Onset of perovskite formation in the catalytic system La2O3/γ-Al2O3

Molecular dynamics simulations of model systems for the surface interaction of lanthanum oxide supported on -alumina have been carried out at 1500 K. The onset of formation of perovskite-like phases has been analysed in samples containing four different concentrations of lanthanum oxide. A mechanism of the formation of perovskite-like polyhedra is proposed. This mechanism essentially involves a displacement of an oxide ion associated to an octahedral aluminum by a lanthanum ion and appears to be independent of La₂O₃ loadings.     

Role of B2O3 in formation of mullite from kaolinite and α-Al2O3 mixtures

Abstract

The microstructure, phase constitution, and physical properties of mullite bodies prepared from α-Al₂O₃- kaolin mixtures with added B₂O₃ were investigated. Densification was found to be enhanced with small additions of B₂O₃. The results indicate that 0.5 wt-% B₂O₃ increases the content and growth rate of the mullite. It was found to be the optimum addition with respect to densification and resulting properties.     

Pixelated sintering of α-Al2O3

The sintering of α-alumina by a brand new and innovative technique, called pixelated sintering (PS), is here studied. Densification and grain growth by PS of perfectly controlled granular compacts are analysed and compared to results obtained using Spark Plasma Sintering (SPS) and Pressure-Less SPS (PL-SPS). Materials are exposed to the same temperature profiles whatever the sintering technique used in order to assess the potential of PS in terms of microstructure control. It is shown that PS can be used as an alternative technique to SPS for fast sintering with the advantages of a much simpler and cost-effective set-up, as well as a better control of the localised heat input. PS also appears to be a very modular technology in the way it controls the temperature gradients allowing its implementation for multi-step sintering approaches, as well as for the fabrication of large and complex parts.     

Sintering of α-Al2O3-seeded nanocrystalline γ-Al2O3 powders

This paper demonstrates that seeding nanocrystalline transition alumina powders is a viable option for producing high quality, alumina-based ceramics. By using α-Al₂O₃ concentrations of ⩾1.25 wt.% α-Al₂O₃ seed particles (equivalent to 5 ×10¹⁴ seeds/cm³ of γ-Al₂O₃) the sintering temperature is reduced from 1600°C for unseeded γ-Al₂O₃ to 1300–1400°C in dry pressed powders. The scale of the sintered microstructure is related to N_v^−1/3 and thus a 100-nm grain size is obtained. It is apparent that seeding is necessary for producing dense, alumina-based ceramics from nanocrystalline transition alumina powders.     

Effects of pretreatment atmospheres on the catalytic performance of Pd/γ-Al2O3 catalyst in benzene degradation

In the current paper, a strategy for catalytic degradation of benzene over Pd/γ-Al₂O₃ catalysts via different atmospheres (H₂, N₂, He and air) pretreatment was carried out in a fixed bed reactor. The experimental results indicated that H₂, N₂, and He pretreatments have a significant positive effect on the initial activity of the catalyst compared to air. We have also investigated the effects of pretreatment atmospheres on the catalytic performance for benzene degradation through the information on the chemical state and crystal structure of the catalysts using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and CO chemisorption measurements techniques. The chemical state of Pd species decreased via H₂ pretreatment, leading to the increase of the initial catalytic activity, while chemical state increased accompanying with a decrease in degradation benzene via air pretreatment. There is no change in the chemical state of Pd species using inert atmosphere (N₂ and/or He) pretreatment, but the initial activity of the catalyst improved significantly due to the modified crystal structure of Pd species in the catalysts, with the crystalline PdO being transformed to amorphous state.     

Investigation of the formation process of highly porous α-Al2O3 via citric acid-assisted sol-gel synthesis

Macroporous α-alumina can be obtained by polymer-induced phase separation during the sol-gel process based on dissolved aluminum salts. Polyethylene oxide (PEO) acts as phase separating agent, propylene oxide as proton scavenger. Calcination at 1200 °C yields the thermodynamically stable α-Al₂O₃. It usually exhibits little to no porosity, with internal surface areas below 5 m²/g. We report on the successful establishment of citric acid as a novel phase separating agent in this process. Macropores ranging from 115 nm up to several μm have been successfully introduced in α-Al₂O₃. Compared to the PEO-route, BET surface areas doubled, going up to 12 m²/g. The materials prepared via the new method also exhibit a significantly different microstructural habitus.¹³C-NMR studies and TG/DTA measurements revealed a complete incorporation of citric acid into the gel network. This implies a change in phase separation mechanism, giving rise to a more particulate structure and hence the formation of larger macropores.     

