Alumina phases

In addition to silica, (Si02)x, alumina, (Al203)x, is among the most common adsorbents in liquid-solid chromatography. Although highly efficient separator columns were developed with spherical beads of small diameter, alumina is of only minor importance in HPLC after the introduction of silica-based, chemically bonded reversed phases. 

The presence of hydroxide groups at the alumina surface was unequivocally confirmed by IR spectroscopy [66], whereas the nmnber and nature of these groups are determined by the thermal preconditioning of the material. The amphoteric character of alumina and its transformation into an anion or cation exchanger (via hydration and subsequent treatment with acid or base) are illustrated in the following reaction scheme  

Accordingly, the anion exchange process occurs between solute anions and OH ions attached to the alumina surface, while cation exchange occurs between solute cations and ions, which are released by dissociation of AI-OH groups. 

Most interesting are the selectivity differences between an alumina phase and a strongly basic anion exchanger with quaternary ammonium groups. In comparison with a conventional anion exchanger, halide ions, for example, are eluted 

In addition to silica, (Si02), aliunina, (AI2O3), is among the most common adsorbents in liquid-solid chromatography. Although highly efficient separator 

The first detailed studies of the retention behavior of inorganic anions and cations on alumina were carried out by Schwab and Ghosh [87]. They confirmed that predominantly ion-exchange processes are responsible for the retention of ionic species on alumina. The ion-exchange model considers the development of an electrical double layer at the alumina surface that is formed by the dissociation of Al-OH groups present at the surface and abstraction of 

10 mmol/L NaOH, step at 0.1 min to 10 mmol/L LiOH flow rate 0.5 mL/min detection suppressed conductivity peaks  

H and OH ions. This process, which is described in a simplified way by the dissociation equilibria (3.30) and (3.31), leads to the formation of positive and negative charges at which, according to Eqs. (3.32) and (3.33), anion or cation exchange occurs with the solute ions X and X, respectively. 

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Dispersion-strengthened copper is made by dispersing a thoria or alumina phase through copper powder. The resulting P/M product retains its strength at elevated temperatures. It is used, for example, as the conductor or lead wine that supports the hot filament inside incandescent lamps. 

Fig. 1. CryoHte—alumina phase diagram from 0 to 18.5% alumina. L, Hquid S, cryoHte S2, comndum O, Hquid , Hquid and soHd , soHd (14).

Fig. 11. Micrographs of (a) a hot-pressed alumina—TiC ceramic showing a white TiC phase and a dark alumina phase (3) and (b) a fracture surface of an...

Alumina is used because it is relatively inert and provides the high surface area needed to efftciendy disperse the expensive active catalytic components. However, no one alumina phase possesses the thermal, physical, and chemical properties ideal for the perfect activated coating layer. A great deal of research has been carried out in search of modifications that can make one or more of the alumina crystalline phases more suitable. Eor instance, components such as ceria, baria, lanthana, or 2irconia are added to enhance the thermal characteristics of the alumina. Eigure 6 shows the thermal performance of an alumina-activated coating material. 

The open nature of the conduction planes allows the Na+ ions to be easily replaced in all these phases. In general, the (3-alumina phase is less flexible to replacement the 3"-alumina more so, and the sodium can be replaced by almost any monovalent, divalent, or trivalent ion. The idealized formulas of these exchanged solids are A+A1h017, Ao A1iiOi7, or Aoj AlnOi . 

The nonstoichiometric nature of the (3-alumina phases lead to considerable difficulties in accurate crystal structure determination. Moreover, the defect structures tend... 

The defect structures of (3-alumina phases containing cations other than Na+ are different in detail. For example, in the silver analog of (3-alumina, the excess silver... 

B. Better tools available, but no consensus on mechanism or active site—1980 to 2006. Rhodes et al.291 published a comprehensive review on the heterogeneously catalyzed water-gas shift mechanism in 1995. Included in that discussion was the copper/zinc oxide/alumina system. The conclusion was that this system appears to be constructed of small metallic islands of copper resting on a zinc oxide alumina phase. Zinc oxide may exert some impact on catalytic activity, but it was suggested in the review that the contribution is small. It was indicated that strong evidence exists to support either a formate or a redox mechanism, and the authors even suggest the possibility that both mechanisms might occur, though insufficient data exist to determine which mechanism predominates. 

