Oxidation State Of Co Research Articles

Synergistic effects of intrinsic cation disorder and electron-deficient substitution on ion and electron conductivity in La1-xSrxCo0.5Mn0.5O3-δ (x = 0, 0.5, and 0.75).

The effects of intrinsic cation disorder and electron-deficient substitution for La1-xSrxCo0.5Mn0.5O3-δ (LSCM, x = 0, 0.5, and 0.75) on oxygen vacancy formation, and their influence on the electrochemical properties, were revealed through a combination of computer simulation and experimental study. First-principles calculations were first performed and found that the tendency of the oxygen vacancy formation energy was Mn(3+)-O*-Mn(4+) < Co(2+)-O*-Co(3+) < Co(2+)-O*-Mn(4+), meaning that antisite defects not only facilitate the formation of oxygen vacancy but introduce the mixed-valent transition-metal pairs for high electrical conductivity. Detailed partial density of states (PDOS) analysis for Mn on Co sites (MnCo) and Co on Mn sites (CoMn) indicate that Co(2+) is prone to being Co(3+) while Mn(4+) is prone to being Mn(3+) when they are on antisites, respectively. Also it was found that the holes introduced by Sr tend to enter the Co sublattice for x = 0.5 and then the O sublattice when x = 0.75, which further promotes oxygen vacancy formation, and these results are confirmed by both the calculated PDOS results and charge-density difference. On the basis of microscopic predictions, we intentionally synthesized a series of pure LSCM compounds and carried out comprehensive characterization. The crystal structures and their stability were characterized via powder X-ray Rietveld refinements and in situ high-temperature X-ray diffraction. X-ray photoelectron spectroscopy testified to the mixed oxidation states of Co(2+)/Co(3+) and Mn(3+)/Mn(4+). The thermal expansion coefficients were found to match the Ce0.8Sm0.2O2-δ electrolyte well. The electrical conductivities were about 41.4, 140.5, and 204.2 S cm(-1) at doping levels of x = 0, 0.5, and 0.75, and the corresponding impedances were 0.041, 0.027, and 0.022 Ω cm(2) at 850 °C, respectively. All of the measured results testify that Sr-doped LaCo0.5Mn0.5O3 compounds are promising cathode materials for intermediate-temperature solid oxide fuel cells.

Steam reforming of methanol over cobalt catalysts: Effect of cobalt oxidation state

Temperature-programmed reduction (TPR) employing hydrogen and methanol as reducing agents and in-situ X-ray diffraction have been employed to investigate the oxidation state of Co–Mn catalysts prepared by oxidative or pyrolytic treatment of fumarate precursors and correlate observations with catalyst activity in methanol reforming. Pyrolytic treatment leads to almost fully reduced catalysts, while catalysts prepared by oxidation are of the spinel structure and get reduced in a stepwise fashion. Both hydrogen and methanol are efficient reducing agents. Interaction of methanol with the catalyst surface occurs more readily with the reduced catalysts.

In SituStudies of Carbon Monoxide Oxidation on Platinum and Platinum–Rhenium Alloy Surfaces

CO oxidation has been investigated by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) on Pt(111), Re films on Pt(111), and a Pt–Re alloy surface. The Pt–Re alloy surface was prepared by annealing Re films on Pt(111) to 1000 K; scanning tunneling microscopy, low energy ion scattering, and X-ray photoelectron spectroscopy studies indicate that this treatment resulted in the diffusion of Re into the Pt(111) surface. Under CO oxidation conditions of 500 mTorr O2/50 mTorr CO, CO remains on the Pt(111) surface at 450 K, whereas CO desorbs from the Pt–Re alloy surface at lower temperatures. Furthermore, the Pt–Re alloy dissociates oxygen more readily than Pt(111) despite the fact that all of the Re atoms are initially in the subsurface region. Mass spectrometer studies show that the Pt–Re alloy, Re film on Pt, and Pt(111) all have similar activities for CO oxidation, with the Pt–Re alloy producing ∼10% more CO2 than Pt(111). The Re film is not stable under CO oxidation conditions at temperatures...

