Showing papers on "Cerium published in 1995"

The Investigation of Cerium as a Cathodic Inhibitor for Aluminum‐Copper Alloys

Abstract: In situ current density mapping, scanning electron microscopy, and energy dispersive spectroscopy were used to study the effects of cerium as a corrosion inhibitor for an aluminum copper alloy (Al 2024-T4) in chloride containing solutions. It was found that cerium inhibits corrosion of this alloy by reducing the rate of the cathodic reaction. This was due to the formation of cerium-rich films over copper containing intermetallics which act as local cathodic sites. Results from tests carried out on an aluminum/copper galvanic couple, which was used to simulate the electrochemical behavior of the copper containing intermetallics, showed that corrosion inhibition was associated with the formation of a Ce-rich film over the copper in agreement with that observed for the alloy.

Preparation and characterization of highly dispersed silica-supported ceria

Abstract: Silica-supported ceria samples of various cerium content ( 0.008 to 11 % Ce ) were prepared either through anchoring cerium acetylacetonate in organic medium or by capillary aqueous impregnation of cerium nitrate. After calcination, ceria particles as small as 2 nm were obtained by the anchoring process while impregnation leads to larger particles (5–9 nm). These samples show significant spectroscopic differences with pure ceria: (i) increase of the X-ray photoelectron spectroscopy binding energies of the Ce 3d and of the O 1s lines, (ii) blue shift of the band gap measured by diffuse reflectance, (iii) observation of narrow UV bands at low cerium content, (iv) increase of the CO stretching vibration of CO-Ce4+ terminal species. These results are ascribed to the coordinative unsaturation of surface cerium and oxygen ions.

Electrochemical Synthesis and Sintering of Nanocrystalline Cerium(IV) Oxide Powders

Abstract: Nanocrystalline CeO{sub 2} powders were prepared electrochemically by the cathodic electrogeneration of base, and their sintering behavior was investigated. X-ray diffraction and transmission electron microscopy revealed that the as-prepared powders were crystalline cerium(IV) oxide with the cubic fluorite structure. The lattice parameter of the electrogenerated material was 0.5419 nm. The powders consisted of nonaggregated, faceted particles. The average crystallite size was a function of the solution temperature. It increased from 10 nm at 29 C to 14 nm at 80 C. Consolidated powders were sintered in air at both a constant heating rate of 10 C/min and under isothermal conditions. The temperature at which sintering started (750 C) for nanocrystalline CeO{sub 2} powders was only about 100 C lower than that of coarser-grained powders (850 C). However, the sintering rate was enhanced. The temperature at which shrinkage stopped was 200--300 C lower with the nanoscale powder than with micrometer-sized powders. A sintered specimen with 99.8% of theoretical density and a grain size of about 350 nm was obtained by sintering at 1,300 C for 2 h.

Thermal stability of doped ceria: experiment and modelling

Abstract: The process of the decrease of the surface area due to crystallite growth in ceria at 943 K is described by a kinetic model involving oxygen and cerium diffusion. The experimentally found variations in the rate of crystallite growth are reported as a function of the content (⩽ 10% cat.) of dopants, which are the cations Ca2+, Mg2+, Al3+, Y3+, Sc3+, Al3+, Th4+, Zr4+ and Si4+. The variations are discussed on the basis of the diffusion of cerium vacancies as the rate-limiting step, and on the basis of calculated expressions of the concentrations of oxygen vacancies, electrons and cerium vacancies vs. the oxygen partial pressure and the dopant content. For cations that are smaller than Ce4+, the comparison between the experimental and theoretical rates asserts the validity of the model and allows the prediction of the efficiency of a cation to stabilize the surface area, from its associations with oxygen vacancies and with the electron-bearing species, Ce′Ce.

Nanophase catalytic oxides. I: Synthesis of doped cerium oxides as oxygen storage promoters

Abstract: Doped CeO2 materials were synthesized with the aim to improve the performance of CeO2 as oxygen storage promoter in gas catalytic reactions. The coprecipitation method was used for the synthesis of fine oxalate precursors of high homogeneity and well defined composition. The chemical and morphological properties of both the coprecipitated oxalates and the calcined oxides were examined. The influence of doping of different metal cations into the CeO2 structure on the oxygen storage capacity in particular was investigated. Some of the doped oxides Ce0.9M0.1 O2 − δ (M = Ca, Nd, Pb, etc.) give an increased oxygen storage capacity, 20–40% higher than the undoped. Their redox activity also remarkably increased.

