Broadband Sum Frequency Generation Studies of Surface Intermediates Involved in Fuel Cell Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 379

2000; Corrigan and Weaver, 1998; Kunimatsu et al., 1985a, b; Korzeniewski et al.,
1986; Tian et al., 1997]. At the onset of chemisorbed CO oxidation, the CO frequency
evidences the anomalous Stark tuning behavior noted by Ross and Markovic and co-
workers (Fig. 12.8), who associated it with compression of CO islands by adsorbed
anions [Stamenkovic et al., 2005].
As mentioned above, careful comparison of the oxidation current and the SFG
signal with a CO-free electrolyte (Fig. 12.6) showed a precise correspondence, so
that SFG was shown for the first time to be capable of quantitative measurements of
CO coverage [Lu et al., 2005]. In particular, we did not see the remarkable features
reported in previous work on Pt electrodes. Baldelli et al. [1999] reported that the
SFG signal vanished 200 mV below the oxidation threshold, which was interpreted
as resulting from the electrochemical generation of an SFG-invisible CO adsorption
state. Dederichs et al. [2000] also reported CO signal disappearance prior to the CO
oxidation threshold, which was attributed to CO depletion in the quite thin electrolyte
layer. Chou et al. [2003a, b] reported that the SFG signal intensity vanished immedi-
ately after the oxidation current reached its maximum. Instead, Fig. 12.6 shows that the
SFG intensity perfectly tracks the oxidation current. However, when the electrolyte
was CO-saturated, the charge and the SFG signal were no longer in correspondence.
(The charge builds up to a considerable extent before the SFG signal starts to drop at
0.44 V; Fig. 12.7.) The current and charge are sensitive to the total number of CO
equivalents oxidized in the electrolyte and on the Pt surface, whereas the SFG
signal is sensitive to the CO stripping process, which reduces the CO surface coverage.
Consequently, with the CO-saturated electrolyte, SFG and voltammetry measure
different things.
Figure 12.7 Comparison (using a 0.1 M H 2 SO 4 CO-saturated electrolyte) of the SFG ampli-
tude of the CO stretch (filled circles) with electric charge (open circles). The charge is computed
by numerical integration of the current shown in Fig. 12.8.