Alumina, Titania, and Zirconia

Aluminum oxide (alumina) and titanium oxide (titania) are not much used as stationary phases in HPLC. The solvent strength on these adsorbents is similar to the solvent strength on silica. Alumina and titania both have basic functions giving strong retention of analytes with acidic groups. In addition, aluminum, titanium. 

Solvent Solvent strength (e°) Boiling point (°C) Viscosity (cP) UV cutoff (nm) 

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The thermograms of Cu reduction in silica-, alumina-, titania- and zirconia-supported catalysts show only one pe the maximum of which is reported in Table 3. The amount of hydrogen consumed by the r uction corresponds, within experimental error, to the theoretical amount required for the reaction ... 

Ordered mesoporous materials of compositions other than silica or silica-alumina are also accessible. Employing the micelle templating route, several oxidic mesostructures have been made. Unfortunately, the pores of many such materials collapse upon template removal by calcination. The oxides in the pore walls are often not very well condensed or suffer from reciystallization of the oxides. In some cases, even changes of the oxidation state of the metals may play a role. Stabilization of the pore walls in post-synthesis results in a material that is rather stable toward calcination. By post-synthetic treatment with phosphoric acid, stable alumina, titania, and zirconia mesophases were obtained (see [27] and references therein). The phosphoric acid results in further condensation of the pore walls and the materials can be calcined with preservation of the pore system. Not only mesoporous oxidic materials but also phosphates, sulfides, and selenides can be obtained by surfactant templating. These materials have pore systems similar to OMS materials. 

Hydrolytically stable inorganic carriers are alumina, titania, and zirconia. Their physical and chemical properties are quite different from that of silica. Their application as a base for bonded phases has been described in literature. 

Chromatographic properties of silica, alumina, titania, and zirconia have been compared... 

Despite the many desirable properties of silica, its limited pH stability (between 2 and 7.5) is also a major issue in NPC when strong acidic or basic mobile-phase additives are used to minimize interactions. Hence, other inorganic materials such as alumina, titania, and zirconia, which not only have the desired physical properties of silica but also are stable over a wide pH range, have been studied. Recently, Unger and co-workers [22] have chosen a completely new approach where they use mesoporous particles based not only on silica but also on titania, alumina, zirconia, and alumosilicates. These materials have been used by the authors to analyze and separate different classes of aromatic amines, phenols, and PAHs (polyaromatic hydrocarbons). 

Two methods for the evaporation of precursors may be employed - resistance heating and electron beam collision. The first method employs a simple alumina crucible that is heated by a W filament. Temperatures as high as 1,800°C may be reached inside the chamber, which is enough for some metals or metal salts to vaporize. Deposition rates for this method are 1-20 A s . The use of an electron beam to assist in the precursor evaporation results in temperatures on the order of 3,000°C, being more suited for the deposition of refractory metals/alloys and metal oxides such as alumina, titania, and zirconia. Since the temperature of the chamber interior is much higher than the walls, the gas-phase ions/atoms/molecules condense on the sidewalls as well as the substrate this may lead to film contamination as the nonselective coating flakes off the chamber walls. 

Effect of heat treating temperature on mean pore diameters of alumina, titania and zirconia membranes... 

The experimental observation that there exists a critical thickness above which cracking occurs cannot easily be explained. Brinker [1] discusses a theory which explains that very thin layers can bear much larger stresses because critical cracks carmot be formed unless a certain critical thickness is surpassed. This thickness is estimated to be equal to or less than 1 pm and Brinker comes to the conclusion that thicker films will always crack. This is certainly not the case for alumina, titania and zirconia films for which much larger (alumina) to larger (titania) thicknesses are observed. As shown in Table 8.2 critical thicknesses of a few pm in single-step dip-coated films occur and critical flaws are smaller than this thickness and so can be present. 

C.H. Chang, R. Gopalan and Y.S. Lin, A comparative study on thermal and hydrothermal stability of alumina, titania and zirconia membranes. /. Membr. Sci., 91 (1994) 27-45. 

