Environmental Engineering Reference
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A more complete analysis of the kinetics on the Pt(111) electrode has been
carried out by Stuve's group [Sriramulu et al., 1999]. In this model, the first step is
a dehydrogenation reaction. From this point, two paths emerge: one proceeds forming
CO, and the second one involves an active intermediate. The model fits the experi-
mental data well, and kinetic constants are obtained. Activation energies have been
determined for polycrystalline electrodes. The apparent activation energies measured
in acid media range between 24 and 74 kJ/mol, depending on the applied potential
[Cohen et al., 2007].
Theoretical studies on Pt(111) surfaces in the presence of water have validated the
above model [Hartnig and Spohr, 2005]. DFT calculations indicated that the oxidation
of methanol starts by the formation of a hydrogen bond between the OH group of the
methanol and a water molecule, followed by the cleavage of a C22H bond close to the
Pt surface. In the next step, a rapid cleavage of the O22H bond leads to the formation of
formaldehyde as a stable intermediate. Other studies indicate that the dehydrogenation
reaction to yield CO takes place at low potentials, whereas the other paths are active at
higher potentials [Cao et al., 2005]. They also suggest that dehydrogenation takes
place by the formation of an adsorbed CH
2
OH, whereas the paths through an active
intermediate have an adsorbed CH
3
O species. This latter path would be more favorable
on steps and defect sites.
Housmans and Koper [2003] have carried out a detailed analysis of the chronoam-
perometric transients on a series of stepped single-crystal electrodes using reaction
models very similar to those used in Franaszczuk et al. [1992] and Jarvi and Stuve
[1998]. The expression that they used to model the current transient was
j(t)
¼
4eN
Pt
k
dec
[1
u
CO
(t)]
2
þ
2eN
Pt
k
ox
u
CO
(t)[1
u
CO
(t)]
þ
j
d
[1
u
CO
(t)] (6
:
18)
with
1
exp(k
ox
t)
1
(1
þ
k
ox
=
k
dec
) exp(k
ox
t)
u
CO
(t)
¼
(6
:
19)
where the first term is rate of methanol decomposition to surface-bonded CO, the
second term is the rate of CO oxidation, and the third term is a very general expression
for the direct oxidation path through formaldehyde and formic acid. The expression for
the time dependence of the CO coverage is obtained by solving the differential
equation for the CO coverage as it follows from the expression for its formation and
subsequent oxidation.
Without the direct pathway contribution, this equation may either yield an increas-
ing or decreasing current transient, depending on the value of k
ox
/k
dec
. If this ratio is
larger than 4, i.e., if methanol decomposition is slow compared with CO oxidation,
then the current is predicted to increase with time. Experimentally, this situation has
been observed for a low methanol concentration and an almost perfect Pt(111) elec-
trode [Housmans and Koper, 2003], which both lead to a low methanol decomposition
rate. Typically, however, current transients decrease with time, suggesting that the rate
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