Structure and Stability of Aluminum Hydroxides: A Theoretical Study

Unveiling interfacial interaction between antimony oxyanions and boehmite nanorods: Spectroscopic evidence and density functional theory analysis

In natural environments, the fate and migratory behavior of metalloid contaminants such as antimony (Sb) significantly depend on the interfacial reactivity of mineral surfaces. Although boehmite (γ-AlOOH) is widely observed in (sub)surface environments, its underlying interaction mechanism with Sb oxyanions at the molecular scale remains unclear. Considering Sb-contaminated environmental conditions in this study, we prepared boehmite under weakly acidic conditions for use in the systematic investigation of interfacial interactions with Sb(III) and Sb(V). The as-synthesized boehmite showed a nanorod morphology and comprised four crystal facets in the following order: 48.4% (010), 27.1% (101), 15.0% (001), and 9.5% (100). The combined results of spectroscopic analyses and theoretical calculations revealed that Sb(III) formed hydrogen bonding outer-sphere complexation on the (100), (010), and (001) facets and that Sb(V) preferred to form bidentate inner-sphere complexation via mononuclear edge-sharing configuration on the (100), (001), and (101) facets and binuclear corner-sharing configuration on the (010) facet. These findings indicate that the facet-mediated thermodynamic stability of the surface complexation determines the interaction affinity toward the Sb species. This work is the first to document the contribution of boehmite to (sub)surface media, improving the ability to forecast the fate and behavior of Sb oxyanions at mineral-water interfaces.

Dynamic Behavior of Platinum Atoms and Clusters in the Native Oxide Layer of Aluminum Nanocrystals

Strong metal-support interactions (SMSIs) are well-known in the field of heterogeneous catalysis to induce the encapsulation of platinum (Pt) group metals by oxide supports through high temperature H2 reduction. However, demonstrations of SMSI overlayers have largely been limited to reducible oxides, such as TiO2 and Nb2O5. Here, we show that the amorphous native surface oxide of plasmonic aluminum nanocrystals (AlNCs) exhibits SMSI-induced encapsulation of Pt following reduction in H2 in a Pt structure dependent manner. Reductive treatment in H2 at 300 °C induces the formation of an AlOx SMSI overlayer on Pt clusters, leaving Pt single-atom sites (Ptiso) exposed available for catalysis. The remaining exposed Ptiso species possess a more uniform local coordination environment than has been observed on other forms of Al2O3, suggesting that the AlOx native oxide of AlNCs presents well-defined anchoring sites for individual Pt atoms. This observation extends our understanding of SMSIs by providing evidence that H2-induced encapsulation can occur for a wider variety of materials and should stimulate expanded studies of this effect to include nonreducible oxides with oxygen defects and the presence of disorder. It also suggests that the single-atom sites created in this manner, when combined with the plasmonic properties of the Al nanocrystal core, may allow for site-specific single-atom plasmonic photocatalysis, providing dynamic control over the light-driven reactivity in these systems.

A Facile Temperature-Controlled "Green" Method to Prepare Multi-kinds of High-Quality Alumina Hydrates via a Ga-In-Sn-Alloyed Aluminum-Water Interface Reaction

The Al sheets alloyed by Ga-In-Sn are generally utilized to react with water for H₂ production, while the valuable byproducts, i.e., alumina hydrates, have not been fully studied. In this work, through controlling the reaction temperature, three types of alumina hydrates, bayerite (40 °C), pseudo-boehmite (PB) (70-120 °C), and boehmite (130-160 °C), were successfully prepared based on a series of interface reactions and structural transformations. These alumina hydrates and their calcined products (alumina) possess high purity with a total impurity element content of <450 ppm, especially an extremely low sodium content (<21 ppm) and iron content (<52 ppm). Significantly, the obtained pseudo-boehmite displays excellent surface properties (specific surface area: 332.7 m² g^-1, pore volume: 0.3 cm³ g^-1, and pore diameter: 3.6 nm), competitive to the current commercial SB powder by Sasol. This work not only deepens the understanding of the byproducts in a Ga-In-Sn-alloyed Al-water reaction but also establishes a facile "green" method oriented to industrial applications, which is promising for the linkage benefits of the hydrogen production industry.

