Aluminum Local Environment and Defects in the Crystalline Structure of Sol−Gel Alumina Catalyst

Purification mechanism of microbial metabolism in kitchen-oil wastewater enhanced by cationic vacancies on γ-Al2O3

The use of catalyst materials to mediate the enhancement of microbial degradation in wastewater is a new economic and energy saving breakthrough in water treatment technology. In this study, γ-Al2O3, which is commonly used as catalyst/carrier, is used as biological filler to treat kitchen-oil wastewater with low biodegradability, and the COD removal rate is about 50 %. It is found that the complexation of cationic vacancies on Al2O3 surface with extracellular polymeric substance (EPS) secreted by microorganisms in wastewater lead to the polarization of electron distribution on biofilm. The efficient degrading bacteria are enriched on reaction interface and obtain electrons to maintain electron dynamic balance by enhancing the transmembrane metabolism of pollutants. The aluminum vacancies on Al2O3 surface accelerate the microbial degradation of pollutants. The cationic vacancies in the structure of catalyst accelerate the acquisition of exogenous electrons by microorganisms without the addition of external energy, which provides a new idea for catalytic fillers to enhance wastewater degradation.

Atomistic Simulations of the Crystalline-to-Amorphous Transformation of γ-Al2O3 Nanoparticles: Delicate Interplay between Lattice Distortions, Stresses, and Space Charges

The size-dependent phase stability of γ-Al₂O₃ was studied by large-scale molecular dynamics simulations over a wide temperature range from 300 to 900 K. For the γ-Al₂O₃ crystal, a bulk transformation to α-Al₂O₃ by an FCC-to-HCP transition of the O sublattice is still kinetically hindered at 900 K. However, local distortions of the FCC O-sublattice by the formation of quasi-octahedral Al local coordination spheres become thermally activated, as driven by the partial covalency of the Al-O bond. On the contrary, spherical γ-Al₂O₃ nanoparticles (NPs) (with sizes of 6 and 10 nm) undergo a crystalline-to-amorphous transformation at 900 K, which starts at the reconstructed surface and propagates into the core through collective displacements of anions and cations, resulting in the formation of 7- and 8-fold local coordination spheres of Al. In parallel, the reconstructed Al-enriched surface is separated from the stoichiometric core by a diffuse Al-depleted transition region. This compositional heterogeneity creates an imbalance of charges inside the NP, which induces a net attractive Coulombic force that is strong enough to reverse the initial stress state in the NP core from compressive to tensile. These findings disclose the delicate interplay between lattice distortions, stresses, and space-charge regions in oxide nanosystems. A fundamental explanation for the reported expansion of metal-oxide NPs with decreasing size is provided, which has significant implications for, e.g., heterogeneous catalysis, NP sintering, and additive manufacturing of NP-reinforced metal matrix composites.

Roles of the structural defects and the combined acidity of H3PW12O40/Zr-MCM-41 catalysts in ultralow sulfur diesel production

This work contributes to the production of ultralow-sulfur diesel using phosphotungstic acid immobilized on Zr-MCM-41 catalysts containing structural defects and combined acidity.

An aluminum lining to the dark cloud of silver resistance: harnessing the power of potent antimicrobial activity of γ-alumina nanoparticles

Although a biologically nonessential element in living organisms, aluminum is notably nontoxic to eukaryotic cells and has a venerable history of medicinal use. We demonstrate that polyethylene glycol-coated γ-alumina nanoparticles (Al₂O₃-NPs) with an average size of 15 nm prepared from a commercial bulk γ-alumina (γ-Al₂O₃) via the top-down sonication technique exhibit antibacterial activity that is comparable to that of AgNPs against both the Gram-negative drug-susceptible Pseudomonas aeruginosa (DSPA) and multidrug-resistant Pseudomonas aeruginosa (DRPA) bacteria, while the antibacterial activity of such Al₂O₃-NPs considerably surpasses that of AgNPs against both the Gram-positive methicillin-susceptible Staphylococcus aureus (DSSA) and methicillin-resistant Staphylococcus aureus (MRSA) bacteria. We also demonstrate that the DSPA bacteria sequentially exposed to Al₂O₃-NPs for 30 days show no indication of resistance development. Furthermore, such Al₂O₃-NPs can completely overcome the drug resistance developed in the conventional antibiotic ciprofloxacin-resistant and AgNP-resistant mutants without developing Al resistance.

