Metalloporphyrin Catalysts of Oxygen Reduction - Fuel Cell Catalysis: A Surface Science Approach - page 644

composition of the film by absorption spectroscopy by exploiting the very high extinc-
tion coefficients of metalloporphyrins around about 400 nm (1 . 10 5 M 21 cm 21 ).
Potential disadvantages are the need for chemical moieties to allow chemisorption
of the catalyst on the electrode surface (e.g., thiols for SAMs on Au), rapid degradation
of the film, and reduction of O 2 on the bare electrode if the quality of the SAM is
limited. Until recently, it was thought that SAMs were, in general, incompatible
with RDEs or RRDEs [Collman et al., 2007b], but a decade-old strategy of attaching
porphyrins to preformed SAMs of terminally functionalized alkyl thiols [Loetzbeyer
et al., 1995] was recently extended to generate high quality SAMs compatible with
RRDEs [Collman et al., 2007b].
In a now classical study by Murray and co-workers [Hutchison et al., 1993] a SAM
of a modified Co tetraphenylporphyrin (see Fig. 18.12 in the next section) was formed.
This SAM was found to catalyze two-electron reduction of O 2 in both acidic and basic
media, similar to the behavior of graphite-adsorbed or dissolved Co(TPP). The selec-
tivity of the SAM was quantified using interdigitated array electrodes. Remarkably,
the catalyst retained its catalytic activity for over 10 5 turnovers, which probably
exceeds the stability of a parent Co(TPP) catalyst absorbed on graphite, although esti-
mates of stabilities of catalysts absorbed on graphite have substantial uncertainty
because of the lack of data on the fraction of the electroactive catalyst that is capable
of participating in the catalysis. Subsequent spectroelectrochemical measurements
[Postlethwaite et al., 1995] demonstrated that (i) the onset of catalysis corresponds
to the reduction of the SAM to the Co II state; (ii) approximately a single monolayer
of the metalloporphyrin is formed with coplanar orientation of the macrocycle relative
to the surface; and (iii) the absorption spectra of the catalyst on the Au surface in both
Co III and Co II states are quite similar to those in a CH 2 Cl 2 solution. However, no infor-
mation was obtained regarding the nature of the axial ligation of the Co ion, which is of
course of primary importance in developing a mechanism of O 2 reduction (see the next
section). This work was later extended to a cofacial Co porphyrin [Hutchison et al.,
1997], a protoheme IX [Loetzbeyer et al., 1995] and a Ru porphyrin [Collman
et al., 1996]. Very recently, O 2 reduction catalysis by highly elaborate Fe porphyrins
that were designed to reproduce the heme/Cu site of cytochrome c oxidase attached to
various SAMs has been reported [Collman et al., 2007b]. In this work, SAMs were
used to control the rate of electron transfer between the electrode and the catalyst.
Several studies of electrocatalytic O 2 reduction by simple metalloporphyrins in
aqueous or mixed organic/aqueous solutions have been reported [Shigehara and
Anson, 1982; Forshey and Kuwana, 1983; LeMest et al., 1997; Su et al., 1990]. In
this setup, the electrode serves as a source of electrons, but both the catalyst and the
substrate are, ideally, in solution. A water-soluble catalyst precludes the use of an
RRDE for detection of partially reduced oxygen, as the oxidation of the catalyst
itself on the ring will generate unacceptably high background currents, and the chemi-
sorption of the catalyst or its decomposition products on the Pt ring may passivate it
toward H 2 O 2 oxidation. As a result, stationary electrodes are typically employed in
such studies. A cofacial bis-Co porphyrin (see Section 18.5) dissolved in 5.6 M
triflic acid was reported to manifest very high stability and high selectivity toward
four-electron

O 2 reduction [LeMest et al., 1997]. Water-soluble Fe-tetrakis