Electronic absorption spectra of nickel dichloride and nickel oxide solutions in the 2CsCl-NaCl and KCl-NaCl metls

The electronic absorption spectra of nickel dichloride and nickel oxide dilute solutions in the 2CsCl-NaCl and KCl-NaCl melts have been measured by ultraviolet reflection absorption spectroscopy in the range 250–2500 nm. The spectroscopic data demonstrate the formation of stable Ni(II)-based tetrahedral groups (NiCl ₄ ^2? ) in solutions of nickel dichloride in the 2CsCl-NaCl melt, whereas in the KCl-NaCl melt, nickel dichloride undergoes thermal dissociation to give Ni(II)-and Ni(I)-based complex groups: NiCl ₄ ^2? , NiCl ₆ ^4? , and NiCl ₄ ^3? . The dissolution of nickel oxide in the 2CsCl-NaCl and KCl-NaCl melts gives mainly NiOCl ₃ ^3? complex groups.     

Crystal growth of Ce2O(CO3)2·H2O in aqueous solutions: Film formation and samarium doping

Crystalline cerium oxide carbonate hydrate (Ce₂O(CO₃)₂·H₂O) was grown in aqueous solutions at a low temperature of 80 °C under ambient pressure. When cerium nitrate was used as a starting material, large Ce₂O(CO₃)₂·H₂O particles were precipitated through homogeneous nucleation and subsequent fast crystal growth. In contrast, the usage of cerium chloride was found to promote the preferential precipitation of Ce₂O(CO₃)₂·H₂O on foreign substrates through heterogeneous nucleation and slow crystal growth. This phenomenon was applied to a chemical bath deposition of Ce₂O(CO₃)₂·H₂O films. Immersion of glass substrates in the solution at 80 °C for typically 24 h resulted in formation of solid films with a unique morphology like a micrometer-scale brush. It was also found that samarium could be incorporated into Ce₂O(CO₃)₂·H₂O during the crystal growth in the solutions, as evidenced by characteristic photoluminescence of Sm³⁺ in heating products of CeO₂. These results suggest that rare-earth oxide carbonate hydrates with a variety of compositions and morphologies can be synthesized from the aqueous solutions.     

Realization of monolayer levels of surface oxidation of nickel by anodization at low temperatures

In order to examine whether monolayer or sub-monolayer extents of surface oxidation can be realized experimentally at Ni prior to onset of bulk-phase oxide formation (as they can for example at Pt, Ru or Au already at room temperature), cyclic voltammetric experiments down to low temperature (−90° C) have been conducted on Ni in solutions of NaOH in 80 mol% methanol with water. The cyclic voltammograms for the first stage of Ni oxidation to α-Ni(OH)₂, and its reduction, show that extents of surface oxidation down to an equivalent monolayer, or less, of Ni(OH)₂ can be realized at sufficiently low temperatures. However, even at these low levels of oxidation of the metal, irreversibility between the processes of Ni oxide formation and reduction is maintained in a way characteristic of the behavior of three-dimensional oxide films. It therefore appears that even at low levels of surface oxidation of Ni which are attainable at low temperature, the oxidation mechanism involves nucleation and growth of the oxide in islands rather than an initial surface-chemical process of OH or O array formation, as at Pt or Au. However, no indications of a dissolution-and precipitation type of oxide formation process, which would involve mass-transport in solution, are given by the present results obtained from experiments in dilute alkali at low temperatures, and at the rotating Ni disc electrode.     

Novel water based cerium acetate precursor solution for the deposition of epitaxial cerium oxide films as HTSC buffers

A novel water based precursor solution using ethylenediaminetetraacetic acid (H₄EDTA) as a complexant and acetic acid and ethylenediamine (EDA) as additional components to obtain CeO₂ buffer layers on Ni (5%W) tapes is described in detail. The influence of complexation behavior in the formation of transparent and homogenous sols and gels by the combination of cerium acetate, acetic acid and H₄EDTA has been studied. The optimal growth conditions for cerium oxide were Ar-5% H₂ gas processing atmosphere, solution concentration levels of 0.2–0.4 M, a dwell time of 60 min at 900 °C and 5–30 min at 1,050 °C. X-ray diffraction, SEM, spectroscopic ellipsometry and pole figures were used to characterize the CeO₂ films. Highly textured CeO₂ layers were obtained.     

