Application of mechanochemical activation for synthesis of uranium–lanthanoid mixed oxides

The applicability of mechanochemistry to produce uranium–lanthanoid mixed oxides is presented. Phase homogeneous uranium–cerium solid solutions of the type Ce _x U_1−x O₂ (x = 0.3 ÷ 0.95) and polyphase systems containing La _y U_1−y O_2+x (y = 0.12) were prepared by mechanochemical activation in air of sol–gel produced precursors. The possibility for synthesis of urania–lanthania solid solution by mechanochemical interaction of La₂O₃ with sol–gel produced U (IV,VI) oxide is established. The crystal structures of the obtained oxides before and after the mechanochemical treatment are analysed by the use of X-ray diffraction method. The size of the crystallites (8–16 nm), lattice parameters, crystallite strains and densities of the oxides are calculated by BRASS program for Rietveld calculation.     

NO Reduction Over Noble Metal Ionic Catalysts

In last 40 years, catalysis for NO _x removal from exhaust gas has received much attention to achieve pollution free environment. CeO₂ has been found to play a major role in the area of exhaust catalysis due to its unique redox properties. In last several years, we have been exploring an entirely new approach of dispersing noble metal ions in CeO₂ and TiO₂ for redox catalysis. We have extensively studied Ce_1−x M _x O_2−δ (M = Pd, Pt, Rh), Ce_1−x−y A _x M _y O_2−δ (A = Ti, Zr, Sn, Fe; M = Pd, Pt) and Ti_1−x M _x O_2−δ (M = Pd, Pt, Rh, Ru) catalysts for exhaust catalysis especially NO reduction and CO oxidation, structure–property relation and mechanism of catalytic reactions. In these catalysts, lower valent noble metal ion substitution in CeO₂ and TiO₂ creates noble metal ionic sites and oxide ion vacancy. NO gets molecularly adsorbed on noble metal ion site and dissociatively adsorbed on oxide ion vacancy site. Dissociative chemisorption of NO on oxide ion vacancy leads to preferential conversion of NO to N₂ instead of N₂O over these catalysts. It has been demonstrated that these new generation noble metal ionic catalysts (NMIC) are much more catalytically active than conventional nano crystalline noble metal catalysts especially for NO reduction.     

Electrochemical intercalation of lithium into nanocrystalline ceria

The specifics of electrochemical lithium intercalation into nanocrystalline ceria were studied. The lithium capacity of CeO_{2 − x} is discovered to increase systematically as the nanoparticle size shifts down, indicating the potential of nanocrystalline ceria for use in electrochromic applications.     

Catalytic soot combustion of α-Fe/Ce–K–O nanocomposites via citrate-gel route

The binary phase, porous, nanocomposite xα-Fe/(1 − x)Ce_0.9–K_0.1–O (x = 0.05–0.2) catalysts and the catalyst-coated honeycomb ceramic device have been prepared by the citrate-gel thermal decomposition-reduction process and the sol–gel assisted dip-coating method, respectively. The nanocomposite of fluorite-type structure CeO₂ nanoparticles about 18–51 nm and α-Fe nanoparticles about 32 nm is obtained at 600 °C for 2 h in a deoxidization atmosphere and the α-Fe in nanocomposite has the suppression effect on grain growth of CeO₂. With Fe content increasing from 0.05 to 0.1, the specific surface area for the nanocomposites increases dramatically from about 4.4 to 43.0 m²/g, reaching a maximum value 57.7 m²/g at x = 0.15, and the pores vary from macropores to micro- or mesopores. Due to the presence of nano α-Fe, all the catalysts exhibit a very high soot catalytic activity, with the lowest T₂₀ (255 °C) and T₅₀ (291 °C) for the nanocomposite with x = 0.15, and it is confirmed by the bench test under practical diesel exhaust gases.     