An alumina-based catalyst will be bound, for the purpose of mechanical strength, with carbon. The alumina-carbon mixture is essentially a composite support for adsorbing the Pt precursor. If it is desired that all metal go onto the alumina phase, which type of carbon (oxidized or unoxidized) and what type of Pt complex should be used and why A sketch of the surface potential vs. pH for alumina and the carbon binder will help. 

This transition produces an isomorphous phase and the resulting y-alumina has the same morphology and texture as its boehmite precursor. With increasing temperature and time the mean pore diameter increases gradually and other phases appear (S-, 6-alumina). Due to the broad XRD lines, the distinction between y- and S-alumina cannot be made 6-alumina occurs at about 900°C while the conversion to the chemically very stable a-alumina phase takes place at T> 1000°C. Some typical results for alumina membranes synthesized without binders are given in Table 2.4. When PVA was used as a binder, thermogravimetric analysis showed that, provided the appropriate binder type was used, the binder could be effectively removed at T > 400°C. The ash residue is of the order of 0.01 wt.%. Mean pore size and... 

Among the several transition alumina phases, y-Al203 is the most important and most studied phase for catalysis [57, 58]. However, even nowadays, several aspects of its structural and surface chemistry are still not well understood, since y-Al203 is a poorly crystalline solid, showing some variation in its structural stoichiometry and a wide range of defects. In the last 50 years, several empirical models for y-AI2O3 surface have been reported, trying to explain the complexity of this surface... 

Most industrial catalysts based on mixed oxides are simply and economically prepared via co-precipitation in aqueous medium.20 For the preparation of hexaaluminates, this method was first reported by S. Matsuda and co-workers.21 La203 xAl203 samples were prepared starting from an aqueous solution of La and Al nitrates. The precipitation was carried out by the addition of NH4OH solution up to pH=8. After it was washed, filtered and dried, the precursor was calcined at different temperatures up to 1400 °C. For a La203/Al203 mole ratio 5/95, the formation of a layered-alumina phase was observed starting from 1000°C and samples with a surface areas of 30 m2/g and 8 m2/g have been obtained upon calcination at 1200 °C and 1400 °C for 2 h respectively. 

Also in Fe-containing materials, promoting effect on the formation of the final layered-alumina phase was observed.24 In the completely substituted material BaFe120i9 a monophasic sample with magnetoplumbite structure was obtained at 700 °C. This low formation temperature was related to the greater mobility of oxygen and Ba ions in the lattice of Fe oxides than in A1 oxides. Indeed, the transition from y-+a alumina occurs in Fe oxides at temperatures hundreds of degrees lower than those required for phase transitions in A1 oxides. 

For composition near the extremes of the formation range of layered-alumina phases, samples with typical px (Ba poor composition) and pri (Ba-rich composition) structure were obtained. For the intermediate composition, BaAl12Oi9,... 

The crystal structure and the sintering behavior of hexaaluminates was widely investigated. The relation of sintering resistance to anisotropic ion diffusion in the layered alumina phase was clarified to a large extent. Other evidence suggests that combustion activity is obtained through a redox mechanism associated with reversible variation of oxidation state of the transition metal ions in the structure. Mn was the best and most stable active component. However, fundamental and applied studies are needed to better clarify the redox mechanism of the reaction and how it is related to the chemical and structural features of the Mn-containing layered-alumina phase. This could also provide useful information for the development of an optimum catalyst composition,... 

Our support precursor having a Mg Al molar ratio of about 3 1 shows an x-ray diffraction pattern typical of hydrotalcite (see Figure la) (10). After calcination at 873 K the resulting diffraction pattern exhibits diffuse peaks corresponding to MgO (Figure lb). No evidence for separate crystalline aluminum phases was found so A1 cations probably remain closely associated with or dissolved in the MgO structure. However, it is possible that amorphous alumina phases, not detectable by x-ray diffraction, may be present in our mixed oxide. 

The catalysts, both fresh and used, were characterized as to BET surface area, pore size distribution, elemental analysis, x-ray diffraction and XPS. Some BET and pore volume data are given in Table 1. The diffraction pattern of Catalyst B gave some indication of a gamma-alumina phase, not well resolved All other peaks were well-resolved, suggesting the absence of amorphous or highly-dispersed phases. 

Pinheiro et al. (2002) C, Co-alumina Phase growth Phase distinction between CNT and whiskers + + + Carbon nanotube growth... 

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