Surface and Bulk Chemistry of Mechanochemically Synthesized Tohdite Nanoparticles

Aluminum oxides, oxyhydroxides, and hydroxides are important in different fields of application due to their many attractive properties. However, among these materials, tohdite (5Al₂O₃·H₂O) is probably the least known because of the harsh conditions required for its synthesis. Herein, we report a straightforward methodology to synthesize tohdite nanopowders (particle diameter ∼13 nm, specific surface area ∼102 m² g^-1) via the mechanochemically induced dehydration of boehmite (γ-AlOOH). High tohdite content (about 80%) is achieved upon mild ball milling (400 rpm for 48 h in a planetary ball mill) without process control agents. The addition of AlF₃ can promote the crystallization of tohdite by preventing the formation of the most stable α-Al₂O₃, resulting in the formation of almost phase-pure tohdite. The availability of easily accessible tohdite samples allowed comprehensive characterization by powder X-ray diffraction, total scattering analysis, solid-state NMR (¹H and ²⁷Al), N₂-sorption, electron microscopy, and simultaneous thermal analysis (TG-DSC). Thermal stability evaluation of the samples combined with structural characterization evidenced a low-temperature transformation sequence: 5Al₂O₃·H₂O → κ-Al₂O₃ → α-Al₂O₃. Surface characterization via DRIFTS, ATR-FTIR, D/H exchange experiments, pyridine-FTIR, and NH₃-TPD provided further insights into the material properties.

A Novel Method for Generating H2 by Activation of the μAl-Water System Using Aluminum Nanoparticles

A method is described for activation of the reaction of room temperature water with micron-scale aluminum particles (μAl) by the addition of poly(epoxyhexane)-capped aluminum nanoparticles (Al NPs). By themselves, Al NPs react vigorously and completely with water at ambient temperatures to produce H2. While pure μAl particles are unreactive toward water, mixtures of the μAl particles comprising 10 to 90% (by mass) of Al NPs, demonstrated appreciable hydrolytic activation. This activation is attributed to the reaction of the Al NPs present with water to produce a basic solution. Speciation modelling, pH studies, and powder X-ray diffraction analysis of the hydrolysis product confirm that the pH change is the key driver for the activation of μAl rather than residual heat from the exothermicity of Al NP hydrolysis. A mechanism is proposed by which the nonreactive aluminum oxide layer of the μAl is eroded under basic conditions. Mixtures 10% by mass of Al NPs can be used to produce the optimal quantity of H2.

Microwave Dewatering of Gibbsite-Type Bauxite Ores: Permittivities, Heating Behavior and Strength Indices

Microwave radiation is a relatively new energy source that is being considered for several applications in mineral processing and extractive metallurgy. In the present research, various gibbsite-type bauxite ores were subjected to microwave radiation. The main objective was to assess the effect of microwave dewatering on the compressive strength indices of the ores and to compare the results obtained to those for conventional heating. Firstly, the fundamental interactions of the microwaves with the ores were evaluated by determining both the real and the imaginary permittivities as a function of temperature, and these were related to the water content. Secondly, the microwave heating behavior was modeled using a 24 factorial statistical analysis. Thirdly, the effect of dewatering by microwave heating on the compressive strength indices of roughly spherical bauxite ore pisoids was studied, and these results were compared to those obtained using conventional heating. Fourthly, the effect of particle size on the compressive strengths of irregular-shaped single particles of bauxite ore was investigated using both heating techniques. Finally, the energy requirements for dewatering of the ores, and hence reducing their compressive strengths, were compared for both processes. On the laboratory scale, the results showed that in comparison to conventional dewatering, microwave dewatering resulted in lower strength indices at both lower moisture removals and energy inputs.

Comparative Study on Electrical Conductivity of CeO2-Doped AlN Ceramics Sintered by Hot-Pressing and Spark Plasma Sintering

Aluminum nitride (AlN) ceramics were prepared by both Hot-pressing (HP) and Spark-Plasma-Sintering (SPS) using cerium oxide as the sintering aid. The characterization of AlN raw powder denoted the presence of an amorphous layer that led to the formation of aluminum oxide. During the sintering process, CeO₂ introduced as a sintering aid was reduced into Ce₂O₃. The latter reacted with aluminum oxide to form a transient liquid phase that promotes sintering by both HP and SPS. A reactional path leading to the formation of secondary phases, such as CeAlO₃ and CeAl₁₁O₁₈, has been proposed according to the pseudo-binary Al₂O₃ – Ce₂O₃. Ceramics obtained from HP and SPS are presented as similar, except for the secondary-phase distribution. The influences of secondary phase composition and distribution on electrical conductivity were evaluated by leakage current measurements. The mechanism of DC conduction and the global conductivity of ceramics were discussed according to the sintering process and the number of secondary phases.

