Metal carbides oxygen dissociation

Vines F, Sousa C, lUas F, Liu P, Rodriguez JA (2007) A systematic density functional study of molecular oxygen adsorption and dissociation on the (001) surface of group IV, V and VI transition metal carbides. J Phys Chem C 111 16982-16989... 

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 revival of interest in Fischer-Tropsch chemistry in the 1970s resulted in new observations that eventually led to the formulation of a modified carbide mechanism, the most widely accepted mechanism at present.202-204,206,214 Most experimental evidence indicates that carbon-carbon bonds are formed through the interaction of oxygen-free, hydrogen-deficient carbon species.206 Ample evidence shows that carbon monoxide undergoes dissociative adsorption on certain metals to form carbon and adsorbed oxygen ... 

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,... 

In order to produce methanol the catalyst should only dissociate the hydrogen but leave the carbon monoxide intact. Metals such as copper (in practice promoted with ZnO) and palladium as well as several alloys based on noble group VIII metals fulfill these requirements. Iron, cobalt, nickel, and ruthenium, on the other hand, are active for the production of hydrocarbons, because in contrast to copper, these metals easily dissociate CO. Nickel is a selective catalyst for methane formation. Carbidic carbon formed on the surface of the catalyst is hydrogenated to methane. The oxygen atoms from dissociated CO react with CO to CO2 or with H-atoms to water. The conversion of CO and H2 to higher hydrocarbons (on Fe, Co, and Ru) is called the Fischer-Tropsch reaction. The Fischer-Tropsch process provides a way to produce liquid fuels from coal or natural gas. 

Upon chemisorption on a metal catalyst the CO molecule dissociates into carbon and oxygen atoms. The surface carbide thus produced is hydrogenated successively to CH, entities, which then link up to afford the products. ... 

---