Direct synthesis of dimethyl carbonate from methanol and carbon dioxide over Ga2O3-CeO2-ZrO2 catalysts prepared by a single-step sol-gel method: Effect of acidity and basicity of the catalysts

XGa₂O₃-CeO₂-ZrO₂ (X=0, 1, 3, 5, 7, and 9) catalysts were prepared by a single-step sol-gel method with a variation of Ga₂O₃ content (X, wt%) for use in the direct synthesis of dimethyl carbonate from methanol and carbon dioxide. The ratio of cerium oxide:zirconium oxide in the XGa₂O₃-CeO₂-ZrO₂ catalysts was fixed to be Ce_0.6Zr_0.4O₂. Effect of acidity and basicity of XGa₂O₃-CeO₂-ZrO₂ on the catalytic performance in the direct synthesis of dimethyl carbonate from methanol and carbon dioxide was investigated using NH₃-TPD and CO₂-TPD experiments, respectively. Experimental results revealed that both acidity and basicity of the catalysts played important roles in determining the catalytic performance in the reaction. The amount of dimethyl carbonate increased with increasing both acidity and basicity of the catalyst. Among the catalysts tested, 3Ga₂O₃-CeO₂-ZrO₂, which retained the largest acidity and basicity, exhibited the best catalytic performance in the direct synthesis of dimethyl carbonate from methanol and carbon dioxide.     

Regulation of Y2O3 addition on structure and properties of Al2O3/ZrO2(Y2O3) directionally solidified eutectic ceramic

Al₂O₃/ZrO₂(Y₂O₃) directionally solidified eutectic ceramics (DSECs) with different Y₂O₃ addition were prepared. Three polymorphs of ZrO₂: m-ZrO₂, t-ZrO₂ and c-ZrO₂ appeared and disappeared in order with the increase of Y₂O₃ addition, the effects of Y₂O₃ addition amount on the mass fraction of ZrO₂ polymorphs were quantitatively analyzed. The gradual decrease of area fraction and average size of colony was attributed to the increase of constitutional undercooling region, the variation of interphase spacing in the colonies was explained via selection mechanism of dendrite tip. The maximum hardness and fracture toughness were 18.35 GPa and 8.55 MPa m^1/2 obtained at 9 mol% and 3 mol% Y₂O₃ addition, respectively. The hardness was determined by the mass fraction of m-ZrO₂ phase, the area fraction of intercolony and the residual compressive stress, and the toughening mechanisms were composed of microcrack toughening, t-m transformation toughening and residual stress toughening.     

Phase equilibria in the La2O3-Sm2O3-ZrO2 system: Experimental studies and thermodynamic modeling

A series of ceramic samples were prepared to experimentally investigate sub-solidus phase relations in the La₂O₃-Sm₂O₃-ZrO₂ system at 1873 K and 1673 K. No ternary compounds have been observed, while the binary La₂Zr₂O₇ and Sm₂Zr₂O₇ pyrochlore phases form a continuous solid solution La_2?xSm_xZr₂O₇ in the ternary system at the selected temperatures. X-ray diffraction and microstructure results demonstrated that the pyrochlore phase is stable in the ZrO₂-rich corner. The homogeneity range of the pyrochlore phase was carefully determined and the phase boundary of the cubic ZrO₂ (fluorite phase) which extends into the ternary system was also constructed via electron probe microanalysis. The as-obtained data were adopted to determine the mixing parameters for the pyrochlore and fluorite phases in the present thermodynamic modeling. A self-consistent database of the La₂O₃-Sm₂O₃-ZrO₂ system was accordingly established for the first time and the calculations agree well with the experimental data in the current work.     

Eutectic structure evolution of Al2O3-ZrO2-Y2O3 system for potential hybrid solar cell application

Abstract

Ternary Al₂O₃-ZrO₂-Y₂O₃ samples with a eutectic composition were prepared by slow cooling. The microstructural evolution was observed with X-ray diffraction (XRD), scanning electron microscopy (SEM).

The SEM observation of the ternary samples agreed with the XRD with a completion of crystallisation by slow cooling. The target materials commonly have ‘cantaloupe skin’ microstructures as shown in the previous studies by Han et al. The nanocomposite may have experienced different cooling rates with two different microstructures, near the surface having experienced optimal conditions for the eutectic reaction during their cooling and thus formed the eutectic microstructure, near the centre having experienced a slower cooling rate. The crystallised eutectic ternary Al₂O₃-ZrO₂-Y₂O₃ system had three different phases with a 3Y₂O₃-5Al₂O₃ (yttrium-aluminium garnet phase), an alumina phase formed by the eutectic reaction, and a solid solution of ZrO₂ and Y₂O₃.     

