Nonporous Alumina Research Articles

Formation of a Porous Crystalline Mg1‒ x Al2O y Overlayer on Metal Catalysts via Controlled Solid-State Reactions for High-temperature Stable Catalysis.

Catalyst deactivation by sintering and coking is a long-standing issue in metal-catalyzed harsh high-temperature hydrocarbon reactions. Ultrathin oxide coatings of metal nanocatalysts have recently appeared attractive to address this issue, while the porosity of the overlayer is difficult to control to preserve the accessibility of embedded metal nanoparticles, thus often leading to a large decrease in activity. Here, we report that a nanometer-thick alumina coating of MgAl2O4-supported metal catalysts followed by high-temperature reduction can transform a nonporous amorphous alumina overlayer into a porous Mg1‒xAl2Oy crystalline spinel structure with a pore size of 2‒3 nm and weakened acidity. The high porosity stems from the restrained Mg migration from the MgAl2O4 support to the alumina overlayer through solid-state reactions at high temperatures. The resulting Ni/MgAl2O4 and Pt/MgAl2O4 catalysts with a porous crystalline Mg1‒xAl2Oy overlayer achieved remarkably high stability while preserving much higher activity than the corresponding alumina-coated Ni and Pt catalysts on MgO and Al2O3 supports in the reactions of dry reforming of methane and propane dehydrogenation, respectively.

Formation of a Porous Crystalline Mg1‐xAl2Oy Overlayer on Metal Catalysts via Controlled Solid‐State Reactions for High‐temperature Stable Catalysis

AbstractCatalyst deactivation by sintering and coking is a long‐standing issue in metal‐catalyzed harsh high‐temperature hydrocarbon reactions. Ultrathin oxide coatings of metal nanocatalysts have recently appeared attractive to address this issue, while the porosity of the overlayer is difficult to control to preserve the accessibility of embedded metal nanoparticles, thus often leading to a large decrease in activity. Here, we report that a nanometer‐thick alumina coating of MgAl2O4‐supported metal catalysts followed by high‐temperature reduction can transform a nonporous amorphous alumina overlayer into a porous Mg1‐xAl2Oy crystalline spinel structure with a pore size of 2–3 nm and weakened acidity. The high porosity stems from the restrained Mg migration from the MgAl2O4 support to the alumina overlayer through solid‐state reactions at high temperatures. The resulting Ni/MgAl2O4 and Pt/MgAl2O4 catalysts with a porous crystalline Mg1‐xAl2Oy overlayer achieved remarkably high stability while preserving much higher activity than the corresponding alumina‐coated Ni and Pt catalysts on MgO and Al2O3 supports in the reactions of dry reforming of methane and propane dehydrogenation, respectively.

Influence of alumina fixed-bed in steam reforming of glycerol for hydrogen production

In this study, hydrogen production by steam reforming was carried out in a tubular fixed-bed reactor. The influence of some operating variables such as temperature, flow rate of the feeding glycerol solution and space velocity on hydrogen production under a non-catalyzed reaction was analyzed. The results showed that the hydrogen yield increased with temperature and decreased with the space velocity. As the feedstock flow rate increased, the hydrogen yield decreased rapidly. Compared with a reference experiment without fixed-bed, the use of non-porous alumina fixed-bed showed better results. Under the best reaction conditions (900 °C and a feedstock flow rate of 0.85 g/min), 17 L/h of syngas were obtained, with a purity of about 55% of hydrogen and a production of 2.7 moles of hydrogen by mol of glycerol.

Fluidized bed chemical vapor deposition of pyrocarbon on various types of powders: Heat and mass transfer analyses and nanotexture characterization

SiC powders coated by pyrocarbon are a promising feedstock for the synthesis of Ceramic-matrix composites. Preparing such powders is possible using low pressure Fluidized-Bed Chemical Vapor Deposition (FB-CVD) with propane as a precursor at 1000 °C. A preliminary study on non-porous and porous alumina powders as substrates has been carried out. With the help of a numerical assessment of heat transfer in the reactor, it has been possible to optimize the setup and achieve excellent deposition yields (up to 100%), even inside very small pores (7 nm), and transpose the protocol to 200-nm diameter SiC powders. The microstructure and texture of the deposits were characterized by TEM, showing dependence to pore size due to a texture-induction effect by the substrate surface. A quantitative assessment of mass transfer indicated that the results obtained here agree with past studies on FB-CVD of other materials and on pyrocarbon CVD on flat substrates.

