Environmental Engineering Reference
In-Depth Information
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Structures of the (2
2)23CO and (
Figure 12.9
)R23.48213CO adlayers
[Villegas and Weaver, 1994; Tolmachev et al., 2004; Wang et al., 2005; Lucas et al., 1999].
In the (2
2)23CO lattice, every atop site is surrounded by six 3-fold sites at a distance of
1.15d. All atop (coverage 0.25) and 3-fold sites (coverage 0.50) are equivalent. In the slightly
(10%) lower packing density, in the (
19
1
p
)R23.48213CO lattice, a single atop site is
surrounded by six near atop neighbors and six pairs of near-bridge dimers. Each near-atop
site has three nearby near-bridge sites at 1.32d and 1.5d. The atop coverage is 7/19 (0.368)
and the bridge coverage is 6/19 (0.316).
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We were also able to measure how the adlayer transition depended on electrolyte-
dissolved CO concentration, on adsorbing (sulfate) versus nonadsorbing ( perchlorate)
supporting electrolyte, and on potential scan rate. Measurements of phase transition
kinetics are summarized in Fig. 12.12, where the potential was switched back and
forth across the phase boundary repeatedly as atop site SFG spectra were obtained
at 5 Hz. Such kinetic measurements are meaningful only if the electrolyte in the
TLE cell is thick enough that its composition remains constant during surface trans-
formations. Figure 12.12 shows only minimal differences in the kinetics between
the first and second cycles of potential switching, verifying that this condition was
met. As shown in Fig. 12.12, the atop site Stark shift (cf. Fig. 12.10) is observed to
occur instantaneously, implying that a new potential is established on the electrode
in ,0.1 s. With a CO-saturated electrolyte, the forward (2
2)
!
(
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)
transformation occurs with a time constant of a few seconds, whereas the back
(
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1
p
)
!
(2
2) transformation is much faster, occurring in ,0.5 s
(Fig. 12.12a). With CO-unsaturated electrolyte, the forward transformation becomes
a bit faster and the back transformation is a bit slower (Fig. 12.12d) than with a
CO-saturated electrolyte.
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