Environmental Engineering Reference
In-Depth Information
This is an irreversible LH reaction (i.e., a second-order reaction between two surface
adsorbates), and generates free sites for the adsorption of OH (Reaction 6.1) or, in the
case of continuous CO oxidation, for the adsorption of CO:
CO
þ
!
CO
ads
(6
:
3)
6.2.1.1 CO Stripping Chronoamperometry
Before discussing experimental
results, let us examine what the LH mechanism predicts for the chronoamperometric
response of an experiment where we start at a potential at which the CO adlayer is
stable and we step to a “final potential” E where the CO adlayer will be oxidized.
We will also assume that the so-called mean field approximation applies, i.e., CO
and OH are well mixed on the surface and the reaction rate can be expressed in
terms of their average coverages u
CO
and u
OH
. The differential equation for the rate
of change of u
CO
with time is
G
m
du
CO
dt
¼
k
0
(E)u
CO
u
OH
¼
k(E)u
CO
(1
u
CO
)
(6
:
4)
where G
m
is the number of surface sites per unit area, and where in the last equality
we have assumed that the formation of OH
ads
through Reaction (6.1) is always in
equilibrium, and its coverage is relatively small, such that u
OH
/ (1 2 u
CO
).
Equation (6.4) may be solved analytically assuming that at t ¼ 0, u
CO
¼ u
in
, with
u
in
the initial CO coverage (which should be ,1), giving as a final expression for the
current density j [Bergelin et al., 1999]
j(t)
¼
Q(k
=
G
m
) exp[
k(t
t
max
)
=
G
m
]
{1
þ
exp[
k(t
t
max
)
=
G
m
]}
2
(6
:
5)
where Q is the charge needed to oxidize the CO adlayer. In an experiment, this charge
has two terms, Q ¼ Q
CO
þ
Q
anions
; Q
CO
is associated with the stripping of CO and
equals 2eG
m
u
in
, and Q
anions
is associated with anion readsorption on the free sites cre-
ated after the CO oxidation. It should be taken into account that anions are always
adsorbed on a clean platinum electrode at the potentials where CO oxidation takes
place. Thus, anions are immediately readsorbed when CO is stripped from the surface,
and this readsorption process is always accompanied with a positive charge transfer.
Equation (6.5) predicts that, for sufficiently high initial CO coverage, the current tran-
sient should exhibit a maximum at t
max
¼ (G
m
/k) ln[u
in
/(1 2 u
in
)]. The reason for this
maximum is the autocatalysis implied by Reactions (6.1) and (6.2), in which every free
site on the surface creates two more. At high initial CO coverage, very few sites are
available for OH adsorption, and the reaction rate is low. As the reaction proceeds,
slowly new free sites are generated through the reaction of OH with CO, increasing
the OH adsorption rate and consequently the overall CO oxidation rate and the current.
The autocatalysis stops when u
CO
¼ 0.5, for which the transient goes through a maxi-
mum. This autocatalytic mechanism for CO oxidation may lead to instabilities under
conditions of continuous CO oxidation, as will be discussed in Section 6.2.1.3.
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