Aluminum oxide, surface area

The catalytic activity of aluminas are mostly related to the Lewis acidity of a small number of low coordination surface aluminum ions, as well as to the high ionicity of the surface Al-O bond [67,92]. The number of such very strong Lewis sites present on aluminum oxide surfaces depends on the dehydroxylation degree and on the particular phase and preparation. Depending on the activation temperature, the density of the strongest Lewis acid sites tends to decrease as the calcination temperature of the alumina increases (i.e., upon the sequence y — 5 —> 9, which is also a sequence of decreasing surface area and increasing catalyst stability). 

On the other hand, removal of layers of water from an aluminum oxide surface in distinct steps has been noted by de Boer (113a). Thus the foregoing explanation cannot be ruled out. since highly hydrated gels made by neutral or low pH and at room temperature generally have specific surface areas in the range 600-800 m g . 

Traditional adsorbents such as sihca [7631 -86-9] Si02 activated alumina [1318-23-6] AI2O2 and activated carbon [7440-44-0], C, exhibit large surface areas and micropore volumes. The surface chemical properties of these adsorbents make them potentially useful for separations by molecular class. However, the micropore size distribution is fairly broad for these materials (45). This characteristic makes them unsuitable for use in separations in which steric hindrance can potentially be exploited (see Aluminum compounds, aluminum oxide (ALUMINA) Silicon compounds, synthetic inorganic silicates). 

The predominate role of the 2inc and aluminum oxides in the ICI catalyst is to reduce the rate of sintering and loss of metallic copper surface area, which, in addition to poisoning, is one of the modes of activity loss with time for this catalyst. 

The small (10 -lm) coating particles are typically aluminum oxide [1344-28-1/, Al O. These particles can have BET surface areas of 100 to 300 m /g. The thermal and physical properties of alumina crystalline phases vary according to the starting phase (aluminum hydroxide or hydrate) and thermal treatment (see ALUMINUM COMPOUNDS, ALUMINUM OXIDE). 

An alternate form of catalyst is pellets. The pellets are available in various diameters or extruded forms. The pellets can have an aluminum oxide coating with a noble metal deposited as the catalyst. The beads are placed in a tray or bed and have a depth of anywhere from 6 to 10 inches. The larger the bead (1/4 inch versus 1/8 inch) the less the pressure drop through the catalyst bed. However, the larger the bead, the less surface area is present in the same volume which translates to less destruction efficiency. Higher pressure drop translates into higher horsepower required for the oxidation system. The noble metal monoliths have a relatively low pressure drop and are typically more expensive than the pellets for the same application. 

The infrared spectra for various aluminum oxides and hydroxides are shown in Figure 3. Figure 3a is a-alumina (Harshaw A13980), ground to a fine powder with a surface area of 4 m /g. The absorption between 550 and 900 cm is due to two overlapping lattice modes, and the low frequency band at 400 cm is due to another set of lattice vibrations. These results are similar to those obtained by reflection measurements, except that the powder does not show as... 

GP 2] [R 3a] The performance of one micro reactor with three kinds of catalyst -construction material silver, sputtered silver (dense) on aluminum alloy (AlMg3), and sputtered silver on anodically oxidized (porous) aluminum alloy (AlMg3) -was compared with three fixed beds with the same catalysts [44]. The fixed beds were built up by hackled silver foils, aluminum wires (silver sputtered) and hack-led aluminum foils (anodically oxidized and silver sputtered), all having the same catalytic surface area as the micro channels. Results were compared at the same flow rate per unit surface area. 

P 17] In order to have a catalyst with a sufficiently high specific surface area, pretreatment of the micro channels made of aluminum was necessary [17], Following a cleaning procedure, an oxide layer with a regular system of nanopores was generated by anodic oxidation (1.5% oxalic acid 25 °C 50 V DC 2 h exposure using an aluminum plate cathode followed by calcination). 

Activated aluminas. Activated alumina is a porous form of aluminum oxide (A1203) with high surface area, manufactured by heating hydrated aluminum oxide to around 400°C in air. Activated aluminas are mainly used to dry gases and liquids, but can be used to adsorb gases and liquids other than water. 

Aluminum foil capacitors occupy an important position in circuit applications due to their unsurpassed volumetric efficiency of capacitance and low cost per unit of capacitance.328 Together with tantalum electrolytic capacitors, they are leaders in the electronic discrete parts market. Large capacitance is provided by the presence of extremely thin oxide layers on anodes and cathodes, and high surface areas of electrodes could be achieved by chemical or electrochemical tunnel etching of aluminum foils. The capacitance of etched eluminum can exceed that of unetched metal by as much as a factor of 50.328... 

Reasonable assumptions on specific surface area for aluminum oxides are 10 m2 g-1 we may further assume 10 4 surface sites g1 and a concentration of 1 g/e.Thus,... 

The 1 1 kaolin structures are chemically simpler the tetrahedral sites are occupied by silicon and the octahedral sites by aluminum. There is a minor amount of substitution, largely of ferric iron for aluminum, but the amounts are generally only a few tenths of a percent by weight of oxide. The kaolin minerals do not expand in the presence of water and their surface area, approximately 10 to 15 m2/g, represents the external area of the crystals. 

When 1,2-dichlorobenzene in hydrogen-saturated deionized water was exposed to a slurry of palladium catalyst (1%) at room temperature, benzene formed via the intermediate chlorobenzene. The reaction rate decreased in the order of MCM-41 (mesoporous oxide having a silicon aluminum ratio of 35) > alumina > Y (dealuminated zeolite having a silicon aluminum ratio of 15). It appeared the reaction rate was directly proportional to the surface area of the support catalyst used (Schiith and Reinhard, 1997). 

F NMR of Fluorine-Doped -Alumina. The samples studied 115) were high surface area aluminum oxides doped with fluorine by addition of aqueous HF to alumina and subsequent dehydration. A suflScient number of paramagnetic impurities were present in the samples to give relaxation times of the order of 0.01 second. The BET surface areas of most of the samples examined were within 20% of 250 meterVgram. 

Most y-alumina powders placed under high vacuum and heated at moderate temperature (373-423 K) undergo complete water removal, and nearly all of the surface aluminum atoms are hydroxylated. Additionally, for hydroxylated alumina thin films prepared via aluminum evaporation/water oxidation cycles, although the surface density of OH groups would be the same as for powders, the specific surface area differs significantly [9]. 

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