Catalytic Fe3+ clusters and complexes in Nafion active in photo-Fenton processes. High-resolution electron microscopy and femtosecond studies

This study presents the detailed nature of iron clusters formed on Fe3+-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) aqua-complexes and H+ or Na+ as counterions of the Nafion SO3- group. Spectroscopic techniques show that iron(III) in the membrane was present mainly as a mononuclear complex of Fe(H2O)(6) and binuclear complexes Fe(H3O2)Fe and Fe-O-Fe. If NaOH or ammonia was added to the former samples prepared by ion exchange, Nafion-Fe membranes with low photocatalytic activity were obtained showing alpha-Fe2O3 and Fe-O-Fe. Detailed high-resolution transmission electron microscopy was carried out for the Nafion-Fe ion-exchariged and also base-treated membranes showing alpha-Fe2O3 nanocrystallites of 3.5-5 nm. Spectral bands were found for iron oxides in the Fe3+-Nafion by femtosecond laser spectroscopy. The alpha-Fe2O3 nanocrystallites in the Nafion exchanged base-treated membranes presented a relaxation dynamics for the excited states close to that observed with alpha-Fe2O3 nanocrystallite colloids taken as reference compounds. Multiexponential transient absorption decay of alpha-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 Fe3+-Nafion membranes with-high activity show different transient dynamics relative to the Fe3+-Nafion with low activity. Correlation of the photocatalytic activity of Fe3+-Nafion with UV-vis, Fourier transform infrared, Mossbauer, and X-ray photoelectron spectroscopic results suggests that the photocatalytic activity correlates with the amount of mononuclear Fe(H2O)(6), binuclear complexes Fe(H3O2)Fe and oxo-bridged Fe-O-Fe found in the membranes.