Utilization of carbon dioxide as soft oxidant for oxydehydrogenation of ethylbenzene to styrene over V2O5–CeO2/TiO2–ZrO2 catalyst

Vanadium oxide and cerium oxide doped titania–zirconia mixed oxides were explored for oxidative dehydrogenation of ethylbenzene to styrene utilizing carbon dioxide as a soft oxidant. The investigated TiO₂–ZrO₂ mixed oxide support with high specific surface area (207 m² g⁻¹) was synthesized by a coprecipitation method. Over the calcined support (550 °C), a monolayer equivalent (15 wt.%) of V₂O₅, CeO₂ or a combination of both were deposited by using wet-impregnation or co-impregnation methods to make the V₂O₅/TiO₂–ZrO₂, CeO₂/TiO₂–ZrO₂ and V₂O₅–CeO₂/TiO₂–ZrO₂ combination catalysts, respectively. These catalysts were characterized using X-ray diffraction (XRD), Raman, scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature preprogrammed reduction (TPR), CO₂ temperature preprogrammed desorption (TPD) and BET surface area methods. All characterization studies revealed that the deposited promoter oxides are in a highly dispersed form over the support, and the combined acid–base and redox properties of the catalysts play a major role in this reaction. The V₂O₅–CeO₂/TiO₂–ZrO₂ catalyst exhibited a better conversion and product selectivity than other combinations. In particular, the addition of CeO₂ to V₂O₅/TiO₂–ZrO₂ prevented catalyst deactivation and helped to maintain a high and stable catalytic activity.     

Preparation and Properties of a Ceramic in the ZrO2 – CeO2 System

The ZrO₂ – CeO₂ solid solution (88 mol.% – 12 mol.%) is synthesized by coprecipitation. The effect of dry mechanical grinding on the dispersity of the solid solution is studied. Combining methods of coprecipitation and mechanical grinding intensifies the sintering process and allows preparation of compact ceramic materials in the ZrO₂ – CeO₂ system.     

Formation and properties of solid solutions of zirconium dioxide with oxides of rare earth elements

Conclusions Interaction of zirconium dioxide with oxides of cerium, yttrium and lanthanum in solid phases occurs at 1400°C with the formation of solid solutions with the cubic structure.Sintering of the specimens may result at 1700–1750°C with a 3-h soak. At 1400°C and a 6-h soak the porosity of the specimens was 30–40%.Complete stabilization of the zirconia is attained by heating to 1700–1750°C with additions of 20 mol.% CeO₂, 15% Y₂O₃ or 25% La₂O₃. An addition of ceria and yttria displaces the effects of polymorphic inversion of the zirconia to the lower temperature region.New highly refractory materials may be obtained from solid solutions of ZrO₂-20% CeO₂, ZrO₂-80% CeO₂, ZrO₂-15% Y₂O₃, ZrO₂-80% Y₂O₃ and ZrO₂-25% La₂O₃ and firing to 1750°C. Some of them have a low coefficient of thermal expansion compared with ZrO₂, stabilized with calcium oxide and magnesium oxide, and apparently better thermal-shock resistance. The advantage in regard to resistance during prolonged heating at 1200°C is possessed by the solution ZrO₂-Y₂O₃. The region of the most effective use of goods made from solid solutions of ZrO₂ with CeO₂, Y₂O₃ and La₂O₃ as highly refractory materials should be determined by extra studies.The possibility of reducing CeO₂ (fusing temperature about 2700°C) to Ce₂O₃ (fusing temperature about 1700°C) limits the use of cerium-containing materials as refractories in chiefly oxidizing conditions.     

Effect of stabilizing additions on the thermal shock resistance of zirconia products

Conclusions To obtain thermal-shock resistant products from zirconia the amount of monoclinic phase in the fired specimens prepared on the basis of granular bodies should be not less than 15%.It is found that with an increase in the content of CaO from 7.0 to 20 mole %, the thermal-shock resistance of the articles made from ZrO₂ is diminished. Introduction of up to 20% monoclinic ZrO₂ in the batch increases the thermal-shock resistance, but less so the higher the content of CaO in the stabilized part of the material. This produces additional stabilization of the zirconia as a result of the migration of the calcium oxide from the stabilized grains into the monoclinic ZrO₂. Additional stabilization of monoclinic zirconia is observed also during cyclic heating in the range 20–1600–20°C.Specimens of zirconia stabilized with CaO possess higher thermal-shock resistance than those made from ZrO₂ stabilized with MgO with the same contents of monoclinic phase.Translated from Ogneupory, No.1, pp.50–55, January, 1967.     

