Electrocatalysis at Platinum and Bimetallic Alloys - Fuel Cell Catalysis: A Surface Science Approach - page 325

value of k app at a pure Pt electrode is the highest in the whole temperature range. Pt-Co
and Pt-Ru exhibited almost comparable k app at 30 - 50 8C. The apparent activation
energies at Pt, Pt-Co, and Pt-Ru at 30 - 70 8C were as low as 3.0, 4.2, and 5.3 kJ
mol 21 , respectively. Hence, elevating the operation temperature does not lead to sig-
nificant enhancement in HOR kinetics. For example, k app for the HOR on Pt at 90 8C
was two times higher than that at 30 8C, whereas k app for the ORR on Pt increased by
about one order of magnitude from 30 to 90 8C [Wakabayashi et al., 2005a], as
described in the next section.
Next, the j k values in the presence of CO ad at these electrodes are compared.
Figure 10.9 shows the current per remaining hydrogen adsorption site free from
CO ad , j H k ¼ j k = (1 u CO ), as a function of u CO . If the HOR active sites are tightly
blocked by the immobile CO ad , the value of j H k should be a constant independent
of u CO . j H k at pure Pt increases slightly up to u CO ¼ 0.6, but it decreases steeply at
high u CO . Such an increase in j H k at low u CO suggests that Pt sites were blocked not
rigidly by CO ad at least for u CO , 0.6, resulting in apparent active sites more than
that expected from u CO . The steep decrease in j k or j H k at high u CO . 3 is certainly
due to a decrease in nearest neighbor CO-free pair sites required for H 2 dissociative
adsorption in the rate-determining Tafel step [Conway and Tilak, 2002; Uchida
et al., 2006].

Figure 10.9 Dependence of j H k ¼ j k /(1 2 u CO ) at 0.020 V on u CO for Pt, Pt 51 Co 49 , and
Pt 54 Ru 46 electrodes in H 2 -saturated 0.1 M HClO 4 solution. (From Uchida et al. [2006], repro-
duced by permission of the American Chemical Society.)