Effect of atomic layer deposition coatings on the surface structure of anodic aluminum oxide membranes

Anodic aluminum oxide (AAO) membranes were characterized by UV Raman and FT-IR spectroscopies before and after coating the entire surface (including the interior pore walls) of the AAO membranes by atomic layer deposition (ALD). UV Raman reveals the presence of aluminum oxalate in bulk AAO, both before and after ALD coating with Al2O3, because of acid anion incorporation during the anodization process used to produce AAO membranes. The aluminum oxalate in AAO exhibits remarkable thermal stability, not totally decomposing in air until exposed to a temperature >900 degrees C. ALD was used to cover the surface of AAO with either Al2O3 or TiO2. Uncoated AAO have FT-IR spectra with two separate types of OH stretches that can be assigned to isolated OH groups and hydrogen-bonded surface OH groups, respectively. In contrast, AAO surfaces coated by ALD with Al2O3 display a single, broad band of hydrogen-bonded OH groups. AAO substrates coated with TiO2 show a more complicated behavior. UV Raman results show that very thin TiO2 coatings (1 nm) are not stable upon annealing to 500 degrees C. In contrast, thicker coatings can totally cover the contaminated alumina surface and are stable at temperatures in excess of 500 degrees C.     

以氨水和碳酸铵为沉淀剂制备氧化铝的对比研究

γ-Al2O3，是一种常用的催化剂载体，它具有比表面大和价廉易得等优点。但对于很多高温反应体系，如汽车尾气催化净化，其热稳定性在很大程度上影响了汽车尾气净化催化剂的活性和稳定性，因此提高γ-Al2O3的高温热稳定性对保持汽车尾气净化催化剂的反应活性、延长催化剂的使用寿命非常重要。     

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

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

Orthophosphate and metaphosphate ion removal from aqueous solution using alum and aluminum hydroxide

The removal of orthophosphates (10(-2) kg P m(-3)), condensed phosphates (10(-2) kg P m(-3)), and mixtures of both (5 x 10(-3) kg P m(-3) as orthophosphate and 5 x 10(-3) kg P m(-3) as metaphosphate) in aqueous solution is studied using alum and aluminum hydroxide. The effects of coagulant dose, pH, temperature, aging of aluminum hydroxide, and presence of different ions are investigated. On the basis of the experimental results, alum is much more efficient in phosphorus removal than aluminum hydroxide even if, in both cases, at the conditions studied, the active coagulant form is Al(OH)(3). The differences then could be due to the higher activity of the in situ formed hydroxide. Orthophosphates and metaphosphates seem to have similar behavior vs pH variation: maximum removal is achieved at pH values 5-6 in all cases. On the other hand, in the simultaneous presence of both P forms, orthophosphate and metaphosphate ions have different affinities for the surface sites of aluminum hydroxide, since for both alum and aluminum hydroxide, orthophosphates are preferentially removed compared to metaphosphates, due probably to orientation effects and the charge per P atom. The presence of sodium, potassium, magnesium, sulfate, chloride, and magnesium, at the concentrations studied and for a pH value of 6, does not influence P removal. Temperature variation, between 25 and 60 degrees C, does not affect alum efficiency but both P forms are increasingly removed with increasing temperature, probably due to polymer Al(OH)(3) breaking, producing new surfaces for adsorption. Aging decreases sorption capacity of Al(OH)(3), while crystallites of increasing size are formed. Finally adsorption of both P forms is best described by the Freundlich isotherm [[K(F)=(49.1-69.1) x 10(-3) (m(3)kg(-1))(1/N), 1/N: 0.14-0.19 for T=25-60 degrees C] and [ K(F)=(1.58-2.79) x 10(-3) (m(3)kg(-1))(1/N), 1/N: 2.17-2.47 for T=25-60 degrees C] for orthophosphate and metaphosphate, respectively.     

High surface area, mesoporous, glassy alumina with a controllable pore size by nanocasting from carbon aerogels

A strategy to synthesize amorphous, mesoporous alumina by nanocasting has been developed, involving carbon aerogel as a hard template and aluminum nitrate solution as an alumina precursor. The alumina generated exhibits small, transparent granules with a 3-6 mm diameter and has inherited the three-dimensional network structure of the carbon template. The mesopore surface area of the alumina can be as high as 365 m2 g(-1), and the pore volume reaches 1.55 cm3 g(-1) after calcination at 600 degrees C in air for 8 h. The pore parameters can be varied within a certain range by variation of the carbon aerogel template and the loading amount of the alumina precursor. At high loadings, the obtained glassy alumina clearly has a bimodal pore size distribution in the mesopore range.     

