Electrocatalysis for the Direct Alcohol Fuel Cell - Fuel Cell Catalysis: A Surface Science Approach - page 352

Figure 11.9 Intensities of the CO L ( A ) and CO 2 ( B ) bands as functions of potential. (a, b)
From the spectra of the species coming from methanol adsorption and oxidation (0.1 M
HClO 4 þ 0.1 M CH 3 OH, 25 8C): (a) Pt/C electrode; (b) Pt 0.8 þ Ru 0.2 /C electrode. (c, d)
From the spectra of the species coming from ethanol adsorption and oxidation (0.1 M
HClO 4 þ 0.1 M C 2 H 5 OH, 25 8C): (c) Pt/C electrode; (d) Pt 0.9 Sn 0.1 /C electrode. The dashed
curves in (c) and (d) show I(E).

et al., 2004b]. Actually, this can be explained by the fact that modification of Pt by Sn
induces two important effects: first, it leads to a decrease in the yield of CO 2 (which is
twice as high with a Pt/C catalyst as with a Pt-Sn/C catalyst); second, it greatly favors
the formation of acetic acid compared with acetaldehyde, as was shown by analyzing
the reaction products in the anode outlet of a DAFC (Table 11.2) [Rousseau et al.,
2006]. The dilution of surface Pt sites by addition of Sn atoms is responsible for the
first effect. The introduction of Sn atoms between Pt atoms decreases, by a statistical
effect, the ability of the catalytic surface to cleave the C - C bond in the adsorption
reaction of ethanol (which needs at least three adjacent Pt atoms). The bifunctionnal
mechanism [Watanabe et Motoo, 1975a] is responsible for the second effect. In this
mechanism, ethanol is adsorbed dissociatively at Pt sites, either via an O-adsorption
or a C-adsorption process [Iwasita and Pastor, 1994a, b; Rightmire et al., 1964] to
form acetaldehyde species, according to the following reactions:

Pt þ CH 3 -CH 2 OH ! Pt (OCH 2 -CH 3 ) ads þ e þ H þ (11 : 13)