Calculated Phase Diagram for the γ ⇌ α Transition in Ce

Abstract: We have calculated the pressure-temperature phase diagram of the $\ensuremath{\gamma}\ensuremath{\rightleftharpoons}\ensuremath{\alpha}$ isostructural transition in Ce on the basis of the Mott transition model. The theory correctly describes the linear variation of the transition temperature with pressure and the existence of a critical point. The quantitative agreement with the experimental diagram is good. The influence of different free energy contributions (configurational, magnetic, and vibrational) on the phase transition in Ce is discussed.

Cerium Content and Cycle Life of Multicomponent AB 5 Hydride Electrodes

Abstract: Multicomponent AB5 hydrides are attractive replacements for the cadmium electrode in nickel-cadmium batteries. The archetype compound of the AB~ alloy class is LaN%, but in a typical battery electrode mischmetal is substituted for La and Ni is substituted in part by various metals. While the effects of Ni substitution have been widely studied, relatively little effort has been focused on the effect of La substitution. Cerium is the predominant rare earth in normal misehmetal, and this paper deals with the effect on cycle life and storage capacity due to the increasing presence of Ce in the alloy series La1_x Ce~ Ni3.~5 Co0.7~Mn0.~Al0.3. Alloys were characterized by the determination of pressure-composition relationships, molar volume of H in the hydride phase and electrode cycle life. The effects due to lattice expansion are taken into account. It was concluded that the rate of loss of electrochemical capacity per charge-discharge cycle due to electrode corrosion was significantly decreased by the presence of Ce.

Cerium impregnated H-mordenite as a catalyst for shape-selective isopropylation of naphthalene. Selective deactivation of acid sites on the external surface

Abstract: The impregnation of cerium is the effective method for the deactivation of external acid sites of H-mordenite. The selectivity of 2,6-DIPN in the isopropylation of naphthalene was enhanced by the impregnation with such a large amount as 30–50 wt.-% of cerium without significant decrease of catalytic activity. The highest selectivity of 2,6-DIPN was achieved up to 70% over a highly dealuminated H-mordenite, (HM(128); SiO 2 /Al 2 O 3 = 128, with 30 wt.-% of cerium. The enhancement of the selectivity is ascribed to the deactivation of external acid sites judging from the activity of the cracking reaction of 1,3,5-triisopropylbenzene. The effective pore radius was not reduced by the modification. The ceria is highly dispersed only on the external surface of H-mordenite without any formation of new kinds of acid sites. The 129 Xe NMR observation suggfests that cerium is not in the pores, but on the external surfaces. The deactivation of the external acid sites is a characteristic property for cerium. Lanthanum and neodymium inhibited catalytic activity of the isopropylation because the pores were narrowed by their impregnation. A possible reason of the deactivation is ascribed to the amphoteric property of ceria.

Effect of chlorine on microstructure and activity of Pd/CeO2 catalysts

Abstract: The evolution of microstructure and activity in benzene hydrogenation of Pd/CeO2 catalysts, prepared from Pd chloride or Pd nitrate, upon reduction at 573–973 K has been studied by X-ray diffraction (XRD), selected-area electron diffraction (SAD), high-resolution transmission electron microscopy (HRTEM) and gas chromatography. The crucial role of chloride from the metal precursor in structural transformations of the ceria support has been established and attributed to its ability to form stable cerium oxychloride at temperatures as low as 573 K. A. rapid decline to zero of the activity of Pd/CeO2 catalysts in benzene hydrogenation was observed when the temperature of reduction was increased from 573 to 773 K. This effect could not be explained by sintering, but XRD data indicated that a metal–support reaction leading to Pd–Ce alloy formation was likely to be the cause. At higher temperature (873 K) the coverage of Pd particles by a thin overlayer (probably CeOx) was observed by HRTEM.

Close coupled catalyst

Abstract: A stable, close-coupled catalyst, an article comprising the close-coupled catalyst and a related method of operation. The close-coupled catalyst comprises a catalyst support and a palladium catalytic component. Preferably and optionally, there are stabilizers including alkaline metal oxide, and rare earth metal components selected from the neodymium and lanthanum components. The close-coupled catalyst composition includes substantially no additional oxygen storage component such as praseodymium or cerium compounds. There is preferably a catalyst such as a three-way catalyst downstream of the close-coupled catalyst. The downstream catalyst preferably includes an oxygen storage component such as cerium oxide or praseodymium oxide.

