Size Effects in Electrocatalysis of Fuel Cell Reactions on Supported Metal Nanoparticles - Fuel Cell Catalysis: A Surface Science Approach - page 523

edges, but with increasing coverage, also occupies particle facets [Maillard et al.,
2004b]. Formation of dense CO ads islands is favored on nanoparticles compared
with extended surfaces [Maillard et al., 2004b].

15.5 INFLUENCE OF PARTICLE SIZE ON ELECTROCATALYTIC
ACTIVITIES: CASE STUDIES

15.5.1 Hydrogen Oxidation Reaction (HOR)

Reactions occurring at the hydrogen electrode were the first electrochemical
reactions to be investigated, and ever since they have been studied as model reactions
in electrochemistry;

2H þ þ 2e ! H 2 (15 : 11)

The hydrogen evolution reaction (HER) is important for water electrolysis, while the
hydrogen oxidation reaction (HOR) occurs at the anode of a PEMFC. The HER and
the HOR as well as related processes of hydrogen adsorption have been reviewed
comprehensively (see, e.g., Frumkin [1961, 1963]; Breiter [2003]; Markovic and
Ross [2002]; Conway [1999]; Conway and Tilak [2002]; Jerkiewicz [1998]; Vetter
[1961]; Petrii and Tsirlina [1994]; Kita [2003] and references therein), and they are,
perhaps, the most studied electrocatalytic reactions, owing to their relatively simple
mechanisms and their practical importance. Investigation of the HOR, especially
with very active Pt catalysts, is hampered by mass transport limitations. Several exper-
imental approaches have been used for Pt electrodes in order to extract kinetic infor-
mation: RDE [Frumkin and Aykazjian, 1955; Breiter and Clamroth, 1954],
electrochemical impedance spectroscopy (EIS) [Conway and Tilak, 2002; Safonov
et al., 1975], S-shaped RDEs [Bagotsky and Osetrova, 1973], and reaction on a surface
partially blocked with adsorbed CO [Breiter, 1975]. The HOR/HER is believed to
occur in a sequence of three elementary steps: the Tafel (T) (15.12), Volmer (V)
(15.13), and Heyrovski (H) (15.14) reactions:

H 2 þ 2 ! 2H ads (15 : 12)

H ads ! þ H þ þ e (15 : 13)

H 2 þ ! H ads þ H þ þ e (15 : 14)

where stands for a free site on the electrode surface. The overall process occurs via
Tafel - Volmer (T - V) and/or Heyrovski - Volmer (H - V) mechanisms [Vetter, 1961;
Conway and Tilak, 2002; Breiter, 2003].
Although the HOR on Pt has been studied very extensively [Markovic and
Ross, 2002; Kita, 2003; Safonov et al., 1975; Bagotsky and Osetrova, 1973;
Breiter, 1975; Chen and Kucernak, 2004a, b; Markovic et al., 1997b; Seto et al.,
1987; Kita et al., 1992; Croissant et al., 1998], there are several disagreements in
the literature concerning the values of the exchange current density, the detailed