Determination Of Oxidation State Research Articles

Oxide- and Zeolite-Supported Molecular Metal Complexes and Clusters: Physical Characterization and Determination of Structure, Bonding, and Metal Oxidation State

This article is a review of the physical characterization of well-defined site-isolated molecular metal complexes and metal clusters supported on metal oxides and zeolites. These surface species are of interest primarily as catalysts; as a consequence of their relatively uniform structures, they can be characterized much more precisely than traditional supported catalysts. The properties discussed in this review include metal nuclearity, oxidation state, and ligand environment, as well as metal-support interactions. These properties are determined by complementary techniques, including transmission electron microscopy; X-ray absorption, infrared, Raman, and NMR spectroscopies; and density functional theory. The strengths and limitations of these techniques are assessed in the context of results characterizing samples that have been investigated thoroughly and with multiple techniques. The depth of understanding of well-defined metal complexes and metal clusters on supports is approaching that attainable for molecular analogues in solution. The results provide a foundation for understanding the more complex materials that are typical of industrial catalysts.

Metal and Ligand K-edge XAS of Titanium−TEMPO Complexes: Determination of Oxidation States and Insights into Ti−O Bond Homolysis

Ti-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) provides a means for generating Ti(III) complexes by homolysis of the Ti-O bond. It has been determined that bis-Cp-Ti-TEMPO complexes readily undergo homolytic cleavage while the mono-Cp-Ti-TEMPO complexes do not. Here Ti K- and Cl K-edge XAS are applied to directly determine the oxidation state of TiCl3TEMPO, TiCpCl2TEMPO, and TiCp2ClTEMPO, with reference to Ti(III) and Ti(IV) complexes of known oxidation state. The Ti K-edge data show that Ti(III) complexes exhibit a pre-edge feature approximately 1 eV lower than any of the Ti(IV) complexes; while the Cl K-edges show that Ti(III) complexes have a Cl K- pre-edge feature to approximately 1 eV higher energy than any of the Ti(IV) complexes. Taken together, the Ti and Cl K-edge data indicate that the Ti-TEMPO complexes are best described as Ti(IV)-TEMPO anions (rather than Ti(III)-nitroxyl radicals). In addition, the Cl K-edges indicate that replacement of Cl by Cp weakens the bonding with the remaining ligands, with the Cl 3p covalency decreasing from 25% to 21% to 17% on going from TiCl3TEMPO to TiCpCl2TEMPO to TiCp2ClTEMPO. DFT calculations also show that the electronic structures of the Ti-TEMPO complexes are modulated by the replacement of chloride by Cp. The effect of the Cp on the ancillary ligation is one factor that contributes to facile Ti-O bond homolysis in TiCp2ClTEMPO. However, the results indicate the primary contribution to the energetics of Ti-O bond homolysis in TiCp2ClTEMPO is stabilization of the three-coordinate product by Cp.

The use of indigenous vegetation for monitoring isotopic composition of lead deposited from the atmosphere in remote upland locations

and preserved in refrigerator afterwards (-18~ XANES measurements were carried out at Photon Factory (Japan) and SPring-8 (Japan). Both As and Fe K-edge XANES spectra were collected in fluorescence mode. Average As concentration in sediments is ca. 3 mg/kg. This is unexceptional value of upper continental crust. According to the previous field surveys, high As groundwater has been observed at 15-40 m depth from ground surface. However, we found that drastic arsenate reduction occurred around the depth of water table (-5 m). Ferric iron reduction, plausibly derived from ferric oxyhydroxide reduction, was observed near the surface as well. Proportions of reduced species are gradually increased toward dept. These results imply that the occurrence of key reactions (FeOOH reduction and arsenate reduction) which have been thought as a trigger of As release cannot fully explain the phenomenon. Conclusions: (1) XANES spectra were successfully measured to determine oxidation states of As in sediments (<10mg/kg). (2) There is a significant difference between the depths where (i) the key reactions (FeOOH and arsenate reductions) occurred and (ii) the concentrations of dissolved As increased.

