Effect of heat treatment on the redox properties of iron porphyrins adsorbed on high area carbon in acid electrolytes: An in situ Fe K-edge X-ray absorption near-edge structure study

Electronic aspects of an iron porphyrin, 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine iron(III) chloride (FeTMPPCl) molecularly dispersed on a high area carbon black (Black Pearls 2000, BP) and then heat-treated (or thermally activated) at 800°C in a flowing inert atmosphere, were investigated in situ in 1.0 M H₃PO₄ by Fe K-edge X-ray near-edge structure (XANES). Profound differences were observed between the ex situ (dry) XANES of electrodes incorporating FeTMPPCl/BP before and after heat treatment and, likewise, in the case of the thermally treated FeTMPPCl/BP before and after immersion of the electrodes in the electrolyte. Monotonic shifts in the absorption edge toward higher energies were observed for heat-treated FeTMPPCl/BP as the potential was increased over a range of over 1 V. The overall magnitude of the shift, ca. 2.5 eV, was virtually the same as that obtained with non-heat-treated (or intact) FeTMPPCl/BP, which occurred in a much narrower potential range, ca. 0.2 V. In contrast to the behavior observed for the intact adsorbed macrocycle, its heat-treated counterpart displayed no affinity for CO, indicating that, to the level of sensitivity of this technique, iron sites in the thermally activated material are quite different from those in the N₄ environment of the intact macrocycle.