The Formation of Cr-Al Spinel under a Reductive Atmosphere

In the present work, for the first time, the possibility of formation of CrAl₂O₄ was shown from the equimolar mixture of co-precipitated Al₂O₃ and Cr₂O₃ oxides under a reductive environment. The crystallographic properties of the formed compound were calculated using the DICVOL procedure. It was determined that it has a cubic crystal structure with space group Fd-3m and a unit cell parameter equal to 8.22(3) Å. The formed CrAl₂O₄ is not stable under ambient conditions and easily undergoes oxidation to α-Al₂O₃ and α-Cr₂O₃. The overall sequence of the phase transformations of co-precipitated oxides leading to the formation of spinel structure is proposed.

Research on High-Pressure Hydrochloric Acid Leaching of Scandium, Aluminum and Other Valuable Components from the Non-Magnetic Tailings Obtained from Red Mud after Iron Removal

Red mud is a hazardous waste of the alumina industry that contains high amounts of iron, aluminum, titanium and rare-earth elements (REEs). One of the promising methods for the extraction of iron from red mud is carbothermic reduction with the addition of sodium salts. This research focuses on the process of hydrochloric high-pressure acid leaching using 10 to 20% HCl of two samples of non-magnetic tailings obtained by 60 min carbothermic roasting of red mud at 1300 °C and the mixture of 84.6 wt.% of red mud and 15.4 wt.% Na2SO4 at 1150 °C, respectively, with subsequent magnetic separation of metallic iron. The influence of temperature, leaching duration, solid-to-liquid-ratio and acid concentration on the dissolution behavior of Al, Ti, Mg, Ca, Si, Fe, Na, La, Ce, Pr, Nd, Sc, Zr was studied. Based on the investigation of the obtained residues, a mechanism for passing valuable elements into the solution was proposed. It has shown that 90% Al, 91% Sc and above 80% of other REEs can be dissolved under optimal conditions; Ti can be extracted into the solution or the residue depending on the leaching temperature and acid concentration. Based on the research results, novel flowsheets for red mud treatment were developed.

The Sealing Step in Aluminum Anodizing: A Focus on Sustainable Strategies for Enhancing Both Energy Efficiency and Corrosion Resistance

Increasing demands for environmental accountability and energy efficiency in industrial practice necessitates significant modification(s) of existing technologies and development of new ones to meet the stringent sustainability demands of the future. Generally, development of required new technologies and appropriate modifications of existing ones need to be premised on in-depth appreciation of existing technologies, their limitations, and desired ideal products or processes. In the light of these, published literature mostly in the past 30 years on the sealing process; the second highest energy consuming step in aluminum anodization and a step with significant environmental impacts has been critical reviewed in this systematic review. Emphasis have been placed on the need to reduce both the energy input in the anodization process and environmental implications. The implications of the nano-porous structure of the anodic oxide on mass transport and chemical reactivity of relevant species during the sealing process is highlighted with a focus on exploiting these peculiarities, in improving the quality of sealed products. In addition, perspective is provided on plausible approaches and important factors to be considered in developing sealing procedures that can minimize the energy input and environmental impact of the sealing step, and ensure a more sustainable aluminum anodization process/industry.

