Environmental Engineering Reference
In-Depth Information
16.7 INTRINSIC PARTICLE SIZE EFFECTS AND SUBSTRATE
MODIFICATIONS
A number of observations can be made concerning the particle size and support depen-
dences of the reactions described above:
†
Oxygen reduction on both carbon- and titania-supported Pt particles is depen-
dent on particle size. A deactivation of the catalytic activity is observed for
decreasing particle size on both supports. In addition, there is no evidence of
any activation of the Pt above that of bulk Pt on either support.
†
Oxygen reduction on both carbon- and titania-supported Au exhibits a similar
dependence on particle size to that observed for Pt, namely, a decrease in
activity with decreasing particle size. This decrease occurs at particle sizes
below about 3 nm. In addition to the decrease in activity, a small increase in
activity is also observed for titania-supported Au nanoparticles.
†
CO electro-oxidation exhibits a strong particle size dependence on both carbon-
and titania-supported Au catalysts: a strong deactivation of the reaction is
observed for particle sizes below about 3 nm. In the case of the titania supports,
however, a distinct activation of the reaction is also evident. This manifests itself
in a strong decrease in the overpotential for the reaction, and an increase in
activity as the particle size decreases in the range 8 - 3 nm. The result is a maxi-
mum in the catalytic activity with particle size.
The results can be understood in terms of the influence of an intrinsic particle size
effect (independent of the support) and a support-induced particle size effect. For both
reactions and both supported metals, the intrinsic effect manifests itself in a decrease in
activity with decreasing particle size below about 3 nm.
The deactivation of the CO electro-oxidation reaction [Hayden et al., 2007a, c], and
the deactivation of the ORR [Guerin et al., 2006b] on both carbon- and titania-sup-
ported Au for particle sizes of 3 nm and below, have led to the suggestion that there
is a common origin to the effect, and the insensitivity to the support indicates that it
is an intrinsic particle size effect. For one of these reactions, namely, oxygen
reduction, it appears in addition that the reaction is also deactivated on small Pt par-
ticles supported both on carbon [Guerin et al., 2004] and on titania [Hayden et al.,
2009]. An explanation for the latter that has been proposed is the increased adsorption
energy of oxygen or OH on small particles, resulting in an inhibition of oxygen
reduction. This is qualitatively consistent with the optimal position of extended Pt
in the theoretically predicted volcano curve for oxygen reduction [Nørskov et al.,
2004]. In the case of Au, however, it is the weak binding of O or OH that is correlated
with the low activity for oxygen reduction, and one may expect qualitatively that an
increasing binding energy on small particles of Au may result in increasing oxygen
reduction activity. If the adsorption energy of oxidizing surface intermediates were
increased sufficiently strongly on Au that small particles were irreversibly oxidized,
this could account for deactivation. There is, however, no evidence that this is the
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