γ-AlOOH Nanomaterials with Regular Shapes: Hydrothermal Fabrication and Cr2O72-Adsorption

Insulating and Robust Ceramic Nanorod Aerogels with High-Temperature Resistance over 1400 °C

Ceramic aerogels, which present a unique combination of low thermal conductivity and excellent high-temperature stability, are attractive for thermal insulation under extreme conditions. However, most ceramic aerogels are constructed by oxide ceramic nanoparticles and thus are usually plagued by their brittleness and structural collapse at elevated temperatures (less than 1000 °C). Despite great progress achieved in this regard recently, it still remains a big challenge to design and fabricate intriguing ceramic aerogels with enhanced mechanical strength and remarkable thermal stability at ultrahigh temperature up to 1400 °C. To this end, we herein report a facile and scalable strategy to manufacture ceramic nanorod aerogels (CNRAs) with hierarchically macroporous and mesoporous structures by the controllable assembly of Al₂O₃ nanorods and SiO₂ nanoparticles. Subsequently, the high-temperature annealing treatment of CNRAs significantly maximizes mechanical strength and promotes thermal tolerance. The obtained CNRAs demonstrate the integrated properties of super-strong heat resistance (up to 1400 °C), low thermal conductivity (0.026 W/m·K at 25 °C and 0.089 W/m·K at 1200 °C), high mechanical robustness (compressive strength 1.5 MPa), and low density (0.146 g/cm³). We envision that this novel nanorod-assembled ceramic aerogels offer considerable advantages than most of the state-of-the-art ceramic aerogels for thermal superinsulation upon exposure to extremely harsh environments.

Simplified waste-free process for synthesis of nanoporous compact alumina under technologically advantageous conditions

Precipitated ammonium aluminium carbonate hydroxide (NH4Al(OH)2CO3) is a promising precursor for preparation of nanostructured Al2O3. However, the experimental conditions, such as the low concentration of Al3+ salt solution, high temperature and/or pressure, long reaction time, and excessive amount of the (NH4)2CO3 precipitating agent, make this process expensive for large-scale production. Here, we report a simpler and cheaper route to prepare nanostructured alumina by partial neutralisation of a nearly saturated aqueous solution of Al(NO3)3 with (NH4)2CO3 as a base at pH < 4. Synthesis in the acidic region led to formation of a polynuclear aluminium cluster (Al13), which is an important "green" solution precursor for large-area preparation of Al2O3 thin films and nanoparticles. Control of the textural properties of the final alumina product during calcination of the prepared aluminium (oxy)hydroxide gel was accomplished by adding low-solubility aluminium acetate hydroxide (Al(OH)(CH3COO)2) as a seed to the Al(NO3)3 solution before neutralisation. The large Brunauer-Emmett-Teller specific surface area (376 m2 g-1) and narrow pore size distribution (2-20 nm) of the prepared compact alumina suggest that the chelating effect of the acetate ions affects the structures of the forming transition aluminas, and the evolved gases produced by decomposition of Al(OH)(CH3COO)2 and NH4NO3 as a by-product of the reaction during calcination prevent particle agglomeration. Other advantages of the proposed process are its versatility and the ability to obtain high purity materials without producing large amounts of by-products without the need for washing and energy saving by using a low processing temperature, and the possibility of recycling the generated CO2 and NH3 gases as the (NH4)2CO3 reagent.

Adsorption of As(V) by boehmite and alumina of different morphologies prepared under hydrothermal conditions

Morphology-controlled materials at the micro- and nanoscale levels are of great significance to the design and application of materials. Stable and well-dispersed boehmite and alumina with different morphologies were fabricated under hydrothermal conditions. The nitrate, chloride, and sulfate aluminum salts yielded nanoplate, microspindle, and microsphere morphologies, respectively. Calcination of the prepared boehmite samples yielded alumina samples with retention of the morphologies. In comparisons of samples with identical morphologies, alumina exhibited better uptake of As(V) than boehmite; the As(V) concentration was analyzed by the standard molybdenum blue method. The adsorption capabilities of the morphologically controlled materials are ranked microspindle > microsphere > nanoplate. The impacts of process parameters, such as reaction time; initial As(V) concentration; solution pH; competing ions (Ca²⁺, Mg²⁺, NO₃^-, PO₄^3-), which are common in most aquatic ecosystems; and co-contaminants (Cr(VI), Pb(II)), on removal efficiencies were examined. A well-defined mesostructure, superior surface area, chemical and electrostatic interaction, and surface charge distribution over the aluminol surface sites could be factors in the uptake of As(V). The design and synthesis of functional hierarchical micro- and nanostructured materials with the desired adsorptive properties, which are suitable for water treatment applications, can be achieved through environmentally benign hydrothermal fabrication.

