Support and Particle Size Effects in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 562

In addition, it sustains CO electro-oxidation at relatively low overpotential, and there
are crystal face dependences for both the ORR and CO oxidation. Since Au is also a
system that exhibits both particle size and support effects in heterogeneous catalysis, it
provides an interesting model system for studying such effects in electrocatalysis.
The ORR on Au was found to be dependent on the crystallographic orientation of
the low index Miller planes in alkaline [Adzic et al., 1984; Markovic et al., 1984], neutral
[Prieto et al., 2003], and acidic [Alvarez-Rizatti and J¨ttner, 1983] media. The activity
decreased with pH in the order (100) .. (110) . (111) [Alvarez-Rizatti and J¨ttner,
1983; Adzic et al., 1984; Markovic et al., 1984; Schmidt et al., 2002; Prieto et al., 2003].
The (100) crystallographic orientation of Au [Adzic and Markovic, 1982; Adzic et al.,
1984; Markovic et al., 1994; Strbac and Adzic, 1996a, b; Blizanac et al., 2004a]
catalyzes oxygen reduction to water in a four-electron process over a pH range of 3 .
pH . 14 [Strbac and Adzic, 1996a, b] at potentials where OH appears to be stable at
the surface. The reasons for these structure and pH dependences are still not fully under-
stood [Kim and Gewirth, 2006; Vassilev and Koper, 2007].
More recently, there have been a number of publications describing the ORR on
supported Au nanoparticle electrodes [Maye et al., 2004; Yagi et al., 2004; Zhang
et al., 2004; El-Deab et al., 2005a, b; Gao et al., 2005; Hern´ndez et al., 2005;
Alexeyeva et al., 2006; Baker et al., 2006]. El-Deab and co-workers studied the
ORR on electrodeposited Au nanoparticles on Au [El-Deab and Ohsaka, 2002a, b]
and glassy carbon [El-Deab et al., 2003, 2005a, b; Gao et al., 2005] supports in
acidic, neutral, and basic solutions.
Very low coverages of electrodeposited Au on an Au electrode result in a lower
overpotential for oxygen reduction in acid electrolytes [El-Deab and Ohsaka, 2002a, b],
with a similar increase in activity observed on glassy carbon-supported Au nano-
particles in both acid [Gao et al., 2005] and alkaline [El-Deab et al., 2005a] media.
These studies also suggest that both Au- and carbon-supported particles can catalyze
the reduction of hydrogen peroxide to water, although in neutral media, the dominant
reaction on glassy carbon-supported small Au particles is hydrogen peroxide for-
mation [El-Deab et al., 2003]. The particles in these studies are relatively large
(10 - 100 nm), and the results can be explained in terms of the different activities
of the exposed facets of the particles. Smaller particle sizes (3.7 - 4.8 nm) of Au
supported on carbon synthesized with preferential growth of the (100) facets
[Hern´ndez et al., 2004, 2005] exhibited enhanced oxygen reduction activity, with
a shift towards a four-electron reduction of oxygen. It has been reported recently
[Baker et al., 2006] that tin dioxide-supported Au nanoparticles of diameter about 1
nm reduce oxygen in 0.1 M HClO 4 to water. Baker et al. speculated that either an elec-
tronic effect (charge transfer from the support to the Au particles) or a “bifunctional”
mechanism might be responsible for this behavior. The latter would involve oxygen
adsorption to form an O 2 2 radical at the support, and the migration of this species to
the perimeter of the Au particles, where further reduction would take place.
16.2.3 CO Electro-Oxidation on Pt
Catalytic oxidation of CO heterogeneously in the gas phase and electrochemically in
solution are important reactions for the removal of CO from reactant stock gases in