Mechanisms of the Oxidation of Carbon Monoxide and Small Organic Molecules at Metal Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 168

Figure 6.7 Stripping voltammetry of saturated CO adlayers on Pt(111) cooled in a hydrogen -
argon atmosphere (dashed line), leading to a well-defined smooth surface, and on Pt(111)
cooled in air (full line), leading to a less well-defined defect-rich surface; sweep rate 50 mV/s.
effect is easily explained by the steps being the active sites for CO oxidation, and a
rapid mobility of the adsorbed CO on the terraces. In situ Fourier transform infrared
(FTIR) spectroscopy showed that the terrace-bonded CO disappears first from the
infrared spectrum, and the more strongly adsorbed step-bonded CO is oxidized last
[Lebedeva et al., 2002b]. This also suggests that CO is being supplied from the terrace
to the step during the oxidation. The high sensitivity to steps and defects was also
demonstrated by an experiment in which the Pt(111) electrode was cooled down
during the preparation procedure in either air or a hydrogen - argon atmosphere.
The former leads to a less perfect Pt(111) surface, as evidenced by very small but
significant changes in blank voltammetry. However, the CO stripping voltammetry
was strongly different between the two surfaces, as shown in Fig. 6.7, with the air-
cooled less perfect Pt(111) being the more active surface.
Under certain circumstances, the CO stripping profile on single-crystal platinum
exhibits more than one peak. Lebedeva and co-workers showed that multiple stripping
peaks may be observed on Pt(111) and stepped Pt surfaces if the initial CO coverage
is below saturation (which is 0.68 monolayer (ML) on Pt(111) if no CO is present in
solution) [Lebedeva et al., 2000a, b]. The origin of these multiple peaks has remained
somewhat elusive, as they cannot be explained by the simple mean field models dis-
cussed above. A special case that has been studied in some detail is the so-called “pre-
peak,” “pre-wave,” or “pre-ignition” peak that is often observed during CO stripping
from a Pt(111) electrode (as well as other Pt surfaces) [Markovic and Ross, 2002].
L ´ pez-Cudero and co-workers showed that a pre-wave is typically observed before
the main oxidation peak if the CO is dosed onto the Pt(111) electrode while maintaining
the potential during dosing within the hydrogen adsorption region (say 0.1 V vs. RHE)