Environmental Engineering Reference
In-Depth Information
higher formaldehyde yields with decreasing potential, at potentials below 0.2 V (up to
30 - 40% at about 0.05 V) for methanol oxidation over a polycrystalline Pt electrode. On
the other hand, despite the high formaldehyde yields, the absolute rates and partial
currents for formaldehyde formation are very low under these conditions, i.e., the
high formaldehyde yields during methanol oxidation result solely from the fact that at
potentials cathodic of 0.5 V, the CO
2
formation rate is even lower than the formaldehyde
formation rate. Considering that the amount of oxidized methanol, and hence also of
formaldehyde formation, during potentiostatic methanol oxidation is of the order of
one nanomole per second at 0.6 V, and even much lower in the H
upd
potential range,
this sequential pathway for CO
ad
formation does not seem to be sufficient to explain
the CO
ad
formation rate upon interaction with methanol. CO
ad
formation by direct
decomposition of methanol has to be possible, although contributions from a pathway
via initial oxidation to (free) formaldehyde and subsequent re-adsorption plus dehydro-
genation to CO
ad
will be present [Olivi et al., 1994].
The reaction sequence of formaldehyde formation and subsequent CO
ad
formation
can proceed either as sequential reactions of adsorbed species, or it can involve
formation and desorption of formaldehyde into the electrolyte and subsequent
re-adsorption and further decomposition of formaldehyde to CO
ad
. Considering the
significant transport and catalyst loading effects discussed above, it is clear that
desorption and subsequent re-adsorption plus dehydrogenation of formaldehyde
will play an important role also for CO
ad
formation, although a direct reaction of
adsorbed RI
ad
species can not be ruled out.
I the context of the third question, for comparison with theory, it would be interest-
ing to know the selectivities for direct CO
2
formation and for desorption of the incom-
plete oxidation products formaldehyde and formic acid during methanol oxidation in a
single adsorption/reaction event. Under steady-state conditions and at 0.6 V, the
measured (overall) selectivity for CO
2
formation (CO
2
current efficiency) reached
values as low as 30% on a Pt/C catalyst for low catalyst loadings (7 mg
Pt
cm
22
)
[Jusys et al., 2003]. For smooth Pt electrodes, this value was even lower [Wang
et al., 2001a]. On the other hand, even on a smooth surface and under enforced elec-
trolyte transport, a molecule desorbing from the surface is likely to undergo several
collisions with the surface, before it finally leaves the diffusion layer into the flowing
electrolyte and is transported away. Each of these collisions can be considered as one
attempt for re-adsorption. For a catalyst layer, this number will be much higher.
Although it is clear from the present data that the selectivity for “direct” CO
2
formation
in a single reaction event, compared with desorption of an incompletely oxidized
reaction product, must be low (of the order of 10% or less), it is not possible at
the moment to deduce quantitative numbers, because of the unknown number of
re-adsorption events.
The much lower amount of formic acid formation during methanol oxidation
compared with formaldehyde oxidation agrees with expectations if we assume that
formic acid is predominantly formed by further oxidation of (free) molecular formal-
dehyde produced in a first step of methanol oxidation. Under the present reaction
conditions, only a very small fraction, about 1 part per thousand, of the total reactant
passing through the cell reacts to give formaldehyde, formic acid, or CO
2
. The rest
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