Aluminium oxide particles

Catalysts can be metals, oxides, sulfides, carbides, nitrides, acids, salts, virtually any type of material. Solid catalysts also come in a multitude of forms and can be loose particles, or small particles on a support. The support can be a porous powder, such as aluminium oxide particles, or a large monolithic structure, such as the ceramics used in the exhaust systems of cars. Clays and zeolites can also be solid catalysts. 

The efficiency with which substances from the surrounding atmosphere are absorbed and distributed in the drop depends on the degree of mixing within the drop. Movement inside the drop can be observed in two ways, namely by suspending solids (e.g. aluminium oxide particles) in the levitated drop and by spiking the drop with a highly concentrated dye solution. In both cases, the speed and the pattern of the distribution inside the drop can be visually observed. 

The development of stress during calcination is shown in Fig. 8.20 for boehmite membranes calcined at 600°C (thickness after calcination is 5 pm). Curve c in Fig. 8.20 represents the curve which is corrected for support effects (see the preceding section on this subject). Three heating and cooling cycles are shown. During the first heating the Al-hydroxide particles of the gel are transformed to boehmite and subsequently to (hydrated) y-aluminium oxide particles and the shape of the first peak of curve c differs from the subsequent peaks. The maximum tensile stress calculated from the deflection amounts about 30 MPa. 

Fig. 6.49. Micrograph of an aluminium reinforced with aluminium-oxide particles [124]...

When choosing an air filter medium, it may be preferable to select one that has been tested using a test dust of non-adhesive free running aluminium oxide particles and proven to have collected this dust without unloading under vibration. 

Increase in concentration of aluminium and phosphoric acid in the liquid serves to slow the reaction. This observation is in line with the above reaction scheme. Increase in the aluminium content will serve to increase the thickness of the coating formed around zinc oxide particles. Increase in phosphoric acid content implies a decrease in water content and an impairment of the hydration reaction. 

Aluminium particles are made with diameters in the range of 20 to 200 nm, with a protective shell of aluminium oxide about 4 nm thick and, mixed with a suitable oxidising agent, are used for rocket propulsion fuels. 

Inserted into the exhaust system of vehicles, catalytic converters can reduce emissions of carbon monoxide and hydrocarbons by up to 90 per cent. The first catalytic converters used mainly platinum, but now palladium is the predominant catalytic metal. The metals are dispersed as tiny particles on a supporting framework of porous aluminium oxide (alumina) (Fig. 18). 

Porous aluminium oxide membranes Au 0.26 pm diameter and 0.3-pm- to 3.0-pm-long gold particles were formed in the pores of aluminium oxide membranes 491... 

The two Windscale piles were fuelled with natural uranium canned in aluminium. Coolant air was blown through the reactor and exhausted from a 120-m stack (Fig. 2.4). Filters were installed at the top of the stack, but were not very effective. Some fuel cans developed pinholes during operation, and others became damaged and lodged in the ducts behind the pile. It is estimated that about 20 kg of irradiated uranium were disseminated to atmosphere as oxide particles from these cans (Stather et al., 1986). The temperature of oxidation was 200-400°C. The particle size, measured at the top of the stack, showed a mass median diameter of 35 fim (Mossop, 1960). 

The following test materials have often been used FCC catalysts, aluminium oxide, silica gel, glass beads, silica or quartz sand, sea sand, coal and coal ash, petroleum coke, metal powders, resin particles, boric acid, and magnesite powder. Mean particle size ranges from 11 /un to 1,041 /rm, and particle density, from 384 kg/m3 to 7,970 kg/m3. According to Geldart s classification (1973), most of these materials belongs to Class A, some to Class B, and a few to Class D or C, as listed in Table II. 

Aluminium and Precipitator Ash. In some Victorian brown coals significant quantities of acid-soluble aluminium are found. This is believed to be present as aluminium hydroxide which is dispersed throughout the water phase of the coal. During combustion of this coal, the refractory aluminium oxide formed takes the shape of the relics of the plant material present in the coal, thus forming an extremely low density ash (approximately 100 kg/m ). Whilst the collection of these particles by electrostatic precipitation is possible, the problem of reentrainment on rapping has necessitated the use of larger sized units than would otherwise be required. It is therefore important to determine the acid soluble aluminium fraction in the coal to determine if precipitation of fly ash is likely to be a problem. 

The most common friction enhancing agents are based on dispersions of silicic acid called sols (Fig. 9.1). About two-thirds of the commercial non-sUp finishing products, listed in the International Textile Auxiliary Buyers Guide, are based on silicic acid sols only a few are aluminium oxide dispersions. These products can be formed in a variety of particle sizes from 5 to 150 pm and yield dispersions that range from clear to milky white. When the sols are deposited on the fibre surface, the surface becomes much rougher and frictional forces between fibres become much larger. 

Conventional thin-layer chromatography (TLC) in our experience, known under the name planar chromatography, uses horizontal or vertical glass or Teflon chambers for the development of chromatograms. As stationary phases, commonly known adsorbents or supports based on silica gel, aluminium oxide, magnesium silica, cellulose, and so forth are used particle sizes are about 20 jitm. The migration of the mobile phase is based on the phenomenon of capillary forces. This chromatographic method is described, in detail, in other sections of this volume. 

A similar but very weak resonance reported at 30 ppm in 7-AI2O3 heated at 600°C has been attributed to 5-coordinated aluminium atoms on the surface of the oxide particles (Pecharroman et al. 1999). Cross-polarisation experiments between Al NMR and the protons from surface-adsorbed pyridine (Morris and Ellis 1989) or ammonia (Coster et al. 1994) have been used to investigate the surface state of cat-alytically active 7-AI2O3. No NMR evidence has been found of the hypothetical 3-coordinated Al theoretically predicted to occur at the alumina surface. Even assuming for this species a 8iso of 95-100 ppm, a Xq of 10 MHz and an t value of one, Coster et al. (1994) found no experimental evidence for its existence, prompting a recent explanation based on density functional calculations that the 3-coordinated surface Al atoms... 

Three minerals (montmorillonite, aluminium oxide and kaolinite) of known particle size and specific surface areas (Zhou et al., 1994) and a sterilised silty clay loam (Champaign, USA) were used as sorbents, along with suspended particulate matter (SPM) collected in polythene carboys from several sites in the Tees Estuary, UK. SPM was collected by high speed continuous flow centrifugation and stored in O.IM NaHCOj at 4 °C. Traces of organics were removed from the three minerals by washing with 0.01 M NaOH. 

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