Aluminum oxide and systems based on it: Properties and applications

The metastable forms of aluminum oxide that exist in the range of 300–800°C are characterized; differences in the microstructures of homogeneous γ-, η-, and χ-Al₂O₃ are demonstrated; and the acid-base properties of the above modifications are compared. The catalytic properties of aluminum oxide in ethanol dehydration and propionitrile ammonolysis were studied. It was found that an increased surface concentration of Lewis acid sites, including strong acid sites (ν(CO) = 2237 cm^?1), is required for preparing an effective catalyst for the dehydration of ethanol, whereas the rate of propionitrile conversion increased proportionally to the surface concentration of Brønsted acid sites. γ-Aluminum oxide was used to prepare catalysts for carbon monoxide oxidation. It was found that the supporting of Pd on γ-Al₂O₃ did not change the support structure. Palladium on the surface of γ-Al₂O₃-550 (T _calcin = 550°C, S _BET = 300 m²/g) occurred as single particles (2–3 nm) and aggregates (～100 nm). The single particles were almost completely covered with a layer of aluminum oxide to form core-shell structures. According to XPS data, they were in atypical states (BE(Pd 3d _5/2) = 336.0 and 338.0 eV), which were not reduced by hydrogen in the range of 15–450°C and were resistant to the action of the reaction mixture. Palladium on the surface of γ-Al₂O₃-800 (S _BET = 160 m²/g) was in the states Pd⁰ and PdO, which are typical of Pd/Al₂O₃, and the proportions of these states can change under the action of the reaction mixture. An increase in the T _calcin of the Pd/Al₂O₃(800)-450 catalyst from 450 to 800 → 1000 → 1200°C led to the agglomeration of palladium particles and to an increase in the temperature of 50% CO conversion from 145 to 152 → 169 → 189°C, respectively. α-Aluminum oxide was used in the preparation of an effective Mn-Bi-O/α-Al₂O₃ supported catalyst for the synthesis of nitrous oxide by the oxidation of ammonia with oxygen: the NH₃ conversion was 95–97% at 84.4% N₂O selectivity.     

Ni catalysts supported on nano-crystalline aluminum oxide prepared by a microemulsion method for dry reforming reaction

Mesoporous nano-crystalline γ-Al₂O₃ with high surface area prepared by a microemulsion (ME) method was employed as carrier for nickel catalysts in dry reforming of methane for syngas production. The structural properties of the catalysts were characterized by X-ray diffraction, Brunauer–Emmett–Teller surface area analysis, temperature programmed reduction and oxidation and scanning electron microscopy techniques. Microemulsion showed it to be a promising way for the production of nano-crystalline aluminum oxide, and the nickel catalysts prepared with this support have significant features and properties to use in the dry reforming reaction. The results revealed that the prepared γ-Al₂O₃ exhibited a nano-crystalline structure (crystal size: c.4.8 nm) with a high specific surface area (308 m² g^?1). In addition, the catalysts with different nickel contents exhibited high catalytic activity in the dry reforming reaction. The results also showed that an increase in Ni loading from 5 to 15 wt% caused a decrease in the specific surface area and nickel dispersion.     

Rapid crystal phase transformation into hexagonally shaped α-alumina using AlF3 seeds

This study focused on synthesizing highly crystalline α-Al₂O₃ at low temperatures. We found that when a small amount of hexagonal structured aluminum fluoride seeds (AlF₃) were added to aluminum precursors, the crystal phase transformation temperature required to form α-Al₂O₃ were considerably lowered and the produced α-Al₂O₃ particles showed hexagonal shapes. Furthermore, when 2.0 mol% of hexagonal structured-AlF₃ particles were added as seed to sol-solutions containing various aluminum precursors such as AIP, AlCl₃, Al(NO₃)₃ or AlOOH, rhombohedral α-Al₂O₃ particles were formed at 750 °C regardless of the use of alumina precursors. Scanning electron microscopy photos showed the α-Al₂O₃ particles produced had hexagonal shapes, and that the morphologies of these particles depended on the primitive structures of the AlF₃ seeds added. Based on the relationship between α-Al₂O₃ crystallites sizes and calcination temperatures, crystal growth in α-Al₂O₃ structure was enhanced by adding AlF₃ seeds.     