Hybrid Phosphate-Alumina Iron-Based Core-Shell Soft Magnetic Composites Fabricated by Sol-Gel Method and Ball Milling Method

Novel Fe-based soft magnetic composites (SMCs) with hybrid phosphate-alumina layers were prepared by both sol–gel and ball-milling methods. The effects of the fabrication methods and the addition of Al2O3 particles on the microstructure and the soft magnetic performance of SMCs were studied. The formation of the hybrid phosphate-Al2O3 shell not only leads to the decrease of the total core loss, but also results in the reduction of the permeability and saturation magnetization. However, the degree of decrease caused by the different methods were not identical. The sample with 8% Al2O3 prepared by the sol–gel method showed the best magnetic performance, exhibiting a high-amplitude permeability (μa) of 85.14 and a low total core loss (Ps) of 202.3 W/kg at 50 mT and 100 kHz. The hysteresis loss factor and the eddy current loss factor were obtained by loss separation. The results showed that the samples with the same Al2O3 content prepared by different methods exhibited almost the same total core loss. However, the contribution of the hysteresis loss and the eddy current loss showed an obvious difference in behavior because of the change of the particle shapes and the refinement of the particle size during the ball-milling process.

High-surface-area corundum by mechanochemically induced phase transformation of boehmite

In its nanoparticulate form, corundum (α-Al2O3) could lead to several applications. However, its production into nanoparticles (NPs) is greatly hampered by the high activation energy barrier for its formation from cubic close-packed oxides and the sporadic nature of its nucleation. We report a simple synthesis of nanometer-sized α-Al2O3 (particle diameter ~13 nm, surface areas ~140 m2 g−1) by the mechanochemical dehydration of boehmite (γ-AlOOH) at room temperature. This transformation is accompanied by severe microstructural rearrangements and might involve the formation of rare mineral phases, diaspore and tohdite, as intermediates. Thermodynamic calculations indicate that this transformation is driven by the shift in stability from boehmite to α-Al2O3 caused by milling impacts on the surface energy. Structural water in boehmite plays a crucial role in generating and stabilizing α-Al2O3 NPs.

A porous geopolymer based on aluminum-waste with acoustic properties

Paval, a solid waste stream from the aluminum industry, is used as a pore generation agent in geopolymers. Paval was mixed with coal combustion fly ash, as a geopolymeric precursor, and activated with alkaline solution with the aim of obtaining porous geopolymers to be used as noise barriers. Both geopolymeric and pore generation reactions happen simultaneously. Aluminum from Paval can react with water and OH¯ from the geopolymerization activating solution, producing hydrogen. The hydrogen gas released generates a highly porous material. The influence of the fly ash-paval proportion and the setting temperature on open porosity, compressive strength and noise-absorbing properties were evaluated. To better understand these influences, the setting time, volume expansion and mineral composition were also studied. The obtained results showed that a higher Paval content (fly ash-Paval ratio 50:50) and setting temperature (70 °C) produced a lower setting time and higher volume expansion, increasing the open porosity and improving acoustic properties, but reducing the compressive strength. The material manufactured under these conditions showed similar amorphous phase content to the non-porous geopolymers made without Paval. On the other hand, the obtained materials did not raise environmental concerns in a normalised leaching test.

Spectrophotometrical analysis for fabrication of pH-independent nano-sized γ-alumina by dealumination of kaolin and precipitation in the presence of surfactant composites

The challenge with γ-Al₂O₃ is its pH-dependent removal efficiency and relatively low adsorption capacity in recovery of wastewaters contaminated by cationic dyes. Therefore; the objective of present investigation was to fabricate a pH-independent nano-sized alumina powder by dealumination of kaolin for uptake of dye. The dealumination of meta-kaolin was carried out by nitric acid solution and the amorphous aluminum hydroxide was precipitated with ammonia in the presence of combined surfactants containing cetyltrimethylammonium bromide (CTAB), polyethylene glycol (PEG) and polyoxyethylene octyl phenyl ether (TX100). The nano-sized gamma alumina particles were fabricated after calcination at 800 °C and then were analyzed based on spectrophotometrical method. The rod-like nano-particles with average width of 5 nm and length of 20 nm were synthesized by precipitation in the presence of admixed surfactant, CTAB-TX100, which improves the removal efficiency due to efficient modification in the interaction of particles. TX100 also showed a significant affinity for increasing the specific surface area because of ability to prevent the particle aggregation which provides a potential for synthesis of alumina adsorbent without foaming, normally observed in the presence of CTAB. Both of mentioned powders indicated relatively pH-independent removal efficiency with higher adsorption capacity. Moreover, adsorption isotherm parameters were determined to verify the improvement of removal efficiency. The presented fabrication technique eventually overcame the pH-dependent recovery problem which is feasible for the development of adsorbents because the starting material is abundant, very low energy is needed, no hazardous waste is generated and applicable in the acidic environments.