Liquid-phase Reductive Deposition as Novel Preparation Method for Highly Dispersive Nickel Catalysts

We have been developing the selective deposition method onto TiO₂ nanoparticles, named as the liquid-phase selective-deposition method, where TiO₂ plays a role of formation center of Ni nanoparticles as well as protection from the aggregative growth of the particles. The concept of this method is to well disperse and stabilize Ni nanoparticles on TiO₂ surface by specific adsorption of Ni precursory complexes and then heterogeneous nucleation on the adsorption sites. The particle size was decreased with increasing the amount of Zn added, thus the catalytically active Ni surface area was increased. The selective deposition onto TiO₂ surface and addition of Zn to the nanoparticle promoted the catalytic activity of Ni–Zn nanoparticle, e.g. the catalytic activity of Ni–Zn/TiO₂ was ca. 10 times higher than that of the unsupported Ni nanoparticles. Ni in the nanocomposite was assigned as metallic, although their surface was oxidized under the atmospheric condition, but Zn and B were deposited as their oxide.     

Effect of Non‐Specifically Adsorbed Ions on the Surface Oxidation of Pt(111)

The oxidation processes of a Pt(111) electrode in alkaline electrolytes depend on non‐specifically adsorbed ions according to in situ X‐ray diffraction and infrared spectroscopic measurements. In an aqueous solution of LiOH, an OH_ad adlayer is formed in the first oxidation step of the Pt(111) electrode as a result of the strong interaction between Li⁺ and OH_ad, whereas Pt oxidation proceeds without OH_ad formation in CsOH solution. Structural analysis by X‐ray diffraction indicates that Li⁺ is strongly protective against surface roughening caused by subsurface oxidation. Although Cs⁺ is situated near the Pt surface, the weak protective effect of Cs⁺ results in irreversible surface roughening due to subsurface oxidation.     

A nitrilo-tri-acetic-acid/acetic acid route for the deposition of epitaxial cerium oxide films as high temperature superconductor buffer layers

A water based cerium oxide precursor solution using nitrilo-tri-acetic-acid (NTA) and acetic acid as complexing agents is described in detail. This precursor solution is used for the deposition of epitaxial CeO₂ layers on Ni-5at%W substrates by dip-coating. The influence of the complexation behavior on the formation of transparent, homogeneous solutions and gels has been studied. It is found that ethylenediamine plays an important role in the gelification. The growth conditions for cerium oxide films were Ar-5% gas processing atmosphere, a solution concentration level of 0.25 M, a dwell time of 60 min at 900 °C and 5-30 min at 1050 °C. X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscopy (AFM), pole figures and spectroscopic ellipsometry were used to characterize the CeO₂ films with different thicknesses. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) was used to determine the carbon residue level in the surface of the cerium oxide film, which was found to be lower than 0.01%. Textured films with a thickness of 50 nm were obtained.     

Solubility and phase behavior of nickel oxide in aqueous sodium phosphate solutions at elevated temperatures

A platinum-lined, flowing autoclave facility was used to investigate the solubility/phase behavior of nickel oxide (NiO) in aqueous sodium phosphate solutions between 290 and 560 K. A layer of hydrous nickel oxide was concluded to exist on the nickel oxide surface below 468 K; only at higher temperatures did the anhydrous nickel oxide phase control the nickel ion solubility behavior. The measured solubility behavior was examined via a nickel(II) ion hydrolysis/complexing model and thermodynamic functions for the hydrolysis/complexing reaction equilibria were obtained from a least-squares analysis of the data. The existence of two new nickel ion complexes are reported for the first time: Ni(OH)₂(HPO₄)⁼ and Ni(OH)₃(H₂PO₄)⁼. The positive entropy change associated with the formation of Ni(OH)₃(H₂PO₄)⁼ leads to its dominance in alkaline phosphate solutions at elevated temperatures.     

