Mechanisms of the Oxidation of Carbon Monoxide and Small Organic Molecules at Metal Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 167

energy obtained are 106 and 125 kJ/mol for the Pt(111) and Pt(100) electrodes,
respectively, in 0.1 M HClO 4 [Herrero et al., 2000, 2004]. In order to compare
these values with those obtained from DFT calculations [Anderson and Albu,
2000], it is necessary to take into account that the DFT values have been calculated
for the elementary Reaction (6.9), whereas the apparent activation energy measured
with voltammetry is a combination of the activation energies of the reversible
Reaction (6.8) in both directions and the Reaction (6.9). When the values of the
activation energies of the reversible Reaction (6.8) are taken into account, a good agree-
ment is obtained between the values (approximately 100 kJ/mol) [Herrero et al., 2004].
The role of the anions of the solution in the oxidation process was also analyzed.
Although the anions are not explicitly considered in the EC mechanism, they play
an important role, since they compete with OH for the adsorption sites. Therefore,
the presence of strongly adsorbed anions hinders CO oxidation. Thus, the stripping
peak is displaced towards more positive potentials as the adsorption strength of the
anion increases. The stripping peak for CO oxidation on the Pt(111) electrode in
0.5 M H 2 SO 4 solution occurs at potentials about 120 - 150 mV more positive than
in 0.1 M HClO 4 . In the same way, the activation energy is 25 kJ/mol higher. The
differences for the Pt(100) electrode are smaller, since (bi)sulfate adsorption on this
electrode is weaker than on the Pt(111) electrode [Herrero et al., 2004].
The potential of the stripping peak, and hence the activity of the electrode for CO
oxidation, also depends on the platinum surface structure in general and on the step
density in particular. Based on the chronoamperometry experiments described in
Section 6.2.1.1, one would expect the stripping peak to shift to lower potential with
increasing step density. That this is indeed the case is shown in Fig. 6.6. Again, this
Figure 6.6 Stripping voltammetry of saturated CO adlayers on Pt(553) (thin full line), Pt(554)
(dashed line) and Pt(111) (thick full line); sweep rate 50 mV/s.