Green Synthesis of Iron Oxides and Phosphates via Thermal Treatment of Iron Polyphenols Synthesized by a Camellia sinensis Extract.

Iron oxide nanoparticles (FeONPs) prepared with plant extracts have been emerging as green and sustainable materials. FeONPs are usually amorphous due to the chelation of the tea polyphenols (TPs) to the iron, and the real nature of the iron compounds is not completely understood. The main goal of this study was to investigate the behavior of the green FeONPs synthesized from an Fe3+ salt and Cammelia sinensis (black tea) extract upon thermal treatment, in order to remove TPs and enable the formation of crystalline materials suitable for a thorough characterization and with the potential for diverse applications. The as-prepared FeONPs were assigned as mixed-valence Fe(III) oxyhydroxides and Fe(II)/Fe(III) ions bound to TPs. A detailed description of the phase transformation upon heating revealed the formation of the rare nano ß-Fe2O3 phase at 400 °C, followed by a transformation to α-Fe2O3 as the temperature increased. Above 600 °C, the unprecedented formation of FePO4 and Fe3PO7 was observed, produced from the reaction of Fe2O3 and free phosphate ions present in the black tea leaves, Fe3PO7 being the major phase obtained at 900 °C. Finally, the catalytic potential of the FeONPs to treat the azo dye methyl orange through a heterogeneous Fenton-like system was investigated.

Removal of methyl orange by heterogeneous Fenton catalysts prepared using glycerol as green reducing agent.

This study aims to prepare environmentally friendly iron catalysts supported on silica, using glycerol as green reducing and stabilizing agent, for application in heterogeneous Fenton degradation of the pollutant dye methyl orange (MO). The catalysts were characterized by X-ray powder diffraction, atomic absorption spectroscopy, scanning electron microscopy, transmission electron microscopy, thermogravimetric analyses, Mössbauer and Fourier transform infrared spectroscopies, which revealed the formation of iron(II)/(III) oxalates from the oxidation of glycerol by the iron(III) nitrate precursor. Besides, iron oxihydroxide nanoparticles with superparamagnetic behavior were also formed. Iron catalysts prepared in the presence of nickel(II) or zinc(II) nitrates lead to the formation of the corresponding oxalates. The catalysts were able to degrade MO, efficiently in 180 min of reaction. Fe/SiO2 furnished higher reaction rates, followed by Zn4Fe2/SiO2, which presented higher iron content as well as the smallest nanoparticles. Reaction parameters such as catalyst dosage, hydrogen peroxide concentration, pH and reaction temperature were investigated.

First-order phase transitions in CaFe2As2 single crystal: a local probe study.

(57)Fe Mössbauer spectroscopy has been used to investigate the structural and magnetic phase transitions of CaFe2As2 (T(N) = 173 K) single crystals. For this compound we found that V(ZZ) is positive and parallel to the c-axis of the tetragonal structure. For CaFe2As2 a magnetic hyperfine field B(hf) was observed at the (57)Fe nucleus below T(N) ~173 K. Analysis of the temperature dependence of B(hf) data using the Bean-Rodbell model shows that the Fe spins undergo a first-order magnetic transition at ~173 K. A collinear antiferromagnetic structure is established below this temperature with the Fe spin lying in the (a, b) plane. Below T(N) the paramagnetic fraction of Fe decreases down to 150 K and for lower temperatures all the Fe spins are magnetically ordered.

Mössbauer study of superconducting NdFeAsO(0.88)F(0.12) and its parent compound NdFeAsO.

(57)Fe Mössbauer spectroscopy has been used to investigate the magnetic order of non-superconducting NdFeAsO (T(N) = 140 K) and superconducting NdFeAsO(0.88)F(0.12) (T(c) = 45 K). A magnetic hyperfine field B(hf) was observed at the (57)Fe nucleus below T(N)∼140 K for NdFeAsO. Below ∼2 K an increase of B(hf) relative to the saturation value was attributed to the transferred B(hf) at the Fe site resulting from the collinear antiferromagnetic (AF) spin structure of the Nd moments. The analysis of the spectra is consistent with a commensurate AF order of Fe spins. No  B(hf) is observed in superconducting NdFeAsO(0.88)F(0.12) down to 1.5 K.