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Figure 16.6 TEM micrographs of titania-supported Au particles. The nominal thickness of
Au was (a) 0.13 nm; (b) 0.78 nm; (c) 1.56 nm; (d) 2.33 nm. The Au deposition rate was
2.6 10 23 nm s 21 . Particle size distributions of Au for various deposition times are shown
in the plot, with the distributions fitted to a normal Gaussian function.
size increases and the distributions become broader. The characteristics of particle
growth on both carbon and titania supports are described in more detail elsewhere
[Guerin et al., 2006c].
16.5 OXYGEN REDUCTION ON SUPPORTED Au
AND Pt NANOPARTICLES
The electrochemical reduction of oxygen has been measured using a chronoampero-
metric methodology in HClO 4 electrolyte on Au-supported nanoparticles [Guerin
et al., 2006b]. The potential was stepped from 0.8 V (with respect to a reversible
hydrogen electrode, RHE) to 0.5, 0.4, and 0.3 V vs. RHE, and the current was recorded
during 90 s holding time. Figure 16.7 shows the TEM surface area normalized currents
of the 0.4 V step (average of the last 20% of the steps) for (a) carbon- and (b) titania-
supported Au particles in 0.5 M HClO 4 . It is evident from these results that the ORR
on supported Au particles is sensitive to particle size on both substrates. On both
carbon and titania substrates, the reaction is strongly deactivated below a critical
particle size of about 3 nm diameter [Guerin et al., 2006b]. Rotating disk electrode
measurements confirmed for both supports that there was no change in reaction mech-
anism, and mainly an indirect reduction to form hydrogen peroxide was promoted
[Guerin et al., 2006b] on all surfaces. In addition, in the case of the titania-supported
Au, there appears to be a small activation of the reaction as particles reduce in size
between 6 and 3 nm.
Recent studies of Pt supported on titania synthesized using the high throughput
method on arrays have also revealed a particle size dependence of the ORR
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