Metalloporphyrin Catalysts of Oxygen Reduction - Fuel Cell Catalysis: A Surface Science Approach - page 670

isotopes and by 1 H- and 19 F-NMR spectroscopy [Collman et al., 2003b]. Similar
results were observed for Co analogues of 2b and 2c, in which Fe is replaced by
Co, whose paramagnetism required characterization by EPR rather than by NMR in
addition to resonance Raman spectroscopy [Collman et al., 2002a]. Although
adduct 2cO 2 is stable at room temperature it undergoes rapid autoxidation in the pre-
sence of H þ sources (compare this with the reversible protonation of the m-superoxo
complexes of cofacial bis-Co porphyrins; Section 18.5). Although Fe and Cu ions in
series 2 metalloporphyrins (Fig. 18.18) are positioned sufficiently close to be bridged
by a peroxide (and indeed PhNO, which is isoelectronic with O 2 , binds series 2
metalloporphyrins in a bridging mode), a bridged peroxide is not formed. It is not
at present known if the preference for a ferric - superoxo/Cu I isomer versus the
Fe III -O-O-Cu II alternative is thermodynamic or kinetic. Notably, O 2 binding to
the heme/Cu site in cytochrome c oxidase also generates a ferric - superoxo/Cu I inter-
mediate (compound A in Fig. 18.5). The distal Cu I ion in 2c, although not interacting
directly with the bound superoxide, had a large effect on the O 2 affinity of metallopor-
phyrin 2c. The Cu-free (Fe-only) form of 2c manifested very low O 2 affinity such that
the O 2 adduct could only be observed at 260 8C, and even at this low temperature the
Fe-only complex rapidly underwent irreversible oxidation.
Compound 1 (Fig. 18.18) reversibly forms an analogous ferric - superoxo/Cu I
adduct at 260 8C, as demonstrated by resonance Raman spectroscopy. However,
warming the sample to 240 8C results in a rapid four-electron reduction of the
bound O 2 ligand, generating a ferryl/Cu II /phenoxyl radical derivative (Fig. 18.18)
[Collman et al., 2007a].
18.6.2 ORR Catalysis by Biomimetic Metalloporphyrins
ORR catalysis by biomimetic metalloporphyrins is typically studied using porphyrins
adsorbed on a graphite electrode in contact with an aqueous electrolyte buffered at
pH 7, to reproduce physiological conditions. For most catalysts, only selectivity at
the plateau of polarization curves at a single pH was reported; in a few cases, the differ-
ence in catalytic selectivity by the bimetallic (FeCu) and monometallic (Fe-only)
forms of porphyrins was also reported. When this difference exists (as in the case
of series 4 catalysts; Fig. 18.17) it provides support that the bimetallic catalyst does
not lose Cu when adsorbed on the electrode. However, when the behavior of the
two forms was identical, or when only the bimetallic catalyst was studied, no evidence
was provided that Cu was indeed present in the adsorbed porphyrin. Overall, the
biomimetic catalysts appear to manifest higher n av values and retain their catalytic
activity for more turnovers than does Fe tetraphenylporphyrin; exceptions are the
bimetallic forms of series 4 porphyrins [Ricard et al., 2001]. Contradictory results
were reported regarding the effect of Cu on n av : ORR catalysis by Cu-free (Fe-only)
forms of series 3 and 5 and by porphyrins 6 - 8 have not been reported [Collman
et al., 2003a]; the bimetallic, FeCu forms of series 4 porphyrins were reported to
manifest lower n av than (Cu-free) Fe-only forms [Ricard et al., 2001]. Distal Cu had
relatively minor effect on ORR catalysis by 1 and series 2 porphyrins when the
electron flux from the electrode was rapid [Boulatov et al., 2002; Boulatov, 2004;