γ-Al2O3 nanorods with tuneable dimensions – a mechanistic understanding of their hydrothermal synthesis

This paper reports mechanistic understanding of the hydrothermal synthesis of alumina (γ-Al₂O₃) nanorods, presenting an economic and reproducible route for their manufacture with tuneable sizes for a wide range of applications.

Synthesis of nanostructured γ-AlOOH and its accelerating behavior on the thermal decomposition of AP

The hydrothermal treatment of γ-AlOOH agglomerates was introduced to synthesize nanostructured γ-AlOOH.

Self-assembled synthesis of urchin-like AlOOH microspheres with large surface area for removal of pollutants

Urchin-like AlOOH microspheres synthesized by a chemical induced solvothermal method revealed high efficiency for the removal of Congo red pollutant.

Efficient removal of hexavalent chromium by high surface area Al2O3rods

High surface area Al₂O₃rods were develped and exhibited high adsorption capacity of 39 mg g⁻¹for hexavalent chromium.

Synthesis of curtain-like crumpled boehmite and γ-alumina nanosheets

Boehmite and γ-alumina nanosheets with a curtain-like crumpled morphology have been synthesized via a template-free hydrothermal process.

Controlling the synthesis and application of nanocrystalline spherical and ordered mesoporous alumina with high thermal stability

In an acetic acid assisted sol–gel system, nanosize spherical mesoporous alumina (Figure Q) and ordered mesoporous alumina (Figure S) were synthesized using different mole ratios of n(acetic acid)/n(Al) and synthesis temperatures.

Synthesis of γ-AlOOH nanocrystals with different morphologies due to the effect of sulfate ions and the corresponding formation mechanism study

The investigation of the metal oxide/inorganic ion interface at the atomic level represents a fundamental issue for the understanding of chemical and physical processes involved in several fields such as catalysis, adsorption, directed synthesis and the mechanistic study of crystal growth. In this paper, a combined hydrothermal synthesis and computational approach based on DFT theory is adopted to investigate the effects of sulfate ions on the final morphology of γ-AlOOH. The quantum mechanical calculations reveal that the sulfate ions interact with γ-AlOOH facets through surface hydroxyls and act as a morphology-directing agent. The adsorption type and chemical bonds between the sulfate ion and γ-AlOOH are discussed. The formation of nanosheets and nanorods of γ-AlOOH is controlled by thermodynamic factors. Moreover, the HR-TEM images reveal the growth directions and exposed planes of boehmite, indicating an oriented-aggregation process which is consistent with the DFT calculations. Overall, all the morphologies of boehmite suggested by the calculations are confirmed by experimental results.

Thin porous alumina sheets as supports for stabilizing gold nanoparticles

Thin porous alumina sheets have been synthesized using a lysine-assisted hydrothermal approach resulting in an extraordinary catalyst support that can stabilize Au nanoparticles at annealing temperatures up to 900 °C. Remarkably, the unique architecture of such an alumina with thin sheets (average thickness ~15 nm and length 680 nm) and rough surface is beneficial to prevent gold nanoparticles from sintering. HRTEM observations clearly showed that the epitaxial growth between Au nanoparticles and alumina support was due to strong interfacial interactions, further explaining the high sinter-stability of the obtained Au/Al2O3 catalyst. Consequently, despite calcination at 700 °C, the catalyst maintains its gold nanoparticles of size predominantly 2 ± 0.8 nm. Surprisingly, catalyst annealed at 900 °C retained the highly dispersed small gold nanoparticles. It was also observed that a few gold particles (6-25 nm) were encapsulated by an alumina layer (thickness less than 1 nm) to minimize the surface energy, revealing a surface restructuring of the gold/support interface. As a typical and size-dependent reaction, CO oxidation is used to evaluate the performance of Au/Al2O3 catalysts. The results obtained demonstrated Au/Al2O3 catalyst calcined at 700 °C exhibited excellent activity with a complete CO conversion at ∼30 °C (T100% = 30 °C), and even after calcination at 900 °C, the catalyst still achieved its T50% at 158 °C. In sharp contrast, Au catalyst prepared using conventional alumina support shows almost no activity under the same preparation and catalytic test conditions.