Mechanochemical Activation of Aluminum: 3. Kinetics of Interaction between Aluminum and Water

Two regimes of oxidation by water are revealed for nanocrystalline aluminum prepared by the mechanical activation of its mixture with graphite and distributed in the matrix of amorphous carbon. At the temperatures 50°C < T < 90°C, nanosized aluminum particles interact with water under quasi-isothermal conditions. The main products are hydrogen and pseudoboehmite AlOOH; a low content of bayerite Al(OH)₃ is also formed. After the induction period, the kinetics of interaction can be satisfactorily described by the law of a diminishing sphere. The effective activation energy of the reaction is equal to 61 ± 10 kJ/mol and is identical for the samples of submicron aluminum prepared by different procedures. At temperatures above 90–95°C, the oxidation of mechanically activated aluminum by water is transformed into a thermally self-accelerated explosion process. Under these conditions, the oxidation of aluminum to α-Al₂O₃ is accompanied by an exothermal reaction between the metal and the carbon matrix during which aluminum carbide Al₄C₃ is formed.     

Preparation and Thermal Decomposition of Synthetic Bayerite

The formation process of bayerite, from an aqueous solution of sodium aluminate through enforced decomposition of aluminate ions by introducing CO₂ gas and aging with mechanical stirring, was investigated by pH measurements of the mother solution during preparation reaction and characterization of precipitates obtained at various stages of preparation. An amorphous precipitate, produced initially by the reaction of introduced CO₂, transformed to bayerite via pseudoboehmite during aging. It was found that the crystalline particle size and morphology of the crystallized bayerite change depending systematically on the preparation conditions. The reaction pathway of the thermal decomposition of the synthesized bayerite was investigated by using thermoanalytical techniques. This revised version was published online in July 2006 with corrections to the Cover Date.     

Crystallization behaviour of TiO2–ZrO2 composite nanoparticles

Nano-composite TiO₂?CZrO₂ materials were prepared via sol?Cgel processes by hydrolysis of mixtures of titanium- and zirconium-containing alkoxides with TiO₂:ZrO₂ ratios of 1:2, 1:1, 2:1, and 10:1. Precipitated powders were dried at room temperature and annealed in ambient air at temperatures between 350 and 500?°C for 4?h. Pure TiO₂ and ZrO₂ powders were synthesized for comparison. Samples were characterized with X-ray diffraction (XRD) and X-ray absorption near edge fine structure (XANES). The detailed analysis of those data provides the crystalline and amorphous phase composition as well as the crystallite particle size. According to XRD and XANES analysis, only the two pure oxide samples and one of the composite samples with a composition TiO₂:ZrO₂?=?10:1, crystallized. Both titania containing powders, the pure TiO₂ and the TiO₂:ZrO₂?=?10:1 composite, were found to crystallize in the anatase structure. ZrO₂ was found to stay amorphous in the composites but crystalline in the pure oxide. In the crystallized composite TiO₂:ZrO₂?=?10:1 sample, the concentration of the amorphous phase remains larger than in pure TiO₂ samples, but the crystallite size was found to be nearly constant with increasing annealing temperature in contrast to the increasing particle size of pure TiO₂-samples. Pure TiO₂ precipitates are amorphous directly after preparation, however they crystallize after 6?month storage at ambient conditions by aging. Such an aging was not observed for the TiO₂:ZrO₂ composites.     

Effect of Solution Chemistry on Preparation of Boehmite by Hydrothermal Assisted Sol-Gel Processing of Aluminum Alkoxides

Various aluminum alkoxide precursors have been used for the preparation of boehmite by hydrothermal assisted sol-gel processing. The coordination status of aluminum in solution for all precursors employed for the preparation of boehmite phase was determined by ²⁷Al NMR and correlation between coordination status of aluminum atoms of precursors and development of boehmite phase has been investigated. Hydrothermal assisted hydrolysis of aluminum alkoxides where the aluminum atoms are four or five coordinated in solution resulted in the formation of boehmite. In contrast, hydrothermal hydrolysis of aluminum alkoxides where the aluminum atoms are six coordinated resulted in the formation of amorphous gel. Development of boehmite phase by hydrothermal hydrolysis of aluminum alkoxides at various temperatures was pursued by X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy.     