Catalytic supports on the base of activated anthracites and synthetic carbons

Selective adsorption of platinum group metals (PMG) on activated carbons from a multi-component model and technological solutions was proposed for the preparation of heterogeneous-supported catalysts. Activated natural anthracites and a nitrogen-containing synthetic carbon are considered as carriers for Pd-supported catalysts. Their catalytic activity was studied in the Pd-catalyzed reactions of hydrogen peroxide decomposition and chloride ions oxidation by manganese(III). On the base of the obtained results, novel high sensitive analytical methods both for direct determination of supported-metal quantity and palladium oxidation states on the surface of spent adsorbents are suggested.

Quantitative determination of platinum oxidation state by XANES analysis

Quantitative determination of platinum oxidation state on supported platinum catalyst was examined by using Pt LIII-edge XANES spectra. The nano-sized platinum particles on the support were partially oxidized under oxidative atmosphere. The atomic ratio of oxygen to platinum (O/Pt) for these particles could be determined by the amount of desorbed oxygen in He carrier (O2-TPD) during elevated temperature. On the other hand, the XANES spectrum of the platinum catalysts oxidized under each corresponding condition was recorded under the same condition, then the area intensity of the white line of these spectra was evaluated by a simple simulation using a set of Gaussian and arctangent functions. A good linear relationship was confirmed between the white line intensity and the atomic ratio of oxygen to platinum, meaning that the oxidation state of the platinum could be quantitatively determined by a simple XANES analysis.

Some Aspects of Defect Chemistry in P-Type Perovskite Conductors

This paper is aimed at addressing some fundamental aspects of defect chemistry in p-type perovskite conductors, including determination of different oxidation states, contribution of various oxidations states to electronic conduction (p type and n type) and magnetic interaction, origin of nonstoichiometry and defect chemistry modeling. Studies on doped and undoped LaMnO3+δ, LaCrO3, LaFeO3 and La(Ni,Fe)O3 indicate that the electron hole conduction in perovskite mixed ionic-electronic conductors (MIECs) results from high oxidation state of B site cations, in which the electron holes are localized. The magnetic interaction, on the other hand, is attributed to the super exchange between B3+ and oxygen ions. The electron conduction in these systems is due to B2+; however some of the perovskites can not hold the structural integrity before B2+ cations are generated.

A decomposition procedure for the determination of copper oxidation states in Cu‐zeolites by XPS

AbstractA procedure to determine the chemical state of copper in differently prepared catalysts is proposed. It is based on a peak decomposition of the main Cu 2p3/2 signal only, which involves both Cu(II) and Cu(I) contributions. The presence of the closest shake‐up satellite is related to the Cu(II) contribution only, in fact a linear dependence of the satellite area versus the Cu(II) area has been found. This method has been applied to characterize variously prepared catalysts and to quantify the different copper states at the surface. A molecular picture is formulated to describe a possible mechanism for copper reduction occurring during the catalyst preparation route at the zeolite surface. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Minimum XPS core-level line widths of insulators, including silicate minerals

A monochromatic AlKα source and magnetic confinement charge compensation yield an As 3d line width of 0.52 eV for the bulk insulator As 2 S 3 (orpiment) which is as narrow as the As 3d line width obtained for the semiconductor FeAsS (arsenopyrite). These results, and the XPS results for silicates, demonstrate that charge broadening of XPS spectra has been overcome for most insulators provided care is taken with sample preparation. XPS core level spectral investigations of these minerals now may be conducted at the same level of detail as have been conducted on semi-conductors and metals. The Si 2p spectrum of a non-conducting orthosilicate (Mg-rich olivine) displays a total line width of 1.36 eV compared to very high resolution synchrotron XPS line widths of 1.1 eV for an Si 2p spectrum of gaseous Si(OCH 3 ) 4 , and 1.42 eV for the Si 2p peak of non-charging SiO 2 thin-films grown on Si metal. These results demonstrate that Si 2p peak broadening of these silicates does not result from differential charging. Comparison of the Si 2p spectrum of Si in olivine which contains non-polymerized, independent SiO 4 tetrahedra, with that of a gaseous analogue, Si(OCH 3 ) 4 , also composed of non-polymerized Si atoms tetrahedrally bonded to O, provides insight into the cause of Si 2p peak broadening for olivine (and probably for all other silicates). The vibrational contributions to the Si 2p spectrum of Si(OCH 3 ) 4 were used to fit the Si 2p peak of olivine, with each vibrational contribution broadened to reflect the minimum line width of the XPS instrument. Although the Si 2p envelope resulting from this procedure yields a reasonable fit to the olivine Si 2p spectrum, additional minor vibrational contributions may be required to reproduce accurately the Si 2p spectrum of olivine. Continued development of the technique should allow determination of transition metal oxidation states and other chemical state properties of non-conductor Al-silicates