Computational fluid dynamics study of the atomic layer deposition process around cylindrical and planar configurations

Computational fluid dynamics (CFD) simulations were used to model atomic layer deposition (ALD) coating processes in the continuum flow regime. CFD model was validated for non-reactive and reactive flows. In ALD CFD simulations, surfaces in various test cases were considered as non-porous aluminum oxide. The effect of trimethylaluminium precursor partial pressure, initial condition, and Reynolds number (Re) on coating timescale were analyzed. The CFD model accurately captured the surface coverage solution for all cases when the domain was initially filled with the precursor. The absence of initial precursor delayed coating time during the surface concentration level development process. As Re increased, coating time significantly decreased due to reduced diffusion limitations and promoted convection swiftly transporting precursors to the surfaces. The provided example showed that 2D polydisperse fixed cylinder bed is fully coated within approximately one flow time through the bed. The benefits of CFD in understanding ALD processes were discussed.

Water adsorption on planar interfaces: Classical density functional study

• A new cDFT based on SAFT with an explicit account for dipole-dipole interactions was formulated. • Our model provides a good description of binodal, liquid-vapor surface tension, latent heat of vaporization, and wetting contact angles. • The energy distribution functions for non-porous silica and alumina were calculated. Theoretical modeling of water adsorption in micro- and mesoporous materials remains a challenging problem for physical chemistry and chemical engineering despite its great importance for industrial applications. Here, we present the theoretical study of bulk and inhomogeneous water within the classical density functional theory. Our model includes all important intermolecular interactions, such as dispersion, association, and dipole-dipole. We validated the model via comparison with existing literature experimental and computer simulation data. Among the others, the following water properties were compared: liquid-vapor binodal (T - ρ ), surface tension, latent heat of vaporization, wetting contact angle, and adsorption isotherms on nonporous silica and alumina. Our computational results demonstrate good agreement with the experimental literature data and show similar trends compared to the several commonly used water models (SPC/E, TIP4P, TIP4P/2005). Because the presented model can accurately describe the thermodynamic and interfacial properties of water then it can be of interest to the adsorption community. Also, the model may serve as a starting point for further works regarding porous adsorbents characterization employing classical density functional theory.

Rod–Airfoil Interaction Noise Reduction Using Gradient Distributed Porous Leading Edges

Rod–airfoil interaction noise is a major concern in several practical industrial and aeronautical applications. In this study, we constructed bio-inspired gradient distributed porous leading edges to reduce rod–airfoil interaction noise. Noise radiations by NACA 0012 airfoils with nonporous aluminum and porous leading edges were experimentally compared in an anechoic wind tunnel by changing the streamwise gap between the upstream rod and the downstream airfoil, as well as the angle of attack of the airfoil. The results of detailed acoustic tests showed that the proposed gradient distributed porous leading edges can significantly reduce noise radiation around and above the peak frequency of the baseline rod–airfoil interaction. Parametric studies on the piecewise porous characteristics showed that rod–airfoil interaction noise reduction is sensitive to the coverage percentage, position, and arrangement order of the porous materials. Porous leading edges with lower pores per inch, larger coverage, and gradually sparse distributed pores better reduced noise. Moreover, the position of the porous material affected the frequency band of noise reduction, and the noise reduction performance was better when it was located in the downstream strips of the porous leading edge.

Functional gradient films on aluminum alloy with high absorption efficiencies and damage thresholds for inertial confinement fusion applications

Functional gradient films with high absorption efficiencies and damage thresholds are crucial for eliminating stray light to decrease laser-induced damage in optical systems. Recent advances related to aluminum-based functional films have attracted much attention. However, the fabrication of large area of metal frame need to satisfy low cost, absorb stray light and high stability. A three-layer functional gradient film is designed here. The bottom barrier layer is non-porous alumina, the middle absorption layer has CdSe-quantum dots embedded in anodic aluminum oxide nanopores, and the anodic aluminum film was sealed by a silica layer at the outer side. It could be mass produced at low cost on large areas on aluminum-alloy substrates and have high damage thresholds. The average measured absorbance of light was over 95% over the range 300–750 nm. Moreover, the prepared process has capability to be operated with large scale and high efficiency in an industrial plant. The aluminum-based functional gradient films has its potential to be used in large sized laser facility manufacturing like building inertial-confinement fusion systems to eliminate stay light and improve system cleanliness.

