Metalloporphyrin Catalysts of Oxygen Reduction - Fuel Cell Catalysis: A Surface Science Approach - page 675

Figure 18.21 (a) FeAc is a simplified version of the biomimetic catalysts in series 1 - 4
(Fig. 18.17); when adsorbed on a graphite electrode, it manifests catalytic behavior comparable
to that of series 2 metalloporphyrins. (b) The simplest structural motif that ensures the axial
imidazole ligation of the Fe II site [Khvostichenko et al., 2007; Yang et al., 2008].

potentials ,0.2 V. One is that it reflects the decrease in the fraction of the ferric -
superoxo intermediate, which would require E per to lie somewhere between about
0.1 and 0.2 V for the ferric - superoxo/ferric - peroxo equilibrium to become shifted
significantly toward the peroxo intermediate at potentials ,0.2 V. The other possi-
bility is that the fraction of the ferric - superoxo intermediate was not perturbed
much at potentials .0.1 V, i.e., that E per , 0.1 V, but the decrease was caused by
recapture of O 2 2 released from one molecule of catalyst by another molecule in the
Fe II state (yielding a ferric - peroxo intermediate that re-enters the cycle;
Fig. 18.20). The probability of such recapture is increased at more reducing potentials
by (i) a decrease in the concentration of O 2 in the catalytic film due to mass transport
limitations (plateau part of polarization curves) and (ii) a shift in the Fe III/II equili-

brium toward Fe II at potentials ,0.1 V.
The combination of selectivity (n av . 3.9), overpotential (about 0.5 V at the turnover
frequency of .2s 21 mol O 2 /mol) and lifetime (.10 4 turnovers) make series 2
metalloporphyrins, even in the monometallic (Fe-only) form, probably the best
among the known metalloporphyrin ORR catalysts. These catalysts perform best in
pH 8 electrolytes rather than in the more popular acidic (pH 0) media. Their unique per-
formance characteristics appear to be largely due to the presence of the imidazole ligand
trans to coordinated O 2 . This assertion is supported by the fact that the metalloporphyrin
FeAc (Fig. 18.21a) is only slightly inferior to series 2 as an ORR catalyst (n av 3.85; a
turnover frequency . 2s 21 was attained at an overpotential of about 0.6 V at pH 7, and
the catalytic performance was retained for at least 10 4 turnovers). Series 2 metallopor-
phyrins have also been shown to be efficient electrocatalysts for reduction of oxohalides
(e.g., ClO 2 2 ), and organic peroxides and peroxoacids [Collman et al., 2002c, 2004b].
Based on the well-studied catalytic mechanism in series 2 metalloporphyrins
(Fig. 18.20), further developments of the heme/imidazole motif in the context of
fuel cell catalysis have to be directed towards the following:

† Simplifying the synthesis of the catalysts (e.g., the highly optimized convergent
synthesis of monometallic series 2 metalloporphyrins required between 14 and
17 steps [Collman et al., 2002d]—FeAc (Fig. 18.21a) is available in 7 steps).