Mechanisms of the Oxidation of Carbon Monoxide and Small Organic Molecules at Metal Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 174

in H 2 SO 4 with that in HClO 4 (Fig. 6.12). In Fig. 6.12a, the stripping voltammetry in
H 2 SO 4 solution again shows two features: a pre-peak corresponding to CO oxidation
at steps, and a main peak corresponding to CO oxidation at terraces. The potentials of
these peaks do not shift with step density, which suggests that these different parts of
the surface do not communicate and that therefore CO mobility is low. On the other
hand, in HClO 4 (Fig. 6.12b), only a single main CO stripping peak is observed, the
peak potential of which shifts to lower values with increasing step density, very similar
to the situation on Pt (see Fig. 6.6). This latter observation can only be explained if we
allow for a finite CO mobility on the terraces. The conclusion must be that the strong
adsorption of the sulfate anion on Rh hampers CO surface mobility. Sulfate is less
strongly adsorbed on Pt than on Rh, and therefore on Pt the influence of the anion
on CO mobility is not noticeable. As a generic conclusion, the effective surface mobi-
lity of any species in an electrocatalytic reaction should always be considered in terms
of the co-adsorption of other species, especially “spectator” species such as anions.

6.2.3 Carbon Monoxide Oxidation on Gold

In the catalysis community, there is considerable interest in the catalytic properties of
oxide-supported nanocrystalline gold, which has been found to be remarkably active
for the oxidation of CO [Haruta, 1997]. In electrochemistry, the ability of gold to
oxidize CO, in the absence of an oxide support, has been known for many years
[Roberts and Sawyer, 1964].
Weaver and co-workers have carried out extensive studies of CO electro-oxidation
on Au single crystals [Chang et al., 1991; Edens et al., 1996]. Continuous oxidation of
CO on Au starts at potentials where the formation of surface oxides or surface-bonded
hydroxyl (OH) is not apparent from voltammetry. Weaver suggested the following
mechanism:

CO þ ! CO ads (6 : 13)

CO ads þ H 2 O ! COOH ads þ H þ þ e (6 : 14)

COOH ads ! CO 2 þ H þ þ e þ (6 : 15)

CO adsorption is considered reversible, as the CO binding to Au is relatively weak
(in the absence of CO in solution, CO does not bind to Au). The observed Tafel
slope (120 mV/dec) is in agreement with Reaction (6.14) as the rate-determining
step, but the pH dependence (60 mV/pH unit) is not, as the rate of the irreversible
Reaction (6.14) should formally not depend on pH. Weaver and co-workers also
reported CO electro-oxidation on gold to be structure-sensitive, with Au(110) being
the most and Au(111) the least active [Edens et al., 1996]. This structure sensitivity
does not reflect the CO adsorption capacity, however, since Au(210) was found to
have a higher CO surface concentration than Au(110), but a lower oxidation capacity
[Chang et al., 1991; Edens et al., 1996].
Markovic and co-workers [Blizanac et al., 2004a, b] have suggested that a small
voltammetric feature near 0.5 V (vs. RHE) observed on clean Au(100) in HClO 4