Non-destructive evaluation of ceramic porosity using terahertz time-domain spectroscopy

Dielectric constants of Al2O3 (alumina) ceramic samples have been measured with non-destructive terahertz time-domain spectroscopy. Using an effective medium approximation, we demonstrated that the relative permittivity can be used to evaluate the porous alumina samples, with porosity fractions ranging from 0% to 20% per volume. This result makes it possible to control sample porosity, a key material parameter related to mechanical strength under compression load, hardness and wear resistance, or thermal stability. We also employed terahertz time-domain spectroscopy to differentiate alumina samples fabricated with different grain sizes, or reveal a small fraction of impurities, such as 5%vol of ZrO2, embedded in non-porous alumina. These results demonstrated the potential of terahertz radiation for non-destructive characterization of low loss high dielectric constant ceramics.

Experimental Study of the Pyrolysis of NH3 under Flow Reactor Conditions.

The possibility of using ammonia (NH3), as a fuel and as an energy carrier with low pollutant emissions, can contribute to the transition to a low-carbon economy. To use ammonia as fuel, knowledge about the NH3 conversion is desired. In particular, the conversion of ammonia under pyrolysis conditions could be determinant in the description of its combustion mechanism. In this work, pyrolysis experiments of ammonia have been performed in both a quartz tubular flow reactor (900–1500 K) and a non-porous alumina tubular flow reactor (900–1800 K) using Ar or N2 as bath gas. An experimental study of the influence of the reactor material (quartz or alumina), the bulk gas (N2 or Ar), the ammonia inlet concentration (1000 and 10 000 ppm), and the gas residence time [2060/T (K)–8239/T (K) s] on the pyrolysis process has been performed. After the reaction, the resulting compounds (NH3, H2, and N2) are analyzed in a gas chromatograph/thermal conductivity detector chromatograph and an infrared continuous analyzer. Results show that H2 and N2 are the main products of the thermal decomposition of ammonia. Under the conditions of the present work, differences between working in a quartz or non-porous alumina reactor are not significant under pyrolysis conditions for temperatures lower than 1400 K. Neither the bath gas nor the ammonia inlet concentration influence the ammonia conversion values. For a given temperature and under all conditions studied, conversion of ammonia increases with an increasing gas residence time, which results into a narrower temperature window for NH3 conversion.

Scanning Nanobeam Diffraction and Energy Dispersive Spectroscopy Characterization of a Model Mn-Promoted Co/Al2O3 Nanosphere Catalyst for Fischer–Tropsch Synthesis

Manganese oxide is an effective promoter for enhancing the activity and selectivity of cobalt-based Fischer–Tropsch synthesis (FTS) catalysts. Previous studies suggest that Co atoms located at the Co–MnO perimeter adsorb CO more strongly and undergo C–O bond cleavage more readily than CO adsorbed on Co sites, which leads to improved C5+ selectivity. Thus, direct characterization of the formation and structure of the active interface between Co nanoparticles and MnO is essential for a full understanding of the FTS promotion effect. Scanning transmission electron microscopic (STEM) methods are well-suited for direct characterization of nanoscale features through imaging, energy dispersive spectroscopy (EDS) elemental mapping, and spatially resolved electron diffraction, a technique known as scanning nanobeam diffraction. However, these techniques require that the catalytic nanoparticles are non-overlapping and well-defined, which is not typical for nanoparticles on porous metal oxide FTS supports. In the present study, we addressed these challenges by synthesizing catalysts composed of cobalt dispersed on nonporous alumina nanospheres (Co/ANS), which yielded isolated nanoparticles that are amenable to elemental and phase mapping by STEM-EDS and scanning nanobeam diffraction. Then, manganese oxide-promoted Co/ANS (Mn–Co/ANS) was characterized using these two techniques, which provided direct evidence for the transformation of a Co–Mn oxide spinel present in the as-prepared catalyst into closely associated Co3O4 and MnO2 nanoparticles in the passivated catalyst after its use for FTS.

Understanding Pore Formation in ALD Alumina Overcoats.

AlOX thin films deposited by atomic layer deposition (ALD) have previously been used to increase both stability and selectivity of supported palladium catalysts and are known to develop nanoscale porosity upon heating. Understanding the factors that affect ALD thin-film porosityenables future design of layered catalytic structures with tunable nanoscale features on industrially-relevant high-surface-area materials. In this study, porous and nonporous aluminum oxide supports with and without palladium nanoparticles were overcoated with thin films of 2-7 nm AlOX by ALD deposited at temperatures of 100, 200, and 300 °C. Hydroxyl loss and changes in surface chemistry were observed upon heating the films, and changes in surface area and pore volume of the annealed films were correlated to AlOX deposition temperature and the presence of Pd. Crystallization of the overcoat to γ-Al2O3 is shown to occur separately from hydroxyl loss and pore formation. A mechanistic understanding of pore formation in AlOX ALD films is obtained by reference to studies of the structural transformations accompanying the formation of transition aluminas from hydroxide precursors. Additionally, a direct and tunable correlation is established between pore development and the overall hydroxyl content of AlOX ALD coatings.

