Support and Particle Size Effects in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 576

case [Hayden et al., 2007b]. An alternative explanation involving the reduced activity
of the different facets exposed at small particles has also been put forward in the case
of the supported Pt [Kinoshita, 1990]; there may indeed be a similar explanation for
the deactivation of reactivity observed for supported Au. A possible alternative expla-
nation for an intrinsic particle size effect proposed for supported Pt [Maillard et al.,
2004a; Mayrhofer et al., 2005b] and Au [Hayden et al., 2007a, c] electrocatalysts is
that there is a change in electronic properties as a result of quantum size effects
below about 3 nm.
Notwithstanding the origin of the deactivation, it is the superposition of this “intrin-
sic” particle size effect with a particle size dependence of the titania-supported acti-
vation that has been suggested to be the primary origin of the maxima observed in
the CO oxidation reaction [Hayden et al., 2007a, c] and the ORR [Guerin et al.,
2006b] for these catalysts. It should be emphasized that the enhanced activity is
unique to titania-supported Au catalysts. In view of the extensive literature on sup-
port-induced activity for titania-supported Au in gas-phase reactions in particular
for CO oxidation, and the particle size dependence of activity with a maximum at
about 3 nm, it seemed reasonable to seek a common explanation for the effects in
heterogeneous catalysis and electrocatalysis [Hayden et al., 2007a, c]. In the case of
heterogeneous catalysis of CO oxidation, the promoting supports are metal oxides,
most notably titania [Haruta et al., 1993; Haruta, 1997; Valden et al., 1998; Bond
and Thompson, 2000; Dat´ and Haruta, 2001; Haruta and Dat´, 2001; Choudhary
and Goodman, 2002; Grisel et al., 2002; Davis, 2003; Dat´ et al., 2004; Lopez
et al., 2004a; Chen and Goodman, 2006], and the specific activity also shows a maxi-
mum using nanoparticles with a mean diameter of 3 nm [Haruta et al., 1993; Valden
et al., 1998; Lopez et al., 2004a; Chen and Goodman, 2006]. This suggested that the
underlying mechanisms responsible for the substrate-induced activity, as well as the
deactivation at small particle sizes, of titania-supported Au in heterogeneous catalysis
and electrocatalysis have common origins [Hayden et al., 2007a, c]. It is important to
note, however, that in the electrochemical case, the oxidant is provided through acti-
vation of water (rather than oxygen) as the surface oxidant in a Langmuir -
Hinshelwood reaction. A number of explanations for the titania-induced activity of
Au in heterogeneous catalysis have been put forward, and these have been considered
in the light of electrocatalytic oxidation.
16.7.1 Titania-Induced Morphology of Au
The increased activity may be the result of a higher binding energy (and hence surface
concentration) of reactant molecules, or the lowering of an oxygen dissociation barrier
through a stronger interaction [Grunwaldt et al., 1999; Lopez and Nørskov, 2002;
Lemire et al., 2004; Lopez et al., 2004b]. For example, it was reported that the heat
of adsorption of CO increased substantially at small titania-supported Au particle diam-
eters [Meier and Goodman, 2004]; the heat of adsorption at bulk Au was reported to be
52 kJ mol 21 , while at particles of about 2.5 nm mean diameter, it was 76 kJ mol 21 .
Also, oxygen seems to interact more strongly with small thin Au particles, as evidenced