Environmental Engineering Reference
In-Depth Information
uncertainty, but with less steep slopes than if the systems followed the Koutecky -
Levich behavior. Application of the Koutecky - Levich equation in such situations
leads to significantly overestimated catalytic selectivities. To test for this situation,
one needs to establish that the Koutecky - Levich slopes are indeed directly pro-
portional to bulk O
2
concentration, [O
2
]
bulk
in as wide a range of O
2
concentrations
as possible.
Although, in theory, the Koutecky - Levich equation can be applied to estimate n
av
and k at any part of the voltammogram (provided that the conditions stated above are
satisfied), for practical reasons only limiting ( plateau) currents can be acquired with
adequate reproducibility to yield suitable Koutecky - Levich plots.
The selectivity of the catalyst can also be assayed using an RRDE (Fig. 18.7a)
[Bard and Faulkner, 2001; Opekar and Beran, 1976]. In this setup, the ring electrode
serves as an electrochemical sensor of partially reduced oxygen species (H
2
O
2
and
O
2
2
/HO
2
) released by the catalytic film at the disk. While the disk potential is scanned,
the (usually) Pt ring is maintained at a fixed oxidizing potential, so that a fraction of
partially reduced oxygen that is hydrodynamically transported from the film - solution
interface to the ring is oxidized to O
2
. This oxidation generates a ring current. In order
to obtain n
av
from this ring current, one needs to know the fraction of the partially
reduced oxygen species generated at the disk that are oxidized at the ring (the so-called
“ring collection efficiency”). When the Pt ring can be set at a potential at which the rate
of the analyte oxidation is much larger than the residence time of the analyte in
the ring's vicinity (i.e., diffusion-limited oxidation), the collection efficiency can be
calculated from the geometric parameters of the RRDE. However, because Pt is
easily passivated toward H
2
O
2
oxidation, sufficient overpotential to achieve diffu-
sion-limited ring oxidation of H
2
O
2
is probably not accessible under most experimen-
tal conditions. As a result, the ring collection efficiency of H
2
O
2
is always lower than
the theoretical value. Experimental methods to maintain collection efficiencies close
to theoretical have been reported [Boulatov et al., 2002]. With such precautions, the
ring current measurements are indispensable for estimating the catalytic selectivity
at potentials where it cannot be obtained from Koutecky - Levich plots (e.g., the
rising parts of the catalytic waves).
It is important to be mindful of the fact that catalytic properties of a film of a
metalloporphyrin can be quite different from those of individual isolated molecules
of the catalyst, thereby complicating mechanistic interpretation of the results. The
major issue is the possibility of stepwise reduction of O
2
at multiple catalytic sites
within a film. For example, the fraction of O
2
reduced to H
2
O
2
by films of Fe tetraphe-
nylporphyrin, Fe(TPP) (see Fig. 18.9 in the next section), decreases as the amount of
the deposited catalyst increases [Collman et al., 2003a]. This dependence suggests that
the putative ferric - hydroperoxo intermediate (see Fig. 18.11 in the next section) may
undergo dissociative decomposition with release of H
2
O
2
faster than reductive
scission of the O - O bond. As a released molecule of H
2
O
2
diffuses through the
catalytic film, it could coordinate to other molecules of the catalyst (particularly if
the concentration of O
2
in the film is low, as it is at the plateau of polarization
curves). Once coordinated, H
2
O
2
can undergo homolysis or heterolysis of the O - O
bond, rather than dissociate from the porphyrin. The thicker the film, the longer the
Search WWH ::
Custom Search