Microstructure evolution and oxidation states of Co in perovskite-type oxide Ba10Co0.7Fe0.2Nb0.1O3–δ annealed in CO2 atmosphere

Ba1.0Co0.7Fe0.2Nb0.1O3–δ (BCFN) oxide with perovskite cubic structure was synthesized by solid state reaction method. CO2 corrosion of BCFN membrane was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFT) and X-ray absorption fine structure spectroscopy (XAFS). Cobalt (Co) K-edge absorption spectra of BCFN annealed in CO2 reveal that the oxidation states of Co in all the samples were larger than +3 and they decreased with the increase of calcination time. At 800 °C, 1% CO2 introduced into He could speed up the reduction of Co cations in comparison with pure He. In addition, sulfate ions in the bulk of BCFN membrane preferred to migrate to the surface under CO2 calcination and form monoclinic Ba(CO3)0.9(SO4)0.1 besides orthorhombic witherite. Moreover, SEM results indicate that the nucleation and growth of carbonates grains started at the grain boundary of the membrane.

Quantitative surface structure determination using in situ high-energy SXRD: Surface oxide formation on Pd(100) during catalytic CO oxidation

We have performed a quantitative structure determination of the 5×5R27° surface oxide, formed on Pd(100) under semi-realistic conditions for catalytic CO oxidation, using in situ high-energy surface X-ray diffraction. We describe the experiment and the extraction of quantitative data in detail. The structural results are in agreement with previous reports of a system consisting of a single layer of PdO(101) formed in pure O2 on top of Pd(100) and studied under ultra-high vacuum conditions.

XAFS study on nanostructured (La;Sr)CoO and (La;Sr)(Co;Fe)O IT-SOFC cathodes

In the last years, extensive research has been devoted to develop novel materials and structures with high electrochemical performance for intermediate-temperatures solid-oxide fuel cells (IT-SOFCs) electrodes. In recent works, we have investigated the structural and electrochemical properties of nanostructured La0.6Sr0.4CoO3(LSCO) and La0.6Sr0.4(Co;Fe)O3(LSCFO) cathodes, finding that they exhibit excellent electrocatalytic properties for the oxygen reduction reaction [1,2]. These materials were prepared by a pore-wetting technique using polycarbonate porous membranes as templates. Two average pore sizes were used: 200 nm and 800 nm. Our scanning electronic microscopy (SEM) study showed that the lower pore size yielded nanorods, while nanotubes were obtained with the bigger pore size. All the samples were calcined at 10000C in order to produce materials with the desired perovskite-type crystal structure. In this work, we analyze the oxidation states of Co and Fe and the local atomic order of LSCO and LSCFO nanotubes and nanowires for various compositions by X-ray absorption spectroscopies. For this purpose we performed XANES and EXAFS studies on both Co and Fe K edges. These measurements were carried out at the D08B-XAFS2 beamline of the Brazilian Synchrotron Light Laboratory (LNLS). XANES spectroscopy showed that Co and Fe only change slightly their oxidation state upon Fe addition. Surprisingly, XANES results indicated that the content of oxygen vacancies is low, even though it is well-known that these materials are mixed ionic-electronic conductors. EXAFS results were consistent with those expected according to the rhombohedral crystal structure determined in previous X-ray powder diffraction investigations.

In-Situ X-Ray Imaging Study on the Cross-Sectional Distribution of Li-Ion Concentration in Electrolyte of Li-Ion Battery

