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.     

SO2 Reactions over γ-Al2O3,Pt/γ-Al2O3,Sn/γ-Al2O3 and Pt–Sn/γ-Al2O3

Platinum and Platinum–tin bimetallic catalysts supported on alumina were prepared by co-impregnation of both metallic precursors on the support and used as catalysts for the oxidation of SO₂. Platinum dispersion was determined by means of H₂–O₂ titration. Tin addition (1 and 2 wt%) only slightly decreased the exposed platinum atoms suggesting that tin is mainly over the support. At temperatures lower than 300 °C, SO₂ did not react with oxygen. Nevertheless, when the temperature was increased, the SO₂ oxidation began. The ignition temperatures for SO₂ oxidation (taken at 50% conversion) were 345 °C for 1% Pt/Al₂O₃ and 520 °C for 1% Pt–2% Sn/Al₂O₃. The strong displacement on activity suggests that tin plays an important role as inhibitor of the SO₂ oxidation reaction.     

N2O Abatement Over γ-Al2O3 Supported Catalysts: Effect of Reducing Agent and Active Phase Nature

A series of metal catalysts (Pd, Rh, Ru, Cu, Fe, In and Ni) supported on γ-Al₂O₃ carrier, were evaluated during N₂O catalytic conversion to N₂ in the absence and presence of excess oxygen and reducing agents (CH₄ or C₃H₈). Among all catalysts tested, Pd-, Ru- and Rh-based samples exhibited the best catalytic performance, in all reaction conditions examined. The reaction was inhibited by O₂, in particular at lower temperatures, while its effect was essentially negligible at higher ones. In the presence of reducing agents and under lean reaction conditions, N₂O conversion was comparably enhanced, with C₃H₈ being more efficient than CH₄; however even in the presence of hydrocarbons N₂O decomposition is the major pathway for N₂O abatement, since reducing agents mainly act as oxygen scavengers reducing and concurrently activating the metal sites. The influence of different co-existing gases (CO, H₂O and SO₂) on the performance of Pd supported catalysts was also investigated, whereas thermal stability tests in the presence of SO₂ indicate a gradual irreversible decrease in activity until a new steady state was established.     

Spark plasma sintering of ultra-porous γ-Al2O3

Nanocrystalline gamma alumina (γ-Al₂O₃) powder with a crystallite size of ~10 nm was synthesized by oxidation of high purity aluminium plate in a humid atmosphere followed by annealing in air. Spark plasma sintering (SPS) at different sintering parameters (temperature, dwell time, heating rate, pressure) were studied for this highly porous γ-Al₂O₃ in correlation with the evolution in microstructure and density of the ceramics. SPS sintering cycles using different heating rates were carried out at 1050–1550 °C with dwell times of 3 min and 20 min under uniaxial pressure of 80 MPa. Alumina sintered at 1550 °C for 20 min reached 99% of the theoretical density and average grain size of 8.5 µm. Significant grain growth was observed in ceramics sintered at temperatures above 1250 °C.     

Triton X-100 directed synthesis of mesoporous γ-Al2O3 from coal-series kaolin

Mesoporous γ-Al₂O₃ has been successfully synthesized by using calcined coal-series kaolin as raw material and Triton X-100 (TX-100) as template. The effect of TX-100/Al^3 + ratio on the structural and textural properties of mesoporous γ-Al₂O₃ was investigated. Physical properties of obtained samples were characterized by X-ray diffraction (XRD), N₂ adsorption–desorption, transmission electron microscopy (TEM), thermogravimetric analysis (TG), scanning electron microscopy (SEM) with energy-dispersive X-ray analysis (EDAX) and Fourier transform infrared spectroscopy (FTIR). The results indicated that the amount of TX-100 influenced the structure and porous properties of mesoporous γ-Al₂O₃ significantly. When TX-100/Al^3 + ratio was in the range of 0.03–0.15, all samples had mesoporous structures with BET surface area of 193.0–261.0 m²/g and pore size of 5.04–6.71 nm. In addition, the reaction mechanism involved in the process was proposed and discussed.     

