Transition Alumina Research Articles

Cu-impregnated alumina/silica bed materials for Chemical Looping Reforming of biomass gasification gas

Raw gas derived from low-temperature biomass gasification usually contains condensable hydrocarbons referred to as “tars” as well as other hydrocarbons such as ethylene and other olefins. Reforming of these compounds via Chemical Looping Reforming has been recently proposed. In this work Cu supported on four different Al2O3/SiO2-based porous support materials were investigated for reforming of ethylene. The investigated particles were manufactured by incipient wetness impregnation of the porous support with a copper nitrate solution followed by calcination, and then tested in a small-scale fluidized bed reactor at temperatures between 600°C and 850°C. The ethylene conversion was found to be strongly inhibited by the presence of aromatics in the gas. However, it was found that Cu supported on commercial PURALOX transition alumina catalyst exhibited superior properties, with high degrees of ethylene conversion achievable even in the presence of aromatic compounds. Under the experimental conditions in this work, up to 90% ethylene conversion was obtained at T=850°C in the presence of high concentrations of benzene. For the other prepared materials, the ethylene conversion was drastically reduced when monoaromatic compounds were present in the feed.

Synchrotron EXAFS and XANES spectroscopy studies of transition aluminas doped with La and Cr for catalytic applications

Transition aluminas doped with Cr find widespread application in the dehydrogenation catalysis industry, while La-stabilized transition aluminas are used extensively for high-temperature application as catalytic supports (Wefers and Misra in Oxides and hydroxides of aluminum, Alcoa Laboratories, Pittsburgh, 1987). In this work, detailed synchrotron XAFS spectroscopy studies were conducted to shed light upon the atomic mechanisms of surface and subsurface reconstructions and/or catalytic support stabilization of doped aluminas. It was demonstrated that in four transition aluminas doped with Cr, it is the atoms which are mostly in the state of oxidation Cr3+ and enter nanoparticles of Cr-bearing phases (Cr2O3 in the case of gamma- and chi-alumina). In the transition series aluminas: “gamma- chi- theta- eta-alumina,” the change of properties (in particular, the dramatic increase in dehydrogenation catalytic activity and catalyst longevity and the coloration of samples) takes place because of the reduction in the average size of Cr clusters and their appearance on the Al2O3 surface, probably responsible for change in catalytic activity. It was demonstrated that in the samples of gamma-alumina doped with La any substantial change in the local coordination of the La atoms takes place only upon heating up to 1400 °C. This makes the La-stabilized gamma-alumina a perfect catalytic support for the numerous applications, e.g., catalytic three-way conversion of automobile exhaust gases. This change manifested itself in the form of increased La–O bond lengths and the La coordination number (from 8 to 12). Furthermore, it was demonstrated that the local environment of La in this new La-bearing phase cannot be explained in terms of the LaAlO3 formation. The absence of the La atoms in the second coordination sphere favors monoatomic distribution of La atoms on grain boundaries, proving that only very small amount of this rare earth material is required to achieve full stabilization. It is inferred that the tendency of La atoms to get surrounded by oxygen atoms, and also the impossibility of going into the alumina bulk, could be a major reason of the increased thermal stability of gamma-alumina doped with lanthanum.

AlF3 reaction mechanism and its influence on α-Al2O3 mineralization

Calcination is an important step in the alumina production process, affecting the final properties of the attained product, such as the surface area, soda and alpha phase contents, morphology and primary crystal size. Over the previous decades, attempts have been made to understand this process aiming to decrease the temperature and time of calcination. Considering this, the use of mineralizers, such as fluorides, is a well-known practice, as these additives act decreasing the activation energy for the transformation from the transition to alpha alumina phase. However, the actual reaction mechanism is not well understood. In this study, the likely mechanism was investigated by DTA-TG-MS for samples containing 3wt% of AlF3 in a transition alumina and a hydrate. The emission of HF gas during the alpha transformation pointed out a gas–solid reaction via the generation of a transition compound (Al–O–F). Physico-chemical properties such as the surface area, alpha content, primary crystal size and morphology were also investigated for 0.45 and 0.6wt% AlF3 addition for the different precursors. Additionally, the influence of the homogenization step in the alpha formation was studied. The results highlighted that a lower amount of AlF3 can be used if there is a good confinement of gases inside the furnace.

