Electrocatalysis for the Direct Alcohol Fuel Cell - Fuel Cell Catalysis: A Surface Science Approach - page 350

11.4 EFFECT OF FOREIGN METALS ALLOYED TO Pt:
COMPARISON BETWEEN ETHANOL AND METHANOL
In situ infrared spectroscopy has shown that the dissociative adsorption of both metha-
nol and ethanol leads to the formation of strongly adsorbed CO species at low poten-
tials [Beden et al., 1987; Perez et al., 1989; Iwasita and Nart, 1991]. Indeed, in both
cases, one can see the existence of an absorption band located close to 2050 cm 21
in the subtractively normalized interfacial Fourier transform infrared (SNIFTIR) spec-
tra (Fig. 11.8). With Pt/C, this species is linearly bonded to the Pt sites and plays the
role of a poison for the electrocatalytic reaction. The surface reaction between this
adsorbed CO species and adsorbed OH (coming from the dissociation of water)
occurs at potentials around 0.4 - 0.5 V for methanol and ethanol with Pt/C. The for-
mation of the final product (CO 2 for both alcohols detected at 2345 cm 21 [Vigier
et al., 2004a; Dubau et al., 2003b, L´ger et al., 2005]) occurs mainly when the cover-
age in adsorbed CO begins to decrease (Fig. 11.9). With Pt-Ru/C, the maximum cov-
erage in CO ads (in the case of methanol) appears at lower potentials in comparison with
Pt/C. With Pt-Sn/C, the formation of CO 2 (in the case of ethanol) is clearly visible
after the disappearance of adsorbed CO, the coverage of which is higher in the low
potential range. Bands corresponding to the formation of acetaldehyde and acetic
acid are clearly observed during ethanol oxidation at Pt-Sn/C electrodes
(Fig. 11.10). The bands located close to 1720, 1370, and 1280 cm 21 are respectively
attributed to the formation of products containing a carbonyl group: a CO stretching
mode from - COOH or - CHO (1370 and 1280 cm 21 ), - C - O symmetric stretching
in adsorbed CH 3 COO 2 , and coupled C - O stretching and OH deformation from
- COOH [Xia et al., 1997]. The bands located between 1280 and 1400 cm 21 probably
correspond to the symmetric CH 3 bending mode of acetic acid [Iwasita et al., 1989].
The absorption band located close to 1720 cm 21 originates from acetic acid and/or
acetaldehyde [Iwasita and Pastor, 1994a; Chang et al., 1990; Beden, 1984].
All these points show clearly that in the case of the oxidation of methanol, the
adsorbed CO species, which is without any doubt a poisoning species at low poten-
tials, is also a reactive intermediate when Pt/C or Pt-Ru/C is used as the electrocata-
lyst. The main advantage in using Ru to modify Pt is the possibility of activating water
at lower potentials and consequently oxidizing adsorbed CO earlier. During ethanol
oxidation, on Pt/C, linearly adsorbed CO is also a poisoning species at low potentials.
Cleavage of the C - C bond is clearly possible in this potential range. The formation of
CO 2 is clearly related to the removal of CO ads from the Pt/C surface, as it coincides
with the maximum of CO ads coverage (similarly to the oxidation of methanol). Pt-
Sn is known to be the best bimetallic catalyst for ethanol electro-oxidation [Delime
et al., 1999; Zhou et al., 2004a, b, 2005]. When Pt-Sn/C is used, the formation of
CO 2 seems to be, at least partially, disconnected from the coverage in adsorbed CO.
The amount of CO 2 formed with Pt-Sn/C, for potentials greater than 0.4 V, should
be related not only to the removal of adsorbed CO, but also to the further oxidation
of acetaldehyde. Two main routes then exist in the overall mechanism of ethanol
oxidation with Pt-Sn/C: the first through the formation of adsorbed CO, involving
electrode poisoning (which occurs strongly with Pt/C); the second via the formation