Georgi Tyuliev

The nature of excess oxygen in Co3O4+ϵ

Abstract A comparative XPS study of CO 3 O 4 powder and oxidized polycrystalline cobalt has been carried out. Lowly charged oxygen in the form of O - and/or OH - species exist not only on the very surface of the oxide, but in deeper layers as well. Two surface models of Co 3 O 4 have been discussed. The lower surface density of cobalt ions as compared with the spinel bulk and the existence of O - /OH - species are responsible for the deviation from stoichiometry.

XPS study of surface composition of polycrystalline CuxCo3−xO4 (0⩽x<1) obtained by thermal decomposition of nitrate mixtures

The composition of surface layers of spinel oxides CuxCo3−xO4 (0⩽x<1), obtained by thermal decomposition of nitrate mixtures, has been studied by means of XPS or ESCA. The surface layer sampled by XPS cannot be described by the bulk formula: the density of Co cations is lower than in the bulk and correspondingly, the oxygen-to-metal ratio and copper-to-cobalt ratio are higher than the mean values for the bulk. The increase in the copper content is accompanied with a decrease of the oxygen-to-metal ratio and an increase in the amount of O- and/or OH- species on the surface of the mixed spinels.

Thin-film coating of Cu-Co oxide catalyst on lanthana/zirconia films electrodeposited on stainless steel

Abstract In this paper we report for the first time that lanthana/zirconia films, electrodeposited on a stainless steel substrate, are suitable for supporting a thin-film coating of Cu-Co oxide catalyst. The catalyst is active towards the O 2 +CO and NO+CO reactions.

Sodium deficient nickel–manganese oxides as intercalation electrodes in lithium ion batteries

Sodium deficient nickel–manganese oxides NaxNi0.5Mn0.5O2 with a layered structure are of interest since they are capable of participating in reactions of intercalation of Li+ and exchange of Na+ with Li+. Taking into account the intercalation properties of these oxides, we provide new data on the direct use of NaxNi0.5Mn0.5O2 as low-cost electrode materials in lithium ion batteries instead of lithium analogues. Sodium deficient nickel–manganese oxides NaxNi0.5Mn0.5O2 are prepared at 700 °C from freeze-dried acetate precursors. The structure of NaxNi0.5Mn0.5O2 is analyzed by means of powder X-ray diffraction, SAED and HRTEM. The oxidation states of nickel and manganese ions are determined by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance spectroscopy (EPR). Model lithium cells are used to monitor the lithium intercalation into NaxNi0.5Mn0.5O2. The surface and composition stability of NaxNi0.5Mn0.5O2 during the electrochemical reaction is monitored by using ex situ XPS and LA-ICPMS. Layered oxides NaxNi0.5Mn0.5O2 exhibit a P3-type of structure, in which the solubility of sodium is limited between 0.5 and 0.75. At 700 °C, NaxNi0.5Mn0.5O2 consists of thin well-crystallized nanoparticles; some of the particles have sizes higher than 100 nm, displaying a trigonal superstructure. For all oxides, manganese ions occur in the oxidation state of +4, while the oxidation state of nickel ions is higher than +2 and depends on the sodium content. The electrochemical reaction occurs within two potential ranges at 3.1 and 3.8 V due to the redox manganese and nickel couples, respectively. During the first discharge, Li+ intercalation and Li+/Na+ exchange reactions take place, while the consecutive charge process includes mainly Li+ and Na+ deintercalation. As a result, all oxides manifest a reversible capacity of about 120–130 mA h g−1, corresponding to 0.5–0.6 moles of Li+. The formation of surface layers in the course of the electrochemical reaction is also discussed.

NiMo/γ-Al2O3 Catalysts from Ni Heteropolyoxomolybdate and Effect of Alumina Modification by B, Co, or Ni

A hydrotreating NiMo/γ-Al2O3 catalyst (12 wt% Mo and 1.1 wt% Ni) was prepared by impregnation of the support with the Anderson-type heteropolyoxomolybdate (NH4)4Ni(OH)6Mo6O18. Before impregnation of the support, it was modified with an aqueous solution of H3BO3, Co(NO3)2, or Ni(NO3)2. The catalysts were investigated using N2 adsorption, O2 chemisorption, X-ray diffraction, UV-Vis spectroscopy, Fourier transform infrared spectroscopy, temperature-programmed reduction, temperature-programmed desorption, and X-ray photoelectron spectroscopy. The addition of Co, Ni, or B influenced the Al2O3 phase composition and gave increased catalytic activity for 1-benzothiophene hydrodesulfurization (HDS). X-ray photoelectron spectroscopy confirmed that the prior loading of Ni, Co or B increased the degree of sulfidation of the NiMo/γ-Al2O3 catalysts. The highest HDS activity was observed with the NiMo/γ-Al2O3 catalyst with prior loaded Ni.