Effects of thermal treatments on the characterisation and utilisation of red mud with sawdust additive.

Extremely large amounts of red mud (bauxite residue) are generated globally every year from alumina refining industries, which are being disposed of on engineered landfills. The objective of this study is to investigate the effects of thermal treatments on red mud for development of utilisation strategies. Thermal treatments of red mud samples and their characterisations were investigated under inert (N2) and oxidative (air) conditions with and without sawdust addition at 200-600°C. After calcination, the resulting samples were analysed using thermogravimetric-infrared spectroscopy (TG-IR) for functional group transformations, thermogravimetric analysis (TGA) for thermal loss profiles and X-ray diffraction (XRD) for mineral transformations. The characterisation results showed that in N2 environment, boehmite in red mud was transferred to transition alumina at around 400°C while losing water from structural components. The addition of sawdust for incubation and calcination of red mud in air increased the surface area, whereas that in nitrogen atmosphere lead to reduction of hematite to magnetite at around 500°C. The incorporated carbon materials played a major role in increasing the surface area especially for pore size less than 2.5 nm. This treated red mud with altered mineral composition and improved properties for binding contaminants can be used for environmental remediation and in the process of metal recovery such as iron.

High field 27Al MAS NMR and TPD studies of active sites in ethanol dehydration using thermally treated transitional aluminas as catalysts

High field quantitative 27Al single pulse (SP) MAS NMR combined with temperature programmed desorption (TPD) of ethanol is used to study the surface of γ-Al2O3 during phase transformation processes induced by calcination in the temperature range of 500–1300°C. Following ethanol adsorption, ethylene is generated during TPD with a desorption temperature above 200°C. The amount of ethylene decreases monotonically with increasing calcination temperature prior to TPD. Significantly, 27Al SP MAS NMR reveals that the amount of penta-coordinated Al3+ ions also decreases with increasing calcination temperature. A quantitative (within experimental error) correlation between the amount of penta-coordinated Al3+ ions and the amount of strongly adsorbed ethanol molecules (i.e., the ones that convert to ethylene during TPD) is obtained. These results provide good evidence for a proposal that the penta-coordinated aluminum sites are the catalytic active sites on alumina surfaces during ethanol dehydration reaction across the entire course of γ-to-α Al2O3 phase transformations.

New ZrO2/Al2O3 Nanocomposite Fabricated from Hybrid Nanoparticles Prepared by CO2 Laser Co-Vaporization.

Alumina toughened zirconia (ATZ) and zirconia toughened alumina (ZTA) are currently the materials of choice to meet the need for tough, strong, and bioinert ceramics for medical devices. However, the mechanical properties of ZrO2/Al2O3 dispersion ceramics could be considerably increased by reducing the corresponding grain sizes and by improving the homogeneity of the phase dispersion. Here, we prepare nanoparticles with an intraparticular phase distribution of Zr(1−x)AlxO(2−x/2) and (γ-, δ-)Al2O3 by the simultaneous gas phase condensation of laser co-vaporized zirconia and alumina raw powders. During subsequent spark plasma sintering the zirconia defect structures and transition alumina phases transform to a homogeneously distributed dispersion of tetragonal ZrO2 (52.4 vol%) and α-Al2O3 (47.6 vol%). Ceramics sintered by spark plasma sintering are completely dense with average grain sizes in the range around 250 nm. Outstanding mechanical properties (flexural strength σf = 1500 MPa, fracture toughness KIc = 6.8 MPa m1/2) together with a high resistance against low temperature degradation make these materials promising candidates for next generation bioceramics in total hip replacements and for dental implants.