Aggregation of Giant Cerium–Bismuth Tungstate Clusters into a 3D Porous Framework with High Proton Conductivity

The exploration of high nuclearity molecular metal oxide clusters and their reactivity is a challenge for chemistry and materials science. Herein, we report an unprecedented giant molecular cerium–bismuth tungstate superstructure formed by self‐assembly from simple metal oxide precursors in aqueous solution. The compound, {[W₁₄Ce^IV₆O₆₁]([W₃Bi₆Ce^III₃(H₂O)₃O₁₄][B‐α‐BiW₉O₃₃]₃)₂}^34? was identified by single‐crystal X‐ray diffraction and features 104 metal centers, a relative molar mass of ca. 24 000 and is ca. 3.0×2.0×1.7 nm³ in size. The cluster anion is assembled around a central {Ce₆} octahedron which is stabilized by several molecular metal oxide shells. Six trilacunary Keggin anions ([B‐α‐BiW₉O₃₃]^9?) cap the superstructure and limit its growth. In the crystal lattice, water‐filled channels with diameters of ca. 0.5 nm are observed, and electrochemical impedance spectroscopy shows pronounced proton conductivity even at low temperature.     

UV absorption of titanium oxide based gels

A spectroscopic study of the gelation and aging of the titanium oxide based gels has been carried out. A careful analysis of measured spectra has shown a permanent growth of the UV absorption in the spectral range of hν⩾E_thr=3.24 eV during all the period of observations. The absorption saturates at times much longer than the gelation time. The observed kinetics is discussed in terms of short oxo-polymer chains nucleation, formation of the primary gel, and gel mass increase under aging. Simple basic models relating the absorption growth to the accumulation of polymeric units successfully fit the experimental curve. The observations suggest using the UV absorption for control of the gel formation.     

The physicochemical and catalytic properties of the Pt-CeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> system

The properties of the Pt-CeO _x system prepared by the oxidation of the Pt₂Ce intermetallic compound were studied. The sample was characterized by X-ray diffraction in situ, thermogravimetry, scanning electron microscopy (with an accessory for energy dispersion analysis), transmission electron microscopy, and temperature-programmed reduction with hydrogen. The catalytic properties of the sample were studied in the model reaction of toluene hydrogenation. The oxidation of the intermetallic compound caused the appearance of metallic platinum and cerium oxide phases and high-dispersity platinum particles encapsulated in cerium oxide. Metallic platinum on the surface of the catalyst experienced rapid deactivation in the presence of hydrogen sulfide; high-dispersity platinum particles encapsulated in cerium oxide exhibited enhanced stability toward sulfur compounds.     

A New Class of Homogeneous Visible‐Light Photocatalysts: Molecular Cerium Vanadium Oxide Clusters