Rheology and MT-DSC studies of the flow properties of ethyl and methyl babassu biodiesel and blends

The synthesis of pigments from the system Ce_1−x O₂–M _x O (M = Cu, Co) was achieved via a polymeric precursors method, Pechini method. Differential scanning calorimetry (DSC) and thermogravimetry (TG) techniques were used to accurately characterize the distinct thermal events occurring during synthesis. The TG and DSC results revealed a series of decomposition temperatures due to different exothermal events, which were identified as H₂O elimination, organic compounds degradation, and phase formation. X-Ray diffraction patterns show the presence of pure cubic CeO₂ phase for the samples with low Cu and Co loading. A decrease of the specific surface area with increasing copper and cobalt content was observed. The UV–visible diffuse reflectance technique was employed to study the optical properties in the 200–800 nm range. Colorimetric coordinates L*, a*, b* were calculated for the pigment powders. The powders presented a variety of colors from yellow for pure CeO₂, to brown for the ones loaded with copper and gray for the ones with cobalt.     

Physicochemical properties of the surface of the (Y,La0.1)Ce x Zr1? x O2?δ( x = 0.1–0.7) and Y0.1Pr0.3Zr0.6O2?δ complex oxide systems

The physicochemical properties of the surface of the Y_0.1Ce _x Zr_1−x O_2−δ, La_0.1Ce _x Zr_1−x O_2−δ (x=0.1–0.7), and Y_0.1Pr_0.3Zr_0.6O_2−δ. complex oxide systems were studied using IR and X-ray photoelectron spectroscopies. An appreciable enrichment of the surface of the solids in rare-earth-metal cations (cerium or praseodymium) during the synthesis was revealed. While cations are uniformly spread over the surface of cerium-zirconium solid solutions, the Y_0.1Pr_0.3Zr_0.6O_2−δ surface is covered by the clusters or even a phase of praseodymia. Reductive treatment in hydrogen with subsequent reoxidation results in the segregation of cerium ions on the Y_0.1Ce_0.3Zr_0.6O_2−δ surface at a temperature as low as 770 K. Original Russian Text ? A.N. Kharlanov, L.N. Ikryannikova, V.V. Lunin, A. Yu. Stakheev, 2007, published in Zhurnal Fizicheskoi Khimii, 2007, Vol. 81, No. 7, pp. 1271–1277.     

Nickel catalysts for the hydrodeoxygenation of biodiesel

A number of nickel and nickel-copper catalysts for the hydrodeoxygenation of fatty acid esters (biodiesel) were studied. The CeO₂ and ZrO₂ oxides and the CeO₂-ZrO₂ binary system were used as supports. The Ni-Cu/CeO₂-ZrO₂ catalyst exhibited the highest activity; it allowed us to quantitatively convert biodiesel into linear alkanes under mild conditions (290–320°C, 1.0 MPa). It was found that the selectivity of the formation of the main product (heptadecane) was 70–80%. The main correlations between the nature of catalysts and their activity under conditions of the target reaction were determined using temperature-programmed reduction, X-ray diffraction analysis, and electron microscopy. It was hypothesized that the high activity of Ni-Cu/CeO₂-ZrO₂ in the test reaction can be explained by the presence of a Ni_{1 − x} Cu _x (x = 0.2–0.3) solid solution as a constituent of the active component of the catalyst.     

Synthesis and structure of quasi-one-dimensional zinc oxide doped with manganese

Nanotubes of manganese-doped zinc oxide Zn_{1 − x} Mn _x O (0 ≤ x ≤ 0.2) were synthesized by heating the Zn_{1 − x} Mn _x (HCOO)(OCH₂CH₂O)_1/2 precursor in air at 500°C. The precursor with extended crystals was synthesized by a solvothermal method based on heat treatment of a mixture of Zn_{1 − x} Mn _x (HCOO)₂ · 2H₂O with an ethylene glycol excess at 100–130°C. The tubular morphology of Zn_{1 − x} Mn _x O particles was identified by transmission electron microscopy. Tubular quasi-one-dimensional particles were shown to have a nanodispersed polycrystalline structure, the size of separate crystallites being from 5 to 20 nm. X-ray photoelectron spectroscopy suggested that the manganese distribution on the outer surface layer of Zn_{1 − x} Mn _x O nanotubes is nonuniform.     

