An in-situ Raman study of the effect of the support for adsorbed iridium-chelates in catalysing oxygen reduction

In-situ Raman spectroscopy was used to obtain some insight into the mechanism of oxygen reduction at adsorbed layers of iridium-octaethylporphyrin (IrOEP), iridium-tetraphenylporphyrin (IrTPP) and iridium-phthalocyanine (IrPc). The selectivity of adsorbed layers of both porphyrins appears to be very sensitive to the support: on gold only hydrogen peroxide is formed whereas on carbon four-electron reduction is observed. On both supports a peculiar, reversible deactivation occurs at low potentials. IrPc gives hydrogen peroxide on both supports and no deactivation. A previously described single site mechanism is discussed in which only out-of-plane iridium can accomplish the four electron reduction to water and deactivation occurs when the iridium is reduced to its 1+ oxidation state. The results of the in-situ Raman study were generally consistent with this mechanism, but were insufficient to establish it beyond reasonable doubt.