Association of Molybdenum Ionic Species with Alumina Surface

The adsorption isotherms at 25, 45, and 65 degrees C of molybdenum solutions of concentration ranges between 10(-3) and 3x10(-2) M(Mo) (pH 4-5) on different alumina samples are investigated. The analysis is conducted using a modified Frumkin isotherm which takes a more realistic account of the lateral interaction between adsorbed species and considers that the adsorption takes place on the most basic OH groups on the surface of alumina. The results are discussed in view of the difference in solutions speciation, and the changes in the pH of the remaining supernatant solutions. The solution temperature, PZC of the used aluminas, the configuration of the basic OH groups on their surface, and the pore structure have been shown to intervene effectively. Copyright 2000 Academic Press.     

The effect of anode surface area on nanoporous oxide formation during anodizing of low purity aluminum (AA1050 alloy)

Porous anodic alumina layers were obtained by a simple two-step anodization of low purity aluminum (99.5 % Al, AA1050 alloy) in a 0.3 M oxalic acid electrolyte at 45 V and 20 °C. The effect of anode surface area on structural features of nanoporous oxide and process of oxide formation was investigated. An ordered structure composed of nanostripes or nanopores was formed on the Al surface during electrochemical polishing in a mixture of perchloric acid and ethanol. This nanopattern is then replicated during the anodic oxide formation. It was found that the pore diameter, interpore distance, and porosity increase slightly with increasing surface area of the aluminum sample exposed to the anodizing electrolyte. On the other hand, a slight decrease in pore density and cell wall thickness was observed with increasing surface area of the sample. The detailed inspection of current density vs. time curves was also performed. The obtained results revealed that the higher surface area of the anode, the local current density minimum, was reached faster during first step of anodization and the increase in current density corresponding to the pore rearrangement process was observed earlier. Finally, a dense array of Pd nanowires (～90 nm in diameter) was synthesized by simple electrodeposition of metal inside the channels of through-hole nanoporous anodic alumina templates with relatively large surface areas (4 cm²).     

Heterogeneous reactions of gaseous HNO3 and NO2 on the clay minerals kaolinite and pyrophyllite

Airborne clay mineral particles have long atmospheric lifetimes due to their relatively small size. To assess their impact on trace atmospheric gases, we investigated heterogeneous reactions on prototype clay minerals. Diffuse reflectance infrared spectroscopy identified surface-adsorbed products formed from the uptake of gaseous nitric acid and nitrogen dioxide on kaolinite and pyrophyllite. For kaolinite, a 1:1 phyllosilicate, HNO3 molecularly adsorbed onto the octahedral aluminum hydroxide and tetrahedral silicon oxide surfaces. Also detected on the aluminum hydroxide surface were irreversibly adsorbed monodentate, bidentate, bridged, and water-coordinated nitrate species as well as surface-adsorbed water. Similar adsorbed products formed during the uptake of NO2 on kaolinite at relative humidity (RH) of 0%, and the reaction was second order with respect to reactive surface sites and 1.5 +/- 0.1 for NO2. Reactive uptake coefficients, calculated using Brunauer, Emmett, and Teller surface areas, increased from (8.0 +/- 0.2) x 10(-8) to (2.3 +/- 0.4) x 10(-7) for NO2 concentrations ranging from 0.56 x 10(13) to 8.8 x 10(13) molecules cm(-3). UV-visible spectroscopy detected gaseous HONO as a product for the reaction of NO2 on wet kaolinite. The uptake of HNO3 on pyrophyllite, a 2:1 phyllosilicate, resulted in stronger signal for nitric acid molecularly adsorbed on the silicon oxide surface compared to kaolinite. Monodentate, bridged, and water-coordinated nitrate species bound to aluminum sites also formed during this reaction indicating that reactive sites on edge facets are important for this system. The uptake of NO2 on pyrophyllite, gammaBET = (7 +/- 1) x 10(-9), was significantly lower than kaolinite because NO2 did not react with the dominant tetrahedral silicon oxide surface. These results highlight general trends regarding the reactivity of tetrahedral silicon oxide and octahedral aluminum hydroxide clay surfaces and indicate that the heterogeneous chemistry of clay aerosols varies with mineralogy and cannot be predicted by elemental analysis.     

