Size Effects in Electrocatalysis of Fuel Cell Reactions on Supported Metal Nanoparticles - Fuel Cell Catalysis: A Surface Science Approach - page 535

[McCallum and Pletcher, 1976] and single-crystalline [Love and Lipkowski, 1988;
Koper et al., 1998; Lebedeva et al., 2002b] Pt were observed. For extended surfaces,
the transient shape was explained by the L - H mechanism [Koper et al., 1998] or the
L - H

mechanism complicated with nucleation and growth of OH ads islands

[McCallum and Pletcher, 1976; Love and Lipkowski, 1988].
Upon decreasing the particle size from above 5 nm to 1.8 nm, CO ads electro-
oxidation kinetics slows down, as manifested by the increase in the time of the current
maxima and the decrease in their amplitude, concomitant with a pronounced current
tailing on the descending slope of the transient, strongly deviating from the transients
reported for extended surfaces [Andreaus et al., 2006; Arenz et al., 2005; Kobayashi
et al., 2005; Maillard et al., 2004a, 2007b]. Most remarkable are the size-dependent
changes in the shape of the transient (Fig. 15.9), which suggest an interplay of
electrochemical and nonelectrochemical steps. Detailed experimental investigations
of the current transients and stripping voltammograms as functions of particle size
and experimental conditions (electrode potential, sweep rate, CO ads coverage, etc.),
together with kinetic modeling, have provided important information concerning
the mechanism of CO monolayer electro-oxidation on carbon-supported Pt nanopar-
ticles, which can be summarized as follows [Andreaus et al., 2006]:

† Water splitting (15.18) is restricted to active sites, whose proportion on the
surface does not correlate with the fraction of edge sites and vertices. Its potential
dependence follows a Tafel law with transfer coefficient a 0.5 and a particle-
size-independent equilibrium potential E e H 2 O = OH 0 : 67 0 : 69 V vs. RHE.
† CO ads surface mobility is essential for the electro-oxidation reaction, with a dif-
fusion coefficient that depends strongly on particle size. For “large” (.5 nm)
particles with multigrained structure, diffusion is fast compared with OH ads for-
mation and CO ads þ OH ads recombination, and does not impose limitations on
the reaction kinetics. The lower limit for D CO for “large” particles was estimated
as over 10 214 cm 2 s 21 . Hence, relatively symmetric current transients are
observed. As the particle size is decreased to 1.8 nm, CO ads diffusivity drops
by at least two orders of magnitude to about 10 216 cm 2 s 21 . Thus, the slow cur-
rent decay for small particles in the whole potential interval studied and for
intermediate size (3.3 nm) particles at high potentials (above about 0.8 V vs.
RHE) can be identified with slow CO ads diffusion to the active sites.
† CO ads þ OH ads recombination occurs in an electrochemical step and follows a
Tafel law with a 0.5, and is particle-size-dependent: when the size decreases
to 1.8 nm, k ox falls by about an order of magnitude.

An important result of this study is the conclusion of a particle-size-dependent
CO ads surface mobility. The value obtained for “large” Pt particles is significantly
smaller than D CO at a solid/gas interface. However, Kobayashi and co-workers,
using solid state NMR, performed measurements of the tracer diffusion coefficient
D CO at the solid/electrolyte interface and for Pt-black particles (about 5 nm grain
size), obtained D CO ¼ 3 : 6 10 13 cm 2 s 1 , which is considerably smaller than the