Hydrocarbons carbidic carbon

Strongly electropositive metals and their amalgams may reduce CO2 to CO, hydrocarbons, carbides, carbon, and oxalates.Similar products are formed if the reduction is carried out by means of these electropositive metals in the presence of transition metal 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... 

Halogenated hydrocarbons Chlorinated paraffins naphthalenes Chlorowax Halo wax Seekay wax Diamond Alkali Co Union Carbide Carbon Co ICI... 

Comparison of the selectivity toward alcohols in the FT reaction of W03, WC, W2C and the XPS and CO adsorption results provide information on the specificity of active sites on each catalyst. W2C favors CO dissociation and formation of hydrocarbons with excess methane and C02 without oxygenate production. It has a more pronounced metallic character than WC perhaps because its metal atoms are in a close-packed arrangement. After pretreatment in flowing H2 at 673 K before FT reaction, part of the surface oxygens (abundant after passivation) probably reacts with carbidic carbon and are removed from the uppermost surface layer. Moreover,... 

According to Eidus (90) carbides formed on cobalt or nickel catalysts are neither intermediate products nor catalysts promoting the formation of hydrocarbons from carbon monoxide and hydrogen. In the absence of hydrogen carbon monoxide poisoned the cobalt catalyst. Despite Eidus results, Braude and Bruns (42) supported Craxford s assumption that the carbide is formed by reaction of the metal (iron) with carbon monoxide and hydrogen. It was pointed out by Eidus (84a) that Braude and Bruns did not clearly distinguish between the carbide and free carbon... 

Preparative methods for carbides of all types include (a) direct union of the elements at high temperature (2200°C and above) (b) heating a compound of the metal, particularly the oxide, with carbon and (c) heating the metal in the vapor of a suitable hydrocarbon. Carbides of Cu, Ag, Au, Zn, and Cd, also commonly called acetylides, are prepared by passing acetylene into solutions of the metal salts with Cu, Ag, and Au, ammoniacal solutions of salts of the unipositive ions are used... 

Studies of the behavior of supported ruthenium systems has been stimulated because of the finding that ruthenium appears to be the most active element, based on exposed surface atoms, for carbon monoxide hydrogenation (ref. 65). A number of workers have studied the dissociation of carbon monoxide and subsequent buildup of carbidic carbon on the various crystallographic faces of ruthenium (refs. 66-70). It has been shown that the carbidic carbon is easily hydrogenated and is thought to be a precursor for the hydrocarbon products, while the less reactive graphitic carbon is associated with catalyst deactivation (refs. 34,71-72). 

We are particularly concerned about assumption (3), as surface spectroscopy has recently shown that in addition to growing chains a substantial amount of carbidic carbon develops on the catalyst surface (35-37). While only part of it might be reaction intermediate under steady state conditions, it would be converted almost completely to methane and small amounts of higher hydrocarbons when the surface is exposed to hydrogen in the absence of CO (37). 

Hydrocarbon synthesis over initially clean transition metal surfaces leads to the rapid buildup of an overlayer of carbidic carbon, often concurrent with an initial rise in the catalytic activity. 

The carbidic carbon is much more reactive towards hydrogen than graphitic carbon. When CO is removed from the CO/H2 feed, methane and minor quantities of higher hydrocarbons are being produced at a rate substantially exceeding the rate of hydrocarbon production from the CO/H2 feed proper. 

Carbon deposits formed by exposure to hydrocarbons and carbon monoxide exhibit, depending primarily on the temperature during deposition, seven reactive carbon states during TPSR with 1-atm Ho, including two very reactive states of chemisorbed carbon, a carbon film, nickel carbide, and two types of filamentary carbon. Filamentary carbon was identified by transmission and scanning electron microscopy, and Nl C was identified from x-ray diffraction measurements. 

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