Matrix-isolated Al2OF6(2-) ion in molten and solid LiF/NaF/KF

A Raman spectrum consistent with that expected from an Al2OF6(2-) ion was observed when Na2O was dissolved in a eutectic LiF/NaF/KF (FLINAK) melt at 500 degrees C, which contained a low concentration of either AlF3 or Na3AlF6. Furthermore, it was possible to trap the Al2OF6(2-) ion in the frozen solid and to measure its Raman and IR spectra at 25 degrees C. A number of bands have been detected; among those, the two most characteristic bands of the Al2OF6(2-) ion at 494 (polarized) and 265 cm-1 in the FLINAK melt at 500 degrees C, and those at 509 and 268 (Raman) and approximately 780 to approximately 900 (IR) cm-1 for the compound matrix isolated in solid FLINAK at 25 degrees C. In the absence of added oxide, the dissolved aluminum fluoride was in the form of the octahedral AlF6(3-) ion, which has characteristic Raman bands at 542 and 325 cm-1 in the FLINAK melt at 500 degrees C. Whereas alumina, Al2O3, was found to be essentially insoluble in FLINAK melts, it was possible to dissolve sufficient amounts of Na2O to convert most of the AlF6(3-) to the oxyfluoroaluminate, Al2OF6(2-). These solutions appeared to be metastable with respect to formation of insoluble alumina at higher temperatures. The present results can be compared to previous measurements on alumina dissolved in pure molten cryolite at much higher temperatures, where alumina solubility is low and broad bands due to oxide species are difficult to detect due to overlap with bands from AlF6(3-) and AlF4-.     

Effect of nickel deposition on hydrogen permeation behavior of mesoporous gamma-alumina composite membranes

Ni/alumina composite membranes were prepared and investigated for hydrogen separation at high temperature. alpha-Alumina-supported gamma-alumina composite membranes were prepared by soaking-rolling method. In order to improve H2 selectivity and permeance of the gamma-alumina membranes, Ni was deposited by a soaking process. As a result of a single gas permeation test of the Ni/alumina composite membranes, hydrogen permeance and H2/N2 selectivity at permeation temperature of 450 degrees C were 6.29 x 10(-7) mol/m2 s Pa and 5.2 which exceeded theoretical Knudsen selectivity. Contribution of surface diffusion was investigated by temperature dependence of H(2) permeance. The surface diffusion was observed at higher temperature above 250 degrees C. The Ni deposition on surface of the gamma-alumina composite membrane led to hydrogen permeation via Knudsen diffusion combined with surface diffusion, which gave high H2 selectivity exceeding the Knudsen diffusion mechanism.     

Ordered mesoporous alumina-supported metal oxides

The one-pot synthesis of alumina-supported metal oxides via self-assembly of a metal precursor and aluminum isopropoxide in the presence of triblock copolymer (as a structure directing agent) is described in detail for nickel oxide. The resulting mesoporous mixed metal oxides possess p6 mm hexagonal symmetry, well-developed mesoporosity, relatively high BET surface area, large pore widths, and crystalline pore walls. In comparison to pure alumina, nickel aluminum oxide samples exhibited larger mesopores and improved thermal stability. Also, long-range ordering of the aforementioned samples was observed for nickel molar percentages as high as 20%. The generality of the recipe used for the synthesis of mesoporous nickel aluminum oxide was demonstrated by preparation of other alumina-supported metal oxides such as MgO, CaO, TiO 2, and Cr 2O 3. This method represents an important step toward the facile and reproducible synthesis of ordered mesoporous alumina-supported materials for various applications where large and accessible pores with high loading of catalytically active metal oxides are needed.     

Transformations of gamma-alumina in aqueous suspensions 1. Alumina chemical weathering studied as a function of pH

Hydration of gamma-Al2O3 is often reported to occur via the superficial transformation of the alumina surface into aluminum hydroxide-like layers. However, very little evidence has been given so far to support this hypothesis. It is demonstrated here by X-ray diffraction, TEM, electron diffraction, and solubility studies that a second process of hydration takes place that involves the dissolution of alumina and subsequent precipitation of well-shaped Al(OH)3 particles from supersaturated alumina aqueous solution. This process can be observed on a macroscopic scale (XRD, TEM) for any pH5, provided that the contact time between alumina and water exceeds 10 h. The least thermodynamically stable phase of aluminum hydroxide, bayerite, becomes favored compared with gibbsite when the pH of the solution is increased. It is assumed that the rate of formation of bayerite germs is greater than that of gibbsite due to variations in aluminum speciation in solution as a function of pH.     