AC TFEL device having a white light emitting multilayer phosphor

Abstract: An AC thin film electroluminescent (TFEL) device includes a multilayer phosphor for emitting white light having improved emission intensity in the blue region of the spectrum. The multilayer stack consists of an inverted structure thin film stack having a red light emitting manganese doped zinc sulfide (ZnS:Mn) layer disposed on a first insulating layer; a blue-green light emitting cerium doped strontium sulfide (SrS:Ce) layer disposed on the red light emitting layer; and a blue light emitting cerium activated thiogallate phosphor (Sr x Ca 1-x Ga 2 S 4 :Ce) layer disposed on the blue-green light emitting layer. The manganese doped zinc sulfide layer acts as a nucleating layer that lowers the threshold voltage, and the cerium activated thiogallate phosphor layer provides a moisture barrier for the hydroscopic cerium doped strontium sulfide layer. The white light from the multilayer phosphor can be appropriately filtered to produce any desired color.

Ortho-selective methylation of phenol over ceo2 catalyst

Abstract: Cerium oxides prepared by thermal decomposition of cerium salts were found to exhibit excellent catalytic activities for the ortho-methylation of phenol without showing any decay.

Cerium/zirconium mixed oxide catalysts having high/stable specific surface areas

Abstract: Cerium/zirconium mixed oxides (optionally including yttrium values), comprising solid solutions thereof, having contents of zirconium of up to 60 atom % and having thermally stable, very high specific surface areas at least greater than 80 m 2 /g, preferably at least 100 m 2 /g and more preferably at least 150 m 2 /g, are well suited as catalysts and/or catalyst supports, notably for the treatment/conversion of vehicular exhaust gases; such Ce x Zr 1-x O 2 particulates are conveniently prepared by thermally treating an aqueous solution of soluble compounds of cerium and zirconium (and optionally yttrium), e.g. the nitrates thereof, present in the desired stoichiometric amounts, and thence recovering and, if appropriate, calcining the reaction product thus formed.

Method for removing nitrogen oxides in exhaust gas by selective catalytic reduction and catalyst for reduction of nitrogen oxides

Abstract: The present invention provides a catalyst for reduction of nitrogen oxides represented by the formula (A a O x .B b O y ).(C c O z .C' c' O z' )/S which is produced by supporting mixed metal oxides represented by the formula (A z O x .B b O y ).(C c O z .C' c' O z' ) in amorphous state on aluminum or silicon-containing support at calcination temperature of 400° to 700° C. in a molar ratio of 0.01:1 to 5:1, and a method for removing nitrogen oxides through selective catalytic reduction which comprises converting 50 to 50,000 ppm of nitrogen oxides in exhaust gas from automobile or from fixed source such as plant turbine and boiler, and other industry in a state of having 0.1 to 20% of excessive oxygen over (A a O x .B b O y ).(C c O z .C' c' O z' )/S catalyst by using 100 to 100,000 ppm of hydrocarbon reducing agent having 1 to 5 carbons under the reaction condition of 200° to 800° C. of reaction temperature, 1 to 10 atmosphere of reaction pressure and 1000 to 100,000/hour of space velocity into nitrogen in which A and B are lanthanide metals, such as lanthanum, cerium, praseodymium or neodymium, and alkali metals or alkaline earth metals, such as sodium, potassium, rubidium, cesium, magnesium, calcium, strontium or barium, C and C' are transition metals of the first period, such as cobalt, copper, nickel, manganese, iron, vanadium, titanium, chromium and zinc, and noble metals, such as platinum, rhodium, iridium, ruthenium, rhenium, palladium and silver, a, b, c and c' have stoichiometrically 0 to 1, provided with a+b=1, c+c'=1 and is in the range of 0.1-3.0:1.0, and S is aluminum or silicon-containing support, and is zeolite, silica, alumina or silica-alumina.

Optical centers of cerium in silica glasses obtained by the sol-gel process

Abstract: A spectral-luminescent study of Ce-containing silica gel-glasses doped at the sol stage and by impregnating porous xerogels with activator chloride salt solutions has been performed. The influence of the degree of doping and redox conditions of vitrification on the activator charge state and the content of hydroxyl ions and chlorine in the glass is considered. The preservation of the glass phase composition and the non-linear increase in the visible absorption intensity with increasing activator concentration from 5 × 1017 to 3.0 × 1020 ions/cm3 are noted. It is concluded that there are two main types of optical centre. It is assumed that they represent Ce(III) oxygen complexes whose nearest surrounding includes quadruply charged (the first type) and triply charged (the second type) ions of cerium. Evidence has been obtained in favor of the superexchange excitation transfer to the central ion of Ce(III) oxygen complexes from photoreduced to the triply charged state of the same ion name of Ce(IV) oxygen complexes.

