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) 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.

XPS investigation of Nafion® membrane degradation

“…Furthermore, with an increase in Fe 2+ concentration, the oxygen binding energy is shifted to lower values. This shift is consistent with the formation of an oxo-bonded iron complex on the membrane surface [18,19] along with the loss of -SO 3 H groups from the membrane. Such results also agree with the detection of SO 4 2− in the exhaust water by ion chromatography.…”

“…First, the main contribution is likely due to the oxo-bonded iron complex. Since the hydrophilic sulfonic acid group in the side chain can move from the membrane bulk to the surface during treatment [8], the oxo-bonded iron complex can form after a proton in -SO 3 H is exchanged with a ferric ion in Fenton's reagent [18]. However, the oxygen concentration in the polymer still increases even after subtracting the maximum number of oxygen atoms bonded to iron (in the form of Fe-OOH).…”

Photo-assisted fenton mineralization of an azo-dye acid black 1 using a modified laponite clay-based Fe nanocomposite as a heterogeneous catalyst

“…In addition, XPS survey spectrum and binding energy of detected elements before and after reaction are shown in figure 2 and table 3, respectively. The binding energy of Fe was determined to be 712 eV, which evidenced the oxidation state of iron was Fe 3+ , and no zero oxidation of Fe was observed [10].…”

Macroscopic Modeling of Polymer-Electrolyte Membranes

“…This is especially true in those cases where the membrane is either chemically or physically altered, resulting in pinhole formation and catalyst layer delamination [130,219], or where the cause of failure can be traced to such issues as gas crossover through the membrane [223]. In addition, the poisoning effects described above in terms of cations in the membrane (including platinum) can lead to performance loss [223][224][225]. In terms of modeling, there has been some work on durability and failure mechanisms, but it is relatively new; understanding failure is one of the main goals of current and future modeling activities.…”