Preparation and characterization of mesoporous γ-Al2O3 membranes

以粒径为0.3～0.4 μm的α-Al₂O₃为原料,通过悬浮液真空抽吸法,制备出平均孔径约为70 nm的完整无缺陷的片状α-Al₂O₃支撑体;以仲丁醇铝为前驱体,采用颗粒溶胶路线制备出稳定的Boehmite溶胶,以此溶胶采用浸浆法,在制备的α-Al₂O₃支撑体上制备出完整无缺陷的γ-Al₂O₃中孔膜,并考察了烧成温度对γ-Al₂O₃中孔膜性能的影响。结果表明,本文制备出的γ-Al₂ O₃膜的孔径约为3 nm,对PEG的截留分子量为2800～5300,纯水渗透通量为11.5～25.9 L.m^-2.h^-1［7.6×10⁵ Pa,（14±1）℃］。说明在孔径为70 nm左右的载体上直接制备孔径为3 nm的完整的中孔膜是可行的。     

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.     

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).     

Microstructural investigations of tubular α-Al2O3-supported γ-Al2O3 membranes and their hydrothermal improvement

γ-Al₂O₃ meso-porous membranes supported by tubular α-Al₂O₃ substrates were prepared by using the sol-gel method and their nanostructural characterizations were performed for the first time with high-resolution transmission electron microscopy (HRTEM) before and after hydrothermal treatment at 500 °C. The HRTEM images and pore size distribution (PSD) analyses revealed that the morphologies as well as the characteristics of the powder and membrane samples prepared from the same boehmite are not identical. γ-Al₂O₃ and La₂O₃-Ga₂O₃ doped-γ-Al₂O₃ (LGA) membranes supported by α-Al₂O₃ were also fabricated and characterized under thermal and hydrothermal conditions for the purpose of comparisons. Finally, two type α-Al₂O₃/γ-Al₂O₃/SiO₂ (AA-SiO₂) and α-Al₂O₃/La₂O₃-Ga₂O₃-γ-Al₂O₃/SiO₂ (ALGA-SiO₂) membranes have been synthesized and the gas permeance of the membrane were measured in the temperature range 100–500 °C.     

Ni- and Zn-promotion of γ-Al2O3 for the hydrolysis of COS under mild conditions

The hydrolysis of COS using γ-Al₂O₃ as catalyst at 30°C is described. The effect of additives (ca. 3% Na, Fe, Co, Ni, Cu, Zn) is described in detail. The additives reduce the surface area of the γ-Al₂O₃ markedly but have no effect on the powder X-ray diffraction pattern. All significantly enhance the initial intrinsic activity (mol COS hydrolysed/m²/h) when compared with unpromoted γ-Al₂O₃. However, this initial performance rapidly decays for the Na-, Fe-, Co- and Cu-modified catalysts. During this initial reaction period, of up to 5 h, the catalyst retains sulphur. In terms of specific activity (mol COS hydrolysed/g/h) only Ni- and Zn-promoted catalysts give a significant increase that is stable throughout the time-scale of these experiments.     

Active Sites of a NiSO4/γ-Al2O3 Catalyst for the Oligomerization of Isobutene

Structural characterization, the mechanism of catalytic activity generation and the nature of active sites of a NiSO₄/γ-Al₂O₃ catalyst for isobutene oligomerization were studied by temperature programmed reduction (TPR), X-ray diffraction (XRD), diffuse reflectance infrared fourier transformed (DRIFTS) and X-ray photoelectron spectroscopy (XPS) techniques. The TPR measurements together with the XRD data indicated that calcination of the catalyst at 500 °C did not form either nickel oxide or nickel aluminate. The presence of only one type of surface nickel species formed by the incorporation of nickel ions into the surface vacant sites of γ-alumina lattice was indicated by XPS with Ar⁺ ions sputtering and TPR measurements. XPS analysis of the calcined catalyst suggested that the oxidation state of nickel ions in the calcined catalyst was (+2) and after calcination the nickel ions were coordinated to relatively more basic ligands. The surface acid centers of the catalyst were found to be only Lewis type. SO₄ ^2? ions were found to be present as a chelating bidentate ligand and enhanced the acidity of metal ( $ {\text{Al}}^{3 + } $ and/or $ {\text{Ni}}^{2 + } $ ) Lewis acid centers. The results suggested that the combined effects of the presence of the bidentate SO₄ ^2? ligand and dehydroxylation generate coordinatively unsaturated $ {\text{Ni}}^{2 + } $ that interact with isobutene during the oligomerization reaction. The formation of lower-valent nickel ions ( $ {\text{Ni}}^{x + } ,x\; \le\; 1 $ ) was demonstrated by in situ DRIFTS using CO as a probe molecule and by XPS measurements. Formation of a binuclear bridging carbonyl complex, $ [{\text{Ni}}({\text{CO}})^{ + } ]_{2} $ suggested that some lower-valent nickel species were formed via in situ reduction by isobutene. Analysis of Ni 2p photolines indicated the appearance of a new lower-valent nickel species ( $ {\text{Ni}}^{x + } ,x \;\le\; 1 $ ) during the course of isobutene oligomerization. Hence it is plausible that lower-valent nickel species might act as the active center for the oligomerization reaction, while the SO₄ ^2? ions enhance the acidity of the Lewis acid sites on the surface and assist in the adsorption of reactant molecules on the surface.     

