Environmental Engineering Reference
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such as methanol oxidation to formaldehyde in a first step, and subsequent
formaldehyde decomposition to CO
ad
(dotted arrow in Fig. 13.8b).
3. What are the probabilities of incomplete oxidation product formation in a single
adsorption/reaction event, without considering effects related to re-adsorption
and subsequent further reaction.
4. At which stage of the reaction process does the branching into the different
parallel pathways occur?
The first question asks essentially the same question as has been raised previously
by Vielstich and Xia for methanol oxidation [Vielstich and Xia, 1995], namely,
whether it is possible to “directly” produce CO
2
along the “direct” pathway (not via
formation and oxidation of CO
ad
), or whether this occurs via formation and sub-
sequent re-adsorption and further oxidation of the incomplete oxidation products
formaldehyde and formic acid. We have shown previously for bulk oxidation of
methanol over Pt/Vulcan thin-film electrodes that the product distribution depends
sensitively on catalyst loading [Jusys et al., 2003]. The fraction of CO
2
increased
with increasing catalyst loading, mainly on the expense of formaldehyde formation,
while the formic acid content in the product distribution changed little. This was
explained by a “desorption - re-adsorption” mechanism, where volatile incomplete
oxidation products can desorb into the diffusion layer and subsequently either leave
the diffusion layer into the flowing bulk electrolyte or adsorb again. In the latter
case, the adsorbed species can either undergo further oxidation or desorb again, to
start the same cycle. This mechanism leads to an increasing fraction of incomplete oxi-
dation products in the exit stream with decreasing catalyst loading [Childers et al.,
1999; Wang et al., 2001b; Jusys et al., 2003] or increasing electrolyte flow [Wang
et al., 2001a]. Similar trends were reported also for ethanol oxidation over polycrystal-
line Pt electrodes and a Pt/C thin-film catalyst electrode, where the amount of acetic
acid increased at the expense of acetaldehyde for the higher surface area Pt/C catalyst
[Wang et al., 2004]. These findings are in perfect agreement with concepts developed
in heterogeneous catalysis, where it is well known that with increasing space velocity
(lower catalyst loading at constant reactant flow, or higher reactant flow at constant
catalyst loading) the system and therefore also the product distribution move
further away from the equilibrium composition [Thomas and Thomas, 1997].
Hence, the present findings clearly indicate that desorption and re-adsorption plus
further reaction of the volatile incomplete oxidation products play an important role
in the reaction process and contribute significantly to the observed CO
2
formation.
Based on the present data, we cannot exclude, however, the possibility that CO
2
can be formed “directly,” without involving the formation of adsorbed or dissolved
formaldehyde and/or formic acid (at zero probability for re-adsorption). Hence, reac-
tion along the direct reaction pathway in Fig. 13.8b could not be confirmed, but also
could not be ruled out.
With regard to the second question, while CO
ad
formation from methanol is
slow at potentials in the H
upd
region, formaldehyde adsorption transients on a Pt
film electrode showed rapid CO
ad
formation under these condition [Chen et al., to
be published]. Furthermore, Korzeniewski and Childers [1998] reported increasingly
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