Correlation between acidity and catalytic activity for the methanol dehydration over various aluminum oxides

Dehydration of methanol into dimethyl ether (DME) was carried out over aluminum oxides with different crystalline phases, viz. η-Al₂O₃, γ-Al₂O₃, θ-Al₂O₃, (χ + γ)-Al₂O₃, δ-Al₂O₃, α-Al₂O₃, and κ-Al₂O₃. The catalytic activity decreased in the following order: η-Al₂O₃ > γ-Al₂O₃ ? θ-Al₂O₃ ? (χ + γ)-Al₂O₃ ? δ-Al₂O₃ > α-Al₂O₃ ≈ κ-Al₂O₃. Several techniques: N₂ physisorption, X-ray diffraction (XRD), temperature-programmed desorption (TPD) of NH₃, and FT-IR spectroscopy after pyridine adsorption were employed to characterize these solid acid catalysts. The good correlation can be found between the catalytic activity and the amount of Lewis acid site determined by the FT-IR spectra after pyridine adsorption.     

Preparation of single-phase α-Fe(III) oxide nanoparticles by thermal decomposition. Influence of the precursor on properties

In this research, we present an experimental procedure to prepare single-phase α-Fe(III) oxide nanoparticles by thermal decomposition of five different precursors including: iron(III) citrate; Fe(C₆H₅O₇), iron(III) acetylacetonate; Fe(C₅H₇O₂)₃, and iron(III) oxalate; Fe(C₂O₄)₃, iron(III) acetate; Fe(C₂H₃O₂)₃, and the thermal curves obtained were analyzed. The influence of the thermal decomposition of precursors on the formation α-Fe₂O₃ was studied by differential thermal gravimetry and thermogravimetry. The synthesized powders were characterized by using X-ray diffraction and scanning electron microscopy. High quality iron oxide nanoparticles with narrow size distribution and average particle size between ca. 2 and 30 nm have been obtained. It was found that the iron precursors affect the temperatures of the pure α-Fe₂O₃ nanoparticle formation with different diameters; iron(III) citrate (29 nm), iron(III) acetylacetonate (37 nm), and iron(III) oxalate (24 nm).     

Effect of chromium(III) oxide on corundum formation from products of shock-wave synthesis of aluminum oxide

Formation of α-Al₂O₃ in thermal treatment of ultradispersed aluminum oxide powders produced by shock-wave synthesis with addition of Cr₂O₃-(NH₄)₂Cr₂O₇ as a precursor was studied. The acceleration of the conversion of δ-and θ-Al₂O₃ to α-Al₂O₃ upon introduction of additives in the stage of synthesis of aluminum oxide was examined.     

An insight into the changes in the thermal analysis curves of boehmite with mechanical activation

This study deals with the changes in the thermal transformation behaviour of boehmite with mechanical activation (MA), carried out in planetary mill. Observed changes in the TG-DTG–DTA curves are: shifting of the desorption of physically adsorbed water to higher temperature, decrease in the γ-Al₂O₃ transformation temperature and its peak area, formation of α-Al₂O₃, not observed for unmilled boehmite upto 1,200 °C, for milling time ≥60 min. Reasons for such changes are explored on the basis of physicochemical changes occurring as a result of high energy milling. Structural degradation is found to increase with increase in milling time. As a consequence of structural changes, Al–OH bonds get stronger, whereas the hydrogen bonds get weaker. Stronger Al–OH bonding and enhanced surface energy increase water affinity and delays its removal. Decreased hydrogen bond strength, easy exit of dehydroxylation product (water) and displacement of Al to tetrahedral positions make the γ-Al₂O₃ transformation easier. Ease of removal of residual hydroxyls from small crystallite transition alumina from MA boehmite, as a result of shorter diffusion path, ensures α-Al₂O₃ transformation at lower temperature.     

MnO x -Al2O3 catalysts for deep oxidation prepared with the use of mechanochemical activation: The effect of synthesis conditions on the phase composition and catalytic properties

