Environmental Engineering Reference
In-Depth Information
oxidation. Both O
ad
/OH
ad
[Lischka et al., 2007] and CO
ad
[Schlapka, 2003] bind
more strongly to Pt
2ML
/Ru(0001) than to Pt
1ML
/Ru(0001), but less strongly than to
Pt(111). Therefore, OH
ad
formation is expected at lower potentials than on Pt
1ML
/
Ru(0001), but at higher potentials than on Pt(111) [Climent et al., 2006]. Similar
effects occur also for CO adsorption, resulting in an increasingly pronounced surface
blocking by CO
ad
in the order on Pt
1ML
/Ru(0001) , on Pt
2ML
/Ru(0001) , on
Pt(111) , on Ru(0001). Obviously, the stabilization of OH
ad
overcompensates the
increased tendency for CO
ad
surface blocking for the Pt
2ML
/Ru(0001) electrode,
while for the even stronger adsorbing Pt(111) electrode, the onset for CO bulk oxi-
dation shifts upwards again. For the electrodes with low and medium Pt contents
(0.05 ML Pt and 0.23 ML Pt), the potentials of the bending points and the maxima
are about equal.
These arguments can be summarized in the following proposed reaction mechan-
ism for CO oxidation at potentials anodic of the bending point:
A(Pt)
þ
CO
!
CO
ad
(Pt)
(14
:
13)
A(Pt)
þ
H
2
O
!
{OH
ad
(Pt)
þ
H
ad
(Pt)}
!
OH
ad
(Pt)
þ
H
þ
þ
e
[see (14
:
8)]
CO
ad
(Pt)
þ
OH
ad
(Pt)
rds
COOH
ad
(Pt)
þ
A(Pt)
(14
:
14)
COOH
ad
(Pt)
þ
H
2
O
fast
CO
2
þ
A(Pt)
þ
H
3
O
þ
þ
e
(14
:
15)
Because of the lower adsorption energies on the Pt monolayer islands, the steady-state
coverage on the islands is relatively low, and surface blocking plays no role.
14.3.2.3 PtRu
/
Ru(0001) Surface Alloys
Potentiodynamic CO oxidation vol-
tammograms recorded on three Pt
x
Ru
12x
/Ru(0001) surface alloys with different Pt
contents are presented in Fig. 14.12a, b, together with a voltammogram for the
Pt
0.23ML
/Ru(0001) surface that was already shown in Fig. 14.10. At the lowest Pt con-
tent (x ¼ 0.07), the general shape of the j - E trace resembles that observed for the Pt
monolayer island-modified electrodes, but with a smaller slope around the onset
potential at 0.55 V, which only becomes steeper at 0.8 V. After passing through a
maximum at 0.9 V, it steadily decreases up to the anodic potential limit at 1.05 V.
For medium (x ¼ 0.25) and high (x ¼ 0.47) Pt contents, the shape of the j - E
curves is rather similar. For the medium (x ¼ 0.25) Pt content electrode, CO oxidation
starts at the same potential as for the low Pt content surface (x ¼ 0.07), increases
slowly, and then rises steeply at E
0.8 V. For the high Pt content (x ¼ 0.47) surface,
CO oxidation starts already at 0.4 V, and the transition to a steep slope occurs at E
0.75 V. Compared with the low Pt content surface, the steep current increase of the j - E
curve is downshifted by about 0.06 V for x ¼ 0.25 and 0.47. In both cases, the current
reaches a plateau at about 0.85 V, where it slowly decreases again with further increas-
ing potential. At the anodic limit, these electrodes still show significantly higher CO
oxidation currents than are found for the Pt
0.07
Ru
0.93
/Ru(0001) electrode. The current
in the plateau, which varied with the electrolyte flow rate, reflects the transport-limited
continuous CO oxidation current.
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