In the previous studies, we observed the three-dimensional distribution of various Co-oxidation states in LiCoO2 (LCO) active materials. It was found that as a LCO electrode is charged, the surface of the electrode is preferentially oxidized than the inside area [1].In this study, we researched the cross-sectional distribution of Li-ion concentration in the electrolyte of Li-ion battery by using an in-situ X-ray imaging method. In order to use this method we first created a special test cell, which models the cross-section of a positive electrode, separator, and negative electrode in a cell. While charging and discharging the cell, we observed how the cross-sectional distribution of Li-ion in the separator and negative electrode changed. Figure 1 illustrates the X-ray image of the cross section structure of the cell: the positive electrode, separator, and negative electrode and their interfaces. Figure 2 shows how the degree of X-ray transmission of the cross section changes before and 15 and 30-second after the cell is charged at 5C current. Before the cell was charged, the transmission degrees of the negative electrode and separator were the same, demonstrating that the Li-ion concentration was equally distributed across them. However, 15-second after the cell was charged, the transmission degrees in the negative electrode increased, and the degree of the electrode surface (the side close to the separator) became higher than that of the inside (the side close to the Cu collector). We expect that charging made the Li-ion concentration in the negative electrode decrease and the concentration of the surface became lower than that of the inside. As for analysis on the separator, the transmission degree on the negative electrode side of the separator became higher than that on the positive electrode side, demonstrating that the Li-ion concentration on the negative electrode side decreased after the cell was charged and that on the positive electrode side increased. Additionally, the concentration was more largely distributed 30-second after the cell was charged than 15-second after.In summary, we succeeded in applying the in-situ X-ray imaging method for identifying the Li-ion distribution in the negative electrode and separator during charging.

Synthesis, Structural, and Magnetic Properties of Nanocrystalline Ti 0.95Co0.05O2-Diluted Magnetic Semiconductors

With a view to investigate the influence of nanometric size on the structural, surface, and magnetic properties of nanocrystalline Ti0.95Co0.05O2-diluted magnetic semiconductors, prepared by a novel simple controllable peroxide-assisted reflux chemical route followed by annealing at different temperatures, a systematic investigation has been undertaken. Structural characterizations such as X-ray diffraction followed by Rietveld refinement, electron diffraction pattern, Fourier transform infrared, Raman scattering, and X-ray photoelectron spectroscopy (XPS) measurements have shown anatase phase formation in nanocrystalline Ti0.95Co0.05O2 without any additional impurity phases. The modified reflux chemical route was effective in obtaining pure phase Ti0.95Co0.05O2 nanoparticles. Surface morphological investigations by using transmission electron microscopy and atomic force microscopy measurements showed the predominant effect of random distribution of nanoparticles on the aggregation behavior and local microstructural changes. The deconvoluted XPS core level Co 2p spectral study manifested the oxidation state of Co as + 2 and is found to be stable with varying particle size and annealing temperature. The ferromagnetic behavior was investigated by vibrating sample magnetometer, magnetic force microscopy, and electron spin resonance measurements. These magnetization studies showed all the samples are ferromagnetic at room temperature without any magnetic clusters. The correlation between structure, surface condition of the nanoparticles and local electronic interactions, and magnetization of the samples was analyzed and explored the origin of ferromagnetism.

Redox-Mediated Reconstruction of Copper during Carbon Monoxide Oxidation

Copper has excellent initial activity for the oxidation of CO, yet it rapidly deactivates under reaction conditions. In an effort to obtain a full picture of the dynamic morphological and chemical changes occurring on the surface of catalysts under CO oxidation conditions, a complementary set of in situ ambient pressure (AP) techniques that include scanning tunneling microscopy, infrared reflection absorption spectroscopy (IRRAS), and X-ray photoelectron spectroscopy were conducted. Herein, we report in situ AP CO oxidation experiments over Cu(111) model catalysts at room temperature. Depending on the CO:O2 ratio, Cu presents different oxidation states, leading to the coexistence of several phases. During CO oxidation, a redox cycle is observed on the substrate’s surface, in which Cu atoms are oxidized and pulled from terraces and step edges and then are reduced and rejoin nearby step edges. IRRAS results confirm the presence of under-coordinated Cu atoms during the reaction. By using control experiments ...

Electrochemical synthesis and characterization of carbon-supported Pt and Pt–Ru nanoparticles with Cu cores for CO and methanol oxidation in polymer electrolyte fuel cells

Pt and Pt–Ru shells on Cu cores supported on Vulcan carbon XC72R have been synthesized and tested as possible anode electrocatalysts for polymer electrolyte fuel cells. Pt(Cu)/C was prepared by Cu electrodeposition on the black carbon support at constant potential followed by Pt deposition on Cu by galvanic exchange, whereas Pt–Ru(Cu)/C was prepared by spontaneous deposition of Ru species on Pt(Cu)/C. The corresponding cyclic voltammograms in 0.5 M H2SO4 solution showed the hydrogen adsorption/desorption peaks and no Cu oxidation. The respective CO stripping peak potentials of Pt(Cu)/C and Pt–Ru(Cu)/C were about 0.1 and 0.2 V more negative than those corresponding to Pt/C and Ru-decorated Pt/C. The best conditions for CO oxidation were found for Cu deposition potentials between −0.2 and −0.4 V vs. Ag/AgCl/KCl(sat). The Pt economy of the Pt–Ru(Cu)/C system was proved for the methanol oxidation, with specific currents more than twice those obtained on the Ru-decorated commercial Pt/C catalysts.

