Broadband Sum Frequency Generation Studies of Surface Intermediates Involved in Fuel Cell Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 392

The BB-SFG thin-layer cell (Fig. 12.2) has the ideal configuration to test the above
prediction, as both vibrational amplitudes and voltammetric currents can be measured
simultaneously (with no IR drop penalty). The matching (to the voltammetric data
shown in Fig. 12.18) SFG spectra are shown in Figs. 12.19 and 12.20 (for details
of spectral assignments of Pt-CO and Ru-CO, see [Friedrich et al., 1996]; as well as
[Villegas and Weaver, 1994; Leung et al., 1988] and [Gutierrez et al., 1991; Oda
et al., 1993], respectively). The data demonstrate that the correct correlation between
spectra and voltammetry has been reached. From the data in Figs. 12.18 and 12.19
alone, one may conclude that the electrochemical and spectroscopic data for Ru-CO
match ideally; that is, there is complete disappearance of the CO amplitude from
Ru at about 0.24 V (Fig. 12.19), correlating with the saddle point of the cyclic volta-
mmogram in Fig. 12.18 (with the common sense deconvolution of the CO stripping
peaks shown in Fig. 12.18). However, there is still a discrepancy between the IR
amplitudes (Fig. 12.19) and the voltammetric stripping (Fig. 12.18) for atop CO on
the Pt sites: instead of the amplitude disappearing near 0.30 V, as expected from
Fig. 12.18, it disappears at a more negative potential, nearly at 0.26 V (Fig. 12.19).
In order to reconcile the inconsistency, we analyzed the spectral disappearance of
all CO surface forms—atop, bridge, and 3-fold—as a function of electrode potential
(Fig. 12.20). While the spectra are noisy, the bridge-bonded CO survives at the Pt
Figure 12.19 Potential-dependent SFG spectra from atop CO on a Pt(111)/Ru electrode in
0.1 M H 2 SO 4 at 1 mV/s in 0.1 M H 2 SO 4 (see Fig. 12.18). The scan potential for each spectrum
is shown on the right. Spectra from Ru-Pt(111)-CO and Pt(111)/Ru-CO are shown, including
the CO phase transition at 0.17 V (vs. Ag/AgCl). The inset shows a blow-up of the SFG spec-
trum from Pt(111)/Ru-CO at 0.17 V.