Environmental Engineering Reference
In-Depth Information
et al. (1977), where the process of methanol dehydrogenation and electro-oxidation to
CO 2 was described schematically by the following sequence of reaction steps:
CH 3 OH ! CH 2 OH ! CHOH ! COH
# # #
CH 2 O ! CHO ! CO
# #
HCOOH ! COOH
#
CO 2
In this reaction scheme, each step includes the release of one electron on the one hand
and the release of a proton or the uptake of a hydroxyl anion—via water uptake and
proton release—on the other. The educt (methanol), final product (CO 2 ), and stable
incomplete oxidation products (formaldehyde and formic acid) are indicated here in
bold to distinguish them from other possible intermediate species adsorbed on the elec-
trode surface. Since both formaldehyde and formic acid can undergo further oxidation,
the product distribution resulting from methanol oxidation (and in a similar way also for
formaldehyde oxidation) will depend not only on the respective reaction rates for the
formation and further oxidation of these species (formaldehyde and formic acid),
but also on transport characteristics of the reaction cell and the catalyst layer/electrode
surface [Jusys et al., 2003]. Desorption and rapid removal of stable, volatile reaction
intermediates (incomplete oxidation products) will increase their contribution to the
product distribution, while the slow, diffusion-controlled removal of these species
(e.g., due to slow electrolyte flow) or an increased tendency for re-adsorption (e.g.,
by an increased electrode surface/porosity) will decrease their contribution and
favor complete oxidation to CO 2 [Jusys et al., 2003]. These predictions are supported
by a number of observations, for instance, by comparing product distributions
measured in model studies with those determined at the exhaust of DMFCs or direct
ethanol fuel cells (DEFCs) [Wasmus et al., 1995; Sanicharane et al., 2002; Tkach
et al., 2004; Seiler et al., 2004; Neergat et al., 2006; Rao et al., 2007], or by comparison
of product yields for different electrode roughnesses [Ota et al., 1984], catalyst loadings
[Childers et al., 1999; Jusys et al., 2003; Bergamaski et al., 2006; Gavrilov et al., 2007;
Islam et al., 2007], reactant concentrations [Wang et al., 2001a; Camara and Iwasita,
2005; Wang et al., 2004], or electrolyte flow rates [Wang et al., 2001a]. Further
mechanistic information has come from studies investigating the influence of
crystallographic orientation [Wang et al., 2001a; Sun and Clavilier, 1987; Shin et al.,
1996; Tarnowski and Korzeniewski, 1997; Sriramulu et al., 1998; Jarvi et al., 1998;
Cuesta, 2006; Housmans et al., 2006; Nakamura et al., 2007; Spendelow et al.,
2007], adsorbed anions [Batista et al., 2003, 2004], or composition of bimetallic cata-
lysts [Shibata et al., 1987; Jusys et al., 2002a, b; Gao et al., 2004; Islam et al., 2007;
Entina et al., 1967; Watanabe and Motoo, 1975; Sun et al., 1988; Gasteiger et al.,
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