Volatile metal compounds

At the Department of Chemical Engineering new catalytically active materials have been produced by burning volatile metal compounds in a flame. This produces an aerosol of very small particles that can be collected on a filter. Especially if the particles are cooled very fast, it is possible to obtain a large area per gram of material. In the following, AI2O3 is produced by this method. 

Examples of the formation of volatile metal compounds or complexes leading to separations based on gas-liquid and gas-solid distribution are much rarer. 

This review has no final conclusions. The whole matter of P.E. spectra of volatile metal compounds is still under investigation the results obtained until now yield only a partial picture, and there are several fundamental problems still open, so generalized conclusions are not warranted at the present stage of research. However, some points regarding the significance of the P.E. spectroscopic technique in coordination chemistry are already self-evident. We shall try to identify open problems, lines of future research, and precautions to be taken both in the experimental research and in the interpretive work, at least in the form and to the extent suggested by the present partial stage of the development of research in this field. 

The reaction of aerosol droplets with gases can also be used to prepare particles of internally mixed composition, such as consisting of different metal oxides. In principle, these powders can be obtained by first cocondensing vapor of the two or more volatile metal compounds (preferably alkoxides), or by nebulizing liquids of mixed composition. In both cases the droplets are then reacted with vapors. 

Belcher, R., Jenkins, C. R., Stephen, W. I., Uden, P. C. Preparative gas chromatography of volatile metal compounds. I. Separation of aluminium, chromium and iron (3-diketonates. Talanta 17, 455 (1970). 

Aerosol-assisted chemical vapor deposition is illustrated by work published by Siadati et al. (2004). An aerosol is a vapor suspension of finely divided particles. The particles can be solid, as in smoke, or liquid, as in fog. A toluene solution of Zr(tfac)4, Y(hfac)3, and Ce(tmhd)4, converted to an aerosol, was used to deliver the metal in a carrier gas of O2. The film deposited was CeC>2-doped Y2O3-stabilized zirconia. The ligands used in this work to produce volatile metal compounds are frequently used in chemical vapor deposition. Trifluoroacety-lacetonate (tfac), hexafluoroacetylacetonate (hfac), and tetramethylheptanedionate (tmhd) are all 3-diketonates, and their structures are shown in Figure 3.22. Like the diethyl dithiocarbamates mentioned above, these precursors could potentially be the source of both the metal and the oxygen in the chemically deposited film. However, to ensure that the metal remains at its highest oxidation state and to avoid a film with mixed valencies on the metal, oxygen is used as the carrier gas. 

Krauskopf, K.B., 1964. The possible role of volatile metal compounds in ore genesis. Econ Geol., 59 22-45. 

Approximately 10 in a million carbon atoms are converted to ions in the FID. The ions produced in the FID carry an electric current from the anode flame to the cathode detector. This electrical charge is the detector signal and response to individual organic compounds is directly proportional to the solute mass and is extremely sensitive. In the absence of CHO+ the current is zero. The FID is widely used as a GC detector and responds to most hydrocarbons that eluate from a GC column. Unfortunately, it is insensitive to hydrocarbons, H2, He, N2, 02, CO, C02, H20, NH3, NO, S, H2S, SiF4 and a host of volatile metallic compounds in gasoline, diesel, distillates, crude oils, natural gas, environmental pollutants and volatile organics in water samples. 

The simplest method of decomposing an organic sample prior to determining the cations it contains is to heat the sample over a flame in an open dish or crucible until all carbonaceous material has been oxidized to carbon dioxide. Red heat is often required to complete the oxidation. Analysis of the nonvolatile components follows dissolution of the residual solid. Unfortunately, there is always substantial uncertainty about the completeness of recovery of supposedly nonvolatile elements from a dry-ashed sample. Some losses probably result fiom the entrainment of finely divided particulate matter in the convection currents around the crucible. In addition, volatile metallic compounds may be lost during the ignition. For example, copper, iron, and vanadium are appreciably volatilized when samples containing porphyrin compounds are ashed. 

