Environmental Engineering Reference
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these alloys is dominantly brought about by promoting the same rate-determining step
(rds) of the first one-electron-transfer step as that of pure Pt. Considering the first-order
dependence of the ORR rate on [O
2
] [Wakabayashi et al., 2005a] and pH [Sepa et al.,
1981], the rds can be written as
!
Pt
x
O
2
H
xPt
þ
O
2
þ
H
þ
þ
e
(10
:
10)
This reaction may consist of the following elementary steps:
Pt
x
yA
ad
þ
O
2
Pt
x
O
2
þ
yA
ad
(A
ad
¼
H
2
O, adsorbed anions, etc
:
)
(10
:
11)
Pt
x
O
2
þ
H
þ
(H
2
O)
z
[Pt
x
O
2
H
þ
(H
2
O)
z
]
(10
:
12)
!
Pt
x
O
2
H
þ
z(H
2
O)
[Pt
x
O
2
H
þ
(H
2
O)
z
]
þ
e
(10
:
13)
where the asterisk denotes the activated complex. Because the activation energy 1
a
on
both alloys and pure Pt is the same, the high ORR activities at the alloys are ascribed to
a large pre-exponential factor Z in the rate constant of (10.9). As stated in the previous
sections, the electronic state at the Pt skin is strongly modified by underlying alloy.
This may increase the coverage of adsorbed oxygen, for example in the reaction
step (10.11), probably assisted by a decrease in the blocking of A
ad
(H
2
O, adsorbed
anions, etc.). The production of H
2
O
2
during the ORR at all the alloy electrodes at
low temperatures also supports such a mechanism [Wakabayashi et al., 2005b].
While an inhibition of “OH poisoning” at the alloy electrodes has been proposed
[Stamenkovic et al., 2002; Paulus et al., 2002; Murthi et al., 2004], it is necessary
to identify the adsorbed species in the ORR on both pure Pt and Pt alloys to find a
clue for the design of high performance cathode catalysts. Such studies are in progress
in our laboratory [Wakisaka et al., 2008].
10.4 CONCLUSIONS
We have demonstrated enhanced ORR activities and excellent CO tolerances in the
HOR at Pt alloyed with nonprecious metals such as Fe, Co, and Ni. It has been
shown that dissolution of the surface alloy layer is followed by rearrangement of
the Pt layer, resulting in a Pt skin layer, which can protect the underlying bulk
alloy. The CO-tolerant HOR activity at Pt skins or Pt-Ru at E , 0.10 V vs. RHE is
ascribed to weaker CO bond strength due to the modification of the Pt electronic struc-
tures, observed as a positive core level shift in Pt4f
7/2
. It has been suggested that such a
modified electronic structure at the Pt skin layer also induces enhanced ORR activity.
To clarify such a mechanism in detail, a number of investigations of the ORR at these
alloys and pure Pt are under progress in our group.
We have also clarified for Pt/CB that ORR activity is not affected by differences in
the particle size. It is very important for highly dispersed alloy catalysts to examine the
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