NH3 TPD study and thermal behaviour of vanadium-exchanged titanium phosphates as catalysts

The ion exchange technique was employed for the preparation of VO²⁺ modified titanium phosphates as catalysts for the selective reduction of NO with NH₃. The samples were prepared by contacting with a vanadyl sulphate solution different precursor materials, amorphous, crystalline or sodium half exchanged titanium phosphate. The vanadium contents of modified phosphates were in the range 0.08–2.3 wt%. XRD and thermal analysis TG/DTA showed that vanadium loading does not cause structural modification in hydrogen titanium phosphate. A vanadyl containing phase was obtained when half sodium titanium phosphate was employed. The NH₃ TPD measurements indicated the presence of a wide distribution of NH₃ adsorbing sites with medium-high strength. Catalytic activity measurements were performed under dilute conditions. It was found that the presence of vanadium even in low amounts strongly promote the catalytic activity.     

Toward a quantitative description of the anodic oxide films on aluminum

A method to quantify the composition of anodic oxide films on aluminum using Infrared Spectroscopic Ellipsometry (IRSE) is proposed. It consists of obtaining the absorption coefficient of the film as a function of wavelength. Using values of the absorption coefficients for the pure components of the film, the percentages (mole or wt%) of each component in the sample can be calculated.The method is demonstrated in a study of the structure of the oxide film on electropolished aluminum and the anodically formed barrier layer film. Both surface oxides were found to be initially a form of amorphous Al₂O₃. While the barrier film is essentially free of water as prepared, the film on electropolished aluminum contained about 25 wt% water. Hydration of both types of films by immersion in boiling water results in the formation of pseudoboehmite (AlOOH). The technique may have more general applicability to the quantitative determination of the composition of corrosion films and other surface layers on metals.     

Acid-base properties of alumina prepared from a hydrated product of centrifugal thermal activation of hydrargillite (cta-product)

Summary Acid-base properties of aluminas prepared by thermal treatment of a hydrated CTA-product at 600°C were studied. The CTA-oxides, representing γ-Al₂O₃, were shown to contain terminal and bridged OH-groups. The concentration of the terminal OH-groups in the CTA-oxides was found to exceed their concentration in γ-Al₂O₃ prepared by dehydration of the “precipitated” pseudoboehmite, whereas the concentration of the bridged OH-groups in the CTA-oxides was lower than that in γ-Al₂O₃ prepared from pseudoboehmite. The total concentration of the surface Lewis acid sites in CTA-oxides varies within the limits of 2.80-4.14 mmol/m² and is essentially above that in g-Al₂O₃ (2.25 mmol/m²). The distinctive feature of the CTA-oxides is that their surface contains strong Lewis acid sites with n_CO = 2220 and 2238 cm^-1. The total concentration of basic sites in the CTA-oxides is lower than that in g-Al₂O₃, however, in contrast to g-Al₂O₃,they contain strong basic sites with n_CDCl3 = 2200 cm^-1.     

Multi‐shaped Amorphous Alloy Ni‐B: Ultrasonically Aided Complexing‐reduction Preparation,Catalytic Ability for NaBH4 Hydrolysis Yielding H2 Gas

The multi‐shaped amorphous alloy (Ni‐B) powders were prepared by complexing reduction route using sodium borohydride (NaBH₄) as reductant with assistance of ultrasonic wave. The selected complexants, i.e. water, ammonia, salicylic acid, and ethylene diamine tetraacetic acid (EDTA) possess sequentially escalating complexation ability. The chemical composition and shapes of the product samples obtained under different conditions were characterized by X‐ray powder differaction, selected area electron diffraction, and transmission electron microscope. The influence of reaction conditions such as the types of Ni‐B, temperatures, NaBH₄ concentrations, and sodium hydroxide (NaOH) content on the hydrogen generation rate of hydrolysis of NaBH₄ solution were investigated in detail. The results show that the as‐prepared Ni‐B powders all belong to amorphous alloy with variable element contents, and the Ni‐B sample prepared from EDTA complexation, possessing the best fineness and dispersity, has the strongest catalytic activity. The mean apparent activation energy of the hydrolysis reaction is 64.90 kJ · mol^–1. The NaBH₄ concentration has little impact on hydrogen generation rate, implying that the catalytic hydrolysis of NaBH₄ solution should be the pseudo zero‐order reaction. Keeping the NaOH content at below 5 % could inhibit the hydrolysis of NaBH₄ solution, but the NaOH contents from 10 % to 15 % will significantly promote the hydrolysis rate of NaBH₄. The hydrolysis reaction mechanisms, especially the effect of NaOH content on the hydrolysis reaction were also analyzed.     