The Microstructure of γ-Alumina

Though γ-Al2O3 has played a central role in heterogeneous catalysis for more than two centuries, its microstructure continues to be debated. Specifically, the positions of Al3+ cations within the crystal lattice have been discussed extensively in the literature. Many authors uphold that the cations primarily occupy spinel sites, while others endorse the occupation of non-spinel sites. The other main point of dispute is whether the structure contains interstitial hydrogen, with some authors supporting a partially hydrated model and others claiming that the structure must be completely dehydrated. The use of different structural models directly affects the predicted geometry of γ-Al2O3 at the surface, which in turn has significant implications for its catalytic utility. A comparison of theoretical data to experimental infrared (IR), X-ray diffraction (XRD), and selected area electron diffraction (SAED) evidence suggests that γ-Al2O3 features cations primarily in spinel positions, while IR and nuclear magnetic resonance (NMR) data indicate that interstitial hydrogen is present within the bulk structure.

Comprehensive Characterization of APTES Surface Modifications of Hydrous Boehmite Nanoparticles

Hydrous boehmite (γ-AlOOH) nanoparticles (BNP) show great potential as nanoscale filler for the fabrication of fiber reinforced nanocomposite materials. Notably, the particle-matrix interaction has been demonstrated to be decisive for improving the matrix-dominant mechanical properties in the past years. Tailoring the surface properties of the nanofiller enables to selectively design the interaction and thus to exploit the benefits of the nanocomposite in an optimal way. Here, an extensive study is presented on the binding of (3-aminopropyl)triethoxysilane (APTES), a common silane surface modifier, on BNP in correlation to different process parameters (concentration, time, temperature, and pH). Furthermore, a comprehensive characterization of the modified BNP was performed by using elemental analysis (EA), thermogravimetric analysis (TGA) coupled with mass spectrometry (TGA-MS), and Kaiser's test (KT). The results show an increasing monolayer formation up to a complete surface coverage with rising APTES concentration, time, and temperature, resulting in a maximal grafting density of 1.3 molecules/nm². Unspecific multilayer formation was solely observed under acidic conditions. Comparison of TGA-MS results with data recorded from EA, TGA, and KT verified that TGA-MS is a convenient and highly suitable method to elucidate the ligand binding in detail.

Greener and sustainable production of bioethylene from bioethanol: current status, opportunities and perspectives

The economic value of bioethylene produced from bioethanol dehydration is remarkable due to its extensive usage in the petrochemical industry. Bioethylene is produced through several routes, such as steam cracking of hydrocarbons from fossil fuel and dehydration of bioethanol, which can be produced through fermentation processes using renewable substrates such as glucose and starch. The rise in oil prices, environmental issues due to toxic emissions caused by the combustion of fossil fuel and depletion of fossil fuel resources have led a demand for an alternative pathway to produce green ethylene. One of the abundant alternative renewable sources for bioethanol production is biomass. Bioethanol produced from biomass is alleged to be a competitive alternative to bioethylene production as it is environmentally friendly and economical. In recent years, many studies have investigated catalysts and new reaction engineering pathways to enhance the bioethylene yield and to lower reaction temperature to drive the technology toward economic feasibility and practicality. This paper critically reviews bioethylene production from bioethanol in the presence of different catalysts, reaction conditions and reactor technologies to achieve a higher yield and selectivity of ethylene. Techno-economic and environmental assessments are performed to further development and commercialization. Finally, key issues and perspectives that require utmost attention to facilitate global penetration of technology are highlighted.

Effects of various morphologies on the optical and electrical properties of boehmite nanostructures

This work reports the variations in the fluorescence and ultra-high dielectric properties of various boehmite nano-structures.

Al2O3-CeO2 Oxides: Synthesis, Characterization and Evaluation as Catalytic Supports in Benzene Combustion. Surface Ceria Effects

Al2O3-CeO2 sol-gel oxides were obtained at ceria loadings of 2, 5, 10, 20 and 50 wt.%, calcined at 700 °C and characterized by N2 physisorption, XRD, TEM, 27Al MAS-NMR, and TPR. All samples present high amounts of surface ceria species and bulk ceria has significant presence from 20 wt.% of ceria. The sample with 50 wt.% presents both the reduction of bulk and interfacial ceria species and X-ray diffraction peaks corresponding to cerianite phase. Surface ceria dispersion degree (surface ceria/total reduced ceria) was determined to be 100 % for samples with 2 and 5 wt.% CeO2, with a slight decrease to ca. 90 % and 75 % for samples with 10 and 20 wt.% CeO2 respectively, and a lower value of 41 % for the sample with 50 wt.% CeO2. It was found the amount of surface ceria has a good correlation with the amount of AlV sites, indicating that incorporated Ce ions distorts the alumina matrix and restricts the amount of surface ceria. Platinum supported on these materials showed that the catalytic activity in benzene combustion for oxygen-depleted environments increased as the surface ceria concentration increased.

