Recent Developments in the Electrocatalysis of the O Reduction Reaction - Fuel Cell Catalysis: A Surface Science Approach - page 284

can leach out under reaction conditions and that it binds oxygen so strongly, the
stoichiometric Pt 3 Co surface cannot be directly responsible for enhanced

ORR activity.
In contrast to the alloy surface, the Pt skin on Pt 3 Co(111) binds atomic and molecu-
lar oxygen less strongly than pure Pt(111) by about 0.3 eV. Moreover, the activation
barrier for O 2 dissociation is increased by 0.16 eV. A comparison of the Pt skin with
the 2% compressed pure Pt surface shows that, in terms of the binding energies of O
and O 2 and the activation energy for O 2 dissociation, the 2% compressed Pt falls
squarely in between the equilibrium Pt and the Pt skin on Pt 3 Co. Thus, the strain
effect and the ligand effect appear to be additive, and each contributes to rendering
the Pt skin less reactive than a normal Pt surface.
The key parameters of the electronic structure of these surfaces are summarized in
Table 9.3. The calculated d-band vacancy of Pt shows no appreciable increase. Instead,
there is a slight charge transfer from Co to Pt, which may be attributable to the differ-
ence in electronegativity of Pt and Co, in apparent contradiction with the substantial
increase in Pt d-band vacancy previously reported [Mukerjee et al., 1995]. What
does change systematically across these surfaces is the d-band center (1 d ) of Pt,
which, as Fig. 9.12 demonstrates, systematically affects the reactivity of the surfaces.
This correlation is consistent with the previous successes [Greeley et al., 2002;
Mavrikakis et al., 1998] of the d-band model in describing the reactivity of various
bimetallic surfaces and the effect of strain. Compressive strain lowers 1 d , which, in
turn, leads to weaker adsorbate - surface interaction, whereas expansive strain has
the opposite effect.
In summary, the Pt skin structure clearly modifies the reactivity of what is otherwise
a pure Pt surface. These findings contradict the original geometric argument that the
Pt - Pt distance needs to contract to be better at activating the O - O bond: a compressed
Pt surface with a shorter Pt - Pt distance is in fact less active for dissociating O 2 . The
ligand effect in this case further destabilizes adsorption and makes O 2 dissociation
more activated. These findings provide seemingly counterintuitive but important
clues for the ORR mechanism: more facile O 2 dissociation does not necessarily

TABLE 9.3 The Electronic Structure of the Pt and Pt 3 Co Alloy
Surfaces. Reprinted with Permission from Xu et al. [2004]

1 d 21 F (eV) s f (%)

Pt(111) 22.52 5.93 93.3

Pt(111) 22% 22.63 6.20 93.2

Pt skin on Pt 3 Co(111) Pt-I 22.58 6.14 93.4

Pt-III 22.79 6.13 93.3

Pt 3 Co(111) Pt 22.69 5.88 93.5

Co 21.45 5.56 79.0

Co(0001) 21.48 5.57 81.3

1 d is the center of the d-band (with respect to the Fermi level 1 F ) or the first
moment of the d-band; s, the spread of the d-band or its second moment; and
f, the filling of the d-band. Co(0001) is included for comparison.