Thermodynamics and kinetics of sintering of Y2O3

Surface energy (γ_S) and grain boundary energy (γ_GB) of yttrium oxide (Y₂O₃) were determined by analyzing the heat of sintering (ΔH_sintering) using differential scanning calorimetry (DSC). The data allowed quantification of sintering driving forces, which when combined with a thorough kinetic analysis of the process, provide better understanding of Y₂O₃ densification as well as insights into effective strategies to improve its sinterability. The quantitative thermodynamic study revealed moderate thermodynamic driving force for densification in Y₂O₃ (as compared to other oxides) represented by a dihedral angle of 152.7° calculated from its surface and grain boundary energies. The activation energy was determined as 307 ± 61 kJ/mol, consistent with activation energies previously reported for processes relevant to sintering of Y₂O_3, such as Y³⁺ diffusion and grain boundary mobility. Finally, we propose that a refined deconvolution study on the DSC curve for Y₂O₃ sintering, combined with the associated material's microstructure evolution, may help identify shifts in sintering mechanisms, and therefore, specific activation energies at increasing temperatures.     

Crystal structure evolution and luminescence property of Ce3+-doped Y2O3-Al2O3-Sc2O3 ternary ceramics

Ce³⁺-doped Y₂O₃-Al₂O₃-Sc₂O₃ ternary ceramics with varying Sc₂O₃ were prepared via solid-state-reactive method. Their constitutions, crystal structures, microstructures and luminescence properties were thoroughly investigated by powder X-ray diffraction, ⁴⁵Sc and ²⁷Al solid-state nuclear magnetic resonance (NMR), scanning electron microscope (SEM) as well as photoluminescence (PL) characterizations. Single-phased YSAG transparent ceramics with garnet structure and homogeneous microstructures can be obtained. The NMR results suggest that partial [AlO₆] octahedrons are converted into [AlO₄] tetrahedrons with increased Sc³⁺. Partial Sc³⁺ occupies the remaining octahedral sites, the other Sc³⁺ occupies the dodecahedral sites. The luminescence spectra of the as-prepared ceramics show broadened emission bands with excessive Sc³⁺ incorporation. More significantly, the solubility of Ce³⁺ in garnet ceramics extensively increases due to the incorporation of Sc³⁺, leading to a considerable red-shift and broadening of emission spectra with little luminescence decline, indicating that the new ceramic phosphors are promising for white-light illumination with improved color rendering index.     

Effect of Y2O3 on the Si3N4-carbon interaction at high temperatures

The method of pulsed reduction extraction in a flow of carrier gas is used to study the effect of Y₂O₃ on the interaction between Si₃N₄ and carbon in the range of 1500–3500°C. High-temperature dependences of the degree of extraction of oxygen (in the form of CO) and nitrogen from Si₃N₄, Y₂O₃, and a proportioned mixture of them (from 2 to 5% Y₂O₃ ) are obtained. It is established that small additives of Y₂O₃ diminish substantially the thermal stability of Si₃N₄. It is assumed that Y₂O₃ has a pseudocatalytic effect on the Si₃N₄ — carbon interaction at high temperatures. The Temkin — Shvartsman method and published values of the Gibbs free energy are used for thermodynamic calculation of the change in the Gibbs free energy for evaluation of the possibility of some hypothetical reactions.     

Separation of Phases by Spinodal Decomposition in the Systems Al2O3−Cr2O3 and Al2O3−Cr2O3−Fe2O3

Separation of phases was investigated in the hexagonal (rhombohedral) systems Al₂O₃−Cr₂O₃ and Al₂O₃−Cr₂O₃−Fe₂O₃. The binary system shows a miscibility gap with a T_c of 950°C; the miscibility gap for the ternary system was determined for a constant Cr₂O₃ content of 16.6 mol%. Dark field transmission electron microscopy of solid solutions annealed within the miscibility gap showed dark and light lamellas ∼50 to 200 Å thick. X-ray diffraction results for the solid solutions in the ternary system indicated that, in the early stages of annealing, broadening occurred only on (hkl) reflections where l≠0. There was no major change in the X-ray diffraction patterns of the annealed solid solutions in the binary system. Electron diffraction results indicated, however, that phase separation in both systems proceeded in the [001] direction. Solid solutions in the binary system separated very slowly; the separation could be enhanced hydrothermally. The mechanism of the separation of phases in both systems is spinodal and proceeds as follows: solid solution→intermediate modulated phase→equilibrium phases.     