Pt mechanical dispersion on non-porous alumina for soot oxidation

A sustainable mechanical method to disperse Pt particles on the surface of non-porous α-Al2O3 microparticles is reported. The hierarchical Pt/α-Al2O3 catalyst possesses a metal specific surface area one order of magnitude lower than the state-of-the-art γ-Al2O3 – supported Pt catalysts of similar metal composition and high alumina specific surface area. The relatively large size of the Pt particles seems to favor the catalytic soot oxidation in spite of their relatively low dispersion. As a result, the temperature of half conversion (T50 = 440 °C) is lower when compared with conventional Pt/γ-Al2O3 prepared by the wet impregnation chemical synthesis route.

Comparison of porous and nonporous alumina bed materials for PAHs formation during plastic waste fluidized-bed incineration

In order to understand adsorption effect of porous materials on PAHs generation during waste fluidized-bed incineration, comparison of porous and nonporous alumina bed materials for PAHs formation was experimentally investigated in a plastic waste fluidized-bed incinerator. Meanwhile, the distributions and contents of gaseous PAHs, particulate associated PAHs, PAHs precursors (monocyclic aromatic hydrocarbons and light hydrocarbon) in the dense region, freeboard region and outlet of the incinerator were also measured. The results indicated that ethylene (C2H4), ethyne (C2H2), propene (C3H6), benzene (C6H6), and styrene (C8H8) were the main precursors for PAHs formation. The 3-ring and 4-ring PAHs were dominant, and the particulate associated PAHs was the main components in flue gas. Meanwhile, the gaseous PAHs and particulate associated PAHs were respectively reduced by 49.20% and 39.41% when the non-porous alumina was replaced by the porous alumina bed materials. Based on the PAHs formation routes, the suppression mechanism for PAHs formation was proposed that the porous alumina adsorbed more PAHs precursors in the dense region due to its higher adsorption capacity, simulateously reduced the evolution rate of the PAHs precursors and prolonged the residence time of PAHs precursors in the freeboard region of the incinerator.

Improved performance of lithium-sulfur batteries at elevated temperature by porous aluminum

With high theoretical capacity and energy density, lithium-sulfur batteries have gained a competitive edge for promising applications, but they still face challenges like poor conductivity of sulfur and volume expansion. Herein, combining the advantages of multidimensional carbon materials, S/C composites consisting of sulfur and Activated Carbon (AC), Carbon Nanotubes (CNTs), Graphene (G) were obtained in a closed system at 155 °C, and 3D electrodes were prepared using porous aluminum (PAl) as current collectors. The as-obtained electrode exhibits a high initial discharge capacity of 1256 mAh/g at 0.1C, about 200 mAh/g higher than that of the electrode using traditional non-porous aluminum (NPAl) current collector. As NPAl is substituted by PAl, the oxidation peak current density of S/C composite is increased from 3.59 to 4.95 A/g, and the charge transfer resistance is decreased from 32.03 to 17.37 Ω. In comparison with the very poor performance of S/C using NPAl at 60 °C, the initial discharge capacity of S/C using PAl can reach 803 mAh/g at 1C rate with a capacity retention rate of 71.7% after 50 cycles, while the corresponding stress decreases to 10.35 MPa. The improved mechanisms of S/C composites using PAl were discussed. Porous aluminum is attractive to improve the electrochemical performance and safety of batteries.

Platinum-Nickel Nanotubes Array as Cathode for PEMFC

Pt-based catalysts such as Pt-Co, Pt-Cu, Pt-Ni etc. have been shown to exhibit higher oxygen reduction reaction (ORR) activity than platinum for Proton Exchange Membrane Fuel Cells. This paper describes the synthesis of a new carbon-free structure of electrodes made of vertically aligned PtNi Nanotubes. The process is based on an electrochemistry route of Ni electrodeposition inside nonporous alumina mold, followed by a step of controlled galvanic displacement, annealing and acid treatment leading to the formation of nanotubes. The latter are transferred onto a Nafion® membrane using an optimized hot pressing process.

