Environmental Engineering Reference
In-Depth Information
Formaldehyde electro-oxidation has been studied much less extensively compared
with methanol and formic acid oxidation [Breiter, 1967; Loucka and Weber, 1968;
Sidheswaran and Lal, 1971; Spasojevic et al., 1980; Beltowska-Brzezinska and
Heitbaum, 1985; Napporn et al., 1995; Nakabayashi, 1998; Nakabayashi et al.,
1998; Mishina, et al., 2002; Okamoto et al., 2005; Mai et al., 2005; Batista and
Iwasita, 2006; Samjesk´ et al., 2007; de Lima et al., 2007]. This may partly be related
to experimental problems: formaldehyde disproportionates to methanol and formic
acid in the absence of methanol (the Canizzarro reaction). Furthermore, a technical
application of formaldehyde as a fuel had not been anticipated, because of its toxicity.
The latter point is also important for DMFC applications, where formaldehyde emis-
sions should be avoided. In potentiodynamic measurements, formaldehyde oxidation
was found to start at significantly more positive potentials than methanol oxidation,
and adsorption of formaldehyde equally results in a reaction-inhibiting CO adlayer
[Sun, 1998; Olivi et al., 1994; Miki et al., 2004; Chen et al., to be published].
Formic acid formation during formaldehyde oxidation was reported and quantified
[Batista and Iwasita, 2006]. Formate species were detected upon formaldehyde
oxidation on Group 1b metals in alkaline solution [Anastasijevic et al., 1993; Jusys,
1994; ten Kortenaar et al., 1999, 2001; Stadler et al., 2002] and as adsorbed species
on Pt electrodes in acidic solution [Miki et al., 2004; Chen et al., to be published].
On the latter substrates [Samjesk´ et al., 2007], they were proposed to act as reaction
intermediates [Miki et al., 2004], similar to methanol oxidation [Chen et al., 2003] and
formic acid oxidation [Miki et al., 2002; Samjesk´ et al., 2007].
In the following, after a brief description of the experimental setup and procedures
(Section 13.2), we will first focus on the adsorption and on the coverage and compo-
sition of the adlayer resulting from adsorption of the respective C
1
molecules at a
potential in the H
upd
range as determined by adsorbate stripping experiments
(Section 13.3.1). Section 13.3.2 deals with bulk oxidation of the respective reactants
and the contribution of the different reaction products to the total reaction current
under continuous electrolyte flow, first in potentiodynamic experiments and then in
potentiostatic reaction transients, after stepping the potential from 0.16 to 0.6 V,
which was chosen as a typical reaction potential. The results are discussed in terms
of a mechanism in which, for methanol and formaldehyde oxidation, the commonly
used dual-pathway mechanism is extended by the possibility that reaction intermedi-
ates can desorb as incomplete oxidation products and also re-adsorb for further oxi-
dation (for the formic acid oxidation mechanism, see [Samjesk´ and Osawa, 2005;
Chen et al., 2006a, b; Miki et al., 2004]).
13.2 EXPERIMENTAL
The DEMS setup and experimental procedures used in this study were the same as
described in more detail elsewhere [Jusys et al., 2001]. Briefly, the DEMS setup
consisted of two differentially pumped chambers, a Balzers QMS 112 quadrupole
mass spectrometer (MS), a Pine Instruments potentiostat, and a computerized data
acquisition system.
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