Preparation and properties of zirconium refractories stabilized by cerium dioxide

Conclusions We studied the reaction of zirconium dioxide with cerium dioxide in mixtures with CeO₂ contents of 6, 8, 10, 12, 15, 18, 20, 33, and 50%. The properties of the samples of these compositions were determined.On the addition of amounts of CeO₂ from 6–12% complete stabilization of ZrO₂ was not achieved by a single sintering at 1750°C. The samples with the composition 88% ZrO₂+12% CeO₂, sintered twice at 1750°C with an intermediate grinding, stabilized almost completely as a solid solution of tetragonal structure.The thermal stress resistance of dense, completely stabilized samples with CeO₂ contents of 15–18%, prepared from finely ground raw oxides, was 3–4 thermal cyclings. It improved when the CeO₂ content was decreased, or when more monoclinic ZrO₂ was added.It was found that the onset and the inversion temperature interval depend on the CeO₂ content, the granular composition of the original oxides, the temperature, and the gaseous sintering medium. We studied the properties of synthesized compositions and their dependence on the reducing or oxidizing conditions of sintering due to a change in the valency of cerium. In order to obtain zirconium-cerium refractories with definite properties it is necessary to have strict control of the gaseous medium during sintering.Translated from Ogneupory, No. 3, pp. 37–44, March, 1969.     

Influence of composition of MgAl2O4 supported NiCeO2ZrO2 catalysts on coke formation and catalyst stability for dry reforming of methane

Dry reforming of methane was studied over Ni catalysts supported on γAl₂O₃, CeO₂, ZrO₂ and MgAl₂O₄ (670 °C, 1.5 bar, 16–20 l CH₄ ml_catalyst⁻¹ h⁻¹). It is shown that MgAl₂O₄ supported Ni catalysts promoted with both CeO₂ and ZrO₂ are promising catalysts for dry reforming of methane with carbon dioxide. Within a certain composition range, the simultaneous promotion with CeO₂ and ZrO₂ has great influence on the amount of coke and the catalyst service time. XRD analyses indicate that formation of crystalline Ce_xZr_1−xO₂ mixed oxide phases occurs on double promotion. In particular, incorporation of low amounts of Zr in the CeO₂ fluorite structure provides stable dry reforming catalysis. As shown with TPR, promotion leads to a higher reduced state of Ni. SEM, XRD and TPR analyses demonstrate that highly dispersed, doubly promoted Ni catalysts with a strong metal-support interaction are essential for stable dry reforming and suppression of the formation of carbon filaments.     

Enhancement of thermal shock and slag corrosion resistance of MgO–ZrO2 ceramics by doping CeO2

The purpose of this paper was to develop ceramics materials with high thermal shock resistance and corrosion resistance for preparing gas blowing components. In this paper, MgO-rich MgO–ZrO₂ ceramics were obtained by using MgO powder and ZrO₂ powder as starting materials and CeO₂ as an additive. Changes in the properties in terms of thermal shock resistance, mechanical properties, and slag corrosion-resistance with chemical compositions were examined correlated to microstructure and phase changes. Especially, the effect of doping CeO₂ on phase transition of zirconia in MgO-rich system was discussed. The results showed that doping amount of CeO₂ significantly improved properties of MgO–ZrO₂ ceramics. Especially when doping amount of CeO₂ was 2 wt%, residual strength ratio was enhanced over 100% after thermal shock testing. In samples doped with CeO₂, ZrO₂ was stable in cubic or tetragonal form due to complete solution of CeO₂, which was important reason for the improvement of various properties of MgO–ZrO₂ ceramics.     

Structural studies of a ZrO2–CeO2 doped system

The local structure around Zr, Ce and dopant atoms (Fe and Ni) in the ZrO₂–CeO₂ system investigated by X-ray absorption spectroscopy (XAS) is reported to better understand the tetragonal phase stabilization process of zirconia. The first coordination shell around Zr atoms is not sensitive to the introduction of dopants or to an increase in the ceria content (from 12 to 20 mol%). Ce ions maintain the eight-fold coordination as in CeO₂, but with an altered bond distance. The formation of vacancies resulting from reduction of Ce atoms can be discarded, because XANES spectra clearly show that Ce ions are preferentially in a tetravalent state. XANES and EXAFS experiments at the Fe K-edge evidence that the local order around Fe is quite different from that of the Fe₂O₃ oxide. On the one hand, ab initio EXAFS calculations show that iron atoms form a solid solution with tetragonal ZrO₂. The EXAFS simulation of the first coordination shell around iron evidences that the substitution of zirconium atoms by iron ones generates oxygen vacancies into the tetragonal network. This is a driven force for the tetragonal phase stabilization process. For Ni doped samples, EXAFS results show that Ni–O mean bond length is similar to the distance found in the oxide material, i.e., NiO compound. Besides this result, no evidence of similar solid solution formation for Ni-doped systems has emerged from the EXAFS analysis.     

