Size Effects in Electrocatalysis of Fuel Cell Reactions on Supported Metal Nanoparticles - Fuel Cell Catalysis: A Surface Science Approach - page 522

have major consequences for electrocatalysis, since the charge required for electro-
oxidation of H UPD will be a function of particle size, and not equal to 210 mC per
cm 2 of Pt, as assumed up to now.
At positive potentials, water splitting occurs on the Pt surface, manifested by an
increase of the current density in the CVs positive of about 0.75 V. According to
Anderson, the equilibrium potential of OH formation on a Pt surface, E e H 2 O = OH
occurs around 0.62 V on bulk Pt [Anderson, 2002]. The onset of surface oxidation,
as well as the charge corresponding to OH/O formation in H 2 SO 4 , do not seem to
depend on the particle size, as confirmed by Table 15.1 and Fig. 15.5 (for further
discussion, see Maillard et al. [2005]). This is in line with the modeling work by
Andreaus and co-workers, which established E e H 2 O = OH 0 : 67-0 : 69 V vs. RHE
regardless of particle size [Andreaus et al., 2006]. Meanwhile, Arenz and co-workers
performed measurements in HClO 4 , and reported a negative shift in the OH ads
formation potential with decreasing Pt particle size, although no clear trend was
observed, because of different metal loadings and thus a contribution from the
pseudocapacitance of the carbon support [Arenz et al., 2005]. The reason for this
discrepancy may be associated with the different electrolytes utilized, but also with
the different approaches to determining Pt surface areas: H UPD [Arenz et al., 2005]
and CO stripping [Maillard et al., 2004a, 2005].
As can be clearly seen from Fig. 15.5, the potential of the oxide reduction peak
systematically shifts negative with decreasing Pt particle size, in agreement with the
previous observations of other authors made both in HClO 4 and H 2 SO 4 electrolytes
[Gloaguen et al., 1994; Kabbabi et al., 1994; Frelink et al., 1995; Takasu et al.,
1996; Genies et al., 1998; Maillard et al., 2004a, 2005]. This, along with the marginal
dependence of the onset of Pt oxidation on particle size, may be attributed to an
increased electrochemical irreversibility of the oxidation of the surface of Pt nanopar-
ticles compared with bulk Pt. Conway attributed the irreversibility in the anodic region
of voltammograms of noble metal electrodes to the so-called “place exchange”
process, resulting in penetration of oxygen in subsurface sites (see the review
[Conway, 1995] and references therein). Recently, application of state-of-the-art
surface science characterization methods has shown that penetration of oxygen in
the metal lattice (e.g., Pd) is facilitated by decreasing particle size [Schalow et al.,
2007]. It would be very interesting to perform in situ spectroscopic investigations
of model supported metal nanoparticles with controlled size and shape in an electro-
chemical environment in order to better understand how their size affects interaction
with oxygen, hydrogen, and other atoms.
CO adsorption on Pt particles supported on Vulcan XC-72 [Park et al., 2001,
2002a; Maillard et al., 2004b] and on low surface area (about 6 m 2 g 21 ) Sibunit
carbon [Park et al., 2001, 2002a; Maillard et al., 2004b] has been investigated
with in situ FTIR spectroscopy. The following conclusions were drawn. On small
(1 - 2 nm) Pt particles, CO is preferentially adsorbed in an atop configuration, but with
increasing particle size, the fraction of bridge-bonded CO grows [Park et al., 2001,
2002a, 2004b]. The vibrational frequency of the C22O bond of atop CO ads redshifts
with decreasing particle size, giving indirect support for stronger CO bonding to
small nanoparticles. At low surface coverages, CO preferentially adsorbs to the particle