Temperature resistant oxide supports for catalytic active phases are necessary for the catalytic combustion of hydrocarbon pollutants in the field of environment as well for energy production through hydrocarbon clean combustion. Four commonly used pure support oxides in catalysis silica, alumina, titania, and zirconia were synthesised by the sol-gel and aerogel methods and, in parallel, the same oxides were doped with yttria. Pure yttria aerogel was also made and characterised. Heat treatments were performed from ambient up to 1200°C and BET surface areas and XRD patterns were recorded after heat treatments at 300, 600, 900 and 1200 °C for pure and doped oxides, respectively. 

However, other derivatization techniques have been developed that are suitable for application to any surface and therefore can be used for the modification of the surfaces of these three oxides. In the following, we will first discuss the properties of the surfaces of alumina, titania, and zirconia, and then proceed to examine some surface modification techniques that have been used to date. 

Attempts have been made to design packings with an expanded pH compatibility compared to silica, but with a hardness comparable to silica. Other inorganic carriers such as alumina, titania, and zirconia have been explored. Indeed, their hardness matches that of silica, and being impervious to small molecules, they also exhibit the same advantageous mass-transfer properties as silica. However, no simple surface modification techniques are available as yet that match the silanization chemistry used for silica. Therefore, polymeric coatings have been used, which then in turn exhibit inferior mass-transfer behavior. 

For alumina, titania, and zirconia, there exists as yet no covalent bonding chemistry that is equivalent to the silanization technique used for silica. Although attempts have been made to silanize these other oxides, the hydrolytic stability of these phases does not match up to the hydrolytic stability of the support itself. Therefore alternative surface modification tet ques have been developed that do not rely on the attachment of the modifier to the surface. The coating can be simply insoluble in the intended mobile phases, or a crosslinked coating can be formed that stretches like a net around the skeleton of the particle. Both techniques are, in principle, independent of the nature of the substrate and can be applied to all inorganic or polymeric packings. 

M. Laniecki, M. M. Grycz, F. Domka, Water-gas shift reaction over sulfided molybdenum catalysts I. Alumina, titania and zirconia-supported catalysts, Appl. CataL A Gen. 196 (2000) 293-303. 

While silica is probably the most frequently encountered oxide surface, other materials particularly alumina, titania and zirconia also have considerable use and spectroscopic characterization is beneficial. One study mentioned above explored the potential for modifying alumina, zirconia, titania and thoria surfaces in a marmer similar to silica [6]. While bonding of the moiety is usually the method of choice, in some cases adsorption is sufficient. For example, polyacrylates adsorbed on alumina are a useful dispersent in the production of certain ceramic products. The successful adsorption of these compounds on aluminia has been monitored by FTIR using the carbonyl stretching frequency for the acrylate species which appears between 1602-1606 cm [15]. Polybutadiene which has been adsorbed on alumina for use as a chromatographic phase can be detected by FTIR [16]. A similar adsorption process has also been tested on zirconia [17,18]. It has also been shown that FTIR can be used to detect Langmuir-Blodgett layers on the metal surfaces of thin-film devices [19]. 

XRD data reveal that alumina particles in the sol are of boehmite crystalline structure and the particles in zircornia and titania sols are of amorphous structure [34], The alumina, titania and zirconia samples obtained from the sols after gelation and calcination at 450°C are respectively in the phases of y-alumina, tetragonal zirconia and anatase. These are thermodynamically metastable phases, and may transform to the thermodynamically stable phases, which are a-alumina, monoclinic zirconia and rutile. The crystallite structure and lattice parameters of these phases are listed in Table 1. 

Alumina, titania and zirconia in their metastable phases will transform to their stable phases. Such phase transformation usually occurs via a nucleation and crystal growth process [34, 37]. Kinetically, however, the phase transformation will not occur or be observed at low temperatures. For alumina, the 7-AI2O3 to a-AhOa (via 5- and 6-aluminas) phase transformation is found to start at temperature around 900°C. This is accompanied with a sharp decrease in the surface area and increase in the pore size of the alumina adsorbent. The surface area of y-AhOs adsorbent will decrease with time at temperatures lower than 900°C due primarily to sintering [34]. 

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