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

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

Mesostructured gamma-Al(2)O(3) with a lathlike framework morphology

A novel three-step assembly pathway is reported for the formation of a mesostructured alumina with framework pore walls made of crystalline, lathlike gamma-Al(2)O(3) nanoparticles. In the initial supramolecular assembly step of the pathway a mesostructured alumina with a wormhole framework morphology and amorphous pore walls is assembled through the hydrolysis of Al(13) oligocations and hydrated aluminum cations in the presence of a nonionic diblock or triblock poly(ethylene oxide) surfactant as the structure-directing porogen. The walls of the initial mesostructure are then transformed in a second hydrolysis step at a higher temperature to a surfactant-boehmite mesophase, denoted MSU-S/B, with a lathlike framework made of boehmite nanoparticles. A final thermal reaction step topochemically converts the intermediate boehmitic mesophase to a mesostructure with crystalline gamma-Al(2)O(3) pore walls, denoted MSU-gamma, with retention of the lathlike framework morphology. The boehmitic MSU-S/B intermediates formed from the chloride salts of aluminum incorporate chloride anions into the mesostructure. Chloride ion incorporation tends to disorder the nanoparticle assembly process, leading to a broadening of the slit-shaped framework pores in the final MSU-gamma phases and to the introduction of intra- and interparticle textural mesopores. However, the well-ordered MSU-gamma phases made from aluminum nitrate as the preferred aluminum reagent exhibit narrow framework pore size distributions and average pore sizes that are independent of the surfactant size and packing parameter, in accord with a lathlike framework assembled from nanoparticles of regular size and connectivity. The high surface areas ( approximately 300-350 m(2)/g) and pore volumes ( approximately 0.45-0.75 cm(3)/g) provided by these mesostructured forms of gamma-Al(2)O(3) should be useful in materials and catalytic applications where the availability of surface Lewis acid sites and the dispersion of supported metal centers govern reactivity.     

Adsorption and desorption of stearic acid self-assembled monolayers on aluminum oxide

Development of coatings to minimize unwanted surface adsorption is extremely important for their use in applications, such as sensors and medical implants. Self-assembled monolayers (SAMs) are an excellent choice for coatings that minimize nonspecific adsorption because they can be uniform and have a very high surface coverage. Another equally important characteristic of such coatings is their stability. In the present study, both the bonding mechanism and the stability of stearic acid SAMs on two aluminum oxides (single-crystal C-plane aluminum oxide (sapphire) and amorphous aluminum oxide (alumina)) are investigated. The adsorption mechanism is investigated by ex situ X-ray photoelectron spectroscopy and infrared (IR) spectroscopy. The results revealed that stearic acid binds to sapphire surfaces via a bidentate interaction of carboxylate with two oxygen atoms while it binds to alumina surfaces via both bidentate and monodentate interactions. Desorption kinetics of stearic acid self-organized on both aluminum oxide surfaces into water is explored by ex situ tapping mode atomic force microscopy, IR spectroscopy, and contact angle measurements. The results exhibit that the SAMs of stearic acid formed on sapphire are not stable in water and are continuously lost through desorption. Water contact angle measurements of SAMs that are immersed in water further indicate that the desorption rate of adsorbates from atomically smooth terrace sites is substantially faster than that of adsorbates from the sites of surface defects due to weaker molecular interaction with the smooth surface. A time-dependent desorption profile of SAMs grown on amorphous alumina reveals that contact angles decrease monotonically without any regional distinction, providing further evidence for the presence of adsorption sites with different types of affinity on the amorphous alumina surface.     

Surface and Texture Characterization of Alumina-Supported Manganese Oxide Catalysts and Their Formate Precursors

Alumina-supported manganese oxide catalysts as well as the parent formates were characterized my means of FTIR spectroscopy and nitrogen physisorption. Two different manganese formate solutions are used for the impregnation of alumina (4.8 and 8.0 mass%). The infrared bands in the high frequency region (OH stretches) indicate that the manganese complexes formed in the solutions are deposited gradually at the basic and acid surface OH groups. Part of the basic OH groups are neutralized with HCOOH (pH 5-5.2). The neutral OH groups remain unchanged during the impregnation. The nitrogen physisorption shows that the initial mesoporous character of the gamma-Al2O3 structure does not change during impregnation. The rFHH values which characterized the adsorbent-adsorbate interaction forces depend on the concentration of the impregnating solutions, i.e., on the type of the manganese complexes deposited on alumina surfaces. On the basis of the analysis of the pore size distribution curves the distribution of the supported formate and oxide phases is discussed. Copyright 1999 Academic Press.     