Photocurrent response and semiconductor characteristics of Ce-Ce2O3-CeO2-modified TiO2 nanotube arrays

We reported Ce and its oxide-modified TiO₂ nanotube arrays (TNTs) and their semiconductor properties. The TNTs were prepared by anodic oxidation on pure Ti and investigated by electrochemical photocurrent response analysis. Then, the TNT electrodes were deposited of Ce by cathodic reduction of Ce(NO₃)₃ 6H₂O. After deposition, the TNT electrodes were fabricated by anodic oxidation at E = 1.0 V_(SCE) for various electricity as Ce-Ce₂O₃-CeO₂ modification. The Ce-deposited TNTs (band gap energy E_g = 2.92 eV) exhibited enhanced photocurrent responses under visible light region and indicated more negative flat band potential (E_fb) compared with the TNTs without deposition. After anodic oxidation, the mixed Ce and its oxide (Ce₂O₃-CeO₂)-modified TNT photoelectrodes exhibited higher photocurrent responses under both visible and UV light regions than the TNTs without deposition. The photocurrent responses and E_fb were found to be strongly dependent on the contents of Ce₂O₃ and CeO₂ deposited on TNTs. A new characteristic of E_g = 2.1 ± 0.1 eV was investigated in the Ce₂O₃- and CeO₂-modified photoelectrodes. X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were also employed to characterize various modified TNTs photoelectrodes.     

Thermodynamic modeling of the K2O-Al2O3 and K2O-MgO-Al2O3 systems with emphasis on β- and βʹʹ-aluminas

A critical evaluation and thermodynamic modeling study including key phase diagram experiments was performed to investigate the K₂O-Al₂O₃ and K₂O-MgO-Al₂O₃ systems. For the first time, potassium β- and β??-alumina solid solutions were described using the Compound Energy Formalism with accurate cation distributions in their sublattices. From the new experimental results, the stability of potassium β??-alumina was assured up to 1600?°C. A large discrepancy reported in the literature, the eutectic temperature between KAlO₂ and β-alumina in the K₂O-Al₂O₃ system, was resolved. A set of self-consistent Gibbs energy functions for all stable phases in the K₂O-MgO-Al₂O₃ system was obtained. As a result, any phase diagram sections and thermodynamic properties of the K₂O-MgO-Al₂O₃ system can be calculated from the optimized Gibbs energy functions. In particular, the cation distribution in the β- and β??-alumina solid solutions is calculated depending on the non-stoichiometry of solution and temperature.     

Some problems in the technology of translucent Y2O3 ceramics

Conclusions The degree of translucence of Y₂O₃ ceramics depends on the amount of additive, the grinding time of the components, and the calcination temperature of the powder. The influence of the technological factors is interrelated; upon them depends the fineness of the powder, the uniformity of the distribution of the additive, and the degree of synthesis of the solid solution.It was established that a poreless high-translucence ceramic can be produced with 6–10 mole % added HfO₂ or ZrO₂. An x-ray phase analysis was used to determine the lattice constant of the solid solution in the Y₂O₃-HfO₂ and Y₂O₃-ZrO₂ systems and the x-ray density of Y₂O₃ ceramics containing up to 20 mole % added HfO₂ or ZrO₂.Translated from Ogneupory, No. 3, pp. 44–48, March, 1978.     

Reactive sintered highly transparent AlON ceramics with Y2O3-MgAl2O4-H3BO3 ternary additive

In this study, highly transparent aluminate oxynitride (AlON) ceramics were prepared via the reactive sintering of Al₂O₃ and AlN powders using a Y₂O₃-MgAl₂O₄-H₃BO₃ ternary sintering additive. The ternary doping process resulted in the efficient preparation of transparent AlON ceramics with small grains and high transmittance as compared to the binary doping (Y₂O₃-MgAl₂O₄) process. The addition of 0.1 wt.% Y₂O₃-0.4 wt.% MgAl₂O₄-0.12 wt.% H₃BO₃ resulted in the formation of a 4-mm-thick AlON ceramic with high transmittance (81% at 600 nm) and low haze (3.46%). This is the best performance in terms of the thickness and transmittance reported for AlON transparent ceramics prepared by the reactive sintering method.     

Combustion synthesis of compounds in the Y2O3-Al2O3 system

In the Y₂O₃-Al₂O₃ system, compounds Y₃Al₅O₁₂ (yttrium aluminum garnet, YAG), YAlO₃ (yttrium aluminum perovskite, YAP), and Y₄Al₂O₉ (yttrium aluminate monoclinic, YAM) are well known. YAG and YAP are of considerable technological importance. Conventional solid-state reaction techniques require high sintering temperatures (above 1800°C) to prepare phase-pure compounds. Though several soft chemical routes have been explored for synthesis of YAG, YAP and YAM, most of these methods are complex. Moreover, phase-pure materials are not obtained in one step and prolonged annealing at temperatures around 1000°C is necessary. In this paper, one-step combustion synthesis of these compounds is reported using a modified procedure and employing mixed (glycine + urea) fuel. Phosphors based on Ce³⁺ activation were also prepared and exhibited characteristic photoluminescence.     