Hydrogenous spinel γ-alumina structure

The structure of γ-Al₂O₃ is still under debate. Here we report a H spinel structure evolved from pseudo-boehmite. A unit cell with two octahedral cationic vacancies and one bulk H was preferential in terms of calculated Gibbs energy, which was well consistent with experimental data. Bulk H was found to migrate out with elevated temperatures. Through calculating the migration barriers of every step, we observed that the "hopping" step was rate-determining. The hopping rates were further estimated by assuming a Boltzmann distribution of energies, and as a result they increased by 2 to 3 orders of magnitude from 500 °C to 800 °C. This investigation will encourage us to study more uncertainties in material structures.

Aluminum (Oxy)Hydroxide Nanosticks Synthesized in Bicontinuous Reverse Microemulsion Have Potent Vaccine Adjuvant Activity

Insoluble aluminum salts such as aluminum (oxy)hydroxide are commonly used as vaccine adjuvants. Recently, there is evidence suggesting that the adjuvant activity of aluminum salt-based materials is tightly related to their physicochemical properties, including nanometer-scale size, shape with long aspect ratio, and low degree of crystallinity. Herein, for the first time, the bicontinuous reverse microemulsion (RM) technique was utilized to synthesize stick-like monodisperse aluminum (oxy)hydroxide nanoparticles with a long aspect ratio of ∼10, length of ∼80 nm, and low degree of crystallinity (denoted as Al-nanosticks). Moreover, the relationship between the physicochemical properties of Al-nanosticks and the bicontinuous RM was discussed. Compared to the commercial Alhydrogel, which contains micrometer-scale aluminum oxyhydroxide particular aggregates with moderate degree of crystallinity, the Al-nanosticks are more effective in adsorbing and delivering antigens (e.g., ovalbumin, OVA) into antigen-presenting cells, activating inflammasomes, and potentiating OVA-specific antibody responses in a mouse model. It is concluded that the aluminum (oxy)hydroxide nanosticks synthesized in the bicontinuous RM are promising new aluminum salt-based vaccine adjuvants.

Role of Alumina Basicity in CO2 Uptake in 3-Aminopropylsilyl-Grafted Alumina Adsorbents

Oxide-supported amine materials are widely known to be effective CO₂ sorbents under simulated flue-gas and direct-air-capture conditions. Most work has focused on amine species loaded onto porous silica supports, though potential stability advantages may be offered through the use of porous alumina supports. Unlike silica materials, which are comparably inert, porous alumina materials can be tuned to have substantial acidity and/or basicity. Owing to their amphoteric nature, alumina supports play a more active role in CO₂ sorption than silica supports, potentially directly participating in the adsorption process. In this work, primary amines associated with 3-aminopropyltriethoxysilane are grafted onto two different mesoporous alumina materials having different levels of basicity. Adsorbent materials with different amine loadings are prepared, and the CO₂ -adsorption behavior of similar amines on the two alumina supports is demonstrated to be different. At low amine loadings, the inherent properties of the support surface play a significant role, whereas at high amine loadings, when the alumina surface is effectively blocked, the sorbents prepared on the two supports behave similarly. At high amine loadings, amine-CO₂ -amine interactions are shown to dominate, leading to adsorbed species that appear similar to the species formed over silica-supported amine materials. The sorbent properties are comprehensively characterized using N₂ physisorption analysis, in situ FTIR spectroscopy, and adsorption microcalorimetry.

Enhanced Phosphorus Locking by Novel Lanthanum/Aluminum-Hydroxide Composite: Implications for Eutrophication Control

Lanthanum (La) bearing materials have been widely used to remove phosphorus (P) in water treatment. However, it remains a challenge to enhance phosphate (PO₄) adsorption capacity and La usage efficiency. In this study, La was coprecipitated with aluminum (Al) to obtain a La/Al-hydroxide composite (LAH) for P adsorption. The maximum PO₄ adsorption capacities of LAH (5.3% La) were 76.3 and 45.3 mg P g^-1 at pH 4.0 and 8.5, which were 8.5 and 5.3 times higher than those of commercially available La-modified bentonite (Phoslock, 5.6% La), respectively. P K-edge X-ray absorption near edge structure analysis showed that PO₄ was preferentially bonded with Al under weakly acid conditions (pH 4.0), while tended to associate with La under alkaline conditions (pH 8.5). La L_III-edge extended X-ray absorption fine structure analysis indicated that PO₄ was bonded on La sites by forming inner sphere bidentate-binuclear complexes and oxygen defects exhibited on LAH surfaces, which could be active adsorption sites for PO₄. The electrostatic interaction, ligand exchange, and oxygen defects on LAH surfaces jointly facilitated PO₄ adsorption but with varied contribution under different pH conditions. The combined contribution of two-component of La and Al may be an important direction for the next generation of commercial products for eutrophication mitigation.

