Broadband Sum Frequency Generation Studies of Surface Intermediates Involved in Fuel Cell Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 396

2002]: as soon as the barrier is crossed, CO oxidation occurs at a high rate, that is, at
low potentials (Fig. 12.18). In support of these concepts, one should notice that the top
of the Ru deposit on Pt is always metallic, and is available for CO chemisorption on all
Pt and Ru sites.
The following are now expected: first, in the CO oxidation electrode potential range
on Pt/Ru surfaces (i.e., at low potentials), entrapped oxygen should be found; second,
the entrapped (subsurface) oxygen concentration between the Ru layers should
increase with increasing multilayer character (and coverage) of the Ru deposit on
Pt. Research focusing on these two issues is planned.
12.6 CONCLUSIONS
We have shown that BB-SFG provides a unique ability to probe the detailed behavior
and kinetics of electrochemical transformations without compromising the conditions
needed for high quality electrochemistry. The high sensitivity and interface selectivity
of SFG allows for the use of an electrolyte layer of 25 mmupto5mV/s, and permits
fast kinetic measurements by improving the response time to applied potential transi-
ents into the millisecond range, and by allowing rapid flow of reagents. Since the
penalties for propagating IR pulses through such a thin layer are small, transparent
electrodes are not needed, and well-defined single-crystal electrodes have been
used, based upon low index Pt(hkl) substrates, including nanostructured mixed
metal electrodes.
Information obtained by this method and reviewed above concerns:
† the coincidence between the IR BB-SFG spectra and the electrochemical beha-
vior of CO chemisorbed on polycrystalline Pt when electro-oxidized to pure
supporting electrolyte;
† evidence that the SFG observables are sensitive to the phase transition of CO
adsorbed on a Pt(111) single-crystal electrode;
† coincidence between vibrational spectra and voltammetry obtained from CO-
covered Pt (111)/Ru surfaces, including the split voltammetric behavior and
all CO chemisorbed forms.
We have also investigated adsorption of acetate on a polycrystalline Pt electrode,
which is of interest in the context of catalysts for ethanol oxidation, where acetate is
an unwanted oxidation intermediate, and have demonstrated the capacity of the BB-
SFG method to examine Pt nanoparticles.
This chapter has provided a quantitative link between the split voltammogram
obtained from CO-covered Pt(111)/Ru surfaces and the IR spectra obtained from
such surfaces. In particular, an interpretation has been given of the disappearance of
the voltammetric split from stripping CO when the Ru coverage becomes higher
than 0.25 ML. We have pointed out that the Ru deposits on Pt and the Pt surface
sites are always available for CO adsorption, showing that neither the Ru sites nor
the Pt sites are covered by oxygen, disregarding the Ru coverage. The physical