Reversible deactivation of a Au/Ce0.62Zr0.38O2 catalyst in CO oxidation: A systematic study of CO2-triggered carbonate inhibition

Highly active supported gold catalysts frequently undergo substantial decrease in activity due to deactivation. The aim of this paper is to investigate a reversible inhibition phenomenon of the CO oxidation in an Au/Ce0.62Zr0.38O2 (Au/CZ) catalyst. The results of a systematic study of CO oxidation conditions (CO, O2, and CO2) are discussed. By identifying CO2 as an Au/CZ poison using isotopic transient analysis (13CO/12CO2), we also investigate the origin of inhibition activity by transient and modulated infrared spectroscopy in DRIFT mode. Results demonstrate that carbonate species are formed by reaction of CO2 with reactive lattice oxygen at the metal–support interphase, which in turn inhibits the replenishment of the vacancies by molecular oxygen. A microkinetic model that accounts for the reaction mechanism and inhibition by CO2 is proposed.

Smoothing out NaxCo0.7Mn0.3O2 ocv Curve – Role of the Oxygen Nonstoichiometry

It is well known that NaxCoO2-y exhibits complicated character of the OCV (Open Circuit Voltage) curve, with several potential plateaus and jumps in 2.0-3.8 V range [1,2]. Origin of such the behavior is commonly attributed to the Na cations ordering [3]. However, often neglected influence of the electronic structure of the cathode material, which manifest itself as an additional energy states present in the main energy gap, seems to play a major role in this case. These states can be directly related to the presence of the oxygen vacancies [4].In this work we show a novel approach, based on the consideration of the influence of cation dopants replacing partially cobalt, which aims to smooth out the OCV characteristic of Na/Na+/NaxCoO2-y battery. On a basis of work [4], one can state that in order to remove most of the potential jumps, the material has to be oxygen stoichiometric (NaxCo0.7Mn0.3O2) and no sodium ordering should be present. Since +4 oxidation state for cobalt cations is not the most favourable one, it gives rise to a presence of the oxygen vacancies, which decrease the average oxidation state of Co. If an appropriate dopant with stable +4 state would be introduced in cobalt positions, the concentration of the oxygen vacancies shall decrease considerable. Furthermore, random distribution of such dopant in Co sublattice will diminish the tendency of Na cations to form ordered structure.As predicted from the considerations shown above, in the case of Mn substitution, OCV curve of Na/Na+/NaxCo0.7Mn0.3O2 is almost completely smoothed out, with only one major potential jump, which is associated with x = 0.5 mol·mol-1. Apart from basic scientific aspect, the obtained result is interesting from the point of view of application, as in commercial batteries voltage cannot show any non-monotonous behaviour.

Modifications in the frustrated magnetism, oxidation state of Co and magnetoelectric coupling effects induced by a partial replacement of Ca by Gd in the spin-chain compound Ca3Co2O6

We have systematically investigated the influence of the gradual replacement of Ca by Gd on the magnetic and complex dielectric properties of the well-known geometrically frustrated spin-chain system Ca3Co2O6 (TN = 24 K with additional magnetic transitions below 12 K), by studying the series Ca3−xGdxCo2O6 (x ≤ 0.7), down to 1.8 K. Heat-capacity measurements establish that the reduction of TN with Gd substitution is much less compared to that by Y substitution. The magnetic moment data reveal that there are changes in the oxidation state of Co as well, unlike for Y substitution, beyond x = 0.2. Thus, despite being isovalent, both these substitutions interestingly differ in changing these magnetic properties in these oxides. We propose that the valence electrons of Y and those of R ions play different roles in deciding the magnetic characteristics of these mixed oxides. It is observed that a small amount (x = 0.3) of Gd substitution for Ca is enough to suppress glassy ac magnetic susceptibility behavior for the peak around 12 K. An additional low-temperature magnetic anomaly close to 5 K gets more prominent with increasing Gd concentration as revealed by heat-capacity data. Trends in temperature dependence of complex dielectric behavior were also tracked with varying composition and a frequency dependence is observed, not only for the transition in the region around 10 K (for some compositions), but also for the 5 K transition which is well resolved for a higher concentration of Gd. Thus, the Gd-substituted Ca3Co2O6 series is shown to reveal interesting magnetic and dielectric behaviors of this family of oxides.