Temperature dependence of Ni3+ quantity in the surface layer of NiO

XPS and EELS measurements reveal that the quantity of Ni3+ ions in the surface layer of NiO increases with temperature. The effect is reversible and can be observed in both UHV and oxygen atmosphere at pressures up to 5 × 10-4 Pa. A possible explanation for the observed temperature changes involves a compensation of the cationic vacancies in the surface layer with Ni3+ ions. These vacancies appear when nickel ions move into the bulk of NiO crystal.

The effect of nickel on the component state and HDS activity of alumina-supported heteropolytungstates

Hydrotreating Ni heteropolytungstate catalysts have been prepared by impregnation of γ-Al2O3-alumina with solutions of H3PW12O40 acid and its Ni salt. The nickel content is varied by adding Ni(NO3)2 salt. The calcined samples are characterized by BET, IR, TPR, and XPS techniques. The catalytic activity is tested for HDS of thiophene. It is shown that the initial heteropolyanion, its lacunary analog, and nickel substituted heteropolyanion are present on the surface as a result of the interaction between the active component and the alumina support. The mixed NiWS phase formed after sulfidation determines the HDS activity of the catalysts.

Hydrodesulfurization activities of NiMo catalysts supported on mechanochemically prepared Al-Ce mixed oxides

Abstract Al2O3-CeO2 supports containing 1-10 wt% Ce were prepared mechanochemically by milling aluminum and/or cerium nitrates with NH4HCO3. Heteropolymolybdate, (NH4)4NiMo6O24, was used as the precursor of the Ni and Mo to prepare NiMo6/Al2O3-CeO2 components in catalysts by impregnation method. The physicochemical properties of the catalysts were determined using chemical analysis, X-ray diffraction, temperature-programmed H2 reduction, temperature-programmed NH3 desorption, X-ray photoelectron spectroscopy (XPS), and the Brunauer-Emmett-Teller method. The catalyst acidity decreased with increasing Ce concentration in the support. XPS showed that the NiS/MoS ratio decreased two-fold for the Ce-modified alumina support. NiMo6/Al2O3, which had the highest acidity, showed the highest activity in hydrodesulfurization of 1-benzothiophene (normalized per weight of catalyst). The concentration of surface MoOxSy species (which is equal to the concentration of Mo5+) gradually decreased to zero for catalysts with Ce concentrations ( 10 wt%. However, the activities of all the catalysts prepared mechanochemically from Al2O3 and Al2O3-CeO2 supports significantly exceeded that of a reference NiMo6/Al2O3 catalyst prepared by impregnation method using the same precursor and with the same composition.

Precursor-based methods for low-temperature synthesis of defectless NaMnPO4 with an olivine- and maricite-type structure

We report precursor-based methods for low-temperature synthesis of two structure modifications of NaMnPO4. The maricite phase is thermodynamically more stable, while the olivine phase is of great interest as a positive-electrode material for lithium and sodium ion batteries. The advantage of synthetic procedures is the formation of defectless NaMnPO4 in the temperature range of 200–400 °C. The structure and morphology characterizations of two modifications are performed by powder XRD, SEM and TEM analyses. The oxidation state of the Mn ions in NaMnPO4 is determined by X-ray photoelectron spectroscopy. The local environment of Na in both structure modifications is assessed by 23Na MAS NMR spectroscopy. The synthesis methods are based on the formation of appropriate precursors that are easily transformed to target NaMnPO4. Thermal decomposition of freeze-dried phosphate–formate precursor yields NaMnPO4 with a maricite structure at 400 °C. KMnPO4·H2O with a dittmarite-type structure acts as a structure-template precursor for the preparation of NaMnPO4 with an olivine structure by an ion exchange reaction. Both olivine and maricite NaMnPO4 do not accommodate any anti-site mixing and Na,Mn deficiency. The morphology of NaMnPO4 consists of nano-sized particles (less than 50 nm) that are closely bound together into aggregates, the shape of the aggregates being dependent on the synthesis procedure used.