In situ X-ray absorption of a carbon monoxide-iron porphyrin adduct adsorbed on high-area carbon in an aqueous electrolyte

The electronic and electrochemical properties of a carbon monoxide-FeTMPP adduct (FeTMPP = (5,10,15,20-tetrakis[4-methoxyphenyl]-21H,23H-porphine)iron(II)) adsorbed on Black Pearls (BP) high-area carbon have been examined in situ in 0.05 M H₂SO₄ by Fe K-edge X-ray absorption near-edge structure (XANES). This adduct was prepared by first polarizing a Teflon bonded electrode, incorporating the high-area FeTMPP/BP material, at a potential at which the adsorbed macrocycle is present in the reduced state, followed by bubbling CO into the electrolyte. The resulting in situ XANES displayed two new preedge features centered at 7112 and 7115 eV, as found for other well-characterized iron macrocycle-CO adducts, as well as a shift of the main absorption edge toward higher values (ca. 1.3 V) indicative of charge transfer from the iron to CO. Dissociation of the adduct could be achieved by polarizing the electrode at more positive potentials to yield the CO-free form of the macrocycle in the ferric state, as has been reported for solution-phase CO-Fe porphyrin adducts in nonaqueous media.