Enhanced thermal stability of palladium oxidation catalysts using phosphate-modified alumina supports

Alumina with 5.0 μmol m⁻² phosphorus additives could retain a higher Pd dispersion after aging, resulting in much higher CO oxidation activity.

Structure and thermal properties of millimeter-scale alumina aerogel beads formed by a modified ball dropping method

We report the formation of alumina aerogel beads with a modified ball dropping method, in which alumina alcogel beads are formed by extruding alumina sol containing a small amount of polyvinyl alcohol through nozzles into ammonia water.

Anodic Oxidation in Aluminum Electrode by Using Hydrated Amorphous Aluminum Oxide Film as Solid Electrolyte under High Electric Field

Dense and nonporous amorphous aluminum oxide (AmAO) film was deposited onto platinized silicon substrate by sol-gel and spin coating technology. The evaporated aluminum film was deposited onto the AmAO film as top electrode. The hydrated AmAO film was utilized as a solid electrolyte for anodic oxidation of the aluminum electrode (Al) film under high electric field. The hydrated AmAO film was a high efficiency electrolyte, where a 45 nm thick Al film was anodized completely on a 210 nm thick hydrated AmAO film. The current-voltage (I-V) characteristics and breakdown phenomena of a dry and hydrated 210 nm thick AmAO film with a 150 nm thick Al electrode pad were studied in this work. Breakdown voltage of the dry and hydrated 210 nm thick AmAO film were 85 ± 3 V (405 ± 14 MV m(-1)) and 160 ± 5 V (762 ± 24 MV m(-1)), respectively. The breakdown voltage of the hydrated AmAO film increased about twice, owing to the self-healing behavior (anodic oxidation reaction). As an intuitive phenomenon of the self-healing behavior, priority anodic oxidation phenomena was observed in a 210 nm thick hydrated AmAO film with a 65 nm thick Al electrode pad. The results suggested that self-healing behavior (anodic oxidation reaction) was occurring nearby the defect regions of the films during I-V test. It was an effective electrical self-healing method, which would be able to extend to many other simple and complex oxide dielectrics and various composite structures.

Preparation of porous alumina microspheres via an oil-in-water emulsion method accompanied by a sol–gel process

Porous alumina microspheres prepared via a facile template-free approach followed by heat-treatment, exhibiting a three-dimensional interconnected structure.

Efficient Synthesis of Amides and Esters from Alcohols under Aerobic Ambient Conditions Catalyzed by a Au/Mesoporous Al2 O3 Nanocatalyst

An efficient heterogeneous Au/mesoporous alumina nanocatalyst has been successfully developed for the synthesis of amides and esters from simple building blocks of readily available alcohols and amines. The processes were simple and were performed at room temperature and atmospheric pressure of O2 to form the desired products with up to 97 % isolated yield. The ability of Au/mesoporous alumina to catalyze these reactions under ambient conditions further enhances the sustainability of these chemical processes. Furthermore, the nanocatalyst was stable to air and water and could be recovered and reused easily. The enhanced catalytic activity of Au/mesoporous alumina might be attributed to the presence of negatively charged Au nanoparticles that could promote oxidation processes as well as the stability of the mesoporous alumina support calcined at a high temperature of 800 °C.

One-pot direct synthesis route to self-assembled highly ordered Zn-decorated mesoporous aluminium oxide toward efficient and sustainable metathesis heterogeneous catalyst design

Zn-modified well-ordered mesoporous alumina successfully synthesizedviaone-pot process and evaluated as support for rhenium-based catalyst for methyl oleate self-metathesis.

Synthesis, characterization and insights into stable and well organized hexagonal mesoporous zinc-doped alumina as promising metathesis catalysts carrier

ZnCl₂-modified hexagonal mesoporous alumina was successfully synthesized, yielding Al–O–Zn–Cl species for supporting methyltrioxorhenium towards methyl oleate self-metathesis.

