Surface Complexes involving other Elements

The reactions of carbons with halogens has been of interest since the First World War, and a large body of information exists in the literature. The reactivity of halogens with carbon blacks decreases in the order Cl Br I, and the mode of action of the halogen depends on temperature. At approximately 1000 °C, addition of chlorine leads to removal of hydrogen as HCl 

Rlvin and J. Aron, Proc. Seventh Conf. Carbon, Cleveland, Pergamon, New York, 

Puri and Kalra have shown that unsaturated sites in carbons play a significant role in catalysing the reaction between hydrogen and bromine. Bromine may substitute hydrogen in the carbon, or may react with added hydrogen in the presence of carbon to form hydrogen bromide. The carbon has no catalytic activity if unsaturated sites are occupied by sulphur or oxygen, and it would seem that catalysis/formation of surface complexes is delicately balanced in the system. 

The formation, nature, and reactions of surface complexes of carbon are obviously open to considerable question, largely because of the difficulty in determining accurately what is happening on a carbonaceous surface. The problem is that, at least in some cases, the nature of the surface complexes may well affect the reactions (catalytic or non-catalytic) of carbons. Keeping the difficulties of analysis of surface complexes in mind, it is convenient to discuss the catalytic activity and reactivity of carbons in the expectation that this can lead to a closer definition of the information that is needed in the context of the surface complexes. 

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Measurements of erosion yields and surface chemistry provide information about the net result of the complex reactions that occur at a surface during extensive exposure to atomic-oxygen and to any other elements of the exposure environment. While inferences may be made about the chemical and physical interactions at the surface, such studies are an insensitive probe of the individual reaction and interaction mechanisms that accumulatively contribute to the net results. Experiments can, however, be designed to study the various steps of the overall erosion process. These steps involve initial interactions of O atoms with a surface (initiation), oxidation of carbon and scission of the hydrocarbon backbone (propagation), and removal of volatile carbon-containing species (material loss). In this section, studies of the initial interactions of O atoms incident on a hydrocarbon surface d40 jjj summarized. 

Although these simple considerations help to frame in a general logic the behavior of these bimetallic surface, there are at present no such simple models to explain the more complex mesoscopic reconstructions, such as the pyramids observed on Pt3Sn(100) or the hill and valley structure observed on PtsSnCl 10). These phenomena are obviously related to the tendency of the system to relax in-plane stress, in turn resulting from the different atomic radius of the elements involved in the presence of concentration gradients. This relaxation appears to take place on the (111) oriented plane simply by an outward relaxation of the tin atoms. On the other two low index surfaces, instead, it takes a more complex route leading to reconstruction phenomena (pyramids on the (100) and hill and valley on the (110)) which are so far unique to the Pt-Sn system. 

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