Synthesis and friction properties of a multifunctional diamond-containing ceramic material

Abstract The paper investigates the fundamentals of the technology of forming multifunctional composite diamond-containing materials with a ceramic matrix of an aluminum oxide. It shows the results of comparative tests of triboengineering properties of the obtained material. The developed technology for creating a billet for a diamond-containing composite ceramic material uses powder metallurgy (aluminum powder and dispersed diamonds). The paper determines optimal modes to form a billet. A sample of a composite mixture has been modified by microarc oxidation. It had a hardened layer of a ceramic matrix on its surface, and a dispersed diamond evenly enclosed in it. There is a description of features of the microarc oxidation technological process of a part from a sintered diamond-aluminum mixture compared with aluminum alloys. It is determined that the thickness of the formed ceramic layer is several times higher compared with the parts made of non-porous aluminum alloys. Moreover, the alkali concentration in the electrolyte and the relative density of a sintered billet have the greatest effect. There are two types of ceramic diamond-containing materials based on the proposed technology. They are: antifriction construction materials for sliding friction units and tool materials for abrasive processing. Triboengineering tests of materials with a small grain size of diamonds revealed their good antifriction properties even without lubricating with liquid materials. It has been established that an abrasive tool made from a material with a high grain size of diamonds has higher wear resistance and productivity compared with traditional abrasive materials.

Application of Si/SiC ceramic filters as support for structural palladium catalysts for the reductive decomposition of aqueous nitrite

A Si/SiC ceramic filter was used to support a structural palladium catalyst in a continuous fixed-bed flow reactor during a reductive decomposition of nitrite. A previous report described the use of non-porous alumina spheres coated with Pd/C, a carbon monolith impregnated with Pd or a structural catalyst consisting of a polyurethane sponge skeleton impregnated with Pd resulted either in negligible activity or a complete stoppage of the flow by increasing the pressure drop in the flow system. The use of a Si/SiC filter impregnated with palladium, however, overcame the demerits and resulted in an 11% conversion of nitrite in 60 min. The use of a Si/SiC filter coated with a char intermediate followed by impregnation with Pd (Pd/C/Si/SiC) resulted in a structural catalyst that enhanced the activity to yield a 33% conversion of nitrite in 60 min. When five Pd/C/Si/SiC structural catalysts were used, however, the complete decomposition was achieved in the same amount of time. Although improvement was previously observed by using an alumina intermediate, use of the char intermediate resulted in a new factor for activity improvement. The characterization results show that it was the affinity between aqueous nitrite and the intermediate rather than palladium dispersion that contributed the most to the improvement of activity.

Reducing polycyclic aromatic hydrocarbon and its mechanism by porous alumina bed material during medical waste incineration

In this paper, porous alumina was used as an alternative bed material to reduce polycyclic aromatic hydrocarbon (PAH) and monocyclic aromatic hydrocarbons (MAH) emission during medical waste incineration in a fluidized bed combustor (FBC). In order to understand the reduction mechanisms of MAH and PAH, porous alumina, nonporous alumina, and silica sand (180–250 μm and 250–320 μm) were used as the bed materials. In comparison to the silica sand (180–250 μm) bed material, the reduction efficiencies of ∑MAH, ∑PAH and ∑PAH toxic equivalent (TEQ) by porous alumina bed material were in sequence as 91.57%, 58.90% and 73.23% during medical waste incineration under 800 °C. There were three mechanisms for the reduction of PAH under porous alumina bed materials. Firstly, the evolution rate of hydrocarbon was reduced by porous alumina due to its low heat transfer coefficient. Secondly, porous alumina bed materials could absorb more gaseous hydrocarbon and prolong the residence time of hydrocarbon in the diluted zone of FBC due to its higher BET. Lastly, the oxidization of the gaseous hydrocarbon was accelerated by porous alumina due to its catalytic effect. Thus, less light hydrocarbon, MAH and PAH were formed during medical waste incineration. The experimental results also indicated that the heat transmission, catalytic effect, and adsorption effect of porous alumina bed materials respectively accounted for 22.8%, 29.2% and 20.9% of the ∑PAH reduction.

Porous and non-porous alumina reinforced magnesium matrix composite through microwave and spark plasma sintering processes

In this study, Alumina reinforced Magnesium Matrix Composites have been prepared by two different sintering method: microwave and spark plasma sintering (SPS) processes. 5 wt% alumina whisker was blended with magnesium powders in ethanol media for 10 min. The sintering temperatures in the spark plasma and microwave furnace were at 420° and 700 °C, respectively. Thermodynamic investigation and XRD results revealed the formation of MgO, Mg2Al3 compounds in both sintered composites. However, microwave sintering led to form a porous composite (53.1% of theoretical density) while SPS resulted to formation of a fully dense composite. The mechanical examinations showed higher bending strength (163 ± 10 MPa) and micro-hardness (101 ± 11 Vickers) of SPS samples rather than microwave samples as it expected. By using two different sintering methods (pressure-less and pressure-assisted) and controlling processing parameters, the porosity of the final products can be changed from porous to fully dense composites.