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Figure 9.17 (a) Calculated most stable configurations for OH or OH รพ O( 2 ML total cover-
age) on seven different (Pt 3 M) ML /Pd(111) surfaces and on a Pt ML /Pd(111) surface, which is
used as reference for the data in (b). In the left and middle panes, A is the attractive (when nega-
tive) or repulsive (when positive) interaction between two OH groups in the unit cell, referenced
to the binding energy of OH at 4 ML coverage; B is the same quantity, but referenced to the cor-
responding value on Pt ML /Pd(111). All energies are in eV. In the right panel, A is the repulsive
interaction between adsorbed O and OH; B is the the same quantity rescaled to the repulsion
between two OH groups on the Pt ML /Pd(111) surface. (b) Measured kinetic current densities
at 0.80 V as a function of the calculated repulsion energy between two OH groups or between
OH and O in a (2 2) unit cell. (Reproduced with permission from Zhang et al. [2005b].)
(Pt 0.8 Re 0.2 ) ML /Pd/C. Although comparing the activity of the Pt/C electrocatalyst
with an average particle size of 3.1 nm with that of Pt ML /Pd/C electrocatalyst of 9
nm [Zhang et al., 2004] does not provide an adequate activity assessment because
of their different surface areas, this does not adversely affect the main conclusions
derived from experiments on single-crystal surfaces.
The ORR activity of Pt ML /Pd/C is much higher compared to that of Pd/C, but,
more importantly, it is indeed also higher than that of Pt/C. The amounts of Pt in
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