Beneficial effect of adding γ-AlOOH to the γ-Al2O3 washcoat of a PdO catalyst for methane oxidation

The beneficial effect of adding γ-AlOOH to the γ-Al₂O₃ washcoat of a ceramic cordierite (2MgO · 2Al₂O₃ · 5SiO₂) monolith, used to support a PdO catalyst, is reported for methane oxidation in the presence of water at low temperature (<500°C). The mini-monolith (400 cells per square inch (CPI), 1 cm diameter × 2.54 cm length; ~52 cells) was washcoated using a suspension of γ-Al₂O₃ plus boehmite (γ-AlOOH), followed by calcination and then deposition of Pd by wet impregnation. An optimum solid content of 25 wt% in the washcoat suspension was used to obtain a ~25 wt% washcoat on the monolith. The presence of γ-AlOOH enhanced the thermal and mechanical stability of the washcoat, provided that the γ-AlOOH content was <8 wt%. Temperature-programmed methane oxidation (TPO) showed that the addition of γ-AlOOH to the γ-Al₂O₃ washcoat decreased the catalyst activity. However, when H₂O (2 vol% and 5 vol%) was present in the feed gas, the γ-AlOOH improved the catalyst activity and stability. A γ-AlOOH content of ~5 wt% in the washcoat was determined to provide the highest catalyst activity and stability for CH₄ oxidation in the presence of water.     

Stabilization of γ-Al2O3 surfaces by additives: insights from computer simulations

Molecular dynamics simulations of the pure and doped surfaces of-Al₂O₃ were performed. Thermal behavior of both the D-layer of the (110) surface and the E-layer of the (001) surface was examined. An abrupt increase in mobility of surface ions was observed at high temperatures. An onset of diffusion occurs for the pure (110) surface at 1200 K. The instability is caused by the cation vacancies adjacent to the surface. Silicon and cerium ions deposited into the sub-surface vacancies reduce the mobility of the surface ions and prevent the onset of diffusion, cerium being more efficient in stabilization than silicon. This forms a microscopic picture of the role of additives in the stabilization of the-Al₂O₃ surfaces.On leave from the Department of Applied Physics, Technical University of Gdansk, Poland.     

[PdCl2(NH2(CH2)12CH3)2] supported on γ-Al2O3 as catalyst for selective hydrogenation

The [PdCl₂(NH₂(CH₂)₁₂CH₃)₂] complex supported on -Al₂O₃ has proved to be a considerably more active, selective and sulfur-resistant catalyst in the selective hydrogenation of styrene to ethylbenzene than a traditional catalyst obtained from acid solutions of PdCl₂, and even than the same complex unsupported. The active species is the complex itself and it is stable under the reaction conditions. Hydrogen treatments above 353K destroy the complex, at least partially, leading to a less active and sulfur resistant catalyst. The higher sulfur resistance, when compared to a conventional Pd/Al₂O₃ catalyst, can be attributed to electronic and geometrical effects.     

