Abstract: A mix-valenced cobalt oxide, CoOx, was prepared from cobalt nitrate aqueous solution through a precipitation with sodium hydroxide and an oxidation by hydrogen peroxide. Further, other pure cobalt oxide species were refined from the CoOx by temperature-programmed reduction (TPR) at 170, 230 and 300 °C (labeled as R-170, R-230 and R-300, respectively). They were characterized by X-ray (XRD), infrared (IR), thermogravimetry (TG) and TPR. The major composition of CoOx is CoO(OH), with a small amount of Co4+ species; R-170 is CoO(OH) with a hexagonal structure; R-230 is Co3O4 with a spinel structure and R-300 is CoO with a cubic structure. Their catalytic activities toward the CO oxidation were further studied in a continuous flow microreactor. The results indicated that the relative activity decreased significantly with the oxidation state of cobalt, i.e., CoO(+2)≳Co3O4(+8/3)≫CoO(OH)( +3)≳CoOx(>+3).

Abstract: The one-electron oxidized species of a Ni(II)-phenolate complex has been shown to be in the Ni(II)-phenoxyl radical state at room temperature and the Ni(III)-phenolate state at < -120 degrees C, indicating that the oxidation state is temperature dependent.

Abstract: The use of XANES spectroscopy, both in classical and in dispersive geometries, is illustrated for the study of copper-based catalysts under in situ or in operando conditions. As case studies, copper-exchanged MFI zeolites and CuCl2/γ-Al2O3 systems are considered. In the former case, in situ XANES spectroscopy was used to characterise well defined complexes (Cu+N2, Cu+(CO)3, Cu+(NH3)(CO) and Cu+(NO)2) formed on copper ions inside the zeolite cavities under controlled conditions. From these results, useful information concerning the symmetry of the formed complexes can readily be gained. The latter case shows how the use of dispersive XANES spectroscopy allows to follow, in real time, the evolution of a system in working conditions. The simultaneous determination of the catalyst activity and of the average oxidation state of copper in the catalyst allows the evolution of a system in working conditions to be followed in real time. The criteria used for the quantification of the Cu(I) and Cu(II) fraction from XANES spectra are discussed in detail.

Abstract: The As sorption capacity of a natural Mn and Fe mineral-containing sample from the Iron Quadrangle province, Brazil, was investigated. A detailed mineralogical identification was obtained by combining X-ray diffraction analyses (with Rietveld refinement), X-ray fluorescence spectroscopy, optical microscopy, and scanning electron microscopy coupled with X-ray energy dispersive spectrometry-EDS. The oxidation state of the adsorbed As species was determined by X-ray absorption near edge structure spectroscopy. The results demonstrate that the presence of naturally occurring Mn oxides promotes the effective oxidation of As (III) to As(V). Also, the Mn minerals show a significant uptake of both the trivalent and pentavalent As species. This study demonstrates that the combined influences of As(III) depletion by oxidation and adsorption on a natural oxide sample consisting of Mn minerals and Fe oxides may effectively contribute to the reduction the As concentration in waters.

Abstract: A total of 119 CeO(n)fragments with n = 3-12 were analyzed by using the bond valence sum, or BVS, method to yield new R(0) values for Ce(III)-O of 2.121(13) A and for Ce(IV)-O of 2.068(12) A. These R(0) values can be used to calculate the oxidation state of Ce in complexes where Ce is bonded only to O ligands. Furthermore, the average R(0) value of 2.094 A gives a good indication of whether the oxidation state of the Ce ion is +3 or +4 from the observed distances without any assumptions. The fact that complexes with coordination numbers of 10-12 are in agreement is significant since this study is the first example which indicates that high coordination numbers also follow BVS rules. The Ce-O distances used in deriving the R(0) values for the +3 and +4 complexes are tabulated as a function of coordination number and have a wide range of values, but the average Ce-O distance reflects the requirement that the BVS must equal the oxidation state. Several examples are given where the oxidation state of the Ce ion is apparently incorrectly assigned, as well as cases where problems with the X-ray structure determinations are indicated by a disagreement between the postulated and calculated oxidation state.

