Electrocatalysis at Platinum and Bimetallic Alloys - Fuel Cell Catalysis: A Surface Science Approach - page 316

We calculated the CO coverage u CO as the site occupation by the following
equation, regardless of the type of adsorbed CO (CO ad ), e.g., linear (on-top),
bridged, etc.:

u CO ¼ 1 DQ H
DQ H

(10 : 2)

where DQ H and DQ H are the hydrogen-desorption charges (from 0.05 V to 0.40 V)
in the CVs with and without CO ad , respectively. It was found that the values of u CO
on all of the CO tolerant alloys were suppressed to less than 0.6 even after 90
minutes, while the less CO-tolerant alloys and pure Pt were almost completely covered
with CO.
The most essential question is why the CO-free sites are secured for H 2 adsorption
and oxidation. Watanabe and Motoo proposed a so-called “bifunctional mechanism”
originally found at Pt electrodes with various oxygen-adsorbing adatoms (e.g., Ru,
Sn, and As), which facilitate the oxidation of adsorbed CO ad at Pt sites [Watanabe
and Motoo, 1975a; Watanabe et al., 1985]. This mechanism has been adopted for
the explanation of CO-tolerant HOR on Pt-Ru, Pt-Sn, and Pt-Mo alloys [Gasteiger
et al., 1994, 1995], and recently confirmed by in situ FTIR spectroscopy [Yajima
et al., 2004]. To investigate the role of such surface sites, we examined the details
of the alloy surface states by various methods.

10.2.2 Surface States of CO-Tolerant Pt Alloys

10.2.2.1 EQCM Analyses It was suggested by XPS analysis that the nonprecious
metals were leached out from the alloy surface during CVs in acidic electrolyte solution.
Therefore, we used an EQCM to measure the mass change at the electrode. The resolution
and stability of our 10 MHz EQCM was +0.1 Hz, i.e., +0.44 ng cm 22

without any

signal averaging.
Figure 10.2 shows the CVs and concomitant mass changes at a working Pt 49 Co 51
alloy EQCM electrode during repetitive potential sweeps in 0.1 M HClO 4 solution.
The CV approaches a steady state after a noticeable decrease in the current density.
The final CV shows a feature similar to that of polycrystalline Pt. The electrode
mass decreases steeply during the several initial sweeps, and reaches a steady value
at the 35th sweep. These results indicate that the Co component was dissolved out
from the alloy and the dissolution was suppressed after several potential sweeps,
i.e., the electrode was electrochemically stabilized. We observed similar behavior
for a Pt-Fe alloy EQCM [Uchida et al., 2002].
If only nonprecious atoms (e.g., Co or Fe) were selectively dissolved from the
alloy surface, the roughness of the resulting Pt surface layer would be increased
with the number of sweeps. However, as seen in Fig. 10.2a, the value of DQ H
(a measure of the electrochemical surface area) rather decreases gradually and reaches
a steady value.