Effect of Al Content in the Mg-Based Alloys on the Composition and Corrosion Resistance of Composite Hydroxide Films Formed by Steam Coating

Mg alloys are expected to be used in fields of the transportation industry because of their lightweight property, however, they show low corrosion resistance. To improve the corrosion resistance, preparation of the protective film on Mg alloys is essential. In this study, composite hydroxide films were prepared on three types of Mg alloys with different aluminum contents—that is, AZ31, AZ61, and AZ91D—by steam coating to investigate the relationship between the Mg-Al layered double hydroxide (LDH) content in the film and the Al content in the Mg alloys. Scanning electron microscopy (SEM) observation demonstrated that films were formed densely on all Mg alloy surfaces. X-ray diffraction (XRD) analyses revealed that all films prepared on AZ61 and AZ91D were composed of Mg(OH)₂, AlOOH, and Mg-Al LDH, while the film containing Mg(OH)₂ and Mg-Al LDH were formed only on AZ31. The Mg-Al LDH content in the film prepared on AZ61 was relatively higher than those prepared on AZ31 and AZ91D. The content of AlOOH in the film increased with an increase in the Al content in the Mg alloys. The film thickness changed depending on the treatment time and type of Mg alloy. Polarization curve measurements in 5 mass% NaCl solution demonstrated that the film prepared on the AZ61 showed complete passive behavior within the potential range of −1.0 to −0.64 V. In addition, immersion tests in 5 mass% NaCl aqueous solution for 480 h demonstrated that the film on the AZ61 had superior durability against 5 mass% NaCl aqueous solution. These results indicated that the film on the AZ61 had the most superior corrosion resistance among all samples. The results obtained in this study suggest that the LDH content in the film could be related to the corrosion resistance of the film.

Alumina: discriminative analysis using 3D correlation of solid-state NMR parameters

Synthetic transition aluminas (χ, κ, θ, γ, δ, η, ρ) exhibit unique adsorptive and catalytic properties leading to numerous practical applications. Generated by thermal transformation of aluminium (oxy)hydroxides, structural differences between them arise from the variability of aluminium coordination numbers and degree of dehydroxylation. Unequivocal identification of these phases using X-ray diffraction has proven to be very difficult. Quadrupolar interactions of 27Al nuclei, highly sensitive to each site symmetry, render advanced 27Al solid-state NMR a unique spectroscopic tool to fingerprint and identify the different phases. In this paper, 27Al NMR spectroscopic data on alumina reported in literature are collected in a comprehensive library. Based on this dataset, a new 3D correlative method of NMR parameters is presented, enabling fingerprinting and identification of such phases. Providing a gold standard from crystalline samples, this approach demonstrates that any sort of crystalline, ill crystallized or amorphous, mixed periodic or aperiodically ordered transition alumina can now be assessed beyond the current limitations of characterisation. Adopting the presented approach as a standard characterisation of alumina samples will readily reveal NMR parameter-structure-property relations suitable to develop new or improved applications of alumina. Methodological guidance is provided to assist consistent implementation of this characterisation throughout the fields involved.

Origin of Nb2 O5 Lewis Acid Catalysis for Activation of Carboxylic Acids in the Presence of a Hard Base

The Nb2 O5 surface catalyzes the amidation of carboxylic acids with amines through Nb5+ Lewis acid activation of the C=O group. In this work, DFT calculations were applied to theoretically investigate the C=O bond activation of a model carboxylic acid (acetic acid) on θ-Al2 O3 (110), anatase TiO2 (101), and T-Nb2 O5 (100) surfaces. The adsorption sites, adsorption energies, reaction energy barriers, electronic properties, and vibrational frequency of acetic acid were examined in detail. It was found that the bond activation of the carbonyl group is most efficient on Nb2 O5 , although the adsorption energy is larger on Al2 O3 and TiO2 . The most efficient C=O bond activation on Nb2 O5 results in the lowest energy barrier of C-N bond formation during amidation. The Nb2 O5 surface also shows larger tolerance to methylamine and water molecules than Al2 O3 and TiO2 surfaces. These crucial factors contribute to the highest amidation catalytic reactivity on Nb2 O5 . Furthermore, the position of the mean density of states of the d-conduction band of the active metal site relative to the Fermi energy level correlates well with the efficiency in the C=O bond activation and, consequently, the catalytic activity for amidation. These results suggest that, unlike a classical understanding of strong acid sites of metal oxide surfaces, interaction of a carbonyl HOMO with an unoccupied metal d-orbital, or, in other words, covalent-like interaction between a carbonyl group and metal adsorption site, is relevant to the present system.

