Environmental Engineering Reference
In-Depth Information
Chronoamperometric transients for CO stripping on polycrystalline platinum were
measured by McCallum and Pletcher [1977]. Love and Lipkowski [1988] were the
first to present chronoamperometric data for CO stripping on single-crystalline plati-
num. However, they interpreted their data on the basis of a different model than the
one discussed above. Love and Lipkowski considered that the oxidation of the CO
adlayer starts at holes or defects in the CO adlayer, where OH adsorbs. These holes
act as nucleation centers for the oxidation reaction, and the holes grow as the CO at
the perimeter of these holes is oxidized away by OH
ads
. This “nucleation and
growth” (N&G) mechanism is fundamentally different from the “mean field”
model presented above, because it does not presume any kind of mixing of CO and
OH [Koper et al., 1998]. Basically, it assumes complete surface immobility of the
chemisorbed CO.
Mathematical expressions for the N&G model can be derived from the classical
theory for the nucleation and growth of two-dimensional films [Schmickler, 1996].
Two regimes are distinguished:
†
instantaneous nucleation, for which the nucleation is fast and holes are occupied
instantaneously
†
progressive nucleation, for which the nucleation rate is slow
The respective expressions are
j(t) / Mk
G
t exp(
pMk
G
t)
(6
:
6)
j(t) / Mk
N
k
G
t
2
exp(
pMk
N
k
G
t
3
=
3)
(6
:
7)
where M is the number of holes, k
N
is the nucleation rate constant, and k
G
is the
growth rate constant. From their experiments, Love and Lipkowski argued that, at
low potential, CO oxidation followed a progressive N&G mechanism, whereas, at
high potential, CO oxidation followed an instantaneous N&G mechanism.
In order to assess the role of the platinum surface structure and of CO surface mobi-
lity on the oxidation kinetics of adsorbed CO, we carried out chronoamperometry
experiments on a series of stepped platinum electrodes of [n(111)
(110)] orientation
[Lebedeva et al., 2002c]. If the (110) steps act as active sites for CO oxidation because
they adsorb OH at a lower potential than the (111) terrace sites, one would expect
that for sufficiently wide terraces and sufficiently slow CO diffusion, the chrono-
amperometric transient would display a Cottrell-like tailing for longer times owing
to slow diffusion of CO from the terrace to the active step site. The mathematical
treatment supporting this conclusion was given in Koper et al. [2002].
Figure 6.2a shows chronoamperometric transients for CO oxidation recorded on
three different stepped electrodes for the same final potential. Clearly, the electrode
with the higher step density is more active, as it oxidizes the CO adlayer in a shorter
period of time. Figure 6.2b shows a fit of a transient obtained on a Pt(15, 15, 14) elec-
trode (terrace 30 atoms wide) by both the mean field model [(6.5), solid line] and the
N&G model [(6.6), dashed line]. The mean field model gives a slightly better fit. More
importantly, the mean field model gives a good fit of all transients on all electrodes,
Search WWH ::
Custom Search