Catalytic Fe³⁺ Clusters and Complexes in Nafion Active in Photo-Fenton Processes. High-Resolution Electron Microscopy and Femtosecond Studies

Abstract

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) aqua-complexes and H + or Na + as counterions of the Nafion SO 3 - group. Spectroscopic techniques show that iron(III) in the membrane was present mainly as a mononuclear complex of [Fe(H 2 O) 6 ] 3+ and binuclear complexes [Fe(H 3 O 2 )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 α-Fe 2 O 3 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 α-Fe 2 O 3 nanocrystallites of 3.5−5 nm. Spectral bands were found for iron oxides in the Fe 3+ −Nafion by femtosecond laser spectroscopy. The α-Fe 2 O 3 nanocrystallites in the Nafion exchanged base-treated membranes presented a relaxation dynamics for the excited states close to that observed with α-Fe 2 O 3 nanocrystallite colloids taken as reference compounds. Multiexponential transient absorption decay of α-Fe 2 O 3 in SO 3 - −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, Mössbauer, and X-ray photoelectron spectroscopic results suggests that the photocatalytic activity correlates with the amount of mononuclear [Fe(H 2 O) 6 ] 3+, binuclear complexes [Fe(H 3 O 2 )Fe] 5+ and oxo-bridged [Fe−O−Fe] 4+ found in the membranes.