The magnetic exchange interactions in chromium chalcogenide spinels

The chromium chalcogenide spinels, MCr2X4 (M = Zn, Cd, Hg; X = O, S, Se), have been the subject of considerable interest in recent years. In each case the crystal structure is that of a normal spinel with the chromium ions exclusively occupying the octahedral (B) sites, so that when diamagnetic ions are located at the tetrahedral (A) sites the only magnetic interactions present are those between B-site ions. Despite such apparently simple circumstances a rich variety of magnetic behaviour is exhibited. For the oxides the ground state spin configurations are antiferromagnetic whilst for the selenides ferromagnetic interactions dominate and several authors have drawn attention to the fact that the nature of the dominant interaction is a function of the nearest neighbour chromium - chromium separation. However, at least two of the compounds exhibit spiral structures and it has been proved difficult to account for the various spin configurations within a unified theory of the magnetic interactions involved. More recently, the possibility of formulating a simplified interpretation of the magnetic interactions has been provided by the discovery that the crystal struture of spinels does not always conform to the centrosymmetrical symmetry Fd3m that has been conventionally assumed. The deviation from this symmetry is associated with small < 111> displacements of the octahedrally coordinated metal ions and the structures so obtained are more correctly referred to the non-centrosymmetrical space group F43m. In the present study, therefore, extensive X-ray diffraction data have been collected from four chromium chalcogenide specimens and used to refine the corresponding structural parameters assuming F43m symmetry and also with conventional symmetry. The diffracted intensities from three of the compounds concerned cannot be satisfactorily accounted for on the basis of conventional symmetry and new locations have been found for the chromium ions in these cases. It is shown, however, that these displacements in chromium positions only partially resolve the difficulties in interpreting the magnetic behaviour. A re-examination of the magnetic data from different authors indicates much greater uncertainty in their measurements than they had claimed. By taking this into consideration it is shown that a unified theory of magnetic behaviour for the chromium chalcogenide spinels is a real possibility.

Effect of sample preparation on the thermal degradation of metal-added biomass

The present study investigates the effect of different sample preparation methods on the pyrolysis behaviour of metal-added biomass; Willow samples were compared in the presence of two salts of zinc and lead containing sulphate and nitrate anions which were added to the wood samples with three different techniques as dry-mixing, impregnation and ion-exchange. The effect of acid and water wash as common demineralisation pre-treatments were also analysed to evaluate their roles in the thermal degradation of the biomass. Results from thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR) and pyrolysis-mass spectrometry (Py-MS) measurements indicated that these pre-treatments change the matrix and the physical-chemical properties of wood. Results suggested that these structural changes increase the thermal stability of cellulose during pyrolysis. Sample preparation was also found to be a crucial factor during pyrolysis; different anions of metal salts changed the weight loss rate curves of wood material, which indicates changes in the primary degradation process of the biomass. Results also showed that dry-mixing, impregnation or ion-exchange influence the thermal behaviour of wood in different ways when a chosen metal salt was and added to the wood material. © 2011 Elsevier B.V. All rights reserved.

A comparative study on the pyrolysis of metal- and ash-enriched wood and the combustion properties of the gained char

This study aims to investigate the pyrolysis behaviour of metal-contaminated wood and the combustion properties of char derived from wood pyrolysis. Seven metals (Na, Mg, Ca, Zn, Cd, Pb and Fe(III)) were introduced to willow in cation form by ion-exchange and the thermal behaviour of demineralised samples and samples with additional ash were also investigated. The results show that the char yield increased from 21% to 24-28% and levoglucosan yield in vapour phase decreased from 88% to 62-29% after the addition of inorganic compounds, even though the metal binding capacity of wood varied from one metal ion to another. While char yield seems to be effected mainly by the concentration of the metal ions, levoglucosan yield was more dependent on the ionic species especially when sodium ions were present. When combustion experiments were carried out with char made of the metal enriched wood, two consecutive steps were observed, both effected by the presence of inorganic compounds. The first step was identified as the release and combustion of volatiles, while the second peak of the burning profile is the actual combustion of the fixed carbon. The burnout temperatures, estimated ignition indices and the conversion indicate that the type and not the amount of metal ions were the determining factors during the second step of combustion. © 2012 Published by Elsevier B.V.

N-Heterocyclic carbene coated metal nanoparticles

N-Heterocyclic carbene coated Au and Pd nanoparticles have been prepared by a ligand exchange reaction; although carbenes quantitatively displaced the thioether and phosphine ligands from the nanoparticle surface, the resultant nanoparticles spontaneously leached metal complexes and aggregated in solution. © 2009 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

The antioxidant potential of modified quercetins and quercetin-metal complexes

Quercetin is a naturally occurring polyphenol compound present in grapes, red wine, tea, apples and some vegetables. Like other flavonoids, it has been found to have antioxidant activity in studies in vitro, although there is still much debate about the bioavailability of flavonoids in the diet and their in vivo antioxidant activity. In general, it is thought that the antioxidant efficiency of polyphenols increases with increasing hydroxylation of the rings, but there have been few studies of other substitutions. We have prepared several derivatives of quercetin, to test the effect of modification on their antioxidant potential. Sodium salts of quercetin-5-sulfonate and quercetin-5,8-sulfonate, and transition metal complexes of quercetin-5-sulfonate were analysed for their total antioxidant potential using the FRAP assay, and compared to unmodified quercetin. It was found that quercetin-5-sulfonate complexes with Zn, Cu(II), Fe(II) and Mg were all significantly better antioxidants than quercetin, quercetin-5-sulfonate was comparable to quercetin, whereas the sodium salt of quercetin-5,8-sulfonate had a decreased total antioxidant potential. Kinetic studies of the FRAP reaction showed no significant differences between quercitin and any of the derivatives. The reaction of all the quercetins in the FRAP assay was found to be slower to reach completion than ascorbate, and appeared to have biphasic characteristics. These results suggest that transition metal ions may facilitate the transfer of electrons from the polyphenol ring system to the oxidant, while substitution with S03 is electron-withdrawing and destabilizes the ring system. This is important both for understanding the antioxidant ability of flavonoids, and for the design of novel antioxidant compounds. Further work is being carried out to assess the ability of the quercetin complexes to protect cultured cells from oxidative stress.