Effects of doping cations on the densification and microstructure of flash-sintered α-Al2O3

The effects of dopants with different valences on the flash sintering behavior of α-Al₂O₃ are investigated. The results indicate that regardless of their valence and ionic radius, all the tested dopants reduce the onset temperature more effectively compared to undoped Al₂O₃. Interestingly, the relative density and grain size of the flash-sintered samples exhibit an inverse linear relationship. The data for the samples doped with non-trivalent cations fall on a different line than those for the samples doped with trivalent cations and the undoped samples. Dopants have an effect on the flash sintering behavior of Al₂O₃ because flash sintering increases the solubility of the dopants in alumina, creating more point defects, and thereby increasing the electrical conductivity of the material. The mechanisms for point defect generation in trivalent and non-trivalent dopants are different.     

Comparative investigations of the catalytic properties of an anodic Al2O3-coated catalyst and of α- and γ-Al2O3 bulk catalysts

The catalytic properties of an Al₂O₃-coated catalyst formed by anodic oxidation of aluminium were compared with those of α- and of γ-Al₂O₃- bulk catalysts in the dehydration of 2-propanol and in the isomerization of n-butenes. In the dehydration reaction both the Al₂O₃-coated catalyst and the γ-Al₂O₃ bulk catalyst show approx. the same activities and activation energies. However, the reaction rate based on the unit of the surface area for the Al₂O₃-coated catalyst is approx. 30 times larger than that for the γ-A1₂O₃ bulk catalyst. In the isomerization of the individual isomeric n-butenes the Al₂O₃-coated catalyst was more active than the γ-Al₂O₃, bulk catalyst and under otherwise equal conditions equilibrium was reached at approx. 80 K lower temperature.     

Improving the dispersion of CeO2 on γ-Al2O3 to enhance the catalytic performances of CuO/CeO2/γ-Al2O3 catalysts for NO removal by CO

Acetic acid (HAc) aqueous was used as solvent in wetness impregnation to prepare CeO₂-modified γ-Al₂O₃ support. With the help of HAc, the dispersion of CeO₂ on γ-Al₂O₃ is significantly improved and the size of CeO₂ nanoparticles can be controlled through tuning the concentration of HAc aqueous. XPS analysis shows that the percentages of Ce^3 + in CeO₂ nanoparticles will vary with the size. Then we load CuO on the as-prepared CeO₂-modified γ-Al₂O₃ support and choose NO reduction with CO as a probe reaction to investigate the influences of impregnation solvent on the catalytic properties. The results demonstrate that the CuO/CeO₂/γ-Al₂O₃ prepared in the solvent with volume ratio of 20:1 (H₂O:HAc) has the highest activity in NO + CO reaction. Combing the structural characterizations and catalytic performances, we think that the size of the CeO₂ nanoparticles should be a key factor that affects the activities of CuO/CeO₂/γ-Al₂O₃. Furthermore, CuO dispersed on CeO₂ nanoparticles with an average size of ca. 5 nm should be the highest active sites for NO + CO reaction.     

Vacancy ordering in the structure of γ-Al2O3

Defect structure of γ-Al₂O₃ prepared by the thermal decomposition of well-crystallized, high purity boehmite (γ-AlOOH) has been studied by HREM. It was shown that the intrinsic feature of γ-alumina structure is a presence of almost hexagonal closed loops formed due to the ordering of cation vacancies over octahedral positions on (110) and (111) planes. These defects are relatively stable; they are preserved, though being changed in shape, in the γ-alumina sample upon its further calcination until the appearance of traces of δ-alumina phase.     