Abstract: The effects of 3 vol.% of CO2 added in the gas feed are investigated in the oxidative dehydrogenation of propane (ODP) to propene on NiMoO4 catalysts. In spite of the low reaction temperature range (400-480degreesC), it turns out that CO2 is not inert during the reaction. CO2 behaves as a strong oxidant with respect to the catalyst. CO2 helps to maintain the catalytic surface in a high oxidation state under conditions for which NiMoO4 undergoes a reduction. CO2 clearly promotes the non-selectivity of the catalyst and can inhibit deactivation. Under particular conditions it can improve propene formation. CO2 is able to oxidize propane to propene. We suggest that CO2 dissociates during the reaction to CO and an active oxygen species, which is responsible for the oxidizing effect of carbon dioxide. This work opens promising perspectives in using CO2 to tune dynamically the oxidation state of "working" oxide surfaces in their most active and selective configurations for oxidation processes. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: Libyan Desert Glass (LDG) is an enigmatic type of glass that occurs in western Egypt in the Libyan Desert Fairly convincing evidence exists to show that it formed by impact, although the source crater is currently unknown Some rare samples present dark-colored streaks with variable amounts of Fe, and they are supposed to contain a meteoritic component We have studied the iron local environment in an LDG sample by means of Fe K-edge high- resolution X-ray absorption near edge structure (XANES) spectroscopy to obtain quantitative data on the Fe oxidation state and coordination number in both the Fe-poor matrix and Fe-rich layers The pre-edge peak of the high-resolution XANES spectra of the sample studied displays small but reproducible variations between Fe-poor matrix and Fe-rich layers, which is indicative of significant changes in the Fe oxidation state and coordination number Comparison with previously obtained data for a very low-Fe sample shows that, while iron is virtually all trivalent and in tetrahedral coordination ( (4) Fe 3+ ) in the low-Fe sample, the sample containing the Fe-rich layers display a mixture of tetra-coordinated trivalent iron ( (4) Fe 3+ ) and penta-coordinated divalent iron ( (5) Fe 2+ ), with the Fe in the Fe-rich layer being more reduced than the matrix From these data, we conclude the following: a) the significant differences in the Fe oxidation state between LDG and tektites, together with the wide intra-sample variations in the Fe-oxidation state, confirm that LDG is an impact glass and not a tektite-like glass; b) the higher Fe content, coupled with the more reduced state of the Fe, in the Fe-rich layers suggests that some or most of the Fe in these layers may be directly derived from the meteoritic projectile and that it is not of terrestrial origin

Abstract: Manganese(II) complexes of mononegative pentadentate N4O ligands [Mn2(mgbpen)2(H2O)2](ClO4)2 (1), (mgbpen− = N-methyl-N′-glycyl-N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine) and [Mn2(bzgbpen)2(H2O)2](ClO4)2 (2), (bzgbpen− = N-benzyl-N′-glycyl-N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine) have been prepared. The crystal structure of the Mn(II)–aqua complex of 1, shows it to be dimeric via (μ-κO)-bridging through one carboxylate oxygen atom of each of the two ligands. The non-coordinated carboxylate oxygen atoms are H-bonded to the water ligands on the adjacent Mn ion. The magnetic coupling interaction is weak and antiferromagnetic, J = −1.3(1) cm−1. The dimeric structures of 1 and 2 are retained in solution and can exist in the gas phase. Complexes 1 and 2 are air stable but can be oxidised by tBuOOH to give unstable mononuclear Mn(III) complexes, or oxo-bridged dimanganese(III) and di-μ-oxo-dimanganese(IV) complexes, depending on solvent. The [Mn(III)–OR]+, R = H or CH3 complexes are generated in water or methanol, respectively, and are potentially useful spectroscopic models for active Mn–lipoxygenases. In acetonitrile, di-μ-oxo-dimanganese(IV) complexes are the highest oxidation state products detected, and these are formed via shorter-lived intermediate μ-oxo-dimanganese(III) compounds. The rate of formation of the various oxidized products is slower in the case of the bzgbpen− systems which contains a bulkier non-coordinating arm. The oxidised complexes were characterised by UV-visible spectroscopy, ESI mass spectrometry and cyclic voltammetry. In addition, III–IV and II–III species were electrochemically generated. Thus the new mononegative pentadentate ligand systems display significant flexibility in the range of Mn oxidation states and species of biological relevance that are accessible: A series of dinuclear compounds with different structures in the five oxidation levels between II–II and IV–IV has been identified. No solid state structures were obtained for high oxidation state species, however it is assumed that in the oxo-bridged compounds the carboxylate groups are terminally ligated in contrast to the starting Mn(II) complexes. Thus the system represents examples of limiting structures in the “carboxylate shift” mechanism proposed to be important in non-heme H2O and O2 activation processes.

