oxidation to CO 2 . Additional factors hampering development of DEFCs include complex reaction, high costs of precious
metal catalysts (Pt and Pt/Ru based catalysts), CO poisoning of Pt catalysts at lower
temperatures in acidic media, and ethanol crossover from the anode to the cathode
[63–66].
Although details of the reaction mechanism still remain unclear, a good agreement exists about reaction pathways that form acetaldehyde (CH 3 CHO) and acetic
acid (CH 3 COOH) as the main products of the oxidation of ethanol at a Pt electrode
in acidic solution. Carbon dioxide (CO 2 ) appears only at very high positive potentials. These products and adsorbed intermediates in acidic solution have been verified using several techniques [67, 68].
The ethanol oxidation reaction (EOR) is a complex reaction occurring in a pattern of parallel reaction pathways. Pure Pt is not capable to catalyze complete oxidation of ethanol. It is, however, a necessary catalyst constituent to provide efficient
adsorption of ethanol, the first reaction step in its oxidation. The efforts to improve
its activity have been focused on bimetallic catalysts, addition of co-catalysts, and
surface modifiers. In most cases, the improvements occur through the bifunctional
electrocatalyst mechanism and the electronic (ligand) effect [69, 70]. Тhе oxyphilic
co-catalyst (e.g. Ru, Sn, Ir, Re, Os, Pb) can provide oxygen atoms at adjacent sites
at a lower potential than that accomplished by pure Pt. The M-OH species can oxidatively remove the strongly adsorbed poisoning species, which are stable at Pt at
that potential. The electronic effect originates from the Pt–co-catalyst interaction,
which modifies the electronic structure of Pt and, as a consequence, the bonding
strength of poisoning species and oxygen-containing species. Decreased Pt content
is another benefit of adding co-catalysts. Pt-Sn and Pt-Ru are considered the most
active binary catalysts [71]. However, despite the increase in overall activity, the
addition of Sn or Ru decreases platinum selectivity towards CO 2 formation.
Total oxidation of ethanol to CO 2 at low potentials cannot be achieved by existing Pt-based binary catalysts. A recent study showed that the ethanol oxidation on
Pt/C yields acetic acid, acetaldehyde, and carbon dioxide in the range of 20–65%,
27–79%, and 0.7–7.5%, respectively, depending on the initial concentration of ethanol [72].
The cleavage of the C–C bond on Pt proceeds mostly through readsorption of
acetaldehyde (Scheme 6.3), yielding adsorbed carbon monoxide CO ads as an intermediate [64]. Carbon-containing species like CO ads are especially strongly adsorbed,
thus blocking Pt sites for further adsorption of reactants at moderately positive
potentials. CO ads can only be removed by oxidation occurring above 0.6 V, when the
Pt surface becomes covered by Pt-OH or Pt-O species by oxidation of water, which
further react with CO ads yielding CO 2 as the final product.
Consequently, pure Pt is not capable to catalyze complete oxidation of ethanol.
However, Pt is a necessary catalyst constituent to provide efficient adsorption of
6 Important Electrocatalytic Reactions
portable electronics, and stationary use. However, the kinetics of ethanol oxidation
reaction is slow even on the best available catalysts, which is the major obstacle that
has hampered commercialization of direct ethanol fuel cells (DEFCs). The major
problem in ethanol oxidation is particularly the difficult breaking of the C–C bond,
which is a necessary step for a complete oxidation to CO 2 . Additional factors hampering development of DEFCs include complex reaction, high costs of precious
metal catalysts (Pt and Pt/Ru based catalysts), CO poisoning of Pt catalysts at lower
temperatures in acidic media, and ethanol crossover from the anode to the cathode
[63–66].
Although details of the reaction mechanism still remain unclear, a good agreement exists about reaction pathways that form acetaldehyde (CH 3 CHO) and acetic
acid (CH 3 COOH) as the main products of the oxidation of ethanol at a Pt electrode
in acidic solution. Carbon dioxide (CO 2 ) appears only at very high positive potentials. These products and adsorbed intermediates in acidic solution have been verified using several techniques [67, 68].
The ethanol oxidation reaction (EOR) is a complex reaction occurring in a pattern of parallel reaction pathways. Pure Pt is not capable to catalyze complete oxidation of ethanol. It is, however, a necessary catalyst constituent to provide efficient
adsorption of ethanol, the first reaction step in its oxidation. The efforts to improve
its activity have been focused on bimetallic catalysts, addition of co-catalysts, and
surface modifiers. In most cases, the improvements occur through the bifunctional
electrocatalyst mechanism and the electronic (ligand) effect [69, 70]. Тhе oxyphilic
co-catalyst (e.g. Ru, Sn, Ir, Re, Os, Pb) can provide oxygen atoms at adjacent sites
at a lower potential than that accomplished by pure Pt. The M-OH species can oxidatively remove the strongly adsorbed poisoning species, which are stable at Pt at
that potential. The electronic effect originates from the Pt–co-catalyst interaction,
which modifies the electronic structure of Pt and, as a consequence, the bonding
strength of poisoning species and oxygen-containing species. Decreased Pt content
is another benefit of adding co-catalysts. Pt-Sn and Pt-Ru are considered the most
active binary catalysts [71]. However, despite the increase in overall activity, the
addition of Sn or Ru decreases platinum selectivity towards CO 2 formation.
Total oxidation of ethanol to CO 2 at low potentials cannot be achieved by existing Pt-based binary catalysts. A recent study showed that the ethanol oxidation on
Pt/C yields acetic acid, acetaldehyde, and carbon dioxide in the range of 20–65%,
27–79%, and 0.7–7.5%, respectively, depending on the initial concentration of ethanol [72].
The cleavage of the C–C bond on Pt proceeds mostly through readsorption of
acetaldehyde (Scheme 6.3), yielding adsorbed carbon monoxide CO ads as an intermediate [64]. Carbon-containing species like CO ads are especially strongly adsorbed,
thus blocking Pt sites for further adsorption of reactants at moderately positive
potentials. CO ads can only be removed by oxidation occurring above 0.6 V, when the
Pt surface becomes covered by Pt-OH or Pt-O species by oxidation of water, which
further react with CO ads yielding CO 2 as the final product.
Consequently, pure Pt is not capable to catalyze complete oxidation of ethanol.
However, Pt is a necessary catalyst constituent to provide efficient adsorption of
6 Important Electrocatalytic Reactions