Mechanical properties of ZrO2 + 12%1CeO2 ceramics with additives of CaO,Y2O3, and Nb2O5

The mechanical properties of transformation-toughened ceramics of the composition ZrO₂ + 12% CeO₂ with additives of CaO, Y₂O₃, and Nb₂O₅ in an amount of 1% (molar fractions) have been studied. Doping increases the strength and cracking resistance of the material. The increase of strength is due to grain refinement in the material. According to their effect in increasing the strength (and in decreasing the average grain size), the doping elements considered can be arranged in the sequence Ca, Y, and Nb. As regards their effect in increasing the critical coefficient of stress intensityK _1c (measure of cracking resistance), the doping elements can be written in the following order: Ca, Y, no additive, and Nb. The level of cracking resistance of the material correlates with the volume of the elementary cell of the tetragonal solid solution. It is shown that cracking resistance increases due to the more intensive destabilizing effect of a doping an additive on the tetragonal solid solution. The highest effect in increasing the strength (from 380 N/mm² for a material without an additive to 680 N/mm² for one with an additive) and the cracking resistance (from 15 to 23 N/mm^3/2) is produced by CaO. Recommendations on the selection of doping elements are given.Translated from Ogneupory, No. 2, pp. 7 – 10, February, 1994.Here and further, molar fractions are given.     

Influence of the crystalline structure of ZrO2 on the metallic properties of Pt in Pt/WO3–ZrO2 catalysts

The influence of the crystalline structure of ZrO₂ on the metallic properties of Pt, when supported on WO₃–ZrO₂, was studied. Pt supported on tetragonal zirconia loses its metallic properties while when supported on monoclinic zirconia it presents good metallic activities. WO₂,^2- deposited on amorphous Zr(OH)₄ before calcination generates an active material for n‐butane isomerization. The larger the fraction of the tetragonal phase of zirconia in this material, the higher the isomerization activity and the lower the metallic activity of Pt. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structure characterization and mechanical properties of CeO2–ZrO2 solid solution system

The measured and calculated lattice parameters, microstructures, and mechanical properties (fracture toughness and microhardness) of CeO₂–ZrO₂ system ceramics are investigated, using CeO₂–ZrO₂ solid solution powder prepared by a microwave-induced combustion process. The CeO₂–ZrO₂ solid solution ceramics were sintered at 1500 °C for 6 h in air; the density of all specimens was greater than 94% of the theoretical density. For Ce_1−xZr_xO₂ (0.00  x  0.50), the measured lattice parameter is in accordance with that of Kim's doped CeO₂ model_. On the other hand, for x  0.50, the measured values fit Kim's doped ZrO₂ model. The fracture toughness and microhardness of CeO₂–ZrO₂ system ceramics with various compositions were investigated with Vickers indentation. The results showed that the crack mode of CeO₂–ZrO₂ solid solution was Palmqvist cracks under loads of 1 kg. Generally, the fracture toughness should increase with grain size at the submicron scale. However, larger grains may lead to spontaneous transformation, which should decrease the potential toughening at room temperature. This behavior was observed in the Ce_0.25Zr_0.75O₂ ceramic, which demonstrated a high fracture toughness that may be ascribed to two causes: (1) fine grain size and (2) transformation toughening.     

Monoclinic phase-free,low temperature spark plasma sintering of CeO2-doped ZrO2 ceramics and its associated benefits on mechanical properties

Consolidating a CeO₂-doped ZrO₂ ceramics, free from monoclinic phase using spark plasma sintering (SPS) is a major challenge faced by previous researchers; Ce⁺⁴ → Ce⁺³ conversion under reducing environments was assigned as the prime factor. We report dense (> 95 % of theoretical density) 20 mol. % CeO₂-doped ZrO₂ ceramics, free from monoclinic phase and any of micro/ macro-cracks via SPS. The sintering temperature (1175 ℃) used for the present work was the lowest compared to previous reports on the same system. Phase analysis revealed a mixture of tetragonal (major phase) and cubic phase (minor). No depletion of cerium (Ce) from the ZrO₂ matrix and no additional/impurity phases were noted after SPS; a common issue that has been observed in most of the previous works. Sintered ceramics showed appreciably high hardness (>11 GPa); the obtained toughness was in-between of tetragonal and cubic CeO₂-ZrO₂ ceramics.     

