Broadband Sum Frequency Generation Studies of Surface Intermediates Involved in Fuel Cell Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 377

(of the order of 1 mm), the scan rate must be kept below a fraction of a millivolt per
second to avoid a significant ohmic drop deformation of the electric current measured
as a function of time. Until recently [Lu et al., 2005; Lagutchev et al., 2006], no data on
the broadband multiplex SFG apparatus [Patterson et al., 2005; Richter et al., 1998]
combined with a TLE cell were reported.
Figures 12.4 - 12.7 show published data from our BB-SFG work on the Pt/CO
system [Lu et al., 2005]. We acquired a time stream of SFG spectra of chemisorbed
CO on a polycrystalline Pt electrode with a 25 mm thick Teflon spacer (Fig. 12.2) in
a 0.1 M H 2 SO 4 electrolyte free of CO (Figs. 12.4 - 12.6). The potential was scanned
at 5 mV/s as spectra were obtained at 5 Hz. The voltammogram is a typical CO strip-
ping voltammogram and is shown in Fig. 12.5. The hydrogen peaks are completely
suppressed, and the CO adlayer is stable from the onset of hydrogen evolution poten-
tial to about 0.4 V (the hydrogen peaks recover on the second scan). The voltammetric
CO oxidation peak begins at 0.42 V and ends at 0.51 V. Using the data in Figs. 12.4 -
12.6 was constructed, and shows that the potential areas of CO stability and oxidation
show excellent fits. The SFG CO amplitude in Fig. 12.6 increases slightly until oxi-
dation begins, whereupon a precipitous drop is seen; the slight deviation between
the charge and SFG amplitude near the end of the oxidation process is attributed to
experimental error. The CO frequency increases with a Stark shift of 28 cm 21 V 21 ,
in good agreement with previous reports [Chou et al., 2003a, b; Dederichs et al.,
Figure 12.4 A series of SFG spectra in the CO stretch region of chemisorbed CO on polycrys-
talline Pt in a CO-free 0.1 M H 2 SO 4 electrolyte. The atop spectra were fit to (12.5) (see text) to
extract the amplitude, frequency, and width [Lu et al., 2005; Lagutchev et al., 2006] (each dis-
played data point is the average of three or five spectra). The electrode potential was swept at a
rate of 5 mV/s, and SFG spectra were obtained every 200 ms. Spectra were obtained at 1 mV
intervals, but, to avoid congestion in the plot, averaged spectra are displayed at 10 mV intervals
in the pre-oxidation region (V , 0.43 V) and at 3.3 mV intervals in the oxidation region (V .
0.43 V) [Lu et al., 2005].