Size Effects in Electrocatalysis of Fuel Cell Reactions on Supported Metal Nanoparticles - Fuel Cell Catalysis: A Surface Science Approach - page 529

facets were the most active in the ORR, then the observed correlation would explain
why the maximum of MA was attained between 3 and 4 nm. Although this idea has
been invoked for explaining PSEs in numerous original and review articles, it does
not seem very convincing in view of the fact that (111) facets are the least active in
the ORR in strongly adsorbing H 2 SO 4 and H 3 PO 4 electrolytes.
An alternative standpoint is to explain the PSEs on the ORR kinetics by electronic
effects concomitant to the decrease of particle size or arising owing to metal - support
interactions. This can be correlated with the increasing concentration of low
coordination atoms, which have low d-band occupancy and are thus prone to strong
adsorption of atoms and molecules [Hanson and Boudart, 1978; Parmigiani et al.,
1990; Kinoshita, 1990, 1992; Giordano et al., 1991; Hwang and Chung, 1993;
Mukerjee and McBreen, 1998]. Stronger adsorption of oxygenated species on
nanoparticles was first reported by Hanson and Boudart in gas-phase catalysis over
Pt/SiO 2 catalysts while studying the H 2 þ O 2 reaction in the interval 273 - 373K
[Hanson and Boudart, 1978]. They reported strong chemisorption of O 2 on Pt nano-
particles with a particle size below 3 nm. In liquid electrolyte, stronger adsorption of
OH ads on smaller Pt/C particles was derived by Mukerjee and McBreen from analysis
of the white line in the in situ X-ray absorption near edge structure (XANES) spectra
[Mukerjee and McBreen, 1998]. They found that the calculated Pt 5d-band vacancy
normalized to the number of surface atoms increased slightly with decreasing particle
size both at 0.0 V and at 0.84 V vs. RHE.
According to Markovic and co-workers and Chen and Kucernak, anions and
OH ads , along with the site-blocking effect (the 1 2 u term in (15.17)), may also
affect the free energy of oxygen adsorption through lateral surface interactions
[Wang et al., 2004; Chen and Kucernak, 2004a, b]. Following the discussion in
Section 15.4, it is likely that for smaller Pt particles, aside from oxygen adsorption
on the surface, its penetration into the subsurface is also possible, strongly affecting
the electrocatalytic properties of the metal. It is well known that the formation of
oxide films on metal electrodes significantly attenuates their activity in the ORR
[Damjanovic and Hudson, 1988; Damjanovic, 1992].
Aside from the studies advocating the influence of particle size on ORR kinetics,
some authors have claimed that the intrinsic catalytic activity of Pt is independent
of particle size and that the observed changes in reaction rate are related to diffusion
[Blurton et al., 1972; Bett et al., 1973; Vogel and Baris, 1977; Watanabe et al., 1988,
1989; Yano et al., 2006; Stonehart, 1990]. In view of the fact that understanding
structure - function relationships is of paramount importance for a fundamental
understanding of electrocatalysis, we will analyze this point of view in some detail.
The idea originally proposed by Stonehart and by Watanabe is that the apparent
correlation between the catalytic activity of nanoparticles and particle size stems
from diffusive interference between Pt crystallites [Stoneheart, 1990; Watanabe
et al., 1989]. If two Pt particles are located in close proximity on the carbon support,
their O 2 diffusion spheres overlap, leading to a decrease in the measured current den-
sity, which translates into decreased apparent kinetic current and SA. These results
were contradicted by Giordano and co-workers in H 3 PO 4 and by Gamez and