Environmental Engineering Reference
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Owing to their strong bond on Ru(0001), mixed CO
ad
þ
OH
ad
/O
ad
layers are very
stable against CO
2
formation, similar to the coadsorption behavior at the solid/gas
interface discussed above. We assume that, for E . 0.55 V, the shift of the equilibrium
between water and adsorbed OH
ad
/O
ad
towards the latter increases the density of the
respective species in the intermixed adlayer, which increases the repulsions between
the adsorbed species and hence leads to more weakly bound OH
ad
/O
ad
and CO
ad
species. These latter species are less stable against COOH
ad
or CO
2
formation, because
of the reduced reaction barrier (“Brønsted - Polanyi - Evans” relation [Bronstedt,
1928]), and can support a reaction via (14.9) or (14.12), respectively, at low rates.
(Note that the total density of the adlayer does not need to remain constant, although
also this is possible.)
14.3.2.2 Pt Monolayer Island-Modified Ru(0001)
The presence of Pt
monolayer islands results in a dramatic increase in CO oxidation activity compared
with a Pt-free Ru(0001) electrode. This is illustrated by potentiodynamic CO oxidation
scans recorded for Ru(0001) surfaces modified by 0.05, 0.23, and 0.9 ML of Pt mono-
layer islands (Fig. 14.10). Already, 0.05 ML Pt (dashed line in Fig. 14.10) is sufficient
to reach at least half of the mass-transport-limited current obtained for the electrodes
with higher Pt coverages, which is more than one order of magnitude higher than the
maximum oxidation current obtained on a Pt-free surface under similar conditions.
The general shape of the scans is comparable for the three different Pt coverages.
Similar to the behavior of the Pt-free surface, CO oxidation starts at about 0.55 V in
the positive-going scan. The current first rises continuously up to a bending point
(G, G
0
,G
00
), and then increases steeply until reaching its maximum value. In the
negative-going scan, the current decreases slightly right after the scan reversal for
the low and medium Pt coverage (0.05 and 0.23 ML Pt) electrodes. For the high Pt cov-
erage sample (0.9 ML Pt), the current remains constant at the mass-transport-limited
value (0.9 ML Pt, dash - dotted curve in Fig 14.10). Starting at E
0.9 V (0.05 and
0.23 ML Pt) or E
0.82 V (0.9 ML Pt), it then decreases continuously, until reaching
zero at about 0.55 V. With increasing amount of Pt, the maximum current grows con-
tinuously, and appears at lower potentials. For 0.9 ML Pt, the mass-transport-limited
current is reached at E
0.9 V. Also, the increase in slope (points G, G
0
,G
00
) shifts
to lower potentials with higher Pt coverage. In the negative-going scan, the activity
is at least equal or even higher than in the positive-going scan at the same potential.
Therefore, we arrive at the same conclusion for the mechanism of CO
ad
oxidation in
the lower potential regime as for Pt-free Ru(0001), postulating that at potentials E ,
0.55 V, only strongly bound OH
ad
/O
ad
species are present in the mixed CO
ad
þ
OH
ad
/O
ad
adlayer, which are not reactive towards CO
2
formation, while for E
0.55 V, additional, weakly adsorbed OH
ad
/O
ad
species are formed, which can react
with the (likewise destabilized) CO
ad
. Similar to CO
ad
oxidation on a Ru(0001) sur-
face, the reaction starts by dissociative adsorption of H
2
O on the Ru(0001) surface
(no shift in the onset potential). In this case, however, the Pt islands can accelerate
the reaction by accepting the H
upd
resulting from a homolytic dissociation process.
Thus, we tentatively propose a mechanism for CO oxidation at potentials between
the reaction onset up to the bending point (see also Lin et al. [1999]), which is
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