Adsorption of aliphatic polyhydroxy carboxylic acids on gibbsite: pH dependency and importance of adsorbate structure

BACKGROUND

Aliphatic (poly)hydroxy carboxylic acids [(P)HCA] occur in natural, e.g. soils, and in technical (waste disposal sites, nuclear waste repositories) compartments . Their distribution, mobility and chemical reactivity, e.g. complex formation with metal ions and radionuclides, depend, among others, on their adsorption onto mineral surfaces. Aluminium hydroxides, e.g. gibbsite [α-Al(OH)₃], are common constituents of related solid materials and mimic the molecular surface properties of clay minerals. Thus, the study was pursued to characterize the adsorption of glycolic, threonic, tartaric, gluconic, and glucaric acids onto gibbsite over a wide pH and (P)HCA concentration range. To consider specific conditions occurring in radioactive wastes, adsorption applying an artificial cement pore water (pH 13.3) as solution phase was investigated additionally.

RESULTS

The sorption of gluconic acid at pH 4, 7, 9, and 12 was best described by the "two-site" Langmuir isotherm, combining "high affinity" sorption sites (adsorption affinity constants [Formula: see text] > 1 L mmol^-1, adsorption capacities < 6.5 mmol kg^-1) with "low affinity" sites ([Formula: see text] < 0.1 L mmol^-1, adsorption capacities ≥ 19 mmol kg^-1). The total adsorption capacities at pH 9 and 12 were roughly tenfold of that at pH 4 and 7. The S-shaped pH sorption edge of gluconic acid was modelled applying a constant capacitance model, considering electrostatic interactions, hydrogen bonding, surface complex formation, and formation of solved polynuclear complexes between Al³⁺ ions and gluconic acid. A Pearson and Spearman rank correlation between (P)HCA molecular properties and adsorption parameters revealed the high importance of the size and the charge of the adsorbates.

CONCLUSIONS

The adsorption behaviour of (P)HCAs is best described by a combination of adsorption properties of carboxylic acids at acidic pH and of polyols at alkaline pH. Depending on the molecular properties of the adsorbates and on pH, electrostatic interactions, hydrogen bonding, and ternary surface complexation contribute in varying degrees to the adsorption process. Linear distribution coefficients K_d between 8.7 and 60.5 L kg^-1 (1 mmol L^-1 initial PHCA concentration) indicate a considerable mineral surface affinity at very high pH, thus lowering the PHCA fraction available for the complexation of metal ions including radionuclides in solution and their subsequent mobilization.

High flux water purification using aluminium hydroxide hydrate gels

Filtration of aqueous liquids has wide implications, for example for provision of clean drinking water. Nevertheless, many people still lack access to safe water and suffer from preventable water-borne microbial diseases. This study reports a new ultrafiltration-range separation technology using a gelatinous layer of aluminium hydroxide polyhydrate as a secondary membrane on a retaining fabric that enables simple and cost-effective production of filtered water. Properties include at least 4-fold higher flux rates than currently available membranes, pressure-resistance, impenetrability to filtered particles, easy cleaning by backwashing and simple, cost-effective replacement by gel injection. Depending on the substrate, filtration is achieved through a packed bed of 1–2 nm hydrate gel globules, partly by mechanical straining with a size exclusion of approx. 10 nm and partly by physical adsorption. As a result, filtration of water (e.g. turbid river water) contaminated with colloids and microorganisms, including viruses, yields clear water that is free of measurable particles or detectable microorganisms. However, small water-soluble molecules (salts, sugars, proteins) remain in the filtrate. The findings demonstrate the potential for wide applicability of hydrate gels in high-flux and low-cost water purification devices.