Synthesis of aluminum oxides from the products of the rapid thermal decomposition of hydrargillite in a centrifugal flash reactor: III. Properties of aluminum hydroxides and oxides obtained via the mild rehydration of the products of the centrifugal thermal activation of hydrargillite

The interaction between the amorphous product of the centrifugal thermal activation of hydrargillite (CTA HG) and aqueous electrolytes (pH 5–11) under mild conditions (15–35°C, atmospheric pressure) has been investigated by a variety of physicochemical methods. This interaction causes material morphologic and phase changes in CTA HG, and the product composition is governed by the pH of the electrolyte and by the hydration temperature and time. The product that forms in a basic medium or water in <24 h contains up to 50% pseudoboehmite. Raising the pH or temperature or extending the hydration time results in the formation of bayerite as the major phase (～80%). An X-ray amorphous hydroxide forms in acid media. The heat treatment of this hydroxide at 550°C yields aluminum oxides differing from alumina prepared via hydroxide reprecipitation. Products with new, unusual properties can thus be obtained.     

Enhancement of the Sorption Ability of Aluminum Oxide Desiccants by Alkaline Modification

The influence of impregnation of aluminum oxide desiccants prepared by centrifugal thermal activation of hydrargillite with alkali (KОН and NaOH) and carbonate (Na₂CO₃ and K₂СО₃) solutions on the physicochemical properties of the products was studied. Impregnation with alkali solutions increases the dynamic capacity of the desiccants by a factor of 2 and more, whereas impregnation with carbonate solutions decreases the sorption characteristics of the desiccants at similar texture characteristics. Introduction of alkaline modifiers leads to a considerable decrease in the concentration of Lewis acid sites on the surface and to an increase in the concentration of strong base sites. Linear correlation was revealed between the concentration of strong base sites on the surface of the desiccants and their dynamic capacity in drying of humid air. The desiccants modified by impregnation exhibit not only high static and dynamic capacity, allowing improvement of the drying efficiency, but also considerably enhanced mechanical strength.     

State of the copper ions in the catalysts for oxidation of carbon monoxide to carbon dioxide

The state of the copper ions in the catalysts for the oxidation of carbon monoxide to carbon dioxide, prepared by dissolving an activated copper-containing aluminum alloy in water followed by calcination (method A) and by impregnation of the support produced by dissolving activated aluminum in water with copper nitrate solution (method B), was investigated by diffuse reflection electronic spectroscopy. It was established that the catalysts contain Cu ²⁺ ions stabilized in fields of octahedral symmetry. The concentration of these ions depends on the method of synthesis of the catalyst, its copper content, and the pretreatment temperature. It is higher in the samples produced by impregnation than in the samples produced by fusion; increase in the amount of copper leads to a decrease while increase in the calcination temperature leads to an increase in the concentration of the above-mentioned ions. Treatment of the oxide systems with the reaction mixture does not affect the state and concentration of Cu ²⁺. The catalytic activity of the samples depends on the method of preparation and increases with decrease in the amount of Cu ²⁺ (O_h) and with increase in the content of the CuO phase in the system.N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Moscow 117913. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 786–790, April, 1992.     