Use of XPS method in determination of chemical environment and oxidation state of sulfur and silver atoms in Ag6S3O4and Ag8S4O4compounds

Use of XPS method in determination of chemical environment and oxidation state of sulfur and silver atoms in Ag₆S₃O₄and Ag₈S₄O₄compounds

Evidence for intestinal oxidative stress in obstructive jaundice-induced gut barrier dysfunction in rats.

An important factor that promotes bacterial and endotoxin translocation in obstructive jaundice is intestinal injury that causes increased permeability. However, little is known of the submicroscopic biochemical events leading to defects of the intestinal barrier. This study was undertaken to investigate the effect of experimental obstructive jaundice on intestinal lipid peroxidation, protein oxidation and thiol redox state. Rats were randomly divided into controls, sham operated and bile duct ligated (BDL). After 10 days, intestinal barrier function was assessed by measuring endotoxin in portal and aortic blood. Tissue samples from the terminal ileum were examined histologically and morphometrically, while other samples were homogenized for the determination of lipid peroxidation, protein oxidation and thiol redox state [reduced glutathione (GSH), oxidized glutathione (GSSG), total non-protein mixed disulphides (NPSSR), protein thiols (PSH) and protein disulphides (PSSP)]. Obstructive jaundice compromised intestinal barrier function leading to significant portal and systemic endotoxaemia. The intestinal mucosa in jaundiced rats was atrophic with significantly decreased villous density and total mucosal thickness. Determination of biochemical parameters of oxidative stress in the intestine showed increased lipid peroxidation and protein oxidation in BDL-rats. Thiol redox state revealed the presence of intestinal oxidative stress in jaundiced rats, indicated by a decrease in GSH and increased GSSG, NPSSR and PSSP. This study shows that experimental obstructive jaundice induces intestinal oxidative stress, which may be a key factor contributing to intestinal injury and leading to endotoxin translocation.

XPS Investigation of ceramic matrixes for disposal of long-living radioactive waste products

The synthesis of ceramic matrixes for the long-term storage of highly active radionuclide wastes and determination of physical and chemical forms of radionuclides in them is one of the important problems in radioecology. It enables to create purpose fully materials for the long-term storage of radionuclides. In the present work the samples of ceramics [CaCe0.9Ti2O6.8(I) and CaCeTi2O7(II}] formed under various conditions were investigated with the X-ray photo electron spectroscopy. It is necessary for synthesis of ceramic matrixes, for the disposal of the plutonium and others tetravalent actinides. A technique was developed for the determination of cerium oxidation state (Ce3+ and Ce4+) on the basis of the X-ray photo electron spectroscopy spectral structure characteristics. It was established that the sample (I) formed at 300 MPa and T = 1400 ?C in the air atmosphere contained on the surface two types of cerium ions in the ratio ? 63 atomic % of Ce3+ and 37 atomic % of Ce4+, and the sample (II) formed at 300 MPa and T= 1300 ?C in the oxygen atmosphere contained on its surface two types of cerium ions also, but in the ratio ? 36 atomic % of Ce3+ and 64 atomic % of Ce4+. It was established that on the surface of the studied ceramics carbonates of calcium and/or cerium could be formed under influence of the environment that leads to the destruction of ceramics.

Detecting and minimizing interferences in uranium oxidation states during dissolution of solid phases

Interferences in uranium oxidation states due to iron during acid dissolution of solid samples were studied. Hydrazine and different carboxylic acids were investigated for their suitability to minimize these interferences. Polyacrylic acid (PAA) was selected for further testing with common iron-containing minerals, pyrite and biotite. The interfering effect of iron decreased with increasing PAA concentration; 2.5% acid concentration was observed to be a suitable level to minimize interferences due to the iron-containing minerals. PAA and polyacrylic-co-maleic acid (PAMA) were tested for their effect on goethite. The oxidation of uranium by goethite was not supressed with the acids but further a slight oxidation was observed. Further experiments are required to find a suitable redox buffer for goethite and iron minerals. However, the promising results for pyrite and biotite widen the range of solid materials that can be considered suitable for the determination of uranium oxidation states.