Physicochemical Properties of Supported γ-Al2O3 and TiO2 Ceramic Membranes

Abstract

Supported γ-Al₂O₃ and TiO₂ ceramic membranes were prepared by sol-gel techniques from alkoxide precursors. Tests were conducted to measure the permeabilities of these membranes to solvent under a variety of operating conditions. Variables studied were feed temperature, length of time on stream, and feed pH. The stabilities of the membranes in harsh chemical environments were also determined. An alternative method for preparing supported ceramic membranes is also suggested.     

Catalytic conversion of CCl2F2 on a γ-Al2O3 catalyst

Reaction of hydrogen with CCl₂F₂ has been studied on a -Al₂O₃ catalyst at 300°C. Pure -Al₂O₃ appears to be unselective in the catalytic conversion of CCl₂F₂, however, halogenated alumina showed high activity in a reaction leading to the formation of disproportionation products.     

Synthesis of γ-Al2O3 nanowires through a boehmite precursor route

By regulating the pH values of the reaction solution, the boehmite (γ-AlOOH) nanowires and nanoflakes were successfully synthesized with a simple hydrothermal route using anhydrous AlCl₃, NaOH and NH₃·H₂O as starting materials. Crystalline γ-Al₂O₃ nanowires with diameter of 10–30 nm and length of several hundreds of nanometer have been prepared by thermal decomposition of γ-AlOOH precursor. X-ray diffraction (XRD), transmission electron microscope (TEM), selected area electron diffraction (SAED), and high resolution transmission electron microscope (HRTEM) were used to characterize morphology and structure feathers of the synthesized samples. The pH values of the solution play important roles in the formation of γ-AlOOH nanowires. After calcinated at 500 °C for 2 h, the obtained γ-Al₂O₃ with a linear structure is similar to the γ-AlOOH precursor.     

Novel synthesis and characterization of nanosized γ-Al2O3 from kaolin

This paper reports the synthesis of nanosized γ-Al₂O₃ from acid-leachates of calcined kaolin. Al (hydr)oxide was precipitated with ammonia from the leachate in the presence of polyethylene glycol. A white powder of nanosized γ-Al₂O₃ particles was obtained after calcination. X-ray diffraction (XRD), different scanning calorimetries and thermogravimetry (DSC-TG), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and magic angle spinning nuclear magnetic resonance (MAS NMR) were used to characterize the samples. Typical nanosized γ-Al₂O₃ particles showed rod-like morphology with width of about 7 nm and length of approximately 20 nm. A possible mechanism from kaolin to nanosized γ-Al₂O₃ is proposed.     

Thermal-assisted cold sintering study of Al2O3 ceramics: Enabled with a soluble γ-Al2O3 intermediate phase

Thermal-assisted cold sintering process (TA-CSP) has been applied to fabricate high dense α-Al₂O₃ ceramics with submicron grain sizes. The α-Al₂O₃ (80 wt%) and γ-Al₂O₃ (20 wt%) powders are firstly mixed and then cold sintered at 300 °C to produce a green bulk with a relatively high density of ～ 86.9 %, and then later a second heat treatment (800–1350 °C) is applied to finally fabricate (～ 98 % dense) α-Al₂O₃ ceramics with grain sizes of 720 nm. A microstructural analysis with XRD and TEM suggests that the TA-CSP samples not only complete the final densification but also drive a phase transition of γ-Al₂O₃ to α-Al₂O₃. To put into perspective the Hardness and Young's modulus of TA-CSP samples reach ～ 14 GPa and ～ 335 GPa, respectively, which is comparable to conventional sintered samples processed at higher temperatures of 1500–1700 °C. Therefore, it is feasible to utilize TA-CSP to prepare α-Al₂O₃ ceramics with small grain sizes at low sintering temperatures.     

Preparation of highly active nickel catalysts supported on mesoporous nanocrystalline γ-Al2O3 for CO2 methanation

In this study, a group of Ni catalysts supported on mesoporous nanocrystalline γ-Al₂O₃ with high surface area and with different Ni contents was prepared and employed in carbon dioxide methanation reaction. The obtained results showed increasing in nickel content from 10 to 25 wt.% decreased the specific surface area of catalysts from 177 to 130 m²/g and increased the nickel crystallite size from 12 to 24 nm. In addition, increasing in nickel content increased the reducibility of catalyst. The catalytic results revealed that the catalyst with 20 wt.% of Ni possessed high activity and stability in CO₂ methanation reaction.