Environmental Engineering Reference
In-Depth Information
desorption and re-adsorption, and further oxidation of the incomplete oxidation product
formaldehyde.
For formaldehyde oxidation, the formaldehyde reactant molecule diffuses reversi-
bly from the bulk electrolyte (HCHO
bulk
, circled in Fig. 13.9) into the catalyst layer
(HCHO
cat
) and finally adsorbs (reversibly). Independent of its origin, from methanol
oxidation or formaldehyde adsorption, adsorbed formaldehyde is in equilibrium with
formaldehyde dissolved in the catalyst layer (HCHO
cat
) and in the bulk (HCHO
bulk
).
On the other hand, further dehydrogenation results in adsorbed formyl species and
finally in adsorbed CO. Both reaction steps are strongly exothermic and therefore irre-
versible (see the references cited above). In aqueous solution, formaldehyde is in equi-
librium with methylene glycol, which is proposed to form adsorbed formate species
upon adsorption (see also Housmans et al. [2006]). These adsorbed species in turn
can desorb as formic acid (upon protonation). Formic acid was shown in previous
studies to dehydrate to CO
ad
or to form CO
2
, either “directly” [Chen et al.,
2006a, b, c] or via the IR spectroscopically detected adsorbed formate [Miki et al.,
2002] (in addition to CO
ad
formation and oxidation via the indirect pathway). Owing
to the additional supply of oxygen via formaldehyde hydration, CO
2
formation
during formaldehyde oxidation is possible without the requirement for OH
ad
formation.
Although the present data do not allow us to decide on the reaction steps leading to
adsorbed formaldehyde during methanol oxidation, the large fraction of formaldehyde
formation at 0.6 V (about 65%) leads us to favor the reaction pathway via C - H
activation rather than via O - H activation (see above), as proposed by Hartnig et al.
[2005, 2007a, b], but in contrast to other theoretical studies [Greeley and
Mavrikakis, 2004; Cao et al., 2005; Taylor and Neurock, 2005]. This is also a
major difference from the very detailed reaction scheme proposed recently by
Housmans et al. [2006]. Since the formation of CO
ad
and CO
2
is irreversible under
present reaction conditions, while most other steps are reversible (see Fig. 13.9), an
increasing probability of re-adsorption and further oxidation of the incomplete oxi-
dation products formaldehyde and formic acid will drive the system more towards
these stable products, as discussed above. The reaction scheme in Fig. 13.9 underlines
that it is indeed possible to produce both CO
2
and CO
ad
via re-adsorption of the incom-
plete methanol oxidation products formaldehyde and formic acid. It is still not poss-
ible, however, to provide unambiguous proof of the existence of a “direct” pathway as
defined in Fig. 13.8b for methanol oxidation (and similarly for formaldehyde
oxidation) that does not proceed via formation of the incomplete oxidation products
formaldehyde and/or formic acid.
In summary, this discussion illustrates the general importance of transport
processes in many (electro)catalytic reactions. These have to be addressed properly
for a detailed (and quantitative) understanding of the molecular-scale mechanism.
Because of the problems associated with the direct identification of the reaction inter-
mediates (see above), experiments on nanostructured model electrodes with a well-
defined distribution of reaction sites of controlled, variable distance and under equally
well-defined transport conditions (first attempts in this direction are described in
[Lindstr ¨m et al., submitted; Schneider et al., 2008]), in combination with detailed
simulations of the ongoing transport processes and theoretical calculations of the
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