Luminescence study of alumina nanopowders prepared by various methods

Mixed nanopowders of transition alumina prepared by combustion synthesis and phase pure ultra-porous α-alumina by oxidation method were investigated using low temperature time-resolved cathodoluminescence and photoluminescence spectroscopy under VUV-XUV excitation. In all samples along with the 7.6 eV emission of self-trapped excitons of α-alumina, luminescence bands due to F, F+ centres with maxima at 3 and 3.8 eV and other UV–visible luminescence bands of intrinsic and extrinsic origin with varying intensity depending on sample preparation method and thermal treatment were studied. In alumina nanopowders the excitonic excitation peak at ∼9.1 eV near fundamental absorption edge is shifted to the higher energies by 0.15 eV in comparison with the same feature in single crystals. The nanostructure of alumina is responsible for this shift.

Synthesis of zirconia toughened alumina (ZTA) fibers for high performance materials

Polycrystalline zirconia toughened alumina (ZTA) fibers were prepared from aqueous solutions of aluminum hydroxide chloride 2.5-hydrate and zirconium oxychloride octahydrate. Fiber processing was accomplished via dry spinning. Poly(vinylpyrrolidone) (PVP) was used as spinning aid. Polycrystalline ZTA fibers were obtained by calcination of the green fibers followed by sintering at defined temperatures in air. Ceramic fibers were 10μm in diameter and had an average tensile strength of 1010 (±363)MPa with peak values reaching 1535MPa. Differential scanning calorimetry/thermogravimetric analysis coupled with mass spectrometry (DSC/TGA-MS) showed an exothermic peak at 1156°C assigned to the crystallization of α-Al2O3 and tetragonal ZrO2 and an overall ceramic yield of 41.7% at 1400°C. X-ray diffraction (XRD) analysis showed that tetragonal ZrO2 can be obtained at 1350°C directly from the amorphous precursor whereas pure-phase α-Al2O3 is formed stepwise via transition alumina phases.

Corrosion and carburization behavior of Al-rich surface layer on Ni-base alloy in supercritical-carbon dioxide environment

In order to improve the corrosion and carburization resistance in a high-temperature supercritical-carbon dioxide (S-CO2) environment, an Al-rich surface layer was developed on Alloy 600 by Al deposition and a subsequent high energy electron beam (EB) remelting. As a result of the EB surface treatment, an Al enriched (5–7wt.%) micro-alloying zone (40μm) was produced. When the EB surface-treated Alloy 600 was corroded in S-CO2 at 600°C (20MPa) for 500h, the surface oxide layer mostly consisted of chromia (Cr2O3) with small amount of transition alumina (Al2O3). In addition, a carburized region of an amorphous C layer inter-mixed with the alumina was observed at the oxide/matrix interface. Meanwhile, when the EB surface-treated specimen was pre-oxidized in helium at 900°C, α-alumina layer was formed on the surface, which showed superior corrosion and carburization resistance in S-CO2 environment. Therefore, it could be said that the presence of Al-rich surface layer alone is not enough to provide sufficient corrosion and carburization resistance in S-CO2 environment at 600°C, unless pre-oxidation at higher temperature is applied to form a more protective α-alumina on the surface.

Preparation and Characterization of Transitional Alumina Obtained through Ammonia Treatment

In this study, different aluminas were obtained by precipitating Al(NO3 )3·9H2O with different concentration of aqueous solution of ammonium carbonate. The obtained aluminas were treated by 50 (vol/vol) % ammonia via low temperature immersing and high temperature hydrothermal treatment, respectively, then, the treated aluminas were calcined again. The crystal types, morphology, textural properties, and pore distribution of the original alumina, ammonia‐treated alumina, and the second calcined alumina were studied by characterizations of X‐ray diffraction, thermogravimetric analyses, nuclear magnetic resonance, Brunauer‐Emmett‐Teller (BET), transmission electron microscopy, and so on. Several modified aluminas with specific morphology, desired pore structure, and pure crystal types were obtained, which can be used in some specific industrial application.

