Support and Particle Size Effects in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 573

densities are similar to those expected for diffusion-controlled oxidation, it is immedi-
ately evident that the voltammograms on the C and TiO x substrates are markedly
different. Most interestingly, the TiO x -supported Au nanoparticles show substantially
higher catalytic activity for CO oxidation at low positive potentials. Indeed, CO
oxidation commences at þ 0.2 V at the TiO x -supported samples, and this compares
with over þ 0.45 V at both of the C-supported nanoparticles. A striking difference
between the two systems under investigation was found at potentials positive to the
onset of Au oxidation. At the C-supported Au nanoparticles, the voltammograms
show features associated with inhibition of CO oxidation by Au oxide formation
and reactivation on the reverse scan at the potential, where Au oxide is reduced. In
contrast, at Au on a TiO x support, inhibition of CO oxidation does not occur at high
anodic potentials. The apparent insensitivity of CO oxidation to anodic potentials
can only be the result of an alternative mechanism of CO oxidation that does not
rely on clean Au sites or modified redox behavior of the titania-supported Au particles
(for further details, see Hayden et al. [2007a, b]).
The voltammograms in Fig. 16.9 clearly show that in both systems the reaction is
strongly dependent on particle size. In order to illustrate this, in Figs. 16.10 and 16.11,
specific activities are plotted against mean particle size for three systems. Figure 16.10
shows the particle size dependence of the CO oxidation activity of the C/Au system
at a fixed potential of 0.8 V, taken from the voltammograms presented in Fig. 16.9.
In the case of carbon-supported Au surfaces, CO electro-oxidation catalysis is
similar to that observed on bulk Au for nanoparticles larger than 3 nm. The voltamme-
try is similar in that the onset of the oxidation takes place at the same overpotential,
and oxidation of the supported Au strongly inhibits the reaction [Hayden et al.,
Figure 16.10 Particle-size-dependent specific activity at 0.8 V vs. RHE for Au nanoparticles
supported on carbon.