Size Effects in Electrocatalysis of Fuel Cell Reactions on Supported Metal Nanoparticles - Fuel Cell Catalysis: A Surface Science Approach - page 536

value previously found for single-crystalline surfaces at a solid/gas interface
[Kobayashi et al., 2005]. The reasons for the lower diffusion coefficient at the
solid/electrolyte compared with the solid/gas interface may be manifold, and include
anion effects, influence of the solvent, and influence of the surface charge density.
Therefore, it would be of interest to explore the influence of anion-specific adsorption
on CO ads electro-oxidation kinetics [Maillard et al., 2004a; Arenz et al., 2005].
The inferred influence of particle size on D CO is in agreement with the 13 CNMR
study by Becerra and co-workers, who reported a considerable increase in the acti-
vation energy for CO ads diffusion with decreasing size of Pt particles supported on
Al 2 O 3 [Becerra et al., 1993]. They explained their data by a model in which surface
diffusion requires the formation of bridge-bonded CO molecules, and by a decrease
in the fraction of the latter with decreasing particle size. A reduced contribution of
bridge-bonded and multiply bonded CO with decreasing size of Pt particles is
supported by FTIR studies [Park et al., 2001]. It should be mentioned, however,
that quantitative determination of the surface coverages of bridge- and atop-bonded
CO molecules from FTIR spectra obtained at high surface coverages is obstructed
by dipole - dipole coupling of the adsorbates.
While size effects on the kinetics of CO ads monolayer electro-oxidation are unequi-
vocal, their origin is still not fully understood. Takasu and co-workers discussed the
changes induced by particle size on particles' electronic properties and on CO bonding
by exploring the UPS spectra of CO over Pd/graphite model catalysts [Takasu et al.,
1984]. Unfortunately, they did not make a distinction between the initial and final state
effects, and thus the validity of their conclusions may be questioned. Mukerjee and
McBreen discussed the influence of d-band vacancies on the strength of adsorbate
bonding [Mukerjee and McBreen, 1998]. It is likely that both geometric (the pro-
portion of low coordinated sites and the types of surface sites) and electronic proper-
ties of Pt nanoparticles change as their size drops below about 4 - 5 nm resulting in a
stronger CO interaction with the surface, increased irreversibility of surface oxidation,
a decrease in the OH ads þ CO ads recombination rate constant, and a strong decrease in
CO ads surface mobility. An increased propensity of small Pt particles to stabilize dense
CO ads islands may also have a negative effect on electro-oxidation kinetics.
An influence of particle size on the kinetics of CO ads electro-oxidation has been
shown by Maillard and co-workers with FTIR spectroscopy. It has been suggested
that the reaction starts on the terraces of “large” ( 3 nm) particles, and then propagates
to the particle edges. Electro-oxidation of CO ads on small (,2 nm) particles
commences at more positive potentials, when CO ads on large particles is oxidized.

15.5.3.2 Formic Acid Oxidation (FOR)

Formic acid electro-oxidation is a

two-electron electrochemical reaction,

HCOOH ! CO 2 þ 2H þ þ 2e (15 : 20)

which on Pt electrodes is known to occur via a so-called “dual pathway” mechanism
involving (i) “dehydrogenation” and (ii) dehydration pathways [Markovic and Ross,
2002]. The former involves the formation of adsorbed CO, which acts as a catalytic