Importance of interlayer H bonding structure to the stability of layered minerals

Layered (oxy) hydroxide minerals often possess out-of-plane hydrogen atoms that form hydrogen bonding networks which stabilize the layered structure. However, less is known about how the ordering of these bonds affects the structural stability and solubility of these minerals. Here, we report a new strategy that uses the focused electron beam to probe the effect of differences in hydrogen bonding networks on mineral solubility. In this regard, the dissolution behavior of boehmite (γ-AlOOH) and gibbsite (γ-Al(OH)3) were compared and contrasted in real time via liquid cell electron microscopy. Under identical such conditions, 2D-nanosheets of boehmite (γ-AlOOH) exfoliated from the bulk and then rapidly dissolved, whereas gibbsite was stable. Further, substitution of only 1% Fe(III) for Al(III) in the structure of boehmite inhibited delamination and dissolution. Factors such as pH, radiolytic species, and knock on damage were systematically studied and eliminated as proximal causes for boehmite dissolution. Instead, the creation of electron/hole pairs was considered to be the mechanism that drove dissolution. The widely disparate behaviors of boehmite, gibbsite, and Fe-doped boehmite are discussed in the context of differences in the OH bond strengths, hydrogen bonding networks, and the presence or absence of electron/hole recombination centers.

Chemically Stable Atomic-Layer-Deposited Al2O3 Films for Processability

Atomic-layer-deposited alumina (ALD Al₂O₃) can be utilized for passivation, structural, and functional purposes in electronics. In all cases, the deposited film is usually expected to maintain chemical stability over the lifetime of the device or during processing. However, as-deposited ALD Al₂O₃ is typically amorphous with poor resistance to chemical attack by aggressive solutions employed in electronics manufacturing. Therefore, such films may not be suitable for further processing as solvent treatments could weaken the protective barrier properties of the film or dissolved material could contaminate the solvent baths, which can cause cross-contamination of a production line used to manufacture different products. On the contrary, heat-treated, crystalline ALD Al₂O₃ has shown resistance to deterioration in solutions, such as standard clean (SC) 1 and 2. In this study, ALD Al₂O₃ was deposited from four different precursor combinations and subsequently annealed either at 600, 800, or 1000 °C for 1 h. Crystalline Al₂O₃ was achieved after the 800 and 1000 °C heat treatments. The crystalline films showed apparent stability in SC-1 and HF solutions. However, ellipsometry and electron microscopy showed that a prolonged exposure (60 min) to SC-1 and HF had induced a decrease in the refractive index and nanocracks in the films annealed at 800 °C. The degradation mechanism of the unstable crystalline film and the microstructure of the film, fully stable in SC-1 and with minor reaction with HF, were studied with transmission electron microscopy. Although both crystallized films had the same alumina transition phase, the film annealed at 800 °C in N₂, with a less developed microstructure such as embedded amorphous regions and an uneven interfacial reaction layer, deteriorates at the amorphous regions and at the substrate-film interface. On the contrary, the stable film annealed at 1000 °C in N₂ had considerably less embedded amorphous regions and a uniform Al-O-Si interfacial layer.

The Effect of Novel Synthetic Methods and Parameters Control on Morphology of Nano-alumina Particles

Alumina is an inorganic material, which is widely used in ceramics, catalysts, catalyst supports, ion exchange and other fields. The micromorphology of alumina determines its application in high tech and value-added industry and its development prospects. This paper gives an overview of the liquid phase synthetic method of alumina preparation, combined with the mechanism of its action. The present work focuses on the effects of various factors such as concentration, temperature, pH, additives, reaction system and methods of calcination on the morphology of alumina during its preparation.

Comparative Study of the Oxidation of NiAl(100) by Molecular Oxygen and Water Vapor Using Ambient-Pressure X-ray Photoelectron Spectroscopy

The oxidation behavior of NiAl(100) by molecular oxygen and water vapor under a near-ambient pressure of 0.2 Torr is monitored using ambient-pressure X-ray photoelectron spectroscopy. O₂ exposure leads to the selective oxidation of Al at temperatures ranging from 40 to 500 °C. By contrast, H₂O exposure results in the selective oxidation of Al at 40 and 200 °C, and increasing the oxidation temperature above 300 °C leads to simultaneous formation of both Al and Ni oxides. These results demonstrate that the O₂ oxidation forms a nearly stoichiometric Al₂O₃ structure that provides improved protection to the metallic substrate by barring the outward diffusion of metals. By contrast, the H₂O oxidation results in the formation of a defective oxide layer that allows outward diffusion of Ni at elevated temperatures for simultaneous NiO formation.