An XPS study of metallic three-way catalysts: The effect of additives on platinum, rhodium, and cerium

Abstract: The effect of Zr, Si, La, and Ba on the thermal aging of Pt, Rh, and Ce in automotive exhaust gas catalysts supported on metal foils was studied with X-ray photoelectron spectroscopy (XPS). Platinum was in metallic state under all circumstances studied in this work. Rhodium was partially (about 30%) oxidized already at room temperature. The decreases in the binding energies of the reduced noble metals due to aging was explained in terms of particle growth. The binding energy of rhodium oxide increased during aging indicating rhodium diffusion into alumina. Cerium was oxidized during aging. It was found that in the lanthanum containing catalysts the binding energies of platinum and rhodium were below the values for bulk metals, and the degree of oxidation of cerium was lower than in the other samples. It was concluded that the presence of lanthanum facilitated the reduction of the noble metals and cerium.

Method of increasing corrosion resistance of metals and alloys by treatment with rare earth elements

Abstract: There is provided a method for treating the surface of metals such as nickel based or high alloy steels, austenitic and ferritic stainless steels, copper and aluminum alloys to increase their corrosion resistance by modification of the metal surfaces to inhibit cathodic corrosion processes. In a single step treatment process the metals are immersed into a heated aqueous composition containing a rare earth salt substantially free of any halide compound. Increased corrosion resistance is obtained using nitrates of yttrium, gadolinium, cerium, europium, terbium, samarium, neodymium, praseodymium, lanthanum, holmium, ytterbium, dysprosium, and erbium nitrates. The rare earth salt is present in the range from about 2% by weight to saturation of the solution. The composition includes a pH-modifying substance such as nitric acid to adjust the pH in the range 0.5 to about 6.5 to attack the surface to remove oxides facilitating deposition of the rare earth. For aluminum alloys the pH is maintained between 4.5 to 6.5, for nickel based alloys and austenitic stainless steels the pH is maintained between 0.5 to 3.5 and between pH 2.0 to 4.5 for ferritic stainless steels. The surface can also be conditioned by abrasion before or during immersion in the composition. Increased corrosion resistance is achieved by immersion for time periods in the range of a few minutes up to one hour with the composition maintained between 60 to 95° C. Cerium, gadolinium, neodymium and praseodymium nitrate when used alone produced the greatest degree of corrosion resistance compared to the other rare earth nitrates. Significant synergistic effects are observed when combinations of two or more rare earth nitrates are used in the compositions. Aqueous treatment solutions based on combinations of cerium nitrate, gadolinium nitrate and lanthanum nitrate are very effective in reducing crevice corrosion.

Thermally stable/highly reducible catalyst compositions comprising alumina and the oxides of cerium and zirconium

Abstract: Thermally stable/highly reducible catalyst compositions, well suited for the catalytic conversion of exhaust gases emanating from internal combustion engines notably vehicular exhaust gases, comprise alumina, the oxides of cerium praseodymium and zirconium, and, optionally, at least one other oxide of bismuth, of a rare earth, or of an element of group VIII of the Periodic Table, and exhibit a stabilized hydrogen-absorption capacity up to a temperature of at least 1,000° C.

High quality CeO2 film grown on Si(111) substrate by using low energy dual ion beam deposition technology

Abstract: By using the mass‐analyzed low energy dual ion beam deposition technique, a high quality epitaxial, insulating cerium dioxide thin film with a thickness of about 2000 A has been grown on a silicon (111) substrate. The component species, cerium and oxygen, are homogeneous in depth, and have the correct stoichiometry for CeO2. X‐ray double‐crystal diffraction shows that the full width at half maximum of the (222) and (111) peaks of the film are less than 23 and 32 s, respectively, confirming that the film is a perfect single crystal.

Quartz glass with reduced ultraviolet radiation transmissivity, and electrical discharge lamp using such glass

Abstract: Ultraviolet (UV) radiation in the UV-C and UV-B bands, which is particularly dangerous, is absorbed and filtered by quartz glass doped with between 0.065% and 3.25%, and preferably between 0.065% and 1.3% by weight, of cerium metal, or cerium as such. Preferably, the cerium is added to quartz sand and/or rock crystal, in form of a fine-grained powder of up to 2 0 μm grain size, in form of cerium aluminate (CeAlO 3 ), present in up to about 5% by weight, and preferably up to about 2%, and melted together in a single step. The quartz glass so obtained is particularly suitable for a metal halide discharge lamp, e.g. as an outer envelope (1), or as the discharge vessel (27) itself, or for halogen incandescent lamps, to form the quartz-glass light bulb or an envelope therefor. A small quantity of titanium oxide, up to about 0.05%, may be added as a further doping agent to the melt to further improve the UV absorption in the B and C bands.