XANES calibrations for the oxidation state of iron in a silicate glass

Fe K-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of anorthite-diopside eutectic glasses containing 1 wt% 57Fe2O3 quenched from melts equilibrated over a range of oxygen fugacities at 1409 °C. The Fe3+/ΣFe ratios were determined previously by 57Fe Mössbauer spectroscopy and vary between 0 (fully reduced) and 1 (fully oxidized). Using the Mössbauer results as a reference, various methods for extracting Fe3+/ΣFe ratios from XANES spectra were investigated. The energy of the 1s → 3d pre-edge transition centroid was found to correlate linearly with the oxidation state. Correlations also exist with the energy of the K absorption edge and the area of peaks in the derivative spectrum associated with the 1s → 4s and crest (1s → 4p) transitions. The Fe3+/ΣFe ratios determined from linear combinations of end-member spectra (Fe3+/ΣFe ~0 and ~1) were found to deviate significantly from the Mössbauer values. This may indicate the susceptibility of this method either to errors arising from the treatment of the background or to changes in Fe2+ or Fe3+ coordination with the Fe3+/ΣFe ratio. The general applicability of any XANES calibration for determining oxidation states is limited by variations in the Fe coordination environment, which affects both the intensity and energy of spectral features. Thus previous calibrations based on mineral spectra are not applicable to silicate glasses. Nevertheless, systematic trends in spectral features suggest that Fe3+/ΣFe values may be obtained from XANES spectra, with an accuracy comparable to Mössbauer spectroscopy, by reference to empirical calibration curves derived from compositionally similar standards.

A furnace design for XANES spectroscopy of silicate melts under controlled oxygen fugacities and temperatures to 1773 K.

A controlled-atmosphere furnace has been constructed for X-ray absorption spectroscopy experiments under imposed oxygen fugacities at temperatures up to 1773 K. The use of the furnace is demonstrated in a study of the oxidation state of Cr in a basaltic silicate melt (mid-ocean ridge basalt) by K-edge XANES spectroscopy. This is the first time the Cr(2+)/Cr(3+) ratio has been identified directly in an Fe-bearing melt. At typical terrestrial oxygen fugacities around half the Cr is present as Cr(2+), even though this oxidation state has never been identified in a terrestrial material and only Cr(3+) is observed after quenching to a glass. Cr(2+) oxidizes to Cr(3+) on cooling in the presence of Fe(3+) according to the electron exchange reaction Cr(2+) + Fe(3+) --> Cr(3+) + Fe(2+). This illustrates the importance of the in situ determination of metal oxidation states in melts.

Evaluation of non-chromatographic approaches for speciation of extractable As(III) and As(V) in environmental solid samples by FI-HGAAS

Non-chromatographic speciation approaches have been developed for determination of water-soluble and phosphate-exchangeable As(III) and As(V) in certified reference materials of coal fly ash and sediments by FI-HGAAS. A 2 IV 6–2 fractional factorial design was employed for screening optimisation of the flow injection manifold. A simple two-stage sequential extraction protocol involving deionized water and a phosphate buffer as extractants was employed. Determination of both oxidation states of As in the extracts could be accomplished following arsine generation under different reaction conditions, namely, (i) selective determination of As(III) in citric acid medium or using soft generation conditions (i.e. low HCl and NaBH 4 concentrations); (ii) determination of total As in each extract using thioglycollic acid as reaction medium or after pre-reduction of As(V) to As(III) with a KI+ascorbic acid mixture. The As(V) content was estimated by difference between both measurements. Reaction conditions were previously optimised and analytical parameters in each reaction medium were established. Overall, the extractable As content was less than 5% in sediment and fly ash CRMs. The LOD of As was around 0.07 μg l −1 for As(III) determination, and 0.06 μg l −1 for total As determination after prereduction. Liquid chromatography coupled to atomic fluorescence spectrometry with post-column hydride generation was used for comparison.

