Oxidation States Of Metal Cations Research Articles

Using collective knowledge to assign oxidation states of metal cations in metal-organic frameworks.

Knowledge of the oxidation state of metal centres in compounds and materials helps in the understanding of their chemical bonding and properties. Chemists have developed theories to predict oxidation states based on electron-counting rules, but these can fail to describe oxidation states in extended crystalline systems such as metal-organic frameworks. Here we propose the use of a machine-learning model, trained on assignments by chemists encoded in the chemical names in the Cambridge Structural Database, to automatically assign oxidation states to the metal ions in metal-organic frameworks. In our approach, only the immediate local environment around a metal centre is considered. We show that the strategy is robust to experimental uncertainties such as incorrect protonation, unbound solvents or changes in bond length. This method gives good accuracy and we show that it can be used to detect incorrect assignments in the Cambridge Structural Database, illustrating how collective knowledge can be captured by machine learning and converted into a useful tool.

Analysis of oxygen bonding with metals of different oxidation states from experimental charge density distribution

Oxygen bonding with metals of different oxidation state of metal cations, ranging from +1 to +6 have been thoroughly analysed using experimental charge density distribution evolved from X-ray diffraction experiments using multipole and MEM models of charge density. Oxygen atoms enacting different bonding characteristics at different symmetries are well defined in the course of study of the (3,-1) bond critical points. In case of low oxidation state of metal cations, as viewed in SrO and ZnO, ionic interactions are pronounced. As the oxidation on metals ascend, combinations of intermediate, polar covalent and eventually strong correlation induced interactions are evidenced.

Effects of Co-substitution on the reactivity of double perovskite oxides LaSrFe2-xCoxO6 for the chemical-looping steam methane reforming

The double perovskite oxides (DPOs) LaSrFe2-xCoxO6 (x = 0, 0.2, 0.4, 0.6, 0.8) were investigated as oxygen carriers for the chemical looping steam methane reforming (CL-SMR). The fresh oxides were prepared by micro-emulsion method and their physical and chemical properties were characterized by X-ray diffraction, H2-temperature programmed reduction and X-ray photoelectron spectroscopy technologies. Meanwhile, isothermal reactions for methane reforming and steam splitting were carried out in a fixed-bed reactor to determine the influences of Co-substitution on the reactivity of LaSrFe2-xCoxO6. The substitution of metal Co has no obvious effect on the crystal structure of double perovskite, but induces a certain degree of Fe/Co disorder generating oxygen vacancies and/or higher oxidation states of metal cations. Synergistic interaction between surface metal ions, such as (Fe4+/Fe5+-O2--Co2+) and (Fe3+-O2--Co3+), plays a positive effect for the dissociation of methane. The activity may be more likely to be associated with the active oxygen species in connection with Co species on the DPOs surface and abundant of syngas was generated due to the concordant of methane dissociation with the lattice oxygen diffusion. Comprehensively considered, an optimal range of the degree of Co substitution is x = 0.4–0.6 for LaSrFe2-xCoxO6, probably converting 70% of CH4 into CO and H2 with molar ratio around 2:1. At the reduced states, the ability of DPOs for steam splitting is primarily associated with the oxygen vacancies after oxygen consumption. The substitution of metal Co slightly enhances the hydrogen production capacity and resistance to carbon formation, achieving the average hydrogen yields at 2.89–3.33 mmol/g oxygen carrier and 1.46–1.61 wt% of carbon depositions.