Abstract: 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 Cr2+/Cr3+ ratio has been identified directly in an Fe-bearing melt. At typical terrestrial oxygen fugacities around half the Cr is present as Cr2+, even though this oxidation state has never been identified in a terrestrial material and only Cr3+ is observed after quenching to a glass. Cr2+ oxidizes to Cr3+ on cooling in the presence of Fe3+ according to the electron exchange reaction Cr2+ + Fe3+ → Cr3+ + Fe2+. This illustrates the importance of the in situ determination of metal oxidation states in melts.

Abstract: In silicate melts, the speciation state of altervalent elements depends on the extension and distribution of polymeric units in the system Since polymerization of silicate melts is driven by acid–base properties of constituting oxides depending on their Lux–Flood reactivity, the bulk optical basicity of molten silicates and glasses allows us to compute the distribution of oxygen among three distinct polarization states (O2−, O−, O0) in conjugation with the polymeric model of Toop and Samis Data available in literature are employed to investigate the mutual interaction between sulfur and iron in silicate melts to constrain the equilibrium between sulfide and sulfate species Results put in evidence (i) the role of bulk melt chemistry, affecting through polymerization, and hence the transfer of electrons among the three polarization states of oxygens, the oxidation state of altervalent elements such as Fe and S and (ii) the limits of the Temkin approach implicit in chemical equations that consider sulfide and sulfate in melts as free anions Improvements of the model’s accuracy for sulfur speciation may be attained by accounting for the various dissociation equilibria of molten sulfide M2/νS, oxide M2/ν and sulfate M2/νSO4 species in their standard state of pure component in the melt phase at temperature and pressure of interest instead of that of complete dissociation required by the Temkin model

Abstract: ScII or mixed oxidation state ScIII/ScI? Reduction of the homoleptic ScIII complex [Sc(P3C2tBu2)3] allows access to the subvalent [{Sc(P3C2tBu2])2}2] complex (see picture), in which the metal centers are formally bivalent. DFT calculations support fomulation of the reduced species as a mixed oxidation state.

Abstract: Ti(III) compounds (halides and n-butoxide) along with several mono-, bi-, and tridentate ligands have been studied for the first time in the atom transfer radical polymerization (ATRP) of styrene. The important advantage of titanium compounds is the white color of the Ti(IV) ions (Ti in the highest and air stable oxidation state) which ensures the absence of coloring in the final polymer. The better control over polymerization was realized when conditions increasing the reactivity of the intermediate Ti(IV) species and decreasing their steady state concentration were maintained. This occurred when chlorides were replaced with bromides (which decreased the bond strength), and when ligands with low donor ability were employed. Using a sulfide complex of Ti(III) chloride and 1,2-bis(hexylthio)ethane allowed styrene polymerization with kinetic and molecular weight characteristics inherent to ATRP.