Effect of silica on the stability of the nanostructure and texture of fine-particle alumina

The effect of the modification of aluminum oxide with silicon oxide on the stability of fine-particle Γ- and δ-Al₂O₃ phases upon heat treatment in the wide temperature range of 550–1500°C was studied. It was found that the Γ- and δ-Al₂O₃ phases modified with silica are thermally stable up to higher temperatures than pure aluminum oxide. This is due to changes in the real structure of the modified samples, specifically, an increase in the concentration of extensive defects stabilized by hydroxyl groups bound to not only aluminum atoms but also silicon atoms. It is likely that Si-OH groups, which are thermally more stable than Al-OH groups, stabilize the microstructure of Γ- and δ-Al₂O₃ to higher temperatures, as compared with aluminum oxide containing no additives. Simultaneously, an increase in the thermal stability of the modified samples is accompanied by the retention of a high specific surface area and a developed pore structure at higher treatment temperatures.     

Solid-state 13C-NMR studies of the aging of poly(vinylidene fluoride)/poly(methyl methacrylate) blends

Solid state ¹³C-NMR was used to investigate the miscibility and subsequent separation of solution-cast blends of poly(vinylidene fluoride) (PVF₂) and poly(methyl methacrylate) (PMMA) with aging for a range of compositions. It was found that one amorphous phase and intimate mixing of the polymer chains in this phase existed for all compositions of the blends, even after 2 months of aging at room temperature as determined by the proton spin lattice relaxation time T_1ρH in the rotating frame, and the time constant T_CH for transfer of magnetization. The T_1ρH is sensitive to the spatial homogeneity of the blend via spin diffusion and would indicate the presence of phases or domains in the amorphous component of the blend larger than approximately 19 Å. The T_CH is proportional to the inverse sixth power of the interatomic distances needed for transfer of magnetization from proton to carbon and would be sensitive to a separation of polymer chains in the amorphous phase with aging on the order of 4–5 Å. There was an increase of the T_1ρH and T_CH values with aging, indicating that a subtle separation between unlike chains in the amorphous phase was occurring although a single amorphous phase was present.     

Effect of the nature of anions of aluminum salts used to synthesize a precursor of the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-ZrO<Subscript>2</Subscript> ceramics on the stabilization of the tetragonal modification of zirconium dioxide

Samples of a precursor for an aluminum oxide ceramics reinforced with zirconium oxide were synthesized by hydrolysis of various aluminum salts in the presence of a ZrO₂ sol under conditions of urea decomposition at 90°C and pH < 4 maintained, with hydrolysis products deposited onto the surface of ZrO₂ sol particles. It was found that the nature of a salt anion affects the interaction of hydrolysis products of the aluminum cation with the surface of ZrO₂ sol particles. The structure of products formed in thermal treatment of samples of a precursor for Al₂O₃-ZrO₂ (T = 1250°C) was characterized by X-ray phase analysis and scanning electron microscopy. The phase transition temperatures of the oxides Al₂O₃ and ZrO₂ contained in the precursor were estimated using the results of thermal analysis of the samples in the temperature range 20–1300°C.     

The effect of aging and natural organic matter on the Th(OH)4 solubility

The solution chemistry of actinides, particularly hydrolysis, is of major importance in the design of nuclear waste repositories and in relation to nuclear fuel reprocessing cycles. In this study the formation and solubility of the Th(OH)₄ solid phase has been investigated as a function of the aging time and the presence of natural organic matter (e.g. humic acid) in 0.1 M NaClO₄, at weak acid pH and under normal atmospheric conditions. Th(OH)₄ has been prepared by alkaline precipitation and characterized by TGA, ATR–FTIR, XRD, and solubility measurements. According to the experimental data Th(OH)₄ is stable and remains the solubility limiting solid phase even in the presence of increased humic acid concentration in solution. Increasing humic acid concentration doesn’t affect the crystallite size and the solubility product of Th(OH)₄. Th(OH)₄ solubility is basically pH depended and governed by the presence of colloidal species. However, solid phase aging, which in absence of humic acid favors crystallinity, affects significantly the Th(OH)₄ solubility.     