Optimization of the sintering thermal treatment and the ceramic ink used in direct ink writing of α-Al2O3: Characterization and catalytic application

Unlike other engineered ceramic products, alumina (Al₂O₃) displays interesting mechanical and physical properties, which makes it an ideal candidate for a wide range of uses in different fields and in particular for catalytic applications. However, the manufacturing of ceramic components has still a major drawback in production of highly complex three-dimensional (3D) shapes, microfeatures or structures with tailored porosity. Direct Ink Writing (DIW), also known as robocasting, is a material extrusion Additive Manufacturing technology and is one of such versatile methods with unique flexibility in material and geometry. In this work, α-Al₂O₃ ceramic materials were designed and produced by DIW to determine the most suitable sintering treatment and ceramic ink composition to design new components for catalytic applications. Several thermal treatments varying sintering temperature and time were tested previously to the preparation of inks with different ceramic loadings, up to 75 wt%. A systematic study of the DIW specimens sintered at the optimal sintering temperature – time combination, in terms of microstructure (density and porosity) and mechanical properties (hardness and indentation fracture toughness), was performed to determine the optimize ceramic loading. Finally, finite element modeling and catalytic experiments conducted for the optimal ceramic ink showed that 3D printed parts with a rectilinear infill pattern and 40% infill density favored catalytic performance.     

Effect of citrate to nitrate ratio on the sol-gel synthesis of nanosized α-Al2O3 powder

This paper studied the effect of the variation of citrate to nitrate ratio on the sol-gel synthesis of nanocrystalline α-alumina powder. The process involves the formation of a sol using aluminium nitrate and citric acid which then transferred to a transparent gel and finally to a powdery mass. The prepared alumina powder was thoroughly characterised by different techniques such as phase analysis, thermal analysis, microstructure study, etc. Variation in citrate to nitrate molar ratio (C/N) influenced the thermal decomposition behaviour, particle size distribution, phase transformation temperature and specific surface area of the developed powder. The complete formation of α- Al₂O₃ was obtained at around 950 °C for C/N=1.5. The unimodal distribution of the particles within a narrow size range and higher particle to particle linking through solid bridging are the characteristics of the powder prepared at C/N=1.5. It was found higher citrate to nitrate ratio in the fuel rich condition is responsible for better properties of synthesised alumina powder.     

Effect of carbon and α-Al2O3 on controlled synthesis of AlON powder

The α-Al₂O₃/C mixtures were prepared by ball milling, and then AlON powders were synthesized by carbothermal reduction and nitridation (CRN). The effects of α-Al₂O₃ particle size, carbon powders morphology and particle size on the morphology and particle size of the synthesized AlON powders were studied. The results showed that as the particle size of α-Al₂O₃ increases, the particle size of the synthesized AlON powders will also increase, but the surface morphology will not be affected. The increase of the carbon particle size will increase the particle size of the synthesized AlON powders. The pore size and number of pores of the synthetic AlON powders were very similar to the morphological characteristics of the carbon powders. In addition, the mechanism of controlling the synthesis of AlON powder with α-Al₂O₃ and carbon as raw materials was also discussed.     

Mechanochemical-molten salt synthesis of α-Al2O3 platelets

The polyaluminium chloride (PACl) precursor was used for a simple and scaled-up mechanochemical-molten salt synthesis of α-Al₂O₃ platelets. PACl, as a low temperature α-Al₂O₃ precursor, was firstly mechanically activated by high-energy ball milling for 5 min, followed by a next 5 min ball milling in the presence of a NaCl–KCl salt mixture. The starting formation temperature of the α-Al₂O₃ phase was 600 °C. In the subsequent annealing in the temperature range of 660–1000 °C, the α-Al₂O₃ phase with a well developed plate-like morphology was obtained. The products were characterized by X-ray powder diffractometry, scanning electron microscopy (SEM), and thermal analysis (DTA, TG) and solution ²⁷Al NMR spectroscopy.     

Phase metastability of nanosized α-Al2O3 crystallites

The reversal of the α- to θ-Al₂O₃ phase transformation and the induced microstructure evolution of boehmite-derived discrete nanosized α-crystallites are examined. Three categories of α-crystallites smaller than 100 nm were examined and found to have similar behavior: (1) pre-existing α-crystallites, (2) α-crystallites formed in situ during the calcination of θ-crystallites of sizes near the critical size, 25 nm, and (3) α-crystallites formed in situ by the thermal treatment of as-received θ-crystallites. The α-crystallite may transform back to the θ-phase above 800 °C. The backwards θ-crystallite may also re-transform to the α-phase again. Because of the density difference between α- and θ-Al₂O₃, the strain involved in the volume expansion and shrinkage during the phase transition eventually results in the formation of a twinned and/or mosaic structure for the θ- and α-crystallites. A strain release model representing the microstructure evolution of the α- to θ-phase and the θ- to α-Al₂O₃ phase transformation is proposed.     

