Sulfur, Phosphorus, and Halogen Functionalities

Sulfur Sulfur is present in carbonaceous materials as elemental sulfur, inorganic species, and organosulfur compounds. Its content varies between 0 and 5%. Carbon-sulfur complexes are extremely stable [121] and are not removed completely from the carbon matrix even at temperatures reaching 1373 K. Only heating in a reducing hydrogen atmosphere is able to remove sulfur completely from the carbon matrix [121]. 

Carbon surfaces can be modified by reactions with sulfur-containing compounds such as H2S, CS2, or SO2 at various temperatures, which results in the formation of sulfur-containing groups [122,123]. It was found that the maximum sulfur uptake takes place at 873 K and that the amount of sulfur fixed depends on the nature of the carbon [124,125]. The reactivity of various types of carbon materials toward sulfur has been investigated by Boehm and Puri [51,52]. 

Puri and co-workers [128,129] found that sulfur is introduced to the carbon matrix by interactions with oxygen-containing functional groups or by addition to unsaturated sites. Thus, the substitution reactions with quinone and phenolic groups lead to thioquinone and thiophenol structures, and the addition reactions result in the formation of sulfide and sulfoxide groups. Moreover, Chang showed the evidence of thiolactones in a number of carbonaceous materials after reaction with various sulfur-containing materials [126]. 

The use of sulfur complexes includes minimizing adsorption of water vapor on charcoals [52] and adsorption of metal ions such as cadmium [130], mercury 

Analogous to chlorine, bromine or iodine form stable halogen-carbon complexes, with the maximum amount fixed at about 773 K. The mechanism of bromine incorporation varies depending on the physical form of bromine (vapors or aqueous solution). When bromine is present in aqueous solution, bromine occupies unsaturated sites on the carbon surface, whereas in reaction with vapor, partial substitution for hydrogen also takes place. The former reaction is used as a measure of surface unsaturation [147,153,154]. The driving force for a partial substitution of hydrogen by bromine is inaccessibility of the small pores to the large bromine molecule. 

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