Carbon on metal surfaces

The accumulation of carbon on metal surfaces when heated in the presence of carbon containing gases is a serious problem encountered in a number of commercial processes. Although carbon appears to deposit on most surfaces there are some materials which are more vulnerable than others since they contain constituents which catalyze carbon formation. The highest catalytic activity is exhibited by the ferromagnetic metals and in particular, iron. Furthermore it is well known that the surface state of such metals can have a dramatic effect on their ability to catalyze the formation of carbon. 

It may be helpful at this point to try to draw together some of the many threads concerning carbon deposition that have appeared many times in the previous chapters a comprehensive and unifying model is not yet available, and indeed it is doubtful if it ever will be, so many are the factors that determine the state of carbon on metal surfaces. Nevertheless it is possible to make a few generalisations, and an attempt to do so is opportune now because the last section of this chapter concerns a constructive use of surface carbon to create useful products. The term carbon will be used as an omnium gatherum for what has been variously named coke, acetylenic residue, carbonaceous deposit and probably other things as well. The following short survey may be amplified by reference to review articles. 

Compound (1) decomposes to form dichloroacetyl chloride, which in the presence of water decomposes to dichloroacetic acid and hydrochloric acid (HCl) with consequent increases in the corrosive action of the solvent on metal surfaces. Compound (2) decomposes to yield phosgene, carbon monoxide, and hydrogen chloride with an increase in the corrosive action on metal surfaces. 

In addition to films that originate at least in part in the corroding metal, there are others that originate in the corrosive solution. These include various salts, such as carbonates and sulfates, which may be precipitated from heated solutions, and insoluble compounds, such as beer stone, which form on metal surfaces in contac t with certain specific products. In addition, there are films of oil and grease that may protect a material from direct contact with corrosive substances. Such oil films may be apphed intentionally or may occur naturally, as in the case of metals submerged in sewage or equipment used for the processing of oily substances. 

Carbon-Carbon Bond Cleavage on Metallic Surfaces. 195... 

There is no clear consensus in the literature, regarding the elementary steps of carbon-carbon bond cleavage and formation on metallic surfaces. 

In some catalytic processes, it is necessary to avoid carbon-carbon bond cleavage. For example, isobutane is mainly transformed into its lower alkane homologues (hydrogenolysis products) on metal surfaces, while it can be converted more and more selectively into isobutene when the Pt catalysts contain an increasing amount of Sn (selective dehydrogenation process) [131]. 

One of the classic examples of an area in which vibrational spectroscopy has contributed to the understanding of the surface chemistry of an adsorbate is that of the molecular adsorption of CO on metallic surfaces. Adsorbed CO usually gives rise to strong absorptions in both the IR and HREELS spectra at the (C-O) stretching frequency. The metal-carbon stretching mode ( 400 cm-1) is usually also accessible to HREELS. 

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... 

Formic acid decomposes generally on metal surfaces to carbon dioxide and hydrogen. A thermal desorption study on Cu(l 10) showed that the reaction... 

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