Solid electrolytes based on CeO<Subscript>2</Subscript> for medium-temperature electrochemical devices

CeO₂-based solid solutions with a fluorite structure are promising materials as electrolytes of medium-temperature electrochemical devices: electrolytic cells, oxygen sensors, and solid oxide fuel cells. In this work, studies are presented of the effect of the dopant cation radius and its concentration on the physico-chemical properties of the Ce_{1 − x} Ln _x O_{2 − δ} solid solutions (x = 0–0.20; Ln = La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb) and also of multicomponent solid solutions of Ce_{1 − x} Ln _x/2Ln′ _x/2O_{2 − δ} (x = 0–0.20; Ln = Sm, La, Gd and Ln′ = Dy, Nd, Y) and Ce_{1 − x − y} Sm _x M _y O_{2 − δ} (M = Ca, Sr, Ba) obtained using the solid-phase synthesis technique. Electric properties of the samples were studied in the temperature range of 623–1173 K and in the oxygen partial pressure range of 0.01–10⁻²² MPa. The values of oxygen critical pressure ( p_O2 ^* )\left( {p_{O_2 }^* } \right) are presented, at which the ionic and electron conductivity values are equal. The values were calculated on the basis of experimental dependences at 1023 K at the assumption that the ionic conductivity value is determined only by the dopant concentration and its effective ionic radius and is independent of the oxygen partial pressure.     

Development of Cocatalysts for Photocatalytic Overall Water Splitting on (Ga1? x Zn x )(N1? x O x ) Solid Solution

The development of cocatalysts promoting overall water splitting on (Ga_1−x Zn _x )(N_1−x O _x ) solid solution photocatalyst is presented. The (Ga_1−x Zn _x )(N_1−x O _x ) is a stable visible-light-driven photocatalyst for stoichiometric water splitting upon loading with a suitable nanoparticulate cocatalyst. Loading with a combination of Cr and Rh oxides, Rh_2−y Cr _y O₃, is demonstrated to raise the quantum efficiency of (Ga_1−x Zn _x )(N_1−x O _x ) for overall water splitting to 2.5% at 420–440 nm. This represents a 10-fold increase in efficiency over the highest efficiency previously obtained using nanoparticulate RuO₂ as a cocatalyst. In addition to the composition, the dispersion and size of cocatalyst nanoparticles are identified as important factors affecting the degree of enhancement for stoichiometric water splitting. Kazuhiko Maeda—Research Fellow of the Japan Society for the Promotion of Science (JSPS).     

Synthesis, crystal and electronic structures, and thermodynamic characteristics of BaCe1 ? xInxO3 ? x/2 solid solutions

New perovskite oxide phases BaCe_{1 − x} In _x O_{3 − x/2} (x = 0.1–0.8) (space group Pbnm) have been synthesized. The unit cell volume of the resulting solid solutions monotonically decreases with an increase in the degree of substitution of indium for cerium due to the contraction of octahedra in perovskite blocks. The thermodynamic stability of the compound BaCe_0.75In_0.25O_2.875 was studied by the solution calorimetry method, and barium cerates were shown to be thermodynamically stable with respect to binary oxides at room temperature. The structure of occupied and vacant states in BaCe_{1 − x} In _x O_{3 − x/2} was determined on the basis of X-ray emission, absorption, and photoelectron spectra, and the energy gap was estimated at ∼2 eV.     

Direct coating of quantum dots with silica shell

This paper describes a method for direct coating of fluorescent semiconductor nanoparticles with silica shell. The fluorescent semiconductor nanoparticles used were CdSe _x Te_1–x nanoparticles coated with ZnS and succeedingly surface-modified with carboxyl groups, or quantum dots (Q-dots). The Q-dots were silica-coated by performing sol–gel reaction of tetraethyl orthosilicate (TEOS) using NaOH as a catalyst in the presence of the Q-dots. Quasi-perfect Q-dots/silica core-shell particles were formed at 5.0 M H₂O and 4.0 × 10⁻⁴ M NaOH. Under these concentrations of H₂O and NaOH, the particle size of Q-dots/silica particles could be varied from 20.1 to 38.1 nm as the TEOS concentration increased from 2.5 × 10⁻⁴ to 50 × 10⁻⁴ M. The Q-dots/silica particles showed fluorescence as well as the uncoated Q-dots.     

