Page 99 - MODERN ASPECTS OF ELECTROCHEMISTRY

Claude Lamy et al.
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of adsorbed CO is high and explains the poisoning phenomena encoun-
tered at a platinum electrode. This poisoning species can be removed [by
oxidation through step (26) into CO ] only at potentials at which oxygen-
2
ated species are present at the electrode surface. For platinum, such
oxygenated species, arising from the dissociation of water, step (22),
appear only for potentials greater than 0.5 to 0.6 V. 37,38 An alternative route
•
is the direct oxidation of ( CHO) into CO by step (23) or through step
ads
2
(24) followed by step (25). In both cases, the reaction again needs the
presence of an oxygen atom, which can be provided only by the dissocia-
tion of water at the platinum surface.
3. How to Increase the Kinetics of the Electrooxidation ofA
Methanol
Platinum is the only acceptable electrocatalyst for most of the primary
intermediate steps in the electrooxidation of methanol. It allows the
dissociation of the methanol molecule by breaking the C–H bonds during
the adsorption steps. However, as seen earlier, this dissociation leads
spontaneously to the formation of CO, which is due to its strong adsorption
on Pt; this species is a catalyst poison for the subsequent steps in the overall
reaction of electrooxidation of CH OH. The adsorption properties of the
3
platinum surface must be modified to improve the kinetics of the overall
reaction and hence to remove the poisoning species. Two different conse-
quences can be envisaged from this modification: prevention of the
formation of the strongly adsorbed species, or increasing the kinetics of
its oxidation. Such a modification will have an effect on the kinetics of
steps (23) and (24) instead of step (21) in the first case and of step (26) in
the second case.
The rate-determining step (rds) of the reaction on platinum is the
oxidation of adsorbed CO with adsorbed hydroxyl species [step (26)]. The
current density of the methanol electrooxidation can be obtained from the
followingequation 39–41 :

j = nF k θ res θ OH exp (α n rds FE/RT) (28)

where θ res and θ OH are the coverages in the adsorbed residues of methanol
are
and in hydroxyl groups (arising from water dissociation), n and n rds
the numbers of electrons involved in the overall reaction and in the
rate-determining step; k is the rate constant, F the Faraday constant, α the
transfer coefficient, and E the electrode potential. The coverages in