The catalytic activity of alumina-manganese catalysts in the oxidation of CO was studied. The MnO _x -Al₂O₃ catalysts were prepared by an extrusion method with the introduction of mechanically activated components (manganese oxide and its mixtures with aluminum oxide, aluminum hydroxide, and a mixture of a manganese salt with aluminum hydroxide) into a paste of aluminum hydroxide followed by thermal treatment in air or argon at 1000°C. In the majority of cases, the catalysts contained a mixture of the phases of β-Mn₃O₄ (Mn₂O₃), α-Al₂O₃, and δ-Al₂O₃. The presence of low-temperature δ-Al₂O₃ suggested the incomplete interaction of manganese and aluminum oxides. It was found that the catalytic activity of MnO _x -Al₂O₃ depends on the degree of interaction of the initial reactants, and its value is correlated with the amount of β-Mn₃O₄ in the active constituent. The intermediate thermal treatment of components at 700°C negatively affects the catalytic activity as a result of the formation of Mn₂O₃ and the coarsening of particles, which levels the results of mechanochemical activation. The greatest degree of interaction between Al- and Mn-containing components was reached in the selection of mechanochemical activation conditions by decreasing the size of grinding bodies, optimizing the time of mechanochemical activation, and using the mechanochemical activation of precursor mixtures. As a result of mechanochemical activation, the initial reactants were dispersed, the amounts of MnO₂ and Mn₂O₃ changed, and defects were formed; this strengthened the interaction of components and increased catalytic activity.     

Formation of Pd–Ag nanoparticles in supported catalysts based on the heterobimetallic complex PdAg<Subscript>2</Subscript>(OAc)<Subscript>4</Subscript>(HOAc)<Subscript>4</Subscript>

The formation of Pd–Ag nanoparticles deposited from the heterobimetallic acetate complex PdAg₂(OAc)₄(HOAc)₄ on α-Al₂O₃, γ-Al₂O₃, and MgAl₂O₄ has been investigated by high-resolution trans-mission electron microscopy, temperature-programmed reduction, and IR spectroscopy of adsorbed CO. The reduction of PdAg₂(OAc)₄(HOAc)₄ supported on γ-Al₂O₃ and MgAl₂O₄ takes place in two steps (at 15–245 and 290–550°C) and yields Pd–Ag particles whose average size is 6–7 nm. The reduction of the Pd–Ag catalyst supported on α-Al₂O₃ occurs in a much narrower temperature range (15–200°C) and yields larger nanoparticles (~10–20 nm). The formation of Pd–Ag alloy nanoparticles in all of the samples is demonstrated by IR spectroscopy of adsorbed CO, which indicates a marked weakening of the absorption band of the bridged form of adsorbed carbon monoxide and a >30-cm^–1 bathochromic shift of the linear adsorbed CO band. IR spectroscopic data for PdAg₂/α-Al₂O₃ suggest that Pd in this sample occurs as isolated atoms on the surface of bimetallic nanoparticles, as is indicated by the almost complete absence of bridged adsorbed CO bands and by a significant weakening of the Pd–CO bond relative to the same bond in the bimetallic samples based on γ-Al₂O₃ and MgAl₂O₄ and in the monometallic reference sample Pd/γ-Al₂O₃.     

Synthesis and characterization of nanocrystalline α-Al2O3 using Al and Fe2O3 (hematite) through mechanical alloying

In this present work, the synthesis of nanocrystalline α-Al₂O₃ using pure aluminum (Al) and Fe₂O₃ (hematite) as the precursors by mechanical alloying technique has been studied. The formation of α-Al₂O₃ nanocrystallites occurs during the solid-state reaction and through the reduction treatment. Also in this paper, effects of milling time on particle size and the lattice strain nanocrystalline α-Al₂O₃ have been investigated. Obtained powders were evaluated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential thermal analysis (DTA), thermal gravimetric analysis (TGA) and X-ray diffraction (XRD). The obtained results indicated that a reduction reaction was completed after 2 h milling in a planetary mill. The crystallite size of obtained α-alumina (α-Al₂O₃) was in general about 12 nm. Finally, the results showed appropriate homogeneity and dispersion of related nanocrystalline.     

Dynamic study of ammonium dawsonite doped with yttrium transformation at elevated temperatures

Co-precipitation of alumina/YAG precursor from aluminum and yttrium nitrates solution with ammonium carbonate results in dawsonite (NH₄Al(OH)₂CO₃). Its crystallographic parameters differ from the compound precipitated without the yttrium additive. It indicates that yttrium ions become incorporated into the dawsonite structure. The DSC/TG and X-ray measurements show decomposition of dawsonite at elevated temperature resulting in γ-Al₂O₃ which transforms to δ and θ modifications at still higher temperatures. The full transformation to α-Al₂O₃ and YAG occurs at temperatures higher than 1,230 °C.     

