Metal carbides forms

The mechanism of the Fischer-Tropsch reactions has been the object of much study (note Eqs. XVI11-55-XV111-57) and the subject of much controversy. Fischer and Tropsch proposed one whose essential feature was that of a metal carbide—patents have been issued on this basis. It is currently believed that a particular form of active adsorbed carbon atoms is involved, which is then methanated through a series of steps such as... 

Hafnium dioxide is formed by ignition of hafnium metal, carbide, tetrachloride, sulfide, boride, nitride, or hydrous oxide. Commercial hafnium oxide, the product of the separation process for zirconium and hafnium, contains 97—99% hafnium oxide. Purer forms, up to 99.99%, are available. 

As previously stated, uranium carbides are used as nuclear fuel (145). Two of the typical reactors fueled by uranium and mixed metal carbides are thermionic, which are continually being developed for space power and propulsion systems, and high temperature gas-cooled reactors (83,146,147). In order to be used as nuclear fuel, carbide microspheres are required. These microspheres have been fabricated by a carbothermic reduction of UO and elemental carbon to form UC (148,149). In addition to these uses, the carbides are also precursors for uranium nitride based fuels. 

Zirconium carbide is inert to most reagents but is dissolved by hydrofluoric acid solutions which also contain nitrate or peroxide ions, and by hot concentrated sulfuric acid. Zirconium carbide reacts exothermically with halogens above 250°C to form zirconium tetrahaHdes, and with oxidizers to zirconium dioxide in ak above 700°C. Zirconium carbide forms soHd solutions with other transition-metal carbides and most of the transition-metal... 

Easily decomposed, volatile metal carbonyls have been used in metal deposition reactions where heating forms the metal and carbon monoxide. Other products such as metal carbides and carbon may also form, depending on the conditions. The commercially important Mond process depends on the thermal decomposition of Ni(CO)4 to form high purity nickel. In a typical vapor deposition process, a purified inert carrier gas is passed over a metal carbonyl containing the metal to be deposited. The carbonyl is volatilized, with or without heat, and carried over a heated substrate. The carbonyl is decomposed and the metal deposited on the substrate. A number of papers have appeared concerning vapor deposition techniques and uses (170—179). 

Carbon forms binary compounds with most elements those with metals are considered in this section whilst those with H, the halogens, O, and the chalcogens are discussed in subsequent sections. Alkali metal fullerides and encapsulated (endohedral) metallafullerenes have already been considered (pp. 285, 288 respectively) and met-allacarbohedrenes (metcars) will be dealt with later in this section (p. 300). Silicon carbide is discussed on p. 334. General methods of preparation of metal carbides are ... 

The formed metal carbide (M-C) is then hydrogenated to a reactive methylene metal species. 

The number of oxides is large since most metallic elements form stable compounds with oxygen, either as single or mixed oxides. However, the CVD of many of these materials has yet to be investigated and generally this area of CVD has lagged behind the CVD of other ceramic materials, such as metals, carbides, or nitrides. The CVD of oxides has been slower to develop than other thin-film processes, particularly in optical applications where evaporation. 

Silicon, like carbon, is relatively inactive at ordinary temperatures. But, when heated, it reacts vigorously with the halogens (fluorine, chlorine, bromine, cmd iodine) to form halides and with certain metals to form silicides. It is unaffected by all acids except hydrofluoric. At red heat, silicon is attacked by water vapor or by oxygen, forming a surface layer of silicon dioxide. When silicon and carbon are combined at electric furnace temperatures of 2,000 to 2,600 °C (3,600 to 4700 °F), they form silicon carbide (Carborundum = SiC), which is an Importeint abrasive. When reacted with hydrogen, silicon forms a series of hydrides, the silanes. Silicon also forms a series of organic silicon compounds called silicones, when reacted with various organic compounds. 

When the metal can form a stable carbide, the product of the carbothermic reduction of its oxide may be a carbide instead of the metal itself. The question as to whether a carbide or the metal forms under standard conditions when the oxide is reduced by carbon is not answered by the Ellingham diagram. To obtain an answer to this question, a more detailed consideration of the thermodynamic properties of the system is necessary. 

The nitrides can be prepared by heating a metal powder in an N2 or NH3 atmosphere to temperatures above 1100°C. The carbides form upon heating mixtures of the metal powders with carbon to temperatures of about 2200 °C. Both the nitrides and carbides can also be made by chemical transport reactions by the van Arkel-de Boer method if the metal deposition takes place in an atmosphere of N2 or a hydrocarbon. Their remarkable properties are ... 

Most forms of carbon, except diamond, which are renowned as supports for precious metal catalysts in certain applications [3], interact strongly with MW [4]. Amorphous carbon and graphite, in their powdered form, irradiated at 2.45 GHz, rapidly (within 1 min) reach very high temperatures (>1300 K). This property has been used to explain MW-assisted syntheses of inorganic solids [5], In these syntheses carbon is either a secondary susceptor which assists the initial heating but does not react with other reactants, or is one of the reactants, e. g. in the synthesis of metal carbides. MW-carbon coupling has also been widely developed ... 

The major use of sodium cyanamide is in the production of sodium cyanide, a compound that is used extensively in preparing solutions for the electroplating of metals. Another use for NaCN is in extraction processes employed to separate gold and silver from ores as a result of their forming complexes with CN . Sodium cyanide, an extremely toxic compound, is also used in the process known as case-hardening of steel. In this process, the object to be hardened is heated and allowed to react with the cyanide to form a layer of metal carbide on the surface. 

In addition to the types of compounds discussed so far, the group IVA elements also form several other interesting compounds. Silicon has enough nonmetallic character that it reacts with many metals to form binary silicides. Some of these compounds can be considered as alloys of silicon and the metal that result in formulas such as Mo3Si and TiSi2. The presence of Si22 ions is indicated by a Si-Si distance that is virtually identical to that found in the element, which has the diamond structure. Calcium carbide contains the C22-, so it is an acetylide that is analogous to the silicon compounds. 

Carbonaceous species on metal surfaces can be formed as a result of interaction of metals with carbon monoxide or hydrocarbons. In the FTS, where CO and H2 are converted to various hydrocarbons, it is generally accepted that an elementary step in the reaction is the dissociation of CO to form surface carbidic carbon and oxygen.1 The latter is removed from the surface through the formation of gaseous H20 and C02 (mostly in the case of Fe catalysts). The surface carbon, if it remains in its carbidic form, is an intermediate in the FTS and can be hydrogenated to form hydrocarbons. However, the surface carbidic carbon may also be converted to other less reactive forms of carbon, which may build up over time and influence the activity of the catalyst.15... 

The earliest theory, advanced by Fischer and Tropsch in 1926 (84), proposed that the reaction proceeded via formation of intermediate metal carbides which react on the catalyst surface to form methylene groups. It was then suggested that these methylene groups polymerize on the surface to form hydrocarbon chains, which desorb as saturated and unsaturated hydrocarbons. In 1939 Craxford and Rideal expanded the carbide theory, proposing (85), for cobalt-based catalysts, the following reaction sequence ... 

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