Recent Developments in the Electrocatalysis of the O Reduction Reaction - Fuel Cell Catalysis: A Surface Science Approach - page 285

Figure 9.12 Binding energies of O versus d-band center (relative to the Fermi level, 1 d - 1 F )
of the Pt and Pt 3 Co alloy surfaces. Labels identify the adsorption sites. The line is the best linear
fit. (Reproduced with permission from Xu et al. [2004].)
entail higher ORR activity; subsequent events (i.e., the hydrogenation/protonation of
oxygen) likely play an important role as well.
9.3.2 Platinum Monolayer Electrocatalysts
The successful use of Pt sub-monolayer on Ru as the electrocatalyst for hydrogen oxi-
dation, with high stability and high CO tolerance, led to extension of the monolayer
catalyst concept to the ORR [Brankovic et al., 2001a, b; Sasaki et al., 2003]. The Pt
monolayers are formed on other pure metals or alloys. The electronic structure of Pt
skins could be altered significantly upon bonding with the substrate metal [Hammer
and Nørskov, 2000; Rodriguez, 1996]. In addition to the possibility of tailoring the cat-
alytic properties of the resultant Pt surface [Kibler et al., 2005], this approach also
allows for a substantial reduction in Pt loading. The catalytic properties of bimetallic
surfaces consisting of metal monolayers on single-crystal metal surfaces have been
extensively studied in UHV [Rodriguez and Goodman, 1992], but to a lesser extent
in electrochemical systems [Adzic, 2002; Baldauf and Kolb, 1996; Naohara et al.,
2000; Schmidt et al., 2002]. Using the technique of displacing the metal adlayer depos-
ited at underpotentials [Brankovic et al., 2001a], Zhang and co-workers prepared Pt
monolayers on Ru(0001), Ir(111), Rh(111), Au(111), and Pd(111) single crystals
and studied their ORR activity using rotating disk measurements and DFT calculations
[Zhang et al., 2005b].
The experiments showed that the properties of the Pt monolayer were modified dif-
ferently by the different supporting metal (Fig. 9.13). Pt ML /Pd(111) and Pt ML /
Ru(0001) are the most and least active of these surfaces, respectively. When the
ORR kinetic currents obtained from Koutecky - Levich plots are plotted against the