Chitosan-mediated synthesis of mesoporous α-Fe2O3 nanoparticles and their applications in catalyzing selective oxidation of cyclohexane

This paper reports the chitosan-mediated synthesis of porous hematite nanoparticles with FeCl₃ as the precursor via a hydrothermal approach at 160 °C. A series of porous chitosan/iron oxide hybrid nanoparticles were obtained via changing the ratio of chitosan to FeCl₃, FeCl₃ concentration and pH value of the reaction solution, and producing porous iron oxide nanoparticles after calcination. The as-prepared samples were characterized by means of X-ray diffraction, transmission electron microscopy, thermal gravimetric analysis, Fourier transform infrared, and N₂ sorption. The particle sizes of these metal oxides were less than 100 nm, and the pore sizes were in the range of 2–16 nm. It was demonstrated that chitosan played a key role in the formation of the porous structures. The resultant α-Fe₂O₃ nanoparticles were used as the support to immobilize Au or Pd nanoparticles, producing Au/α-Fe₂O₃ or Pd/α-Fe₂O₃ nanoparticles. The as-prepared α-Fe₂O₃ nanocatalyst exhibited high selectivity towards cyclohexanone and cyclohexanol for catalyzing cyclohexane oxidation with O₂ at 150°C.     

Preparation of Mesoporous γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> from Aluminum Hydroxide Peptized with Organic Acids

The relationships of the formation of aluminum oxide from aluminum hydroxide peptized with organic acids (propionic, maleic, malonic, tartaric) were studied. The pore structure parameters of the hydroxide samples and of aluminum oxide samples obtained from them are strongly influenced by acid peptization. The nature of the acid influences the extent of defectiveness of the γ-Al₂O₃ structure, manifested in the density of aluminum oxide, measured by helium pycnometry. The possibility and conditions for preparing mesoporous γ-Al₂O₃ suitable for use in chemical and petrochemical processes were determined.     

Active component of supported vanadium catalysts in the selective oxidation of methanol

The structure of catalysts based on vanadium oxide supported on different oxides (SiO₂, γ-Al₂O₃, ZrO₂, and TiO₂) was investigated. Their catalytic properties in the selective oxidation of methanol in a temperature range of 100–250°C were studied. It was shown that the nature of the support determines the structure of the oxide forms of vanadium. The supporting of vanadium on SiO₂ and γ-Al₂O₃ leads to the preferred formation of crystalline V₂O₅; the surface monomeric and polymeric forms of VO_x are additionally formed on ZrO₂ and TiO₂. It was established that the crystalline V₂O₅ oxide is least active in the selective oxidation of methanol; the polymeric forms are more active than monomeric ones. The mechanism of the selective oxidation of methanol to dimethoxymethane and methyl formate on the vanadium oxide catalysts is considered.     

Thermal decomposition of nickel aluminium mixed hydroxides and formation of nickel aluminate spinel

Various nickel aluminium mixed hydroxide samples of different compositions were prepared by co-precipitation from their nitrate solutions using dilute NH₄OH. Additional samples were prepared by impregnation of hydrated Al₂O₃, preheated at 600 and 900°C, with nickel nitrate solution in an equimolar ratio. The thermal decomposition of different mixed solids was studied using DTA. The X-ray investigation of thermal products of the mixed solids was also studied.The results obtained revealed that the presence of NiO up to 33.3 mole % with aluminium oxide much enhanced the degree of crystallinity of the γ-Al₂O₃ phase. In contrast, the presence of Al₂O₃ much retarded the crystallization process of the NiO phase. With the exception of samples containing 20 mole% NiO, all the mixed hydroxide samples, when heated in air at 900°C, led to the formation of well-crystalline Ni Al₂O₄ spinel, alone, or together with either NiO or γ-Al₂O₃, depending on the composition of the mixed oxide samples. The solid containing 20% NiO and heated at 900°C was constituted of amorphous NiO dispersed in γ-Al₂O₃. Heating the nickel nitrate-impregnated Al₂O₃ in air at 800–1000°C led to the formation of Ni Al₂O₄ together with non-reacted NiO and γ-Al₂O₃. The degree of crystallinity of the spinel was found to increase by increasing the calcination temperature of the impregnated solids from 800 to 1000°C and by increasing the preheating temperature of the hydrated Al₂O₃ employed from 600 to 900°C.     

