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This study presents the detailed nature of iron clusters formed on Fe 3+ -Nafion membranes. The catalytic nature of these clusters during immobilized Fenton processes was observed to be a function of the deposition method of Fe ions on the Nafion. The nonbiodegradable azo-dye Orange II and 2-propanol were utilized as convenient organic model compounds in photoassisted Fenton degradation processes. The highest photocatalytic activity was observed when samples were prepared by ion exchange between iron(III) aquacomplexes and H + or Na + as counterions of the Nafion SO3group. Spectroscopic techniques show that iron(III) in the membrane was present mainly as a mononuclear complex of [Fe(H2O)6] 3+ and binuclear complexes [Fe(H3O2)Fe] 5+ and [Fe-O-Fe] 4+ . If NaOH or ammonia was added to the former samples prepared by ion exchange, Nafion-Fe membranes with low photocatalytic activity were obtained showing R-Fe2O3 and [Fe-O-Fe] 4+ . Detailed high-resolution transmission electron microscopy was carried out for the Nafion-Fe ion-exchanged and also base-treated membranes showing R-Fe2O3 nanocrystallites of 3.5-5 nm. Spectral bands were found for iron oxides in the Fe 3+ -Nafion by femtosecond laser spectroscopy. The R-Fe2O3 nanocrystallites in the Nafion exchanged base-treated membranes presented a relaxation dynamics for the excited states close to that observed with R-Fe2O3 nanocrystallite colloids taken as reference compounds. Multiexponential transient absorption decay of R-Fe2O3 in SO3 --water clusters was observed with time constants close to 320 fs, 1.5 ps, and 31 ps after the excitation pulse. Samples of Fe 3+ -Nafion membranes with high activity show different transient dynamics relative to the Fe 3+ -Nafion with low activity. Correlation of the photocatalytic activity of Fe 3+ -Nafion with UV-vis, Fourier transform infrared, Mo ¨ssbauer, and X-ray photoelectron spectroscopic results suggests that the photocatalytic activity correlates with the amount of mononuclear [Fe(H2O)6 ] 3+ , binuclear complexes [Fe(H3O2)Fe] 5+ and oxo-bridged [Fe-O-Fe] 4+ found in the membranes.

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