The effect of calcination temperature on physicochemical properties of alumina as a support for acetylene selective hydrogenation catalyst

The influence of the presence of transition alumina phases on the catalytic properties of Pd–Ag/Al2O3 was investigated for selective hydrogenation of acetylene in operating conditions near a commercial plant. Transition alumina samples were prepared by calcination in the temperature range of 1080–1150 °C. It was observed that by increasing the α-phase content of alumina sample, acidity and surface area were decreased while average pore diameter was increased. From the viewpoint of catalytic performance, increasing the calcination temperature caused acetylene conversion to be decreased and ethylene selectivity to be increased. The higher the α-phase content in alumina support, the better its selectivity.

On the selectivity to higher hydrocarbons in Co-based Fischer–Tropsch synthesis

Impact of process variables on selectivity to C5+ products during cobalt Fischer–Tropsch synthesis (FTS) are summarized and discussed, comprising temperature; pressure; synthesis gas composition; transport limitations of synthesis gas and products; conversion of CO; and effect of water. Further, effect of catalyst formulation, preparation and activation is included, specifically catalyst synthesis; cobalt crystallite size; crystal structure of cobalt; distribution of crystallites; support materials; pore structure; acidity; surface modifications; reduction and pretreatment. Other topics comprise promoters with focus on reduction promoters including rhenium and nickel; impurities like alkali and sulphur; deactivation and regeneration. Discussion is based on mechanistic understanding, specifically kinetics based on hydrogen assisted CO dissociation; a linear relationship between CH4 and C5+ products; possible reinsertion of olefins; hydrogenation of olefins; and surface coverage based on SSITKA data. It is concluded that the majority of reported data on FTS can be rationalized in terms of a CHx pool mechanism on cobalt where x shifts toward monomers and high polymerization probability (α-value); or methane and light products formation, depending on synthesis conditions and catalyst properties.Selectivity responses are illustrated mainly based on cobalt catalysts on transition alumina supports. How the support in itself affects C5+ selectivity is challenging. There clearly is a change in the composition of the cobalt surface pool of reaction intermediates, but how this change originates in support properties remains to be illuminated. So far there is no direct evidence for the termination process; β-hydrogen abstraction, to play a major role in determining FTS selectivity.

Towards Atomic Level Understanding of Transition Alumina Phases and Their Phase Transformations

Towards Atomic Level Understanding of Transition Alumina Phases and Their Phase Transformations

Exploring the Complex Porosity of Transition Aluminas by 129Xe NMR Spectroscopy

Crystalline mesoporous γ-, δ-, and θ-alumina samples with complex porosity and various surface areas (70 to 330 m2/g) were characterized by 129Xe NMR spectroscopy and nitrogen adsorption. Experimental conditions have been optimized to measure 129Xe NMR chemical shifts independent of the nature of the surface and solely dependent on the pore size. Xenon adsorption constants have been determined from xenon adsorption isotherms and from the fit of the chemical shift versus volume to surface ratio (V/S) curves with a simple exchange model. The discrepancy observed between those values has been attributed to the rugosity of the alumina pore surface. A direct correlation between the observed chemical shift and the alumina pore diameter was obtained for the whole series of alumina samples.

Solid state 27Al NMR and X-ray diffraction study of alumina–carbon composites

The structural and chemical transformations occurring in alumina–carbon composites upon heat treatment were investigated by using a combination of X-ray diffraction (XRD) and solid-state 27Al nuclear magnetic resonance (NMR) spectroscopy. Two different carbon precursors were employed: a commercial activated carbon and a char obtained by carbonization of the endocarp of babassu coconut at 700°C. The alumina–carbon composites were prepared by aqueous impregnation of the carbon supports with aluminum nitrate and, after filtering and drying, the as-synthesized powders were heat-treated under argon flow at temperatures up to 1000°C. The Al compounds present in the as-synthesized samples were identified by XRD and solid-state 27Al NMR as nanocrystalline aluminum oxyhydroxides or hydroxides, depending on the synthesis conditions. All Al-containing phases were XRD-amorphous in the char-derived nanocomposites, with the presence of a distribution of AlO6 (octahedral Al site), AlO5 (pentacoordinated Al) and AlO4 (tetrahedral Al site) units revealed by solid-state 27Al NMR spectroscopy. The heat treatments caused the formation of transition aluminas dispersed over the carbon supports, with the occurrence of different amounts of AlO6, AlO5 and AlO4 units depending on the heat treatment temperature and on the type of carbon precursor used for the preparation of the composites.