Study of CuCl2 Supported on SiO2 and Al2O3

The nature and stability of surface species of CuCl₂ supported on α-Al₂O₃, γ-Al₂O₃, and SiO₂ were investigated by using X-ray diffraction techniques and reflectance spectroscopy. No specific chemical interaction of CuCl₂ is observed on an inert α-Al₂O₃ support, as opposed to hydrated carriers as SiO₂ and γ-Al₂O₃. On these supports the coordination sphere of Cu²⁺ consists of surface groups (OH^? or O^? at drying and activation, resp.), H₂O and Cl^?, with the H₂O ligands decreasing in concentration in the process of impregnation, drying and calcination. γ-Al₂O₃ samples, calcined at 400°C, show γ-Cu₂(OH)₃Cl as opposed to CuAl₂O₄ at higher temperatures. The absence of Cu₂(OH)₃Cl on SiO₂-supported samples is related to the acid-base characteristics of the carriers. The various supports can be arranged in the following order of stability of the complexes formed: γ-Al₂O₃ > SiO₂ ? -Al₂O₃.     

Preparation of aluminum nitride fibers from alumina gel fibers by nitridation in ammonia

Aluminum nitride (AlN) fibers were prepared from alumina gel fibers and by heat-treatment in ammonia. The influence of silica on the formation of AlN was investigated. It was shown that phase transformation of alumina (γ-Al₂O₃ to α-Al₂O₃) and nitridation reaction took place above 1,100 °C for pure alumina fiber. The addition of a small amount of silica (3 wt%) suppressed the formation of α-Al₂O₃ and preserved the highly reactive metastable alumina, and nitridation rate was enhanced. Fine grain (~20 nm) AlN fibers were obtained for pyrolysis at 1,150–1,250 °C for 3 h in ammonia, and AlN was identified as the sole crystalline phase.     

Synthesis of α-alumina from a less common raw material

A nanostructured α-Al₂O₃ with particle size lower than 100 nm was obtained from a hazardous waste generated in slag milling process by the aluminium industry. The route developed to synthesize alumina consisted of two steps: in the first one, a precursor of alumina, boehmite, γ-AlOOH was obtained by a sol–gel method. In the second step, the alumina was obtained by calcination of the precursor boehmite (xerogel). Calcination in air was performed at two different temperatures, i.e. 1,300 and 1,400 °C, to determine the influence of this parameter on the quality of resulting alumina. X-Ray diffraction patterns and transmission electron microscopy images of calcined powers revealed beside corundum the presence of transition aluminas and some rest of amorphous phase in the sample prepared at 1,300 °C. The increase of the calcinations temperature to 1,400 °C favors the formation of an almost single-phase corundum powder. The transition of θ- to α-Al₂O₃ was followed by means of infrared spectroscopy, since it is accompanied by the disappearance of the IR band frequencies associated with tetrahedral sites (AlO₄ sites), giving rise to a spectrum dominated by Al³⁺ ions in octahedral sites (AlO₆) characteristic of corundum.     

Thermal decomposition of managanese oxyhydroxide

Temperature-programmed thermal decomposition of γ- and α-manganese oxyhydroxide has been studied between 20 and 670°C under vacuum and under a low pressure (10 Torr) of oxygen. Solid products at various temperatures have been analyzed by X-ray diffractometry. Under vacuum γ-MnOOH decomposed below 400°C to a mixture of Mn₅O₈, α-Mn₃O₄, and water according to the reaction scheme: 8MnOOH → Mn₅O₈ + Mn₃O₄ + 4H₂O. Above this temperature Mn₅O₈ was converted to α-Mn₃O₄ as a result of oxygen removal. The vacuum dehydration at 250°C of oxyhydroxide rich in α-MnOOH led to the formation of a new modification of Mn₂O₃ isostructural with corundum (α-Al₂O₃). In oxygen both oxyhydroxides decomposed to β-MnO₂. γ-MnOOH transformed directly to β-MnO₂ while α-MnOOH appeared to transform via corundum-phase Mn₂O₃ as an intermediate.     

Alumina nanofibers obtained via electrospinning of pseudo-boehmite sol/PVP solution

Alumina nanofibers were fabricated by calcination of the polyvinylpyrrolidone (PVP)/pseudo-boehmite nanocomposite precursor fibers formed by electrospinning PVP/ethanol solution of dispersed pseudo-boehmite nanoparticles with and without additive of silica. The evolution of the phase, mechanical property and morphological features of the calcined fibers were studied and the effect of adding SiO₂ on the phase transformation of alumina was discussed. Adding SiO₂ can retard the phase transformation of γ-Al₂O₃ to α-Al₂O₃ and therefore inhibit the growth of alumina grains during calcination. Upon calcining the precursor fibers with 4 wt% SiO₂ additive at 1,300 °C, continuous alumina nanofibers with diameter ranging from 300 to 800 nm were obtained. These continuous nanofibers exhibited good flexibility and could be very promising for applications in filtration and catalyst support.