Abstract: In situ X-ray absorption fine structure (EXAFS) spectroscopy combined with on-line gas analysis was used to get insight into the oxidation state of the constituents and of the structure of a Pd/ZrO2 catalyst during activation and catalytic combustion of methane. The sample was prepared from an amorphous Pd33Zr67-alloy by oxidation at 400 °C. In situ XANES/EXAFS together with thermal analysis showed that the Pd and Zr constituents were oxidized at the same time. Catalytic combustion activity was higher if the catalyst was previously reduced by hydrogen than if the fully oxidized sample was used. According to EXAFS complete reduction of PdO/ZrO2 to Pd/ZrO2 was achieved by hydrogen exposure at room temperature. In situ EXAFS studies during temperature programmed reaction in methane/oxygen and under reaction conditions uncovered that palladium is mainly in an oxidized state. However, during methane combustion in 1% CH4/4% O2/He at 500–550 °C the active catalyst contains some metallic Pd, as evidenced by in situ EXAFS, which seems to be beneficial for the activity. The catalyst can be fully reduced by methane and re-oxidized by oxygen in separate steps at this temperature.

Abstract: XANES and EXAFS spectroscopic studies at the Mn–K- and Br–K-edge of reaction products of (S,S)-(+)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride ([(salen)MnIIICl], 1) and (S,S)-(+)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) bromide ([(salen)MnIIIBr], 2) with 4-phenylpyridine N-oxide (4-PPNO) and 3-chloroperoxybenzoic acid (MCPBA) are reported. The reaction of the MnIII complexes with two equivalents of 4-PPNO leads to a hexacoordinated compound, in which the manganese atom is octahedrally coordinated by four oxygen/nitrogen atoms of the salen ligand at an average distance of ≈1.90 A and two additional, axially bonded oxygen atoms of the 4-PPNO at 2.25 A. The oxidation state of this complex was determined as ≈+IV by a comparative study of MnIII and MnV reference compounds. The green intermediate obtained in reactions of MCPBA and solutions of 1 or 2 in acetonitrile was investigated with XANES, EXAFS, UV/Vis, and Raman spectroscopy, and an increase of the coordination number of the manganese atoms from 4 to 5 and the complete abstraction of the halide was observed. A formal oxidation state of IV was deduced from the relative position of the pre-edge 1s→3d feature of the X-ray absorption spectrum of the complex. The broad UV/Vis band of this complex in acetonitrile with λmax=648 nm was consistent with a radical cation structure, in which a MCPBA molecule was bound to the MnIV central atom. An oxomanganese(V) or a dimeric manganese(IV) species was not detected.

Abstract: A set of Ce3+ activated phosphate glasses is studied. Dependence of the absorption band-edge position on the activator content and glass composition is discussed with respect to variation of the so-called optical basicity. XPS analysis of the samples is performed to get information about the oxidation state of the Ce activator cations in the glasses.

Abstract: The reactions of ciprofloxacin (CIP) and norfloxacin (NOR) with iron(II) and iron(III) perchlorate have been investigated. The optical spectra support the formation of four complexes for each oxidation state with 1 : 1, 1 : 2, 1 : 3 and 1 : 4 metal to ligand molar ratios. The electrical conductivity and magnetic susceptibility measurements show that the isolated complexes are high spin and the Fe(ClO 4 ) 2 and Fe(ClO 4 ) 3 complexes behave as 1 : 2 and 1 : 3 electrolytes, respectively. The IR spectra indicate that CIP and NOR bind to the iron ion as bidentate ligands through the carbonyl oxygen atom and one of the oxygen atoms of the carboxylate group.

