Environmental Engineering Reference
In-Depth Information
very significant amount of incompletely oxidized reaction products in addition to the
complete oxidation product CO
2
. This is in perfect agreement with the formal reaction
scheme introduced by Bagotzky et al. (1977) and also later work [Petukhova et al.,
1977; Ota et al., 1984; Iwasita and Vielstich, 1986; Korzeniewski and Childers,
1998; Childers et al., 1999; Wang et al., 2001a; Jusys and Behm, 2001; Jusys et al.,
2003; Housmans et al., 2006]. Formic acid is produced in both reactions; methanol
oxidation also results in formaldehyde formation. For formic acid oxidation, CO
2
is
the only reaction product. Under the present reaction conditions, CO
2
formation con-
tributes about 50% of the faradaic current (or about 27% of the reaction products)
during methanol oxidation at 0.6 V; for formaldehyde oxidation under similar reaction
conditions, CO
2
is only a minority reaction product.
While the data provide clear evidence for the formation of incomplete oxidation
products, and help to identify the nature of the stable adsorbate(s) formed upon inter-
action with the respective C
1
molecules, the molecular-scale information on the actual
reaction mechanism and the main reaction intermediates is very indirect. Also, the
reaction step(s) at which branching into the different reaction pathways occurs (e.g.,
“direct” versus “indirect” pathway, or complete oxidation versus incomplete oxidation)
cannot be identified directly from these data. Nevertheless, by combining these and
the many previous experimental data, as well as theoretical results, conclusions on
the molecular-scale mechanism are possible, and are substantiated by a solid data base.
Before going into more detail, we will briefly summarize the different mechanistic
proposals
and
the
experimental
information
available
on
possible
reaction
intermediates.
1. For both methanol oxidation and formic acid oxidation, a “dual-pathway mech-
anism” has been proposed (for methanol oxidation, see Lamy et al. [1983]; Jarvi
and Stuve [1998]; Cuesta [2006]; Housmans et al. [2006]; Iwasita [2003]; for
formic acid oxidation, see Parsons and VanderNoot [1988]; Sun et al. [1988];
Willsau and Heitbaum [1986]; Miki et al. [2002]; Samjesk´ and Osawa
[2005]; Chen et al. [2006a, b, c]; Samjesk´ et al. [2005, 2006]; Miki et al.
[2004], Chang et al. [1989]), in which one reaction pathway proceeds via for-
mation and subsequent oxidation of CO
ad
(P, “indirect pathway”), while the
other leads, via one or more reaction intermediates RI, “directly” to CO
2
(“direct pathway”) (Fig. 13.8a).
2. A similar reaction scheme is also likely for formaldehyde oxidation [Loucka
and Weber, 1968].
3. Whereas in the indirect pathway, CO
ad
is clearly identified as a reaction intermedi-
ate, the specific nature of the intermediate(s) in the direct pathway is under debate.
For methanol oxidation, species such as COH [Xia et al., 1997; Iwasita et al.,
1987, 1992; Iwasita and Nart, 1997], CHO [Zhu et al., 2001; Willsau and
Heitbaum, 1986; Wilhelm et al., 1987], COOH [Zhu et al., 2001], and adsorbed
formate species [Chen et al., 2003] have been proposed. Adsorbed formate
species were identified during formaldehyde oxidation [Samjesk´ et al.,
2007], methanol oxidation [Nakamura et al., 2007; Chen et al., 2003, unpub-
lished], and formic acid oxidation [Miki et al., 2002, 2004; Samjesk´ and
Osawa, 2005; Chen et al., 2006a, b, c; Samjesk´ et al., 2005, 2006].
Search WWH ::
Custom Search