Support and Particle Size Effects in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 574

Figure 16.11 Titania (TiO x )-supported (a) and niobium-doped titania (Nb 0.08 Ti 0.92 O x /Au)-
supported (b) Au particle-size-dependent specific activities for—CO oxidation at a potential
step of 0.5 V vs. RHE. The current densities have been corrected for the TEM-determined
surface areas.
2007a]. It is evident from the particle size dependence in Fig. 16.10, however, that at
particle sizes below 3 nm, the activation shows a sharp fall with decreasing particle
size. This result is similar to those obtained for the particle size dependence of the
ORR on carbon-supported Au, which is also characterized by a sharp de-activation
of the reaction for nanoparticles smaller than 3 nm at 0.4 and 0.3 V vs. RHE
[Guerin et al., 2006b].
Figure 16.11a shows the results of chronoamperometric measurements on TiO x /
Au electrodes, where the potential was stepped from initially 0 V vs. RHE in 50
mV steps to 0.5 V. The results shown are for the real surface-area-corrected current
densities at 0.5 V vs. RHE. A similar particle size dependence is also observed at
0.3 V vs. RHE [Hayden et al., 2007c]. In the case of the titania support, the conduc-
tivity of the oxide derives from the oxygen defects incorporated in the bulk during
synthesis [Guerin et al., 2006c]. Since the stability of such substrates in oxidative con-
ditions may be poor, with oxidation leading to the formation of a more stoichiometric
material with low conductivity, experiments have been carried out on niobium-
doped titania, which provides materials with a dopant-induced conductivity, which
should lead to greater stability. Figure 16.11b shows the results of CO electro-
oxidation on niobium-doped titania-supported Au nanoparticle (Nb 0.08 Ti 0.92 O x /Au)
electrodes. The measurements were made under identical conditions to those shown
for titania-supported Au in Fig. 16.11a. Both the doped and substoichiometric tita-
nia-supported Au nanoparticles exhibit a maximum in the activity for CO oxidation
at a particle size of 3 nm. At a potential of 0.5 V vs. RHE, the carbon-supported
nanoparticles show no activity. This particle-dependent behavior (an increase in
addition to a decrease in specific activity), taken together with the strong lowering
of the overpotential for the reaction, is strong evidence for a support-induced modifi-
cation of Au activity.