Environmental Engineering Reference
In-Depth Information
(N-methyl-4-pyridyl)porphyrin cation (FeTMPyP; see Fig. 18.9 in the next section)
was also found [Forshey and Kuwana, 1983] to reduce O
2
to H
2
O over a wide
range of pH. The proposed mechanism involves the intermediacy of the free H
2
O
2
,
and was derived on the basis of mathematical simulation of the experimental cyclic
voltammograms in the presence of O
2
. However, because many simple metallopor-
phyrin catalysts adsorb strongly to graphite, which is commonly used as the working
electrode in these studies, it is often difficult to rule out the possibility that the observed
catalytic behavior arises from the electrode-adsorbed catalyst [Su et al., 1990].
Finally, studies of metalloporphyrin-catalyzed O
2
reduction by a soluble reductant,
such as a ferrocene derivative, cobaltocene, or an organic macrocycle, have been
reported [Fukuzumi et al., 1990, 2004; Collman et al., 1997; Anson, 1985]. Co por-
phyrins have been used in all but one study, since Fe
II
porphyrins seem to degrade
much faster under these conditions. The advantages of such a homogeneous setup
include the capacity to monitor the reaction spectroscopically, which in favorable
cases may allow identification of the dominant species during the catalysis.
Likewise, the rates of catalytic reduction of H
2
O
2
under anaerobic conditions are
not influenced by the rate at which it traverses the solution/catalytic film interface,
as is the case of surface-confined catalysts. Establishing that a catalyst reduces
H
2
O
2
much more slowly than O
2
under these circumstances can serve as convincing
evidence against the stepwise four-electron reduction mechanism of O
2
through free
H
2
O
2
. In this setup, determining the potential dependence of catalytic rates is more
difficult. Also, reactions in which two molecules of the catalyst participate in reduction
of a single O
2
molecule, such as the formation of m-peroxo and m-oxo dimers of
simple Fe porphyrins, ( por)Fe - O
x
- Fe( por), x ¼ 1, 2 as kinetically important inter-
mediates, are likely. Finally, whereas electron transfer from the electrode to a catalyst
in physical contact with the electrode is probably always very rapid, reduction of cat-
alytic intermediates by a dissolved reductant is limited at least by diffusion, potentially
lowering the selectivity and/or lifetime of the catalyst.
Although all these methods allow one to determine n
av
, they do not provide any
direct information about the nature of the partially reduced oxygen byproducts that
are responsible for n
av
, 4. It is typical to assume that the byproduct is H
2
O
2
, but
superoxide (O
2
or its conjugate acid, hydroperoxyl radical, HO
2
,pK(HO
2
) ¼ 4.5)
and especially hydroxyl radical can also be generated. The nature of the partially
reduced oxygen byproducts may affect the stability of the catalyst (see below) and
determine stereoelectronic modifications of the catalyst that may lead to an increased
n
av
.HO
2
is generated by autoxidation of Fe porphyrin/O
2
complexes; it undergoes
rapid outer-sphere reduction to HO
2
2
, whereas its conjugate base, O
2
2
is a fairly
reactive and strong one-electron outer-sphere reductant, forming O
2
. If the local
concentration of HO
2
/O
2
2
is high, this species could undergo bimolecular dispropor-
tionation to O
2
and H
2
O
2
. At low overpotentials, thermodynamics limits the fraction of
O
2
that can be reduced to HO
2
/O
2
2
.
Hydroxyl radical is a strong indiscriminate outer-sphere oxidant (generating OH
2
)
and H-atom abstractor (generating H
2
O) [Huie and Neta, 1999]. Simple Fe porphyrins
are known to promote O - O bond homolysis in reaction with H
2
O
2
[Watanabe, 2000].
Because of its high reactivity, once generated,
†
OH probably reacts with the
Search WWH ::
Custom Search