In-situ XANES and XPD studies of NiO/Ce0.9Zr0.1O2 IT-SOFCs anode nanomaterial as catalyst in the CPOM reaction

The aim of this paper is to study the oxidation states of metal cations and the changes in the crystalline phases in NiO/Ce0.9Zr0.1O2 nanocatalysts under typical conditions of methane catalytic partial oxidation reaction by in-situ X ray absorption near edge spectroscopy (XANES) and X-ray powder diffraction (XPD) studies. The Ce0.9Zr0.1O2 mixed oxide was obtained by glycine-nitrate-combustion method, being the nickel incorporated by incipient wetness impregnation. The evolution of the crystalline structure with temperature and operating conditions was followed by in-situ XPD experiments and the oxidation states of Ce and Ni cations by in-situ XANES experiments at the Ce-LIII and Ni-K absorption edges. It was observed that NiO is completely reduced to Ni° at temperatures above 650°C while Ce0.9Zr0.1O2 fluorite-like phase showed changes in lattice parameter due to cerium reduction and crystallite growth, but no phase transformations or segregations were observed. It was confirmed that Ni° is the active centre to activate methane molecule while Ce4+/Ce3+ ratio is strongly related with CO/CO2 concentration in the exhaust gas flow.

Synergistic improvements in stability and performance of the double perovskite-type oxides La2−xSrxFeCoO6 for chemical looping steam methane reforming

Chemical looping steam methane reforming (CL-SMR) is a potential route to efficiently co-produce syngas and hydrogen. Development of oxygen carrier with high activity, good recyclability, strong resistance to carbon deposition and excellence capacity for steam splitting is highly desired for this process. The article investigated a novel and unique structure of double perovskite-type oxides La1−xSrxFeCoO6 (x=0, 0.2, 0.4, 0.6, 0.8, 1.0) as oxygen carrier. XRD, XPS and H2-TPR technologies were adopted to characterize the physical and chemical properties of them. Meanwhile, isothermal reactions and cyclic redox reactions were carried out in a fixed-bed reactor to determine the influences of Sr-substitution on the reactivity of La1−xSrxFeCoO6. XRD results confirmed the formation of double perovskite crystal structure for all the samples, while substitution of Sr induced a certain degree of Fe/Co disorder generating oxygen vacancies and/or higher oxidation states of metal cations. Synergistic interactions between surface metal ions, such as Fe4+/Fe5+ with Co3+ which were detected by XPS, strongly enhance the reducibility of oxygen carriers. Three zones including total oxidation of methane by surface oxygen, partial oxidation of methane by lattice oxygen and carbon deposition were divided. Among the six samples with different substitution of Sr, La0.6Sr0.4FeCoO6 exhibited the best oxygen transport ability, thermal stability, as well as capacity for hydrogen generation. A stable CH4 conversion at ∼90% with desired H2/CO ratio at 2.0–2.5 in the methane reduction stage, and an average hydrogen yield at ∼5.9mmol/g oxygen carrier with ∼93.8% of hydrogen concentration in the steam oxidation stage were obtained during twenty successive redox reactions, which made them very attractive for the purpose of chemical looping partial oxidation of carbon fuel in real applications.

Determination of the surface electronic structure of Fe3O4(1 1 1) by soft X-ray spectroscopy

The determination of surface terminations in transition metal oxides is not trivial because many structural configurations could be possible. They exhibit various terminations depending on the oxidation states of metal cations exposed to the surface. Fe3O4 is one example in which octahedrally and tetrahedrally coordinated Fe2+ and Fe3+ cations coexists with oxygen anions. For the identification of the surface termination of Fe3O4(111) grown on Pt(111) we have employed surface sensitive synchrotron based X-ray photoelectron and absorption spectroscopy. It has been shown that the topmost surface is octahedrally coordinated Fe3+ rich.

Synthesis and Structure of New Cd−Bi−S Homologous Series: A Study in Intergrowth and the Control of Twinning Patterns