The effect of toluene-insoluble fraction of coal on catalytic activities of a Ni-Mo-γ-Al2O3 catalyst in the hydrotreating of coal liquids

Feedstocks prepared with 723 K⁺ distillation residues obtained from the Australian brown coalderived oil and hydrogenated creosote oil were hydrotreated to elucidate the effects of toluene-insoluble fractions of coal on the catalytic activities of a Ni-Mo-γ-Al₂O₃ catalyst. The toluene-insoluble fractions exerted harmful effects on the catalyst by deactivation due to carbonaceous deposits formed on the catalyst surfaces. The deactivation of the catalyst was more significant in hydroconverting of 623 K⁺ residues to 623 K⁻ oil fractions and hydrodenitrogenation reactions than in hydrodesulfurization reaction.The analyses of carbonaceous deposits on the spent catalyst surfaces by using an EPMA and a CP/MAS ¹³C-NMR indicated that the toluene-insoluble fractions were too refractory to be hydrogenated on the catalyst surfaces and hindered the reactant molecules from accessing to the active sites compared to asphaltenes.     

Using modified Pechini method to synthesize α-Al2O3 nanoparticles of high surface area

A modified Pechini method for the preparation of a high surface area α-alumina is proposed. The synthesis of these nanoparticles was carried out using a polymer as a chelating agent. The polymer was prepared from citric acid and acrylic acid by the melt blending method. The resulting α-alumina (98.16%) after calcination at 900 °C consisted of cylindrical nanoparticles of 100–200 nm in length and <25 nm in diameter with a relatively high surface area (18 m² g^?1).     

A critical role of pH in the colloidal synthesis and phase transformation of nano size α-Al2O3 with high surface area

This paper demonstrates the critical role of pH in the colloidal (i.e. sol-gel) synthesis of nano size α-Al₂O₃. Investigation, based on X-ray diffraction pattern, indicates that the transformation of α-Al₂O₃ from gels obtained at high pH⩾7, undergoes via formation of boehmite (gel→AlOOH→γ-Al₂O₃→α-Al₂O₃). On the other hand, the transformation of α-Al₂O₃ at low pH (⩽6) follows the gel→γ-Al₂O₃ →α-Al₂O₃ sequence. The transmission electron microscopy reveals that the particle size of the α-Al₂O₃ decreases from ≈750 to ≈70 nm with decreasing pH from 12 to 2.5, respectively. The α-Al₂O₃ with large surface area (≈130 m²/g) is obtained when it is processed at low pH (2.5). The true powder density of the α-Al₂O₃ sample derived at pH=2.5 is observed to be 3.92 g/cm³ after sintering at 1450 °C, due to fine particles. The isoelectric point (iep) and ξ of the α-Al₂O₃ are found to be 8.7 and 9.2 when synthesized at pH 2.5 and 12, respectively.     

Mechanochemical synthesis of α-Al2O3-Cr3+ (Ruby) and χ-Al2O3

In this work we present a unique method to synthesize χ-Al₂O₃ and α-Al₂O₃ doped with Cr³⁺ (ruby). The ruby is synthesized by mechanical milling of pseudoboehmite that is doped in-situ with chromium. The doping is carried by adding chromium sulfate hydrate to an aqueous solution rich in aluminum sulfate hydrate. The pH in the solution is controlled to be between 9 and 10 by using ammonia, which induces the pseudoboehmite precipitation. The Cr³⁺ is added for its remarkable effects on the detectability of ruby emitting luminescent R₁ and R₂ bands that are traceable in Raman spectroscopy. The formation of ruby is detected at milling times as short as 5 hours and increased with the milling time. Ruby phase is further confirmed by means of true atomic resolution Transmission Electron Microscopy (TEM).