Anisotropic Chemical Expansion of La1–xSrxCoO3−δ

The crystal structure and the thermal and chemical expansion of rhombohedral La1–xSrxCoO3−δ (x = 0.3, 0.4) perovskite were investigated by in situ high-temperature X-ray diffraction in pure oxygen and nitrogen atmospheres. The crystal structure was confirmed to change from rhombohedral (R3̅c) to cubic symmetry (Pm3̅m) with increasing temperature due to a second-order ferroelastic to paraelastic phase transition. The rhombohedral distortion at low temperature causes crystallographic anisotropy, and anisotropic thermal expansion, caused by rectification of the antiferrodistortive tilting and a minor decompression of the CoO6/2 octahedra is reported. The onset of chemical expansion due to reduction of the oxidation state of Co was observed below the phase transition temperature, which enabled investigation of the crystallographic anisotropy of the chemical expansion. The chemical expansion was significantly larger along the c-axis relative to the a-axis, demonstrating the strong damping of the octahedral tilting with increasing oxygen vacancies induced by the thermal reduction of Co. This phenomenon caused a strong depletion of the ferroelastic phase transition temperature with decreasing partial pressure of oxygen.

Surface Segregation of Pd from TiO2‐Supported AuPd Nanoalloys under CO Oxidation Conditions Observed In situ by ETEM and DRIFTS

AbstractA TiO2‐supported AuPd bimetallic catalyst with an Au/Pd atomic ratio of 8 was prepared by deposition‐precipitation with urea, and its activity in CO oxidation at room temperature was compared to the one of a monometallic Au/TiO2 catalyst. X‐ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses suggest that Au‐Pd/TiO2 contains bimetallic nanoparticles after reduction under H2 at 500 °C although the presence of monometallic Au particles cannot be totally excluded. The evolution of the AuPd nanoparticles surface composition during exposure to O2 and CO/O2 was studied in situ by environmental high resolution electron microscopy (ETEM) and DRIFTS. Pd segregation at the surface of the bimetallic nanoparticles was evidenced by DRIFTS and directly observed by ETEM under O2 and CO/O2 with the formation of Aucore‐Pdshell structure. The changes in the surface composition of the Au‐Pd nanoparticles under CO/O2 was paralleled with the higher rate of deactivation in the first reaction stages observed for Au‐Pd/TiO2 compared to Au/TiO2, which could be related to the possible replacement of Au in low coordination sites, at the origin of the high activity in CO oxidation, by Pd atoms. These results noticeably underline the modifications induced by the reactant that can undergo a bimetallic AuPd catalyst.

RYB tri-colour electrochromism based on a molecular cobaloxime

The three oxidation states of Co in a molecular cobaloxime were used to realise an electrochromic device displaying the red, yellow, blue (RYB) set of subtractive primary colours. A facile method for the lithographic patterning of a several micrometre thick indium tin oxide (ITO) mesoporous layer was developed, which served as the scaffold for cobaloxime adsorption.