Four compounds, CdBi2S4, CdBi4S7, Cd2.8Bi8.1S15, and Cd2Bi6S11, have been synthesized using a RbCl/LiCl flux. Single-crystal X-ray structure refinements were made for CdBi2S4 and Cd2.8Bi8.1S15. The final R values for these two crystals were respectively 3.05% and 5.53%. The compounds CdBi2S4 and CdBi4S7 are isotypic with respectively HgBi2S4 and Y5S7. Cd2.8Bi8.1S15 can be considered as an ordered intergrowth between the two phases CdBi2S4 and CdBi4S7 and belongs to a new structure type as does Cd2Bi6S11. All four structures can be viewed as chemically twinned NaCl superstructures where the twinning plane is the (113) plane. We find there is segregation of the larger Bi atoms toward the twinning planes, whereas smaller Cd atoms occupy the interior sites of the NaCl domain. We also find that the average oxidation state of the cations controls the twinning patterns even in the presence of metal disorder. We propose the possible twinning patterns for Cd2Bi6S11 based on the observed average oxidation state of metal cations. The space groups of CdBi2S4, CdBi4S7, and Cd2.8Bi8.1S15 are C2/m for all three crystals. Cell axes for the three phases are respectively a = 13.095(2) Å, b = 3.9792(7) Å, c = 14.611(3) Å, β = 116.30(1)°; a = 13.112(2) Å, b = 4.0032(7) Å, c = 11.765(2) Å, β = 105.21(1)°; and a = 13.111(2) Å, b = 3.9885(8) Å, c = 24.710(5) Å, β = 97.84(2)°. Cell axes for Cd2Bi6S11 are a = 13.112(3) Å, b = 3.997(1) Å, c = 35.84(1) Å, and β = 90.39(2)°.

Surface Chemistry of Nanocrystalline Cerium Oxide

ABSTRACTNanocrystalline cerium oxide was synthesized by magnetron sputtering of a metallic target, followed by controlled post-oxidation. The resulting cerium oxide clusters were <10 nm in size, and highly non-stoichiometric in nature. The oxygen deficiency of such materials was associated with the unusual catalytic activity in oxidation and redox reactions. This paper compares the surface chemistry of nanocrystalline CeO2−x with stoichiometric CeO2. It further explores the promoting effect of Cu-doping on surface reduction and oxidation.The oxidation states of metal cations were examined with X-ray photoelectron spectroscopy after various oxidizing and reducing heat treatments in a connected reaction chamber. Isothermal pulsed reduction and oxidation of the samples were investigated by thermogravimetric analysis. Reduction properties of the different materials are discussed in terms of their microstructure, oxygen deficiency and chemical composition. These studies will help to understand the importance of bulk defects and synergistic effects in multicomponent and multiphase materials for high surface reactivity.

Structural environments and oxidation states of metal cations in bismuth cerium molybdate solid solutions by x-ray absorption spectroscopy

Bismuth and cerium L/sub 3/-edge and molybdenum K-edge X-ray absorption investigations of silica-supported bismuth cerium molybdates were undertaken to probe the local structural environments and oxidation states of the metal cations. A total of 10 single-phase compositions in the bimodal Bi/sub 2-x/Ce/sub x/(MoO/sub 4/)/sub 3/ solid solution (for 0 less than or equal to x less than or equal to 0.25 and 1 less than or equal to x less than or equal to 1.5) were examined; namely, x = 0, 0.025, 0.075, 0.125, 0.2, 0.25, 1, 1.125, 1.25, and 1.5. For low cerium concentrations (x less than or equal to 0.25), the Bi/sub 2-x/Ce/sub x/(MeO/sub 4/)/sub 3/ materials possess severely distorted defect-scheelite structure with cerium cations dissolved in the Bi/sub 2/(MoO/sub 4/)/sub 3/ (x = 0) lattice, whereas for high cerium concentrations (x greater than or equal to 1), they possess undistorted defect-scheelite structures with bismuth cations dissolved in the Ce/sub 2/(MoO/sub 4/)/sub 3/ (x = 2) lattice. The EXAFS results provide the first quantitative structural information about the individual Bi/sup 3 +/ and Ce/sup 3 +/ sites in the Bi/sub 2-x/Ce/sub x/(MoO/sub 4/)/sub 3/ compositions for 0.025 less than or equal to x less than or equal tomore » 1.5; only site-averaged metrical parameters for an admixture of bismuth and cerium cation positions are available from X-ray and neutron diffraction measurements.« less