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surface was investigated theoretically [Desai et al., 2002; Greeley and Mavrikakis,
2002, 2004; Okamoto et al., 2003; Cao et al., 2005; Taylor and Neurock, 2005;
Hartnig and Spohr, 2005; Hartnig et al., 2007b]. It had already been proposed in the
early 1970s [Capon and Parsons, 1973a, b, c] that formic acid oxidation can proceed
via different pathways (the “dual-pathway mechanism”), where one pathway, the “indir-
ect pathway,” includes formation and subsequent oxidation of CO
ad
, while the other, the
“direct pathway,” does not involve CO
ad
formation, but proceeds in another, often unspe-
cified, way directly to CO
2
(see also Fig. 13.8a). A similar dual-pathway mechanism was
proposed later also for methanol oxidation [Parsons and VanderNoot, 1988; Jarvi and
Stuve, 1998; Leung and Weaver, 1990; Lopes et al., 1991; Herrero et al., 1994, 1995].
It should be noted that in these earlier studies, both pathways were assumed to lead to
CO
2
as final product, whereas in some later studies, the formation and desorption of
the incomplete oxidation products formaldehyde and formic acid was considered as
the second pathway [Cao et al., 2005] (see also the discussion in Section 13.4.1).
Based on in situ IR and DEMS observations, COH [Xia et al., 1997; Iwasita et al.,
1987, 1992; Iwasita and Nart, 1997], CHO [Willsau and Heitbaum, 1986; Wilhelm
et al., 1987], COOH [Zhu et al., 2001], and HCOO [Chen et al., 2003] have been pro-
posed as possible adsorbed reaction intermediates. The role of the last of these as a reac-
tive intermediate [Chen et al., 2003] is still under debate [Nakamura et al., 2007].
Formic acid oxidation on Pt catalysts and metal electrodes has equally been inten-
sively studied, for example, by Parsons and co-workers and by other groups
[Parsons and VanderNoot, 1988; Capon and Parsons, 1973; Anastasijevic et al.,
1989; Wolter et al., 1985; Sun et al., 1988, 1994; Iwasita et al., 1994; Pastor
et al., 1996; Xia, 1999; Sun and Yang, 1999; Schmidt et al., 2000; Yang and
Sun, 2002; Jiang and Kucernak, 2002; Okamoto et al., 2004]. Detailed reviews
are given in [Jarvi and Stuve, 1998; Markovic and Ross, 2002]. As mentioned
abovee, a dual-pathway mechanism was proposed, involving a direct oxidation
path via a reactive intermediate (the direct pathway) in addition to oxidation via
decomposition to CO
ad
and its subsequent oxidation (the indirect pathway) (see
also Fig. 13.8a) [Parsons and VanderNoot, 1988; Sun et al., 1988; Iwasita et al.,
1996; Lu et al., 1999; Miki et al., 2002; Samjesk´ and Osawa, 2005; Chen et al.,
2006a, b, c]. An adsorbed bridge-bonded formate was detected during the reaction
by in situ IR spectroscopy studies in an attenuated total reflection (ATR) configuration
[Miki et al., 2002; Samjesk´ and Osawa, 2005; Chen et al., 2006a, b, c; Samjesk´
et al., 2005, 2006]. This species was proposed to act as active reaction intermediate
[Miki et al., 2002; Samjesk´ and Osawa, 2005; Samjesk´ et al., 2005, 2006].
Based on a quantitative evaluation of spectro-electrochemical data for formic acid oxi-
dation over a Pt film electrode, which were measured under continuous mass transport
conditions as a function of formic acid concentration [Chen et al., 2006a, b] and reac-
tion temperature [Chen et al., 2006c], we favored the direct dehydrogenation of formic
acid to CO
2
as the major pathway, rather than a pathway via the adsorbed formate
species detected by IR spectroscopy. This interpretation, where the IR-detected
adsorbed formates act as “spectator species” rather than as reactive intermediates,
was also supported by recent density functional theory (DFT) calculations [Hartnig
et al., 2007a].
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