Environmental Engineering Reference
In-Depth Information
20.02 V on bare Ru(0001) (Figs. 14.2 and 14.5f ) to
þ
0.05 V on Pt-modified
Ru(0001) (Fig. 14.5 g - i) shows that the Pt monolayer islands act as catalyst for the
replacement process, as described above.
In the positive-going scan, the Pt monolayer islands also catalyze the H
upd
!
OH
ad
exchange reaction, reducing the onset potential of peak A from 0.15 V on Pt-free
Ru(0001) to 0.1 V in the presence of the Pt islands. At these potentials positive of
the equilibrium potential for H
upd
$
OH
ad
exchange, H
upd
removal is driven by the
energy gain due to OH
ad
adsorption. On the Pt island-modified surface, the mechan-
ism for exchange of H
upd
by OH
ad
must be such that the Pt islands can act as catalysts.
This makes a heterolytic process as described by (14.3) and (14.4) unlikely, since
(14.3), which controls the onset of the exchange reaction, should not be affected by
the presence of the Pt monolayer islands. A catalytic acceleration of the exchange pro-
cess is possible, however, if the exchange is initiated by spillover of (small amounts
of ) H
upd
from the Ru(0001) areas to Pt islands, (14.6), where it becomes oxidized,
(14.7), in combination with dissociative H
2
O adsorption/OH
ad
formation at the inter-
face between the Ru substrate and the Pt islands, (8):
H
upd
(Ru)
þ
A(Pt)
!
H
upd
(Pt)
þ
A(Ru)
(14
:
6)
H
upd
(Pt)
þ
H
2
O
!
A(Pt)
þ
H
3
O
þ
þ
e
(14
:
7)
A(Ru)
þ
A(Pt)
þ
H
2
O
!
OH
ad
(Ru)
þ
H
upd
(Pt)
(14
:
8)
Neither reaction step/sequence is hindered by the dense H adlayer on the Ru(0001)
areas, and can therefore proceed faster than without Pt. The dissociation of water is
supported by the higher binding energy of the H
upd
species at the edge of the Pt mono-
layer islands compared with H
upd
adsorption on the Pt islands [Hammer et al., 1997].
Furthermore, the spillover of H
upd
from the Ru(0001) terraces to the Pt islands, (14.5),
which on first view is in contrast to the much higher adsorption energy of H
upd
on the
Ru(0001) substrate than on the Pt monolayer islands, is facilitated by the high (local)
H
upd
coverage (the adlayer on the Ru areas is essentially saturated throughout the
exchange process, until complete replacement of H
upd
by OH
ad
is reached), which
reduces the H
upd
bond strength on the Ru areas owing to repulsive interactions.
Finally, it is important to note that the ongoing reaction requires mobility of the
OH
ad
and H
upd
species in the adlayer on the Ru areas.
At higher potentials, positive of the H
upd
$
OH
ad
exchange, the CVs of the Pt island-
modified Ru(0001) surface closely resemble those of the Pt-free Ru(0001) electrode,
except for the lower currents/charges in the characteristic features. This simply reflects
the fact that at these potentials, the surface reactivity is dominated by the electrochemical
properties of the remaining exposed Ru surface. As already mentioned, the Pt monolayer
islands themselves contribute only little to the voltammetric behavior, which is due to the
weak bonding and hence low adsorbate coverages on these islands.
14.3.1.3 Pt
x
Ru
1
2
x
Surface Alloys on Ru(0001)
Surface Structure and Adsorption Properties at the Solid
/
Vacuum
Interface
Heating Ru(0001) surfaces covered by submonolayer amounts of Pt
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