Size Effects in Electrocatalysis of Fuel Cell Reactions on Supported Metal Nanoparticles - Fuel Cell Catalysis: A Surface Science Approach - page 500

per unit surface area of the active component) does not depend on the size of the
particles, the reaction is classified as “facile” or “structure-insensitive” by analogy
with the definition given by Boudart for heterogeneous catalysis [Boudart, 1969].
For this kind of reaction, dispersion may be increased effectively until the metal -
insulator transition is reached, with the mass activity (i.e., the activity per gram of
the active component) increasing in inverse proportion to the particle size. On the
other hand, if the specific electrocatalytic activity of nanoparticles depends on the par-
ticle size, the reaction is classified as “demanding” or “surface-sensitive” [Boudart,
1969]. This holds especially true for multielectron reactions occurring in PEMFCs
at temperatures below about 130 8C. Indeed, it is now well established that the electro-
catalytic activity of Pt for the ORR [Kabbabi et al., 1994; Takasu et al., 1996; Gamez
et al., 1996; Genies et al., 1998], the ORR in methanol-containing electrolytes
[Maillard et al., 2002], the MOR [Yahikozawa et al., 1991; Kabbabi et al., 1994;
Frelink et al., 1995; Takasu et al., 2000; Cherstiouk et al., 2003a] and CO electro-
oxidation [Takasu et al., 1997, 2000; Cherstiouk et al., 2003a, b; Maillard et al.,
2004a, b, 2005, 2007b; Mayrhofer et al., 2005b] is strongly dependent on the particle
size. In this case, the following questions arise:

† How do particle size and structure influence catalytic activity?
† What are the physical reasons for this dependence?
† Ultimately, can catalytic activity and stability be purposely tuned via modifi-
cation of particle size and nanostructure?

These questions are of both applied and fundamental importance, since answering
them will further our understanding of electrocatalysis.
This chapter is organized as follows. In Section 15.2, we will briefly discuss struc-
tural and electronic properties characteristic of supported metal nanoparticles, and will
provide the reader with references where more information can be found. In Section
15.3, we will analyze various model systems currently utilized for studying particle
size effects (PSEs) in electrocatalysis, along with their advantages and limitations.
Sections 15.4 and 15.5 will be devoted to the adsorption and electrocatalytic properties
of supported metal nanoparticles. Owing to obvious space constraints, it will not be
possible to give a comprehensive treatment of all the published data, so we will
rather present a selective review essentially pinpointing studies in which we have
been directly involved. In Section 15.6, we will attempt to summarize the current
state of understanding of PSEs in electrocatalysis, and will present our vision of further
developments in the vitally important area of electrocatalysis concerned with size and
structural effects.

15.2 STRUCTURE AND PROPERTIES OF METAL NANOPARTICLES

An inherent property of nanoparticulate materials is their high dispersion D, which for
spheres scales with the inverse diameter 1/d and with N 1 = 3
total

. For example, for a