Studying deformation of fine grain zirconia green products during sintering

Conclusions Deformation during the sintering of specimens of finely grained zirconia, prestabilized with calcium oxide, and its mixtures with monoclinic modifications can exceed the deformation of prestabilized zirconia; furthermore the composition containing 30% monoclinic phase will deform to a greater extent at 1300–1400°C.The deformation of specimens with an addition of CeO₂ and especially of Y₂O₃ is less than with an addition of CaO even during partial stabilization of the zirconia, although in the temperature range studied the open porosity is reduced to zero.We investigated the effect of the content of active zirconium dioxide in mixtures with industrial zirconia on the nonsteady rate of deformation of specimens in the range 1100–1500°C with a staged heating cycle.The method of preparing the products from finely grained bodies, in which we combine stabilization by additives of CaO, Y₂O₃ and CeO₂ with sintering, does not increase deformation compared with the method of preparing the articles from prestabilized (completely or partially) material.Translated from Ogneupory, No. 6, pp. 46–51, June, 1969.     

Effect of High‐Temperature Aging on the Fracture Toughness of 40 mol% Ceria‐Stabilized Zirconia

The 40 mol% CeO₂‐stabilized ZrO₂ ceramic was synthesized by the sol‐spray pyrolysis method and aged at 1400°C–1600°C. The effects of high‐temperature aging on its fracture toughness were investigated after heat treatments at 1500°C for 6–150 h in air. Characterization results indicated that the activation energy for grain growth of 40 mol% CeO₂‐stabilized ZrO₂ was 593 ± 47 kJ/mol. The average grain size of this ceramic varied from 1.4 to 5.6 μm within the aging condition of 1500°C for 6–150 h. The Ce‐lean tetragonal phase has a constant tetragonality (ratio of the c‐axis to a‐axis of the crystal lattice) of 1.0178 during the aging process. It was found that the fracture toughness of 40 mol% CeO₂‐stabilized ZrO₂ was determined to be 2.0 ± 0.1 MPa·m^1/2, which did not vary significantly with prolonging aging time. Since no monoclinic zirconia was detected in the regions around the indentation crack‐middle and crack‐tip, the high fracture toughness maintained after high‐temperature aging can be attributed to the remarkable stability of the tetragonal phase in 40 mol% CeO₂‐stabilized ZrO₂ composition.     

Phase Transformation Behavior of a Pyrochlore‐Type CeO2–ZrO2 Binary Compound

The phase transformation behavior of the superlattice structure of a CeO₂–ZrO₂ pyrochlore‐type binary compound (CP) was investigated so as to better understand how to improve the thermal stability of such a system. CP was synthesized through high‐temperature reduction of a conventional CeO₂–ZrO₂ solid solution with a 1:1 molar ratio of Ce and Zr. High‐resolution transmission electron microscopy and selected‐area electron diffraction clearly revealed that the pyrochlore structure of CP transformed to the standard disordered cubic fluorite or tetragonal zirconia structure after having been subjected to a high‐temperature durability test; moreover, it was determined that this phase transformation moves inward from the crystallite surface. This discovery suggests a new method by which to improve upon this material for practical applications.     

Volatilization of components from zirconia ceramics

Conclusions We investigated volatilization in isothermal conditions of solid solutions of oxides of calcium and yttrium in ZrO₂.The calcium oxide is intensely sublimited from the solid solution of zirconium dioxide, stabilized with CaO at 2000–2100°C. However, during soaking and rapid cooling, destabilization does not occur, even during sublimation of 70–75% CaO. With rise in temperature to 2400–2500°C and a 4 h soak, only 0.5% CaO is preserved in the solid solution, which leads to destabilization and conversion of a large part of the cubic form of ZrO₂ with calcium oxide by fusion, we note sublimation of part of the CaO, and the remaining quantity (3%) is adequate for complete conversion of the zirconia into the stable cubic form.Volatilization of the stabilizing additive occurs in the form of YO at substantially higher temperatures than volatilization of the calcium oxide from the solid solution of zirconium dioxide, stabilized with yttrium oxide. Simultaneously with this we note volatilization of zirconium dioxide in the form of ZrO and ZrO₂.For the use of zirconium dioxide at elevated service temperatures, we would recommend yttrium oxide as a stabilizing additive.Translated from Ogneupory, No. 1, pp. 49–52, January, 1968.     