Thermodynamic modeling and experimental investigation of the MgO-Y2O3-ZrO2 system

Solid-state phase equilibria in the MgO-Y₂O₃-ZrO₂ system as well as the equilibria including liquid were investigated in the whole-compositional range using high-temperature differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDX). Isothermal sections at 1493, 1573, 1693, and 1923 K were constructed based on experimental studies. The presence of tie line between MgO and Y₄Zr₃O₁₂ in the temperature range between 1493 and 1573 K was confirmed. The eutectic melting in the MgO-Y₂O₃-ZrO₂ system was established using DTA followed by SEM/EDX microstructure investigation. Based on the obtained experimental results, the thermodynamic database was derived.     

On the thermodynamics of concentrated strong electrolytes aqueous solutions the system NaNO3—NaCl—H2O

A simple analytical function is proposed for the calculation of molar excess Gibbs energy of binary solutions of strong electrolytes in water. Empirical parameters can be easily determined from experimental data reported in the literature. The treatment is extended to ternary solutions and to solid liquid equilibria using an empirical equation. Absence of solid solutions is assumed. The correlations are applied to the common ion ternary system NaNO₃-NaCl-H₂O in the temperature range from 0 to 100°C. Predictions of activity coefficients of the binary systems and saturation curves of the ternary system are in good agreement with experimental data.     

Phase diagrams of the systems Al2O3-ZrO2-Ln(Y)2O3 as a source of multiphase eutectics for creating composite structural and functional materials

Phase equilibria in the ternary systems Al₂O₃-ZrO₂-Ln₂O₃ (Ln = La, Nd, Sm, Gd, Er, Yb and Y) were investigated by identical methods in all range of concentrations and 1250-2700 °C temperature range using differential and derivative in solar furnace thermal analysis in controlled medias, X-ray diffraction phase analysis, local X-ray spectral analysis, chemical analysis, electron and optical microscopy, petrography. The phase diagrams of the systems studied are presented as isothermal at 1650 °C sections and melting diagrams. The interaction in the systems is characterized by the absence of ternary compounds and regions of appreciable solid solutions based on the binary compounds and components. Only narrow regions of ternary solid solutions were discovered at high temperatures by CALPHAD method and because of existing small solubility on the base of ZrO₂ in the binary bounding system Al₂O₃-ZrO₂. The phase equilibria in the systems are determined by zirconia as the most stable compound. Solidification in the systems is completed in eutectic reactions. The established interaction regularities allowed to forecast interaction and phase diagrams construction in systems with other lanthanides (Ce, Pr, Pm, Eu, Tb, Dy, Ho, Tm, Lu). New 13 quasibinary and 26 ternary eutectics were found for the first time. Their temperatures rise from 1660 °C for the La₂O₃ system to 1840 °C for the Lu₂O₃ system. Fluorite-type phases equilibrate with garnet-type phases for the systems from Tb₂O₃ to Lu₂O₃. In the systems from Pr₂O₃ to Gd₂O₃ they equilibrate with perovskite-type phases and β-Al₂O₃. On the base of microstructure investigations it was established that three-phase alumina-rich eutectics crystallizes according to the mechanism of cooperative growth. It opens up possibilities to obtain composite materials using directional solidification method.     

Effect of gaseous medium on ZrO2 and CeO2 reactions and the fired material properties

Conclusions Cerium dioxide mixed with ZrO₂ and in the form of tetragonal solid solution with ZrO₂, preliminarily synthesized in an oxidizing atmosphere, is reduced in vacuum and in a reducing atmosphere to Ce₂O₃, and in this state forms solid solutions with ZrO₂; CeO₂ in vacuum and in a reducing atmosphere is partially reduced to the metal.At 1750–2000°C and with concentrations of 5.55–16.65 mol% Ce₂O₃ forms with ZrO₂ cubic solid solutions with a crystal lattice of the fluorite type which during rapid cooling to room temperature preserves its structure, and with sufficiently slow cooling decomposes into a solid solution of Ce₂Zr₂O₇-ZrO₂ of the pyrochlore type, and monoclinic solid solution ZrO₂-Ce₂O₃.During combined stabilization and sintering the composition ZrO₂-Ce₂O₃ sinters badly; and furthermore the density and strength of the fired products diminish with increase in the content of stabilizing oxides. Dense and strong articles of composition ZrO₂-Ce₂O₃ are obtained from presynthesized solid solutions ZrO₂-CeO₂ by intermediate fine grinding.Solid solutions ZrO₂-Ce₂O₃ with an increase in temperature are oxidized in air, which leads to destruction of dense articles. The high (15% or more) porosity of the specimens contributes to the compensation of the volume changes in the changes during oxidation, so there are no essential changes in the strength of the articles.Translated from Ogneupory, No.1, pp. 40–46, January, 1970.