Support and Particle Size Effects in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 578

16.7.3 Reactions at the Titania-Au Perimeter
It has been suggested that spillover of oxygen from the support at the particle - support
interface, or active sites at the particle perimeter itself, may be responsible for enhance-
ment of CO oxidation [Boccuzzi et al., 2001; Liu et al., 2003; Molina et al., 2004]. The
results here suggest that a mechanism based on the reaction of a substrate-accommo-
dated oxidant diffusing and reacting at the perimeter of the nanoparticles is unlikely
[Haruta et al., 1993; Boccuzzi et al., 2001], since there is no evidence that titania
can activate water to produce such an oxidant (OH) at these low potentials.
However, a high activity induced by the activation of water at perimeter sites
cannot be ruled out [Hayden et al., 2007a, c]. A correlation between the total length
of the particle perimeters and the activity in CO electro-oxidation as a function of
Au nanoparticle size supported on titania has been suggested [Hayden et al.,
2007c], at least in so far as the maximum in activity is predicted at about 2.5 nm.
However, the decrease in activity observed experimentally is much steeper below 3
nm than the decrease in perimeter length, although this could be explained by the
additional contribution of intrinsic particle size affects.
16.8 CONCLUSIONS
There are clearly two important contributions that must be considered in evaluating the
influence of particle size on electrocatalytic reactivity: intrinsic particle size effects
and support-induced particle effects. The model electrocatalyst studies reveal a deac-
tivation of Au and Pt particle activity when particles are 3 nm and smaller. These
studies also reveal that the support can strongly influence the activity of the catalyst,
and that there are strong similarities between such effects in heterogeneous catalysis
and electrocatalysis. The decrease in the activity of small particles as a result of
some intrinsic (support-independent) effect clearly will act to limit the advantage
gained by dispersion in optimizing mass activity. It is evident, however, that gains
in specific and mass activity can be achieved by varying the support material. This
provides a powerful method of optimization for supported metal electrocatalysts.
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