The first systematic access to molecular cerium vanadium oxides is presented. A family of structurally related, di‐cerium‐functionalized vanadium oxide clusters and their use as visible‐light‐driven photooxidation catalysts is reported. Comparative analyses show that photocatalytic activity is controlled by the cluster architecture. Increased photoreactivity of the cerium vanadium oxides in the visible range compared with nonfunctionalized vanadates is observed. Based on the recent discovery of the first molecular cerium vanadate cluster, (nBu₄N)₂[(Ce(dmso)₃)₂V₁₂O₃₃Cl] ? 2 DMSO ( 1 ), two new di‐cerium‐containing vanadium oxide clusters [(Ce(dmso)₄)₂V₁₁O₃₀Cl] ? DMSO ( 2 ) and [(Ce(nmp)₄)₂V₁₂O₃₂Cl] ? NMP ? Me₂CO ( 3 ; NMP=N‐methyl‐2‐pyrrolidone) were obtained by using a novel fragmentation and reassembly route. Pentagonal building units {(V)M₅} (M=V, Ce) reminiscent of “Müller‐type” pentagons are observed in 2 and 3 . Compounds 1 – 3 feature high visible‐light photooxidative activity, and quantum efficiencies >10 % for indigo photooxidation are observed. Photocatalytic performance increases in the order 1 < 3 < 2 . Mechanistic studies show that the irradiation wavelength and the presence of oxygen strongly affect photoreactivity. Initial findings suggest that the photooxidation mechanism proceeds by intermediate formation of hydroxyl radicals. The findings open new avenues for the bottom‐up design of sunlight‐driven photocatalysts.     

Investigation of Fe−Ni Mixed‐Oxide Catalysts for the Reduction of NO by CO: Physicochemical Properties and Catalytic Performance

A series of Fe?Ni mixed‐oxide catalysts were synthesized by using the sol–gel method for the reduction of NO by CO. These Fe?Ni mixed‐oxide catalysts exhibited tremendously enhanced catalytic performance compared to monometallic catalysts that were prepared by using the same method. The effects of Fe/Ni molar ratio and calcination temperature on the catalytic activity were examined and the physicochemical properties of the catalysts were characterized by using XRD, Raman spectroscopy, N₂‐adsorption/‐desorption isotherms, temperature‐programmed reduction with hydrogen (H₂‐TPR), temperature‐programmed desorption of nitric oxide (NO‐TPD), and X‐ray photoelectron spectroscopy (XPS). The results indicated that the reduction behavior, surface oxygen species, and surface chemical valence states of iron and nickel in the catalysts were the key factors in the NO elimination. Fe_0.5Ni_0.5O_x that was calcined at 250 °C exhibited excellent catalytic activity of 100 % NO conversion at 130 °C and a lifetime of more than 40 hours. A plausible mechanism for the reduction of NO by CO over the Fe?Ni mixed‐oxide catalysts is proposed, based on XPS and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses.     

Oxygen exchange properties in the new pyrochlore solid solution Ce2Sn2O7-Ce2Sn2O8

The phase-pure cerium stannate pyrochlore (Ce₂Sn₂O₇) has been prepared for the first time. The structure and oxidation states of both cations were carefully reviewed, and the compound was unambiguously replaced within the rare-earth stannate series. As a consequence of the low stability of trivalent cerium in oxide phases, one oxygen per formula unit could be intercalated by calcination under O₂ at 400 °C, leading to the new Ce₂Sn₂O₈ pyrochlore. This latter phase is subject to oxygen under-stoichiometry from 400 to 700 °C. However, oxygen deintercalation seems to be in competition with cerium oxide segregation at high temperature, leading to the formation of cerium deficient pyrochlore phases.     

Conditional solubility versus pO2− of cerium(III) oxide in molten equimolar NaCl+KCl at 727°C

Reactions between cerium trichloride and oxide ions were studied in NaCl+KCl (1/1) at 1000°K, by potentiometry with a calcia-stabilized zirconia membrane electrode. Titration curves clearly demonstrated the existence of soluble cerium oxychloride (CeO⁺) and precipitated cerium oxide (Ce₂O₃), with respective dissociation constants 10^?11 and 10^?30 (molality scale). The corresponding conditional solubility diagram {log S (Ce^III)=f(pO^2?)} is presented and discussed.     