Kinetic Study of Oxygen Adsorption over Nanosized Au/γ-Al2O3 Supported Catalysts under Selective CO Oxidation Conditions

O2 adsorption is a key process for further understanding the mechanism of selective CO oxidation (SCO) on gold catalysts. Rate constants related to the elementary steps of O2 adsorption, desorption and surface bonding, as well as the respective activation energies, over a nanosized Au/γ-Al2O3 catalyst, were determined by Reversed-Flow Inverse Gas Chromatography (RF-IGC). The present study, carried-out in a wide temperature range (50–300 °C), both in excess as well as in the absence of H2, resulted in mechanistic insights and kinetic as well as energetic comparisons, on the sorption processes of SCO reactants. In the absence of H2, the rate of O2 binding, over Au/γ-Al2O3, drastically changes with rising temperature, indicating possible O2 dissociation at elevated temperatures. H2 facilitates stronger O2 bonding at higher temperatures, while low temperature binding remains practically unaffected. The lower energy barriers observed, under H2 rich conditions, can be correlated to O2 dissociation after hydrogenation. Although, H2 enhances both selective CO reactant’s desorption, O2 desorption is more favored than that of CO, in agreement with the well-known mild bonding of SCO reactant’s at lower temperatures. The experimentally observed drastic change in the strength of CO and O2 binding is consistent both with well-known high activity of SCO at ambient temperatures, as well as with the loss of selectivity at higher temperatures.

Effects of Co doping on structure and optical properties of TiO2 thin films prepared by sol–gel method

Co-doped TiO2 thin films were synthesized on quartz substrates by sol–gel method. Atomic force microscopy results indicate that the surfaces of the Ti1−xCoxO2 (0≤x≤0.10) films become smooth and compact with increasing Co content. X-ray diffraction results show that all the films are rutile phase structure and Co doping leads to lattice contraction. X-ray photoelectron spectroscopy results reveal that the predominant oxidation state of Co is divalent. Peak positions of Raman-active modes (B2g, A1g and Eg) shift to lower frequency with increasing Co content. The refractive index n at 670nm from transmittance spectra increases with increasing Co content. The OBG varies between 3.10 and 3.26eV. Note that optical band gap (OBG) first increases and then decreases with increasing Co content, reaching its maximum value when x is 0.03. These results suggest that the increasing mechanism of OBG is related to the decrease of grain size, compressive stress, and reduction of rutile TiO2, and the decreasing mechanism of OBG is involved with defect and impurity. The competition of the two mechanisms leads to the strange change of OBG.

Oxygen permeation and oxidation states of transition metals in (Fe, Nb)-doped BaCoO 3 − δ perovskites

The influence of cation substitution on the transition metal oxidation states, oxygen nonstoichiometry and oxygen transport properties of Ba(Co, Fe, Nb)O 3 − δ perovskites was investigated with room temperature XRD, X-ray absorption near edge structure and high temperature oxygen permeation measurements. Results show that substitution of Fe for Co is accompanied by a decrease in the lattice constant, whereas Nb substitution leads to the opposite trend. With increasing Fe and/or Nb content, the oxygen permeation flux decreases, whereas the apparent activation energy of the oxygen permeation increases. The oxidation state of Co is smaller than +3 and susceptible to the variation of cation composition. In contrast, the oxidation states of Fe and Nb are larger than +3 and close to +5, respectively, and almost independent of the composition. The influence of Fe and Nb contents on the oxygen transport properties is discussed in terms of the oxygen stoichiometry and metal–oxygen bonding.

Structural and optical properties of Co-doped ZnO nanocrystallites prepared by a one-step solution route

Zinc oxide (ZnO) nanocrystallites with different Co-doping levels were successfully synthesized by a simple one-step solution route at low temperature (95 °C) in this study. The structure and morphology of the samples thus obtained were characterized by XRD, EDS, XPS and FESEM. Results show that cobalt ions, in the oxidation state of Co 2+, replace Zn 2+ ions in the ZnO lattice without changing its wurtzite structure. The dopant content varies from 0.59% to 5.39%, based on Co-doping levels. The pure ZnO particles exhibit well-defined 3D flower-like morphology with an average size of 550 nm, while the particles obtained after Co-doping are mostly cauliflower-like nanoclusters with an average size of 120 nm. Both the flower-like pure ZnO and the cauliflower-like Co:ZnO nanoclusters are composed of densely arrayed nanorods. The optical properties of the ZnO nanocrystallites following Co-doping were also investigated by UV–Visible absorption and Photoluminescence spectra. Our results indicate that Co-doping can change the energy-band structure and effectively adjust the luminescence properties of ZnO nanocrystallites.