Environmental Engineering Reference
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et al., 2000]. Utilization of metal oxides (e.g., TiO
2
) as supports for model and fuel cell
electrocatalysts is a very interesting novel direction [Guerin et al., 2006a, b; Hayden
et al., 2007]. TiO
2
interacts with metal particles much more strongly than do carbon
materials, which on the one hand allows tuning of electrochemical properties of
metal particles [Hayden et al., 2007] and on the other stabilizes them against coalesc-
ence. Application of metal electrodes as supports for particle deposition may result in
alloy formation. Model nanoparticles on flat nonporous substrates offer better control
over the particle distribution and allow internal diffusion effects inside the pores to be
avoided. A further improvement in creating model systems has been suggested in a
number of publications [Kasemo et al., 2000; Gustavsson et al., 2004; Kumar and
Zou, 2006], and is based on the use of ordered nanoparticle arrays. This emerging
approach seems very promising, and in the future will hopefully allow one to separate
effects arising from the influence of particle size on intrinsic catalytic activities from
effects of interparticle separation on the overlap of diffusion zones. When considering
various model approaches towards understanding properties of nanosized particles,
one must not leave out the investigation of single-crystal electrodes. The motivation
for this approach is the analogy between low coordinated sites on high index (stepped)
crystals and particle edges and vertices. The influence of steps of specific geometry
was demonstrated in studies of the ORR on Au [Strbac et al., 1994] and Pt [Macia
et al., 2004; Kuzume et al., 2007], CO adsorption [Kim et al., 1993] and electro-
oxidation [Lebedeva et al., 2000, 2002a], and electro-oxidation of organic molecules
on Pt [Housmans and Koper, 2003; Sun and Yang, 1999; Tarnowski and
Korzeniewski, 1997]. Feliu and co-workers have documented negative shifts of the
potential of total and free zero charge with increasing step density [Attard et al.,
2004; Climent et al., 2006]. Application of single-crystalline surfaces resulted in sig-
nificant progress in the understanding of the roles of different types of sites in various
electrocatalytic processes. However, it should be kept in mind that some essential
characteristics of supported metal nanoparticles cannot be simulated with single-
crystal surfaces. The first concerns the influence of support, which may affect
geometric ( particle shape, lattice parameter, etc.) and electronic (DOS, shift of the
d-band, etc.) properties of metal nanoparticles, and, if reagents adsorb on the support,
supply them via surface diffusion [Zhdanov and Kasemo, 2000]. The second charac-
teristic is size confinement. Indeed, as outlined in Section 15.2, metal nanoparticles
may have different chemical potentials, Fermi energies, and DOS owing to their
small dimensions. Zhdanov and Kasemo have pointed out that different electric
field distributions on nanoparticles compared with flat surfaces may add to the differ-
ences in the rates of electrochemical reactions [Zhdanov and Kasemo, 2002]. The
third factor is related to the interplay of reaction kinetics on crystal facets of different
configurations [Zhdanov and Kasemo, 2000]. Komanicky and co-workers investi-
gated the ORR at “nanofaceted” Pt electrodes with alternating (111) - (100) plane
edges [Komanicky et al., 2005]. They demonstrated that the catalytic activity of “nano-
faceted” electrodes is higher than the activities of individual (111) and (100) planes,
which was attributed to the kinetic interplay between them.
We would also like to point out some essential differences in site geometry between
particle edges and monoatomic steps, which may be crucial for some catalytic
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