Flame hydrolysis

The crystallinity of the same material composition varies depending on the type of flame that is used. The intrinsically high temperature of the H2/O2 flame leads to more crystalline materials compared to colder methane-based flames [71]. 

The productivity of the method is low, for example compared to FSS. The use of aqueous solutions is the major drawback. Contrary to FSS where the liquid is directly sprayed into the flame (see below), the aqueous solution needs to be nebulized into the flame with low flow rates to avoid extinction of the flame by water and to keep the flame temperature high. Together with the use of organic precursors, such as acetates and addition of citric acid, this ensures a stable H2/ 

O2 flame, therefore stable and homogeneous production. Enhanced and efficient collection of the nanosize particles can be realized using an electrostatic precipitator equipped with a stainless steel multipin effluviator [74], 

A compromise between the concentration of the precursor solution, the flow rate of air fed to the nebulizer, and the orifice of the nebulizer needs to be met in order to obtain crystalline materials with nanometric particle size. A further optimization parameter is the humidity degree of the air fed to the nebulizer, which needs also to be controlled as it ensures a constant precursor concentration during nebulization thus providing constant production rate and particle size [71], 

FH synthesis of Lao.gCeo.iCoOs+s afforded spherical primary particles of about 20-80 run, one order of magnitude smaller than those obtained with the citrate method for the same composition [75]. The specific surface area ranges from 

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III flame hydrolysis <20 600-1200 50-100 <1 170 Corning 7940, Dynasil 1000, Shinetsu P-10, SpectrosH, SuprasH, NSG-ES... 

The manufacture of vitreous sihca by flame hydrolysis can be a single- or multistep process. In the single-step process, sihca particles are formed and... 

A high purity titanium dioxide of poorly defined crystal form (ca 80% anatase, 20% mtile) is made commercially by flame hydrolysis of titanium tetrachloride. This product is used extensively for academic photocatalytic studies (70). The gas-phase oxidation of titanium tetrachloride, the basis of the chloride process for the production of titanium dioxide pigments, can be used for the production of high purity titanium dioxide, but, as with flame hydrolysis, the product is of poorly defined crystalline form unless special dopants are added to the principal reactants (71). 

Silica is the support of choice for catalysts used in processes operated at relatively low temperatures (below about 300 °C), such as hydrogenations, polymerizations or some oxidations. Its properties, such as pore size, particle size and surface area are easy to adjust to meet the specific requirements of particular applications. Compared with alumina, silica possesses lower thermal stability, and its propensity to form volatile hydroxides in steam at elevated temperatures also limits its applicability as a support. Most silica supports are made by one of two different preparation routes sol-gel precipitation to produce silica xerogels and flame hydrolysis to give so-called fumed silica. 

The second preparation route uses flame hydrolysis, a versatile way to produce all kinds of oxides with high specific surface areas. The advantages of fumed silica over xerogels are the better mechanical properties and higher purity of the former. 

Flambant see coal grade (France), 6 713t Flame agents, 5 814, 824-825 Flame arrestors, for acetylene, 1 185 Flame attenuation process, 13 387 Flame detectors, 21 851 Flame heating, case hardening by, 16 199 Flame hydrolysis, in vitreous silica manufacture, 22 412, 413-414 Flame ionization detector (FID)... 

Further chemical evidence for the existence of hydroxyl groups on anatase was obtained by Herrmann (305). Anatase made by flame hydrolysis of TiCl4 was used. Its surface area was 60 m /gm. The... 

Leanza, R Rossetti, 1 Fabbrini, L Oliva, C Fomi, L. Perovskite catalysts for the catalytic flameless combustion of methane Preparation by flame-hydrolysis and charaeterisation by TPD-TPR-MS and EPR. Appl. Catal, B Environmental, 2000, Volume 28, Issue 1, 55-64. 

Other methods including hydrothermal precipitation, flame hydrolysis, thermal decomposition of Fe(CO)s and high temperature reaction of Fe " chloride with iron, are used only on a small scale to obtain specialty products (see Chap. 19). 

Hydrothermal processes, i. e. the heating of suspensions of ferrihydrite in alkaline media under pressure, have been used to produce large platy crystals of hematite. This process gives vell formed crystals, but is expensive. The crystals can be reduced to produce isomorphous magnetite plates. Flame hydrolysis involves burning Fe " chloride at 400-800 °C to iron oxide. Owing to the many technical difficulties associated with this process, it is not commercially important. 

The production of vitreous silica from chemical precursors was first described in patents filed in 1934, including a fabrication method in which fine, high purity powders were produced by decomposing silanes (39). Forms were then cast from aqueous slips. More importandy, a flame hydrolysis process which used SiCl4 as the chemical precursor was described (40). This latter approach led to a marked improvement in glass purity and served as the basis for the processes used in the 1990s to make synthetic vitreous silica. 

Modern Manufacturing Techniques. Manufacturing techniques for making bulk vitreous silica are for the most part improved variations of the historical processes. The main exception is the sol—gel process (see Sol-gel technology). All processes involve the fusion or viscous sintering of silica particles. The particles can be in the form of a loose powder or a porous preform. The powders can be made from natural quartz or from the decomposition of chemical precursors, such as silicon tetrachloride, and tetraethylorthosilicate (1 EOS). In some approaches, such as flame hydrolysis, the powder is produced and fused in a single step. The improvements made to these techniques deal mainly with the procedures used to prepare the powders, that is, to control purity and particle size, and the specific conditions under which the powders are consolidated. 

Microfine TiOz with predominant anatase structure can also be manufactured by reductive flame hydrolysis of TiCl4 at < 700°C [5.287] ... 

Fig. 2. Practical absorptivity1 spectra for transition elements in fused silica prepared by flame hydrolysis absorptivity is given in terms of ppbw metal4 . (Reproduced by permission of The American Ceramic Society)...

The practical absorptivities indicate the overall effect of the transition element when it is incorporated into fused silica by the flame hydrolysis method. The absorptivities do not reveal the absorbing power of the individual valence state (i.e. true absorptivity). 

Ruano, J.M., Benoit, V., Aitchison, J.S., Cooper, J.M., Flame hydrolysis deposition of glass on silicon for the integration of optical and microfluidic devices. Anal. Chem. 2000, 72, 1093-1097. 

Aerosils , flame hydrolysis products of SiCl4 very pure materials. 

III SiCl4 Flame hydrolysis Metals <1 1000 Suprasil Spectrosil CGW 7940 Dynasil Synsil ... 

Of all these processes only flame hydrolysis is used to produce oxides with high surface areas on a large industrial scale Therefore, the emphasis of this article lies on this subject In general, this process is suitable for any volatile compound that can be decomposed at a high temperature to form an oxide Possible precursor compounds that can be used are metal chlorides carbonyls such as Ni(CO)4 or Fe(CO)s, and volatile alcoholates Reactions that have been studied are, for example, the manufacture of AUOj from A1CU [3]... 

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