Feldmann, J. Volatile Metal Compounds of Biogenic Origin. In Handbook of Elemental Speciation H Species in the Environment, Food, Medicine... 

Other special methods for growing metal single crystals are recrystallization with alternate mechanical deformation and annealing (systematically and successfully used till now with Al, Mg, and Fe) as well as a crystallization process in which a volatile metal compound decomposes thermally on a high-temperature filament and the metal deposits on it. This procedure may be used also for the preparation of some metals and very pure simple metal compounds. Some of these cannot be obtained as pure by any other method (Ti, Zr, Hf, Nb, Ta, etc.) (Koref Van Arkel Agte Burgers De Boer 0.2]). 

Thermal decomposition of volatile metal compounds, especially carbonyls (see carbonyl iron, p. 1636). 

Metal deposits which are useful for preparative purposes may be obtained by thermal decomposition of suitable volatile metal compounds (hydrides, carbonyls) on hot surfaces. For Instance, decomposition of Ni(CO)4 on Pyrex glass wool at 150°C produces a deposit of very finely divided nickel, which is an excellent catalyst for gas-phase hydrogenation of olefinic double bonds [4]. 

Thermal or photochemical decomposition of volatilized metal compounds in the presence of oxygen. Metal compoxmds that... 

Chemical vapor deposition (CVD) employs a gas mixture that contains a volatile chemical compound of the metal to be deposited and a reducing agent, such as hydrogen or methane. When the gas mixture is introduced into the reaction chamber, the volatile metal compound is catalytically reduced at the surface of the substrate and a thin deposit forms. For example, titanium carbide can be deposited at temperatures between 800 and 1100 °C according to the reaction ... 

Other flame types such as the counter-diffusion flame can be used [73]. In all cases, particles are formed once the volatile metal compounds are converted to stable metal oxide or metal nanoparticles in the high-temperature environment of the flame. 

Keller reviewed the literature on the gas chromatography of volatile metal compounds and in this discusses the chromatography of stannic chloride using squalane, n-octadecane, silicone oil, paraffin wax and Apiezon grease as partitioning agents supported on Chromosorb at temperatures between 100 and 200 C. 

Inorganic chemistry provides many of the materials—from concrete to silicon chips—that are indispensable to modem life. The need for designed materials with special properties will continue to grow in the new century. Oiganometallic chemistry is beginning to contribute in several ways. For example, in metal organic chemical vapor deposition (MOCVD). a volatile metal compound is decomposed on a hot surface to deposit a film of metal. A typical example is the use of Cr(C6H6)2 to deposit Cr. ... 

An alternative method is the decomposition of vapours of volatile metal compounds on the base metal surface, e.g., tungsten is coated by the decomposition of its hexachloride and nickel by decomposition of nickel carbonyl. 

Sampling in Tedlar bags has been employed when gas samples were taken in remote areas. Different volatile metal compounds (VMCs) of arsenic, tin and antimony were shown to be stable for at least a few days, provided they were stored in the dark to prevent UV-mediated decomposition. ... 

Feldmann, J., Naels, L., and Haas, K. (2001) Cryotrapping of C02-rich atmospheres for the analysis of volatile metal compounds using capillary GC-ICP-MS. J. Anal. At. Spectrom.,... 

Ultra fine M0O3 particles were prepared in a flame reactor which has already been described (ref. 6). It should just be recalled that the oxide aerosol is generated from the vapor of a volatile metallic compound (M0O2CI2 or M0Q5) which is injected into the burner fed with hydrogen and oxygen and then decomposed in the flame either by hydrolysis or by oxidation. The oxide issued ftom the flame reactor was treated at 673 K in air for 24 h (standard treatment) in order to eliminate residual chlorine and to transform all the M0O3 into the orthorhombic phase (see below). 

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