Microstructural origin of time-dependent changes in alumina sol–gel-coated Inconel 718 exposed to NaCl solution

Inconel 718 was spin coated twice and fourfold with a sol to obtain 200- and 400-nm-thick transition alumina films on the surface. Bare and sol–gel alumina-coated Inconel 718 samples were exposed to NaCl solution to study their corrosion behavior by means of electrochemical impedance spectroscopy. In combination with scanning electron microscopy, it was shown that bare Inconel 718 is after initial passivation prone to pitting corrosion. For the coated Inconel 718 samples, an improvement in the protective effect of the coatings with time was observed. This observation is in contradiction to the results of other authors who observed degradation of alumina coatings exposed to NaCl solution with time. Transmission electron microscopy revealed a formation of (1) a compacted region at the coating surface as well as (2) a chromium-rich region at the interface between coating and alloy during the contact with a NaCl solution. The last one mainly contributes to the observed chronological sequence of electrochemical characteristics of the samples.

Changes in the Thermal Behavior and Surface Area of Transitional Alumina Induced by the Inclusion of Metallic Ions

Transitional alumina was synthesized from economical precursors and by precipitation route. The precipitation was carried out by the addition of sodium hydroxide drop by drop to an aqueous solution of aluminum sulfate. The powder obtained was aged for 24 h and then annealed at 600 and 800°C with the purpose to study the changes of surface area and porosity of the material. The textural characteristics of this material were compared with solid containing copper, zinc and iron species. The powder characterization was carried out by X-ray diffraction, thermal analysis, infrared spectroscopy, and N2 adsorption at 77 K.

Chemical vapour synthesis and characterisation of Al2O3 nanopowders

Direct synthesis of transition alumina (Al2O3) nanopowders in gas phase by a chemical reaction of precursor vapour (aluminium-tri-sec-butoxide) with oxygen is reported. The synthesis method is a modified chemical vapour deposition process known as a chemical vapour synthesis (CVS). Reactor pressure and carrier gas (He) flow were varied in order to examine their influence on the quality of the as-synthesised nanocrystalline powder. Characterisation of the obtained powders was performed by means of XRD, FT-IR, SEM, TEM, thermogravimetry and low temperature nitrogen adsorption.

Atomic structure of a spinel-like transition Al2O3(100) surface.

We study a crystalline epitaxial alumina thin film with the characteristics of a spinel-type transition Al2O3(100) surface by using atom-resolved noncontact atomic force microscopy and density functional theory. It is shown that the films are terminated by an Al-O layer rich in Al vacancies, exhibiting a strong preference for surface hydroxyl group formation in two configurations. The transition alumina films are crystalline and perfectly stable in ambient atmospheres, a quality which is expected to open the door to new fundamental studies of the surfaces of transition aluminas.

Structure of δ-Alumina: Toward the Atomic Level Understanding of Transition Alumina Phases

Transition Al2O3 derived from thermal decomposition of AlOOH Boehmite have complex structures and to a large extent remain poorly understood. Here, we report a detailed atomic level analysis of δ-Al2O3 for the first time using a combination of high-angle annular dark field electron microscopy imaging, X-ray diffraction refinement, and density functional theory calculations. We show that the structure of δ-Al2O3 represents a complex structural intergrowth from two main crystallographic variants, which are identified as δ1-Al2O3 and δ2-Al2O3. The two main variants are fully structurally described, and in addition, we also derive their energy of formation. On the basis of comparison with other relevant transition Al2O3 phases, it is shown how energetic degeneracy leads to the structural disorder and complex intergrowths among several transition Al2O3. The results of the work have important implications for understanding thermodynamic stability and transformation processes in transition alumina.