Recent Developments in the Electrocatalysis of the O Reduction Reaction - Fuel Cell Catalysis: A Surface Science Approach - page 276

Figure 9.6 Visual representation of the platinum oxide growth mechanism. (a) Interaction of
H 2 O molecules with the Pt electrode occurring in the 0.27 V E 0.85 V range. (b) Discharge
of 2 ML of H 2 O molecules and formation of 2 ML of chemisorbed oxygen (O chem ). (c) Discharge
of the second 2 ML of H 2 O molecules; the process is accompanied by the development of repul-
sive interactions between (Pt - Pt) d þ -O chem

d2 surface species that stimulate an interfacial place
exchange of O chem and Pt surface atoms. (d) Quasi-3D surface PtO lattice, comprising Pt 2 þ
and O 22 moieties, that forms through the place-exchange process. (Reproduced with permission
from Jerkiewicz et al. [2004].)

1

indicated that a

2 ML of chemisorbed O ads , instead of OH ads , was formed at 0.85 -
1.15-V through the discharge of H 2 O. Further discharge of H 2 O results in the for-
mation of the second

1 1

2 ML of O ads undergoing place-
exchange with interfacial Pt atoms to form a quasi-3D near-surface Pt - O lattice
[Jerkiewicz et al., 2004], as shown in Fig. 9.6. You and co-workers found that only
0.3 ML of Pt atoms exchanged places with oxygen-containing species [Nagy and
You, 2002; You et al., 2000]. The place-exchange mechanism exposes Pt to the elec-
trolyte and allows further oxidation and dissolution [Wang XP et al., 2006].
Furthermore, Nagy and You showed that PtO is mobile and can diffuse to energeti-
cally favorable sites on the Pt surface, further exposing underlying Pt atoms to the elec-
trolyte [Nagy and You, 2002]. At higher potentials, the PtO film can be oxidized to
PtO 2 , which is also mobile [You et al., 2000]. This was studied using surface X-ray
scattering (SXS) by You and co-workers, who showed that the reversible oxidation
involved a place-exchange mechanism up to 1.25 V, while irreversible oxidation
(restructuring) occurred above that potential [You et al., 1994].
OH adsorption on Ru is a key factor that makes this metal the major component of
various bimetallic catalysts for anode reactions. Ru - OH causes a significant inhibition
of the ORR [Inoue et al., 2002]. In situ SXS data for the oxidation of Ru(0001) in acid

2 ML of O ads , with the first