Dopant effects on the structural, low temperature Raman scattering and electrical transport properties in SrTi1?xFexO3 nanoparticles synthesized by sol-gel method

We investigate effects of Fe dopant concentration on the structure, as well as low temperature Raman scattering and electrical transport properties in SrTi_1−x Fe _x O₃ (x = 0.00, 0.10, 0.20, 0.30, 0.40) nanoparticles prepared by sol-gel method. The results show an average particle size of powder is about 30 nm, and the lattice parameters decrease as increasing the Fe content. In the Raman spectra, a broad structure in the region 200–500 cm⁻¹ is almost absent and the peaks in the region 600–800 cm⁻¹ show different weights with respect to SrTiO₃, relating to structural changes with increasing dopant concentration in conjunction with increasing grain boundary contribution to the impedance. The abrupt change in Raman peak position as function of temperature suggests a phase transition in our samples in the range of 110–150 K. These results indicate that the Fe ion has replaced the site of Ti in unit cell. These results also demonstrate the feasibility of synthesizing the compound with low annealing temperature.     

Master sintering curve for Gd-doped CeO<Subscript>2</Subscript> solid electrolytes

The sintering behavior of gadolinia-doped ceria powders was studied by the master sintering curve (MSC). Dilatometric analyses of powders produced by a soft chemical method were performed to provide the experimental data set for the construction of the MSC. The assumed model provided good fittings of the MSC and the activation energy for the sintering of Ce_1−x Gd _x O_3−δ, with x = 0, 0.05, 0.1, and 0.2 were found to be in the 218–325 KJ/mol range, depending on the dopant content. The results supported that both the nanometric size of the particles and the difference in ionic radii between Gd³⁺ and Ce⁴⁺ affects the sintering of Gd-doped CeO₂.     

Structural and low temperature Raman scattering studies in SrTi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> nanoparticles synthesized by sol–gel method

We investigate effects of Co dopant concentration on the structure and low temperature Raman scattering properties in SrTi_1−x Co _x O₃ (x = 0.00, 0.10, 0.20, 0.30) nanoparticles prepared by sol–gel method. The dopant induced changes are studied by XRD, and Raman scattering measurements. The results show an average particle size of about 20 nm depending on the Co content and the lattice parameters decrease as increasing the Co content. In the Raman spectra, a broad structure in the region 100–500 cm⁻¹ is almost absent and the peaks in the region 600–800 cm⁻¹ show different weights with respect to SrTiO₃, relating to structural changes. The anomalous change in the area ratio of Raman peaks as function of temperature suggests a phase transition in our samples in the range of 110–130 K. These results indicate that the Co ion has replaced the site of Ti in unit cell. This novel route also demonstrates the advantage of synthesizing the compound with low annealing temperature.     

Highly Dispersed Ce x Zr1−x O2 Nano-Oxides Over Alumina, Silica and Titania Supports for Catalytic Applications

We have been exploring the utilization of supported ceria and ceria–zirconia nano-oxides for different catalytic applications. In this comprehensive investigation, a series of Ce _x Zr_1−x O₂/Al₂O₃, Ce _x Zr_1−x O₂/SiO₂ and Ce _x Zr_1−x O₂/TiO₂ composite oxide catalysts were synthesized and subjected to thermal treatments from 773 to 1073 K to examine the influence of support on thermal stability, textural properties and catalytic activity of the ceria–zirconia solid solutions. The physicochemical characterization studies were performed using X-ray diffraction (XRD), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HREM), thermogravimetry and BET surface area methods. To evaluate the catalytic properties, oxygen storage/release capacity (OSC) and CO oxidation activity measurements were carried out. The XRD analyses revealed the formation of Ce_0.75Zr_0.25O₂, Ce_0.6Zr_0.4O₂, Ce_0.16Zr_0.84O₂ and Ce_0.5Zr_0.5O₂ phases depending on the nature of support and calcination temperature employed. Raman spectroscopy measurements in corroboration with XRD results suggested enrichment of zirconium in the Ce _x Zr_1−x O₂ solid solutions with increasing calcination temperature thereby resulting in the formation of oxygen vacancies, lattice defects and oxygen ion displacement from the ideal cubic lattice positions. The HREM results indicated a well-dispersed cubic Ce _x Zr_1−x O₂ phase of the size around 5 nm over all supports at 773 K and there was no appreciable increase in the size after treatment at 1073 K. The XPS studies revealed the presence of cerium in both Ce⁴⁺ and Ce³⁺ oxidation states in different proportions depending on the nature of support and the treatment temperature applied. All characterization techniques indicated absence of pure ZrO₂ and crystalline inactive phases between Ce–Al, Ce–Si and Ce–Ti oxides. Among the three supports employed, silica was found to stabilize more effectively the nanosized Ce _x Zr_1−x O₂ oxides by retarding the sintering phenomenon during high temperature treatments, followed by alumina and titania. Interestingly, the alumina supported samples exhibited highest OSC and CO oxidation activity followed by titania and silica. Details of these findings are consolidated in this review.     

