Support and Particle Size Effects in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 564

inhibits CO oxidation. The literature, however, implies that the mechanism at Au is
similar to that at other precious metals, but adsorbed CO and adsorbed OH species
are present only at very low coverages. This may implicate specific, low coordinate
sites on the Au surface as critical to the oxidation reaction, and also suggests that
the intermediates are both formed and react very rapidly [Burke and Nugent, 1997,
1998; Burke, 2004]. A structure sensitivity of the CO oxidation activity is indeed
evident on single-crystal Au electrodes [Chang et al., 1991; Blizanac et al., 2004b],
with the activity decreasing in the order Au(110) Au(100) . Au(111). This also
supports the notion that defect sites at surfaces may dominate the electro-oxidation
of CO. Recent studies of structurally induced changes in activity on Au(111) single
crystals have also been explained by an increased number of low coordinated sites
during the lifting of the surface reconstruction [Gallagher et al., 2005]. Several
groups have reported studies of CO oxidation in acid solution at single-crystal Au
surfaces [Chang et al., 1990, 1991; Edens et al., 1996; Blizanac et al., 2004a, b],
although the voltammetry is similar to polycrystalline Au, the conclusions are some-
what different. Blizanac and co-workers have provided evidence that oxidation of the
Au surface initiates at potentials as low as þ 0.3 V vs. SHE [Blizanac et al., 2004b],
and have highlighted competition for surface sites between 'OH' and anions of the
electrolyte [Blizanac et al., 2004a]. Also, infrared spectroscopy of the surface confirms
the adsorption of CO, although there is disagreement about the extent of adsorption
[Chang et al., 1990, 1991; Edens et al., 1996; Blizanac et al., 2004a, b]. Weaver
and co-workers concluded that the coverage is never above 0.1 monolayer in acid sol-
ution [Chang et al., 1990, 1991], while Blizanac and co-workers reported almost full
monolayer coverage as well as well-formed CO stripping peaks on cyclic voltammo-
grams [Blizanac et al., 2004b]. The differences between the data at polycrystalline and
single-crystal surfaces are very surprising, and explanation must await further studies.
There have been even fewer studies of CO electro-oxidation on supported Au
particles than on supported Pt. One may expect, however, on the basis of the studies
on extended Au surfaces, that there will be a structure sensitivity to the CO electro-
oxidation reaction on supported Au particles.
16.3 SYNTHESIS AND SCREENING OF MODEL SUPPORTED
ELECTROCATALYSTS
16.3.1 High Throughput Synthesis of Supported Nanoparticles
Electrocatalytic activity of supported metal particles has been investigated on surfaces
prepared in an ultrahigh vacuum (UHV) molecular beam epitaxy system (DCA
Instruments) modified to allow high throughput ( parallel) synthesis of thin-film
materials [Guerin and Hayden, 2006]. The system is shown in Fig. 16.1, and consisted
of two physical vapor deposition (PVD) chambers, a sputtering chamber, and a sur-
face characterization chamber (CC), all interconnected by a transfer chamber (TC).
The entire system was maintained at UHV, with a base pressure of 10 210 mbar.
Sample access was achieved through a load lock, and samples could be transferred