How does α-FePc catalysts dispersed onto high specific surface carbon support work towards oxygen reduction reaction (orr)?

A study by XRD of iron phthalocyanine (FePc) leads to conclude that as received FePc was under a phase whereas heat-treated FePc was under P phase. The activity of alpha- and beta-FePc towards oxygen reduction reaction (orr) was compared. The alpha-FePc catalyst displayed an overpotential towards the oxygen reduction reaction 100 mV lower than that with beta-FePc. Tafel slopes were drawn from electrochemical data and two Tafel slopes of -65 mV/decade and -121 mV/decade were obtained with alpha-FePc and only one (-63 mV/decade for beta-FePc). The differences in catalytic behaviour between both catalysts were discussed in terms of structure differences. It was shown, by using electrochemical quartz crystal microbalance (EQCM) technique, that alpha-FePc likely formed mu-oxo dimers at potentials higher than 700 mV vs. RHE. These mu-oxo dimers are reduced at the same potential than the hydroxy monomer of alpha-FePc. When orr was carried out at alpha-FePc based electrode, two reduction waves were detected. The first reduction wave (called pre-wave) at low overpotentials was related to the formation of dimer and the second wave at higher overpotentials to the adsorption and reduction of oxygen mainly at the monomeric form of FePc. From these results and on the basis of Tafel slopes analysis, different mechanisms were proposed depending on the potential and on the Tafel slope values: one involving exclusively the production of H2O via the formation of dimers, and the second involving simultaneously the production of H2O and H2O2 via the formation of hydroxo monomers and dimers. The proposed mechanism of orr for the low potentials range considering the first charge transfer as determining step was confirmed by "in situ" IR reflectance spectroscopy at alpha-FePc/C electrode. (c) 2006 Elsevier B.V. All rights reserved.