Low-temperature selective catalytic reduction of NO on CeO2-CuO/Al2O3 catalysts prepared by different methods

CeO2-CuO/Al2O3 catalysts were prepared by three different methods and their activities for selective catalytic reduction (SCR) of NO with NH3 were investigated. As can be seen from the experimental results, the catalyst prepared by the single-step sol-gel (SG) method showed the best SCR activity and resistance to SO2 and H2O. In order to investigate the relationship between the preparation method and the performance of SCR catalysts, the catalysts were characterized by using Brunauer-Emmett-Teller, X-ray diffraction, temperature programmed reduction with hydrogen, temperature programmed desorption with ammonia, X-ray photoelectron spectroscopy, Fourier transform infrared and thermo-gravimetric analysis techniques. It was found that the excellent performance of CeO2-CuO/Al2O3 catalyst prepared by the single-step SG method should be resulted from its large surface area, low crystallinity, high oxygen storage capacity, high NH3 adsorption capacity, high concentration of surface chemisorbed oxygen, weak sulphation process and weak water absorption.

Fabrication and enhanced light-trapping properties of three-dimensional silicon nanostructures for photovoltaic applications

In order to make photovoltaics an economically viable energy solution, next-generation solar cells with higher energy conversion efficiencies and lower costs are urgently desired. Among many possible solutions, three-dimensional (3D) silicon nanostructures with excellent light-trapping properties are one of the promising candidates and have recently attracted considerable attention for cost-effective photovoltaic applications. This is because their enhanced light-trapping characteristics and high carrier collection efficiencies can enable the use of cheaper and thinner silicon materials. In this review, recent developments in the controllable fabrication of 3D silicon nanostructures are summarized, followed by the investigation of optical properties on a number of different nanostructures, including nanowires, nanopillars, nanocones, nanopencils, and nanopyramids, etc. Even though nanostructures with radial p-n junction demonstrate excellent photon management properties and enhanced photo-carrier collection efficiencies, the photovoltaic performance of nanostructure-based solar cells is still significantly limited due to the high surface recombination effect, which is induced by high-density surface defects as well as the large surface area in high-aspect-ratio nanostructures. In this regard, various approaches in reducing the surface recombination are discussed and an overall geometrical consideration of both light-trapping and recombination effects to yield the best photovoltaic properties are emphasized.

Low Concentration Fe-Doped Alumina Catalysts Using Sol-Gel and Impregnation Methods: The Synthesis, Characterization and Catalytic Performance during the Combustion of Trichloroethylene

The role of iron in two modes of integration into alumina catalysts was studied at 0.39 wt% Fe and tested in trichloroethylene combustion. One modified alumina was synthesized using the sol-gel method with Fe added in situ during hydrolysis; another modification was performed using calcined alumina, prepared using the sol-gel method and impregnated with Fe. Several characterization techniques were used to study the level of Fe modification in the γ-Al₂O₃ phase formed and to correlate the catalytic properties during trichloroethylene (TCE) combustion. The introduction of Fe in situ during the sol-gel process influenced the crystallite size, and three iron species were generated, namely, magnetite, maghemite and hematite. The impregnated Fe-alumina formed hematite and maghemite, which were highly dispersed on the γ-Al₂O₃ surface. The X-ray photoelectron spectra (XPS), FT-IR and Mössbauer spectroscopy analyses revealed how Fe interacted with the γ-Al₂O₃ lattice in both catalysts. The impregnated Fe-catalyst showed the best catalytic performance compared to the catalyst that was Fe-doped in situ by the sol-gel method; both had better catalytic activity than pure alumina. This difference in activity was correlated with the accessibility of the reactants to the hematite iron species on the surface. The chlorine poisoning for all three catalysts was less than 1.8%.

Enhancing catalytic activity and stability for CO2 methanation on Ni–Ru/γ-Al2O3via modulating impregnation sequence and controlling surface active species

Co-impregnation and sequential impregnation were used to prepare a series of Ni–Ru/γ-Al₂O₃ catalysts for CO₂ methanation. By comparing the structure, surface species and catalytic performances, we uncovered the surface species segregation of Ru in Ni–Ru catalysts, and its effect on catalytic properties.

Sol–gel synthesis of highly pure α-Al2O3 nano-rods from a new class of precursors of salicylaldehyde-modified aluminum(iii) isopropoxide. Crystal and molecular structure of [Al(OC6H4CHO)3]

Synthesis and characterization of salicylaldehyde-modified aluminum(III) isopropoxide, [(OPrⁱ)_3−nAl(OC₆H₄CHO)_n] precursors used for sol–gel synthesis of α-Al₂O₃ nano-rods.