Abstract: The polyoxometalates are intermediate species between the mononuclear oxo/hydroxometalates [MOx(OH)y]n− and polymeric metallic oxides and are usually known with transition metals in a high oxidation state. They are obtained by condensation reactions that occur when a solution of the metal anion is acidified

Abstract: A 1 bismuth-doped VPO catalyst was prepared by refluxing Bi(NO3)3 and VOPO4 ⋅ 2H2O in isobutanol. The incorporation of Bi into the VPO lattice lowered the overall V oxidation state from 4.24 to 4.08. It also lowered both the peak maximum temperature for the desorption of oxygen from the lattice from 1001K (undoped) to 964K with a shoulder at 912K and the peak maxima for H2 temperature-programed reduction from 863, 1011 and 1143K (undoped) to 798, 906 and 1151K. The total oxygen desorbed from the Bi-doped catalyst was only one-fourth that of the undoped catalyst, while the amount of oxygen removed by TPR was roughly the same for both catalysts. These results suggest that in anaerobic oxidation, the Bi-doped catalyst will have roughly the same activity as in undoped catalyst in C4 hydrocarbon oxidation but will have a higher selectivity to products such as olefins and maleic anhydride.

Abstract: Periodic ab initio QM calculations are employed in order to study the structure and redox properties of Cr, Mn, Fe. and Co trivalent transition metal dopants in AlPO-34. Our results show that the local environment of Mn and Co dopants is a distorted tetrahedron due to Jahn-Teller distortions. The bonding between the Me-III dopants and the neighboring oxygens is ionic in nature, and explains the Lewis acidity of the Me-III ions. The replacement energy DeltaE(III) of the 3+ dopant ions in AlPO-34 increases linearly as a function of the Me-O bond distance. Finally, the calculated redox energies of the Me-II/Me-III couples indicate that, among the transition metal ions investigated, Fe is the most stable in the 3+ oxidation state, whereas Mn is the most stable as 2+ ion. Cr and Co, instead, have intermediate behavior and can switch more easily between the two oxidation states. These results contribute to elucidate the mechanistic details of catalytic processes occurring in MeAlPOs.

Abstract: Vanadium and mixed vanadium and iron antimonates with rutile-type structures have been studied by XANES at Sb L1 edge, 57Fe Mossbauer spectroscopy and ESR spectroscopy at 77 K. The results showed that both antimony and iron remained at their highest oxidation state, i.e. SbV and FeIII, whereas vanadium was present as VIII and VIV. Two types of VIV species were distinguished corresponding to well-isolated vanadyl species in distorted octahedral coordination and vanadyl species in the same coordination but close to each other and in a dipole–dipole interaction. Both VIII and total VIV concentrations decreased when the iron content increased, whereas isolated VIV concentration increased first and then decreased, with a maximum for x = 0.2 in FexV1-xSbO4. The observed variations in cationic composition are discussed in relation with the catalytic properties of the compounds in the ammoxidation of propane. Isolated VIV-O moieties appeared to be the most active and selective catalytic sites.

Abstract: In view of an understanding of catalytic efficiency the electronic and magnetic state of iron in Fe/KL and the influence of Pt on these properties in Pt-Fe/KL were investigated by superconducting quantum interference device (SQUID) magnetization measurements and Mossbauer spectroscopy. After calcination in air, iron is present only as Fe3+ ions in both samples, mainly in a paramagnetic state. There is also a small antiferromagnetic contribution, probably due to iron oxide and hydroxide particles which precipitate on the outer surface of the zeolite crystallites during preparation. After reduction in H2, the behavior of both samples is predominantly superparamagnetic, but the presence of Pt increases both the saturation magnetization and the size of the residual hysteresis loop. At the same there remains a pronounced paramagnetic contribution which appears to be even slightly higher than before reduction. The interpretation is that alloy particles of metallic Pt–Fe are formed on the outer surface. Since small clusters inside the zeolite crystallites are most active for catalysis the alloy formation seems to be not of relevance for an enhancement of sulfur resistance. It was also shown that during reduction a fraction of Fe2+ is formed. The coordination of the Fe2+ ions is clearly influenced by Pt and it seems very likely that Fe2+ ions are in close contact with Pt clusters. This interaction is probably the origin of the enhanced sulfur resistance and catalytic activity in the reforming reactions.