Environmental Engineering Reference
In-Depth Information
TABLE 9.1 Calculated Parameters for Free O
2
2
Molecule and Adsorbed O
2
2
on Pt(111) at Different Coverages. Reprinted with Permission from Shao et al. [2006b]
d
O-O
(
˚
)
n
O-O
(cm
21
)
ML
BE (eV)
Site
O
2
2
1.27
1123
1/9
20.60
1.39
b-f-t
796
1/9
20.76
1.37
t-b-t
875
O
2
2
/Pt(111)
1/4
20.68
1.36
t-b-t
896
1/2
20.39
1.34
t-b-t
1051
1
.0
1.27
t-b-t
1292
absorption spectroscopy (SEIRAS) in an attenuated total reflection mode (ATR) to
study the ORR on a Pt electrode in alkaline solution [Shao et al., 2006a]. Spectral
and voltammetry data (Fig. 9.2), together with the vibrational frequencies calculated
using DFT (Table 9.1), provide evidence for a superoxide intermediate (O
2
2
) present at
medium to high coverage. Further evidence supporting this assignment includes simi-
lar spectra in acetonitrile solutions and a lack of the n
O-O
signature in the 1200 - 1000
cm
21
range in the absence of O
2
. Gewirth and co-workers, using surface-enhanced
Raman spectroscopy (SERS), assigned a band at 1162 cm
21
to adsorbed HO
2
(per-
oxyl), the superoxide intermediate in acid solutions, on a BiOH-modified Au(111)
surface [Li and Gewirth, 2005].
Experimentally, the activation energy for the thermal dissociation of adsorbed O
2
has been estimated to be about 0.30 eV, which means that O
2
dissociates on Pt(111) at
low temperature. However, this barrier increases significantly with increasing oxygen
coverage. Using the climbing-image nudged elastic band (cNEB) method [Henkelman
and J
´
nsson, 2000; Henkelman et al., 2000] the activation energy for O
2
dissociation
is calculated to be 0.77 eV at
4
ML of O
2
[Xu et al., 2004]. A cartoon of the reaction
sequence is shown in Fig. 9.3. Thus, a high coverage of co-adsorbed O or OH would
substantially hinder O
2
dissociation. The dissociated product, atomic O, prefers to
adsorb in the fcc threefold hollow site, with a binding energy of 23.88 eV on
Pt(111), followed by the hcp hollow, which is 0.4 eV less stable. The binding
Figure 9.3 Cross-section and top views of selected states along the O
2
dissociation path on
Pt(111), from the initial state (t-b-t) to the dissociated product state (fcc
2). The third
image is the transition state. (Reproduced with permission from Xu et al. [2004].)
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