Modification of a phase-inhomogeneous alumina support of a palladium catalyst. Part I: effect of the amorphous phase on the textural and acidic characteristics of alumina and methods for controlling its phase homogeneity

The contribution of the amorphous phase in aluminum hydroxide and alumina to their properties was investigated in this work. It was shown that the amorphous phase in aluminum hydroxide is stabilized by alumoxanes, and it is a source of a finely porous component and capable of increasing the surface area by ~78%. In γ-Al₂O₃, amorphous alumina raises the surface area and the acidity. It was established that the method of chemical modification does not change the phase inhomogeneity but allows adjusting the acid properties while maintaining the high specific surface area. It was shown that the amorphous phase is more reactive when processing phase-inhomogeneous aluminum hydroxide with acetic acid. Crystallization of amorphous alumina as a result of high-temperature treatment of phase-inhomogeneous aluminum hydroxide is accompanied by significant decrease in specific surface area and acidity.     

Thermal Stability of Pt/Al2O3 Catalysts Prepared by Sol-Gel

A series of Pt/Al₂O₃ catalysts were prepared using a sol-gel method. The influence of several parameters used in the synthesis including: metal content, identity of the metal precursor, and the water/alkoxide ratio on the structural properties of the fresh (dried) and calcined samples were studied. It was found that the BET surface area decreased with an increase in the platinum content. A surface area of 500 m²/g was obtained following calcination at 773 K. The structure of fresh samples as determined by FTIR corresponded to that of a pseudoboehmite structure. Samples prepared using a water/alkoxide ratio (H₂O/ATB) of 9 showed a well-defined, uniform pore size distribution following calcination at 773 K. Metal dispersions comparable to those obtained using impregnation methods were obtained. Aging studies (calcination at 873 K for 24 h) performed on these catalysts, exhibited sintering behavior which were similar to Pt/Al₂O₃ catalysts prepared by other methods. The sample prepared using a H₂O/ATB ratio of 9 had the highest surface area and was more thermally resistant towards metal sintering. A bimodal metal particle size distribution was observed: some particles exhibited sintering while others of similar size showed a greater thermal stability to sintering. The sample having the largest surface area and the highest thermal stability following thermal treatment was a consequence of a more condensed structure and a higher pore roughness obtained after drying the gel. This enabled the formation of an alumina structure which was more amorphous and limited aggregation of platinum particles due to surface diffusion within the pore structure.     

Study on forced hydrolysis reaction of acidic Fe2(SO4)3 solution—structure and properties of precipitates

The phase composition, morphology, and inner structure of the particles precipitated from a forced hydrolysis reaction of acidic Fe₂(SO₄)₃ solution were examined. The shape and size of precipitated particles strongly depended on the concentration of the reactants in the starting solution, particularly on that of Fe₂(SO₄)₃. The relatively lower concentration of Fe₂(SO₄)₃ produced acicular or rod-like multi-domainic goethite particles, while the higher one provided irregularly shaped hydronium jarosite particles. This difference strongly depended on the pH of the aging solution; acicular goethite particles were produced at pH≥1.6 though jarosite was produced at pH≤1.5. The large goethite and jarosite particles exhibited a high microporosity. The time resolved transmission electron microscope (TEM) observation technique revealed that the rate of hydrolysis reaction for producing goethite was fast and the particle growth was almost terminated after aging for 1 day. On the contrary, the system favoring precipitation of jarosite exhibited a phase transformation from cubic and hexagonal basic iron(III) sulfate, produced in the early stage of the reaction until 10 h, to large irregular jarosite particles.     

Specific features of phase transformations of amorphous aluminum hydroxide produced by ammoniation of potassium alum

A novel method for neutralization of aluminum salts was suggested for the example of treatment of crystalline potassium alum with gaseous ammonia. An assumption was made that the structure of the forming primary particles is specific. The process of formation of a pseudoboehmite structure was studied in relation to the aging conditions of the aluminum hydroxide precipitate.     

Preparation and Characterization of Niobium Oxide Coated Cellulose Fiber

The Nb₂O₅/cellulose composite was prepared by reacting α-cellulose with NbCl_5-n(OC₂H₅)_n, in nonaqueous solvent, under nitrogen atmosphere and submitting the obtained material to hydrolysis. An increase in the crystallinity degree is observed in the composite material because the precursor reagent reacts with the amorphous phase of the cellulose fibers. Loadings between 4.5 and 16.0% of the oxide were achieved and in every case the oxide particles uniformly cover the fiber surface. Lewis and Brønsted acid sites were determined by using pyridine as the basic molecular probe.