Stability and functional properties of Sr0.7Ce0.3MnO3 − δ as cathode material for solid oxide fuel cells

Studies of oxygen diffusion, interphase exchange, specific electric conductivity, and thermal expansion showed that perovskite-like Sr_0.7Ce_0.3MnO_{3 ? δ} (SCMO) as a potential cathode material for solid oxide fuel cells (SOFCs) has considerable advantages over the conventional materials based on lanthanum-strontium manganites. To prevent the interactions of SCMO with solid electrolyte membranes of stabilized zirconia and lanthanum gallate, it is necessary to deposit protective layers of solid solutions based on cerium oxide, which do not form new phases in contact with SCMO and electrolytes. The trials of model SOFCs with porous SCMO-based cathodes demonstrated satisfactory electrochemical and endurance characteristics of these electrodes.     

Synthesis and acid functionalization of cerium oxide nanoparticles

Colloidal cerium oxide has been obtained by controlled oxidation of soluble Ce(III) salts in hydrothermal conditions. The homogeneous nucleation of CeO₂ through thermolysis of this oxidizing solution allows the formation of well dispersed colloidal particles. Under optimal conditions, well crystallized particles with an average size of 8 nm are obtained. The surface is terminated by acetate groups which can be substituted by grafting phosphonic acids or phosphoric acids. Particularly, the grafting of 2-carboxyethylphosphonic acid or phosphonoacetic acid increases the acidic character of the surface as observed by impedance spectroscopy.     

Understanding the formation and evolution of ceria nanoparticles under hydrothermal conditions

Supercritical growth: The formation and evolution of ceria nanoparticles during hydrothermal synthesis was investigated by in?situ total scattering and powder diffraction. The nucleation of pristine crystalline ceria nanoparticles originated from previously unknown cerium dimer complexes. The nanoparticle growth was highly accelerated under supercritical conditions.     

AFM study of morphological changes associated with electrochemical solid–solid transformation of three-dimensional crystals of TCNQ to metal derivatives (metal = Cu, Co, Ni; TCNQ = tetracyanoquinodimethane)

In situ atomic force microscopy (AFM) allows images from the upper face and sides of TCNQ crystals to be monitored during the course of the electrochemical solid–solid state conversion of 50 × 50 μm² three-dimensional drop cast crystals of TCNQ to CuTCNQ or M[TCNQ]₂(H₂O)₂ (M = Co, Ni). Ex situ images obtained by scanning electron microscopy (SEM) also allow the bottom face of the TCNQ crystals, in contact with the indium tin oxide or gold electrode surface and aqueous metal electrolyte solution, to be examined. Results show that by carefully controlling the reaction conditions, nearly mono-dispersed, rod-like phase I CuTCNQ or M[TCNQ]₂(H₂O)₂ can be achieved on all faces. However, CuTCNQ has two different phases, and the transformation of rod-like phase 1 to rhombic-like phase 2 achieved under conditions of cyclic voltammetry was monitored in situ by AFM. The similarity of in situ AFM results with ex situ SEM studies accomplished previously implies that the morphology of the samples remains unchanged when the solvent environment is removed. In the process of crystal transformation, the triple phase solid∣electrode∣electrolyte junction is confirmed to be the initial nucleation site. Raman spectra and AFM images suggest that 100% interconversion is not always achieved, even after extended electrolysis of large 50 × 50 μm² TCNQ crystals.     

Oxidizing valve metals: effect of electronic properties of the oxide films

Photocurrents emerging during the formation of anodic oxide films (AOF) on such valve metals as W, Ti, Zr, Nb, Ta are measured during an increase (direct run) and a decrease (reverse run of voltammetric curves) in the anodic potential. Capacitances of AOF formed at certain potentials are measured at potentials below the AOF formation potential. Effect of semiconductor properties on the AOF growth is considered through the formation of a Schottky barrier at the oxide/electrolyte interface. Calculated thicknesses of AOF and the depleted layer are compared. The donor-concentration drop in AOF with the distance from the metal/oxide interface is a condition for the growth of thick semiconductor oxide films. The measured potential dependence of the semiconductor-film capacitance is used to plot the donor concentration drop as a function of the distance from the Nb₂O₅/Nb interface in the oxide layer on a niobium electrode. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.