Support and Particle Size Effects in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 577

by an increase in desorption temperature at TiO x /Au surfaces [Bondzie et al., 1999]. In
this regard, there is indeed strong evidence of a surface structural dependence in CO
electrocatalytic oxidation on Au [Blizanac et al., 2004b].
In the case of the electro-oxidation of CO, the increased activity may also be due to
an enhancement in CO coverage, or a lowering of the activation energy for water in the
provision of adsorbed OH. Increased CO binding energy, and hence CO coverages,
would result in the possibility of observing adsorbed CO in CO stripping voltammetry,
as on single-crystal surfaces [Bondzie et al., 1999], and no such stripping peaks on
titania-supported Au could be observed [Hayden et al., 2007a, c]. It is also true to
say that if the increased activity of CO electro-oxidation on titania-supported Au
were a result of modifications in CO adsorption, this would not provide an explanation
for the apparent activation in the ORR [Guerin et al., 2006b]. One cannot so easily rule
out the possibility of the enhancement of water activation, particularly since this may
also influence the ORR. There is no evidence for different morphologies of Au
particles

on carbon and titania supports—at least from TEM characterization

[Guerin et al., 2006c].

16.7.2 Titania-Induced Electronic Modification of Au

The most active Au particle size for CO oxidation was observed where a band gap of
0.2 - 0.6 eV was measured by scanning tunneling spectroscopy (STS); these are
centers about two atomic layers thick and about 3 nm in diameter [Valden et al.,
1998]. In a study of monolayers of Au supported on thin TiO x films, it was shown
that a “bilayer” of Au was the most active structure (and it was suggested that there
is no influence of the perimeter) [Chen and Goodman, 2004]. It is also apparent
that structural and electronic modifications associated with substrate-induced strain
in the Au particle [Mavrikakis et al., 2000; Xu and Mavrikakis, 2003] or charge trans-
fer may contribute to the activity of Au d þ nanoparticles on titania substrates [Sanchez
et al., 1999; Okazawa et al., 2006]. Further evidence for the CO oxidation activity of
positively charged Au þ derives from the apparent activity of partially oxidized Au
clusters [Park and Lee, 1999; Guzman and Gates, 2004], and, in the limit of atomic
dispersion, positively charged Au ions were suggested to be crucial for the activity
of titania-supported Au in the water gas shift reaction, where zero-valent Au atoms
were found to be inactive [Fu et al., 2003].
The activation observed in titania-supported Au electrocatalysts is unlikely to arise
from electronic effects in monolayer or bilayer Au [Valden et al., 1998; Chen and
Goodman, 2004], since the electrocatalytic activity was correlated with the size of
three-dimensional titania-supported Au particles [Guerin et al., 2006b; Hayden
et al., 2007a, c]. The possibility that titania-induced electronic modification of
three-dimensional particles below 6.5 nm is responsible for the induced activity, how-
ever, could not be excluded. It was pointed out, though, that such electronic effects
should dominate for the smaller particle regime (,3 nm), where deactivation of the
Au is observed on all supports.