Preparation and characterization of porous cordierite for potential use in cellular ceramics

Cordierite porous ceramics Z, X, and K were prepared using three mixtures of clay minerals: Z from kaolinite, talc, and aluminum hydroxide, X from kaolinite, talc, vermiculite, and aluminum hydroxide, and K from kaolinite, talc, and magnesium oxide. Ceramics were different in porosity, specific surface area, cordierite polymorphs, and secondary crystalline phases. Vermiculite influenced textural architecture of calcined cordierite ceramics X and predestinated crystallization of the high-temperature hexagonal α-cordierite with secondary minerals enstatite, spinel and corundum. Ceramics Z contained low-temperature orthorhombic β-cordierite, enstatite, and corundum, K was diphase of β-cordierite and forsterite. Total pore area (TPA) and specific surface area (SSA) of X, in spite of the higher porosity and the pore size distribution in the range of 300–1000 nm, were smaller in comparison with TPA and SSA of Z. Ceramics K retained high porosity, two maxima at 300–1000 nm and 50–200 nm in the pores size distribution, and the highest TPA and SSA compared to those observed in ceramics Z and X. Presented at the 8th Conference on Solid State Chemistry, 6-11 July 2008, Bratislava, Slovak Republic.     

Molecular layering of phosphorus oxide structures on the surface of gamma alumina

Phosphorus oxide structures were synthesized on the surface of porous gamma alumina, with phosphorus oxychloride and water vapors used as reagents for successive and alternating (up to four times) treatment of a solid-phase matrix by the molecular layering method with in situ monitoring of the process. It was shown that this procedure yields a double-phase system constituted by a core (aluminum oxide) and shell (amorphous phosphorus oxide layer) with a prescribed chemical composition and structure with characteristics dependent on the number of molecular layering cycles. With increasing treatment rate (from 1 to 4) of the solid-phase matrix with halide and water vapors, the concentration of phosphorus in the samples steadily grows. In the process, the specific surface area, pore volume, and pore size in the double-phase system being formed become smaller. The results obtained in the study are of interest for development of catalytic, sorption, and other functional systems of the core–shell type.     

Nickel based electrocatalysts for oxygen evolution in high current density, alkaline water electrolysers

A number of nickel based materials are investigated as potential oxygen evolution catalysts under conditions close to those met in modern, high current density alkaline water electrolysers. Microelectrodes are used to avoid distortion of voltammetric data by IR drop even at the high current densities employed in such water electrolysers. High surface area nickel metal oxides prepared by cathodic deposition and mixed oxides prepared by thermal methods are considered. A mixed Ni/Fe oxide is the preferred electrocatalyst. The influence of hydroxide ion concentration and temperature on the voltammetry is defined. Preliminary stability tests in a zero gap cell with an OH(-) conducting membrane show no significant increase in overpotential during 10 days operation in 4 M NaOH electrolyte at a current density of 1 A cm(-2) at 333 K.     

Hexamethylenetetramine directed synthesis and properties of a new family of alpha-nickel hydroxide organic-inorganic hybrid materials with high chemical stability

A new family of organic-inorganic hybrid material of alpha-nickel hydroxide formulated as Ni(OH)2-x(An-)x/n-(C6H12N4)y.zH2O (A=Cl-, CH3COO-, SO4(2-), NO3-; x=0.05-0.18, y=0.09-0.11, z=0.36-0.43) with high stability and adjustable interlayer spacing ranging from 7.21 to 15.12 A has been successfully prepared by a simple hydrothermal method. The effects of various anions and hexamethylenetetramine (HMT) on the d values of alpha-nickel hydroxide have been systematically investigated. This family of hybrid materials is of such high stability that they can stand more than 40 days in 6 M KOH. The product with a formula Ni(OH)1.95(C6H12N4)0.11(Cl-)0.05(H2O)0.36 has a high surface area of about 299.26 m2/g and an average pore diameter of about 45.1 A. The coercivity (Hc) value is ca. 2000 Oe for the sample with a d spacing of 13.14 A. Moreover, the prepared alpha-Ni(OH)2 in our experiment is of high stability in strong alkali solution. Such high stability could be derived from strong chelating interactions between the Ni ions and HMT molecules with the interlayers. This high chemical stability could make this material more suitable for the applications.