Mechanisms of the Oxidation of Carbon Monoxide and Small Organic Molecules at Metal Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 173

Figure 6.11 Stripping chronoamperometric current transients on stepped Rh single-crystal
surfaces. (a) Full transients obtained on the four different surfaces at E ¼ 0.65 V (with respect
to an RHE), in 0.5 M H 2 SO 4 . (b) Initial parts of the transients in (a).
absent on Rh(111). Our interpretation of these transients is that the pre-peak
corresponds to the oxidation of CO at or near the step sites, and that the main peak
corresponds to CO oxidation on terraces. The fact that these processes can be observed
separately suggests a low mobility of CO on the Rh(111) terraces. Kinetic Monte Carlo
simulations of a model incorporating these different reactions indeed show that in the
limit of low CO mobility, two separate peaks can be observed in the
chronoamperometry, which behave qualitatively similar to the experimental transients
[Housmans et al., 2007].
In a later communication [Housmans and Koper, 2005b], it was shown that the low
CO mobility on Rh(111) is not an intrinsic feature of this surface, but is strongly influ-
enced by the co-adsorbing anion. Whereas the experiments above were carried out in
H 2 SO 4 , the same experiments in HClO 4 showed a very different behavior. CO oxi-
dation on Rh in HClO 4 looks qualitatively similar to CO oxidation on Pt in H 2 SO 4
or HClO 4 . The chronoamperometry shows only one peak in HClO 4 , in contrast
to the two features observed in H 2 SO 4 . The difference is most clearly illustrated by
comparing the CO stripping voltammetry on a series of Rh stepped single crystals
Figure 6.12 Stripping voltammetry of a saturated monolayer of adsorbed CO on Rh(111),
Rh(554), and Rh(553) in (a) 0.5 M H 2 SO 4 and (b) 0.1 M HClO 4 ; scan rate 20 mV/s.