The long-term high-temperature behavior of refractories produced from ZrO2 stabilized with Nd2O3

Conclusions A technology was developed for the production of zirconia refractories from ZrO₂ stabilized with Nd₂O₃. The thermal strength of the product is adequate for long-term service at a large (200–300°C/mm) temperature gradient. Products based on a zirconia — neodymium solid solution can be used several times.It was established that no appreciable Nd₂O₃ vaporization and, consequently, no appreciable destabilization of the ZrO₂ develops in a neutral medium at 2100–2500°C.The solid-phase processes developing at 2100–2500°C in products from a mixture of 70% cubic solid solution (88 mole % ZrO₂+12 mole % Nd₂O₃) and 30% unstabilized ZrO₂ fired at 1750°C consist of the redistribution of the Nd₂O₃ between the cubic solid solution Nd₂Zr₂O₇-ZrO₂ and the unstabilized ZrO₂, and the diffusion of some of the Nd₂O₃ from the cooler to the working zone.Translated from Ogneupory, No. 3, pp. 52–55, March, 1976.     

Influence of crystal lattice defectiveness on the properties of solid solutions based on ZrO2

Conclusions It is shown that the parameter of the crystal lattice of cubic solid solutions based on zirconium dioxide, stabilized with oxides of yttrium, ytterbium, and calcium, is increased with a rise in their concentrations, and hardly alters for the solid solution ZrO₂-Sc₂O₃.In the solid solutions of Y₂O₃, Sc₂O₃, and CaO in zirconium dioxide we note a reduction in the x-ray densities with an increase in the concentration of stabilizing additives. For the solid solutions ZrO₂-Yb₂O₃ we note an increase in the x-ray density with rise in the concentration of ytterbium oxide.The introduction into the zirconia of 9–13% stabilizing oxides ensures the highest density characteristics for the ceramics.Translated from Ogneupory, No. 1, pp. 12–15, January, 1987.     

TiO2–ZrO2 mixed oxide as a support for hydrotreating catalyst

Pure TiO₂, ZrO₂ and TiO₂–ZrO₂ mixed oxides are prepared by urea hydrolysis. Hydrotreating catalysts containing 12 wt% molybdenum are prepared using these oxides and characterized by BET surface area, pore volume, XRD and oxygen chemisorption. It is observed that oxides produced by the method of urea hydrolysis have higher surface area as compared to those available commercially. With increasing zirconia content in the mixed oxide, the surface area increases and a maximum value is obtained for a mixed oxide having Ti and Zr molar ratio of 65/35. XRD results indicate that mixed oxides are poorly crystalline in nature. Thiophene hydrodesulfurization, cyclohexene hydrogenation and tetrahydrofuran hydrodeoxygenation are taken as model reactions for evaluating catalytic activities. It is found that both O₂ uptake and catalytic activities increase with increasing zirconia content in mixed oxide and reach maximum values for the 12 wt% Mo/TiO₂–ZrO₂ (65/35) catalyst. With further increases in zirconia content, O₂ uptake and catalytic activities decrease and the lowest values are observed for the pure ZrO₂ supported catalyst.     

The effect of long-term firing at 2100–2200°C on the concentration and microstructure of solid solutions in the systems ZrO2-Y2O3-CaO and ZrO2-Y2O3-MgO

Conclusions Solid solutions in the system ZrO₂-MgO fired at a high temperature (above 2000°C) in vacuo (5 · 10^–4mm Hg) for 5 h decompose as a result of the complete vaporization of the MgO. Solid solutions of ZrO₂-CaO undergo partial decomposition when fired in these conditions. The addition of 2 mole % or more yttrium oxide to the solid solutions ZrO₂-CaO and ZrO₂-MgO results in significantly lower CaO and MgO vaporization.The long-term exposure of solid solutions in the system ZrO₂-CaO and ZrO₂-MgO to 2200°C in a helium atmosphere results in the formation of an intercrystalline layer in which not only the stabilizing oxide but also the impurities are concentrated.In three-component solid solutions which contain yttrium oxide the degree of vaporization is lower and the intergranular secondary phase less developed so that the degree of collective recrystallization is lower.Translated from Ogneupory, No. 8, pp. 44–48, August, 1976.