Apports des phyllosilicates dans la différenciation entre altération hypogène et altération supergène dans le basalte triasique du Moyen Atlas (Maroc)

Abstract Triassic basalt of the Middle Atlas has been subject to metamorphic transformation then weathering. Occurrence in both metabasalt and saprolite of ubiquitous clay minerals, such as smectite and mixed layers chlorite–smectite, makes it difficult to distinguish between the two alteration facies and explains the interest of complementary sources of information. In the Bhallil weathering profile, petrographical and mineralogical analyses of primary igneous minerals and their alteration products coupled with Fe oxidation state determination in clay fractions allow to identify three alteration facies: ( i ) metamorphic basalt, where iron occurs mainly as the ferrous form; ( ii) the lower part of saprolite, where iron is partially oxidized to its ferric form; ( iii ) the upper part of saprolite, where iron is completely oxidized. To cite this article: A. Dekayir et al., C. R. Geoscience 334 (2002) 877–884.

In Situ Determination of Local Ce Oxidation States During Redox Reactions

In Situ Determination of Local Ce Oxidation States During Redox Reactions

ENVIRONMENTAL AND BIOLOGICAL APPLICATIONS OF EXTENDED X-RAY ABSORPTION FINE STRUCTURE (EXAFS) AND X-RAY ABSORPTION NEAR EDGE STRUCTURE (XANES) SPECTROSCOPIES

ABSTRACT XAS (X-ray absorption spectroscopy) has proven to be a powerful technique in several fields including the biological and environmental sciences. It has enabled scientists to analyze samples that could not be analyzed using classical techniques such as XRD (X-ray diffraction). In addition, it allows for the direct determination of elemental oxidation states, where the use of other methods is time consuming, cumbersome, and can lead to false results. XAS experimentation has allowed the direct determination of the local coordination environments in many different samples. This review article briefly explains some of the concepts related to two of the most important techniques of XAS, which are EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure). An explanation of some of the parameters used to obtain XAS data and data reduction from raw spectroscopic data is discussed. Finally, some applied examples are given that has been obtained though the use of XAS in the biological and the environmental fields.

Organosulfur/Conducting Polymer Composite Cathodes : II. Spectroscopic Determination of the Protonation and Oxidation States of 2,5-Dimercapto-1,3,4-thiadiazole

The dimercaptan 2,5-dimercapto-1,3,4-thiadiazole (DMcT) has been employed as a component in secondary lithium battery cathodes. However, spectroscopic studies of its behavior have been impeded by general uncertainty regarding its vibrational band character. The purpose of this study is to provide a spectroscopic basis for the unambiguous determination of protonation and oxidation state of DMcT. The first part of this paper is concerned with structural and spectroscopic assignment of DMcT and some derivatives. Several structural and spectroscopic assignments are intrinsically interesting, including (i) the suggestion that the thioamide-containing compounds adopt the thione tautomer structure whenever the opposite nitrogen is unsubstituted and (ii) assignment of an ring mode for the first time for these compounds. The second part of this paper uses that resource to examine the in situ Raman spectroelectrochemistry of DMcT in a nonaqueous solvent. The results confirm, in part, the redox wave assignments made previously in the literature. Specifically, Raman spectroelectrochemistry of solutions of DMcT and the lithium salt of its first conjugate base (LiDMcT) show that at ca. 0.8 V LiDMcT is oxidized while DMcT is not oxidized, and that at +1.9 V both species were oxidized. Results regarding oxidation-initiated precipitation of oligomers are also reported. © 2002 The Electrochemical Society.

Application of XANES for the Determination of Oxidation States of Co and Pb in Natural Ferromanganese Nodules

Abstract Cobalt K-edge and Pb LIII-edge XANES spectra were obtained by fluorescent XANES using solid state detector (SSD) for Co and Pb in natural rocks, ferromanganese nodules. The XANES spectra suggest that Co(III) and Pb(II) are their main oxidation states in ferromanganese nodules. Based on the results, enrichment mechanisms of Co and Pb in ferromanganese nodules were discussed.