Environmental Engineering Reference
In-Depth Information
mixed adlayers is inferred from voltammetric and coulometric analysis of CO oxi-
dation and corroborated by Fourier transform infrared (FTIR) measurements. Two
possible effects have to be considered. First, there is the electronic effect that would
alter the stability of the CO adlayer. This is related to the modification of the electronic
backdonation from the metal to the CO molecule and also to the existence of lateral
interactions between CO and the adatom. On the other hand, the existence of a bifunc-
tional mechanism with the adatoms acting as donors of oxygenated species also needs
to be considered. The changes that the adatom exerts on the CO adlayer on Pt(111) can
be summarized as follows:
†
In general, the adatoms exert a significant effect on the CO adlayer structure, dis-
placing it from multifolded adsorption sites to on-top sites. The fact that, for the
same adatom coverage, the degree of blocking of hydrogen adsorption is more
important than that of CO adsorption suggests that the adatoms adsorb on hol-
lows, blocking hydrogen multifolded adsorption but allowing on-top adsorption
of CO [Chang and Weaver, 1991; Herrero et al., 1995a, d; Herrero et al., 1996;
Kizhakevariam and Weaver, 1994].
†
In the presence of Bi or Te, the C;;O bond is weakened, as concluded from the
displacement of the CO stretching band to lower wavenumbers. There is also a
change in the dependence of the band frequency on electrode potential, with
the slope dn/dE increasing for the adatom-modified surfaces. These changes
indicate that the adatom alters the electronic properties of the surface, increasing
the amount of electronic backdonation and stabilizing the adsorbed CO mol-
ecule. No catalytic enhancement is expected from this effect.
†
In the presence of As, a displacement of the vibrational band towards higher
wavenumbers indicates a weakening of the backdonation mechanism, decreasing
the stability of CO on the electrode surface.
†
Almost no displacement of the vibrational band is observed in the presence of Sb
or Se, indicating that electronic effects are very weak in these cases.
†
For Bi, As, and Sb, the CO oxidation undergoes bifunctional catalysis through an
adatom-meditated oxygen transfer. This conclusion can be obtained from the
following observations based on experiments similar to that shown in Fig. 7.11.
When CO oxidation on a Bi-modified and on a clean surface are compared, a
slight decrease in the onset potential for CO oxidation is observed. The oxidation
of the adatom is also severely affected, and takes place coupled with the CO
oxidation. The reduction of the adatom in the first negative scan after the CO
stripping attests that the adatoms were oxidized together with CO oxidation.
For Bi, however (Fig. 7.11), the charge under the first negative scan is lower
than in subsequent cycles. This has been taken as an indication that adatoms
donate oxygenated species to adjacent CO molecules to promote their oxidation.
In this way, after CO oxidation, the Bi adlayer ends up partially reduced.
Chronoamperometric measurements of CO oxidation at different constant poten-
tials demonstrate that the catalytic enhancement only takes place in the potential
region where the adatom is oxidized.
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