Support and Particle Size Effects in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 563

PEMFC applications, since CO poisons Pt-based anode catalysts. Almost a monolayer
of CO is adsorbed if traces of CO (about 10 ppm) are present in the feedstock gas
hydrogen at the PEMFC anode at temperatures below 100 8C [Li et al., 2003]. The
alloying of a second, or even third, metal component in Pt has been the favored
method of providing CO-tolerant anode catalysts, and to date the most active catalytic
system for both applications is Pt/Ru alloy [Wasmus and Kuver, 1999]. The efficacy
of the alloying component in providing CO tolerance has been attributed to either low-
ering of the overpotential for CO electro-oxidation and oxidation of CO through a
“bifunctional” mechanism [Watanabe and Motoo, 1975a, b] or an electronic effect
of Ru on Pt that reduces the CO binding energy on the Pt [Hammer and Nørskov,
1995; Hammer et al., 1996; Shubina and Koper, 2002; Tsuda and Kasai, 2006], con-
comitantly reducing the mean coverages of CO.
The overall electrocatalytic oxidation of CO can be expressed as
CO þ H 2 O ! CO 2 þ 2H þ þ 2e

(16 : 1)

with a standard potential E o ¼ 20.106 V (with respect to a standard hydrogen elec-
trode, SHE) [Galus, 1985]. A Langmuir - Hinshelwood mechanism for this reaction
is widely accepted for polycrystalline as well as single-crystal Pt surfaces
[McCallum and Pletcher, 1976; Markovic and Ross, 2002]; the overpotential for
the surface reaction is associated with activation of water to produce the surface
oxidant, and the surface reaction may take place through a hydroxycarbonyl inter-
mediate [Herrero et al., 2000].
Both the binding energy of CO and the activation of water may be expected to
be structure-sensitive, and hence the CO tolerance of Pt may also be expected to be
dependent on particle size and particle morphology. There is an effect of Pt particle
size on the stripping voltammetry of CO (see Guerin et al. [2004]; Maillard et al.
[2004a, b, 2005]; Arenz et al. [2005]; Mayrhofer et al. [2005a, b] and references
therein). Generally, a displacement of the CO stripping peak is observed towards
higher potentials at particles below about 3 nm [Guerin et al., 2004; Maillard et al.,
2004a, b]. This can be attributed to either an increasing overpotential for water acti-
vation at small particles or changes in the mobility of the reactants in the subsequent
Langmuir - Hinshelwood reaction over particle facets.

16.2.4 CO Electro-Oxidation on Au

The mechanism of anodic oxidation of CO at polycrystalline Au remains uncertain.
Several groups have reported that the voltammetry of Au in acidic electrolytes is
straightforward, with a well-formed oxidation wave/peak [Stonehart, 1966; Gibbs
et al., 1977; Kita et al., 1985; Sun et al., 1999]. There is, however, no voltammetric
evidence for the adsorption of CO on the Au surface, and spectroscopic studies indi-
cate only a weak interaction of CO with polycrystalline Au surfaces in acidic solutions
[Kunimatsu et al., 1986; Cuesta et al., 2003]. Moreover, there is little evidence for the
formation of oxidizing species at the potential where the oxidation process is observed.
Certainly, the oxidation of CO occurs at a potential over 500 mV less positive than that
where bulk Au oxide is formed, and, indeed, the formation of this oxide strongly