H₂O Dissociation-Induced Aluminum Oxide Growth on Oxidized Al(111) Surfaces

The interaction of water vapor with amorphous aluminum oxide films on Al(111) is studied using X-ray photoelectron spectroscopy to elucidate the passivation mechanism of the oxidized Al(111) surfaces. Exposure of the aluminum oxide film to water vapor results in self-limiting Al2O3/Al(OH)3 bilayer film growth via counter-diffusion of both ions, Al outward and OH inward, where a thinner starting aluminum oxide film is more reactive toward H2O dissociation-induced oxide growth because of the thickness-dependent ionic transport in the aluminum oxide film. The aluminum oxide film exhibits reactivity toward H2O dissociation in both low-vapor pressure [p(H2O) = 1 × 10(-6) Torr] and intermediate-vapor pressure [p(H2O) = 5 Torr] regimes. Compared to the oxide film growth by exposure to a p(H2O) of 1 × 10(-6) Torr, the exposure to a p(H2O) of 5 Torr results in the formation of a more open structure of the inner Al(OH)3 layer and a more compact outer Al2O3 layer, demonstrating the vapor-pressure-dependent atomic structure in the passivating layer.

Phosphate alteration of chloride behavior at the boehmite-water interface: New insights from ion-probe flow adsorption microcalorimetry

Surface complexation of phosphate to aluminum oxyhydroxides can alter surface reactivity depending on the time-scale and mode of attachment. The effects of phosphate adsorption on reactivity of boehmite (γ-AlOOH) particles were investigated using ion-probe flow adsorption microcalorimetry (ipFAMC). Consistent with previous studies on adsorption energetics, probing the surface of pristine γ-AlOOH with chloride ions yielded endothermically unimodal temperature signals with a measured molar heat of exchange (ΔH(exc)) of -3.1 kJ/mol. However, when the surface of γ-AlOOH was probed with chloride following phosphate complexation, significant changes in surface reactivity resulted. Irrespective of phosphate loading, the typical endothermic response of the chloride-surface hydroxyl interaction was replaced with a multi-modal energy signature consisting of exothermic and endothermic features. These features indicate that in the presence of phosphate, the overall nature of the interaction of chloride with specific surface hydroxyls located on different exposed planes and their subsequent reactivity was transformed to a more complex environment accompanied by two or more short-lived secondary reactions. It was also shown that phosphate-promoted surface alteration of γ-AlOOH was highly selective to probing with chloride since no changes in reactivity were observed when nitrate was employed as the primary ion probe under identical experimental conditions.

Tuning the size and shape of nano-boehmites by a free-additive hydrothermal method

Nano-boehmites with a range of controlled sizes and morphologies exhibiting high dispersability in water were synthesized through a free-additive hydrothermal methodology.

Effect of Isomorphous Substitution on the Thermal Decomposition Mechanism of Hydrotalcites

Hydrotalcites have many important applications in catalysis, wastewater treatment, gene delivery and polymer stabilization, all depending on preparation history and treatment scenarios. In catalysis and polymer stabilization, thermal decomposition is of great importance. Hydrotalcites form easily with atmospheric carbon dioxide and often interfere with the study of other anion containing systems, particularly if formed at room temperature. The dehydroxylation and decomposition of carbonate occurs simultaneously, making it difficult to distinguish the dehydroxylation mechanisms directly. To date, the majority of work on understanding the decomposition mechanism has utilized hydrotalcite precipitated at room temperature. In this study, evolved gas analysis combined with thermal analysis has been used to show that CO₂ contamination is problematic in materials being formed at RT that are poorly crystalline. This has led to some dispute as to the nature of the dehydroxylation mechanism. In this paper, data for the thermal decomposition of the chloride form of hydrotalcite are reported. In addition, carbonate-free hydrotalcites have been synthesized with different charge densities and at different growth temperatures. This combination of parameters has allowed a better understanding of the mechanism of dehydroxylation and the role that isomorphous substitution plays in these mechanisms to be delineated. In addition, the effect of anion type on thermal stability is also reported. A stepwise dehydroxylation model is proposed that is mediated by the level of aluminum substitution.