The effect of ball-milling on the thermal behavior of anatase-doped hematite ceramic system

High energy ball-milling methods were employed in the synthesis of anatase-doped hematite xTiO₂(a) · (1−x)α-Fe₂O₃ (x = 0.1, 0.5, and 0.9) ceramic system. The thermal behavior of as obtained ceramic system was characterized by simultaneous DSC–TG. The pure anatase phase was found to be stable below 800 °C, but there is a 10.36% mass loss due to the water content. Two exothermic peaks on DSC curves of pure anatase indicate the different crystallization rates. The pure hematite partially decomposed upon heating under argon atmosphere. Ball-milling has a strong effect on the thermal behaviors of both anatase and hematite phases. For x = 0.1 and 0.5, there is gradual Ti substitution of Fe in hematite lattice, and the decomposition of hematite is enhanced due to the smaller particle size after ball-milling. The crystallization of hematite was suppressed as the enthalpy values decreased due to the anatase-hematite solid–solid interaction. For x = 0.9, most of the anatase phase converted to rutile phase after long milling time. The thermal behavior of xTiO₂(a) · (1−x)α-Fe₂O₃ showed smaller enthalpy value of the hematite transformation to magnetite and anatase crystallization due to the small fraction of hematite phase in the system and hematite–anatase interaction, while the mass loss upon heating increased as a function of milling time due to more water content absorbed by the smaller particle size.     

Structural,optical and photoluminescence properties of Zn<Subscript>1−x</Subscript>Ce<Subscript>x</Subscript>O (x = 0, 0.05 and 0.1) nanoparticles by sol–gel method annealed under Ar atmosphere

Ce doped ZnO nanoparticles (Zn_1−xCe_xO, x = 0.0, 0.05 and 0.1) have been synthesized by sol–gel method at annealing temperature of 500 °C for 1 h under Ar atmosphere. The synthesized samples have been characterized by powder X-ray diffraction (XRD), energy dispersive X-ray studies, UV–Visible spectrophotometer and fourier transform infrared (FTIR) spectroscopy. The XRD measurements indicate that the prepared nanoparticles have a hexagonal wurtzite structure and CeO₂ crystallites. The calculated average crystalline varied from 21.97 to 15.62 nm with increase in Ce concentrations. The increase in lattice parameters reveals the substitution of Ce into ZnO lattice. The presence of functional groups and the chemical bonding is confirmed by FTIR spectra. PL spectra of the Zn_1−xCe_xO system show that the shift in near band edge emission from 386 to 363 nm and a shift in blue band emission from 517 to 485 nm which confirms the substitution of Ce into the ZnO lattice.     

Phase transitions and ion transport in NASICON materials of composition Li1 + xZr2 ? xInx(PO4)3(x = 0–1)

NASICON materials of composition Li_{1 + x} Zr_{2 − x} In _x (PO₄)₃(x = 0–1) were synthesized. The phase constitution, particle size, and conductivity of these materials were studied as s function of synthesis temperature. High-temperature X-ray powder diffraction was used to study phase transitions in the materials synthesized. Low levels (x ≤ 0.1) partial substitution of indium for zirconium considerably increase the lithium ion conductivity and reduce the activation energy for conduction compared to the parent compound.