Electrocatalysis for the Direct Alcohol Fuel Cell - Fuel Cell Catalysis: A Surface Science Approach - page 345

[Waszczuk et al., 2001b; Tong et al., 2002]. Because Ru is deposited as nanosized Ru
islands of monoatomic height, the Ru coverage of Pt could be determined accurately.
In that case, the best activity with regard to methanol oxidation was found for a Ru
coverage close to 40 - 50% at 0.3 and 0.5 V vs. RHE. However, the structure of
such catalysts and the conditions of study are far from those used in DMFCs.
Moreover, the surface composition of a bimetallic catalyst likely depends on the
method of preparation of the catalyst [Caillard et al., 2006] and on the potential
[Blasini et al., 2006].
Using the colloidal Pt (12x) þ Ru x /C catalysts described above, the optimal atomic
ratio depends upon methanol concentration, cell temperature, and applied potential, as
shown by the Tafel plots recorded with methanol concentrations of 1.0 and 0.1 M at
T ¼ 298K (Fig. 11.4) and 318K (Fig. 11.5). Some authors have stated that for poten-
tials between 0.35 and 0.6 V vs. RHE, the slow reaction rate between adsorbed CO and
adsorbed OH species must be responsible for the rate of the overall process [Iwasita
et al., 2000]. From these results, it can be underlined that, at a given constant potential
lower than 0.45 - 0.5 V vs. RHE, an increase in temperature requires an increase in Ru
content to enhance the rate of methanol oxidation, and that, at a given constant poten-
tial greater than 0.5 V vs. RHE, an increase in temperature requires a decrease in Ru
content to enhance the rate of methanol oxidation.
Increasing the temperature above 40 8C gives to Ru the ability to adsorb and dehy-
drogenate methanol [Chu and Gilman, 1996]. Moreover, Ru allows the activation of
water molecules at lower potentials than Pt. According to the bifunctional theory of
electrocatalysis for the complete oxidation of methanol [Watanabe and Motoo,
1975b], the presence of a large amount of Ru in the Pt 0.5 þ Ru 0.5 /C catalyst can
explain the higher activity of this catalyst at lower potentials. At higher potentials,
above 0.5 V vs. RHE, Pt-rich catalysts become more active. In the limiting current
range, the catalytic surface is blocked by adsorbed oxygen species [Watanabe and
Motoo, 1975a, b], which makes the adsorption of organic species more difficult.
Because Ru adsorbs oxygen species at more negative potentials, increasing the content
of Ru causes the limiting current to decrease. On the other hand, some authors have
shown that in the temperature range 25 - 60 8C, pure Pt displays greater activity than
Ru for methanol oxidation for potentials higher than 0.5 V vs. RHE [Gasteiger
et al., 1994]. The combination of both effects may explain the decrease in limiting cur-
rent with an increase in the Ru atomic ratio.
Arrhenius plots recorded with different catalyst compositions (Fig. 11.6) allow
determination of the apparent activation energy DH at 0.5 V vs. RHE. Although
radioactive labeling measurements indicated that the presence of Ru leads to an
increase in the rate of methanol adsorption/dehydrogenation at Pt for potentials
lower than 0.5 V vs. RHE (Fig. 11.1), infrared studies have shown that the coverage
by CO-adsorbed species remains small for Ru-rich catalysts [Bett et al., 1998],
which indicates that the limiting step may still be the adsorption/dehydrogenation
of methanol, in agreement with the proposition of Lei and co-workers from deuterium
isotope analysis of methanol oxidation on a Pt 0.5 Ru 0.5 catalyst [Lei et al., 2002].
Therefore, a value close to 60 kJ mol 21 of the apparent activation energy is reasonable.
For Ru-poor catalysts, another rate-determining step than that proposed for Ru-rich
catalysts seems to be involved because of the low value of the apparent energy of