Carbon-sulfur surface compounds

Incompletely Characterized Carbon Sulfides. A poorly characterized black soHd, known as carsul, occurs as a residue ia sulfur distillation or as a precipitate ia molten Frasch sulfur (12,13). Although this material may approach the composition of a carbon sulfide, it is more likely also to contain some chemically bound hydrogen and possibly other elements. Carbon—sulfur surface compounds of the formula C S, where xis greater than 4,... 

Carbon storage, of hydrogen, 23 786 Carbon sulfides, 23 621, incompletely characterized, 23 62 Carbon sulfotelluride, 24 419 Carbon-sulfur surface compounds, 23 621 Carbon tetrabromide, 4 348 Carbon tetrachloride, 6 249 acrylamide solubility in, 2 290t... 

Figure 2.3 Carbon-sulfur surface compounds as described in the literature (a) sulfide (b) thiophenol (c) disulfide (d) thioquinone (e) sulfoxide (f) thiolactone [126,128,129],...

Chemisorption of the gases resulting in the formation of carbon-sulfur surface compounds... 

The physically-adsorbed gases and vapors can be recovered from the carbon surface by evacuation or by passing an inert gas over the surface of the carbon the elemental sulfur can be removed by extraction with various solvents. The chenusorbed sulfur that is present in the form of very stable carbon-sulfur surface compounds is difficult to recover. It cannot be removed by extraction or by heat treatment alone. It can, however, be conveniently removed by heat treatment in hydrogen gas at 500°C. The formation and properties of carbon-sulfur surface compounds formed on carbons as a result of treatment with sulfur dioxide, hydrogen sulfide, or carbon disulfide is beyond the scope of this book, but they have been dealt with in great detail in our earlier books. Active Carbon and Carbon BlackP... 

Another part of the sulfur formed by the reaction between the carbon surface and SO2 gets bonded to the carbon surface in the form of stable carbon-sulfur surface compounds that can be neither desorbed by extraction with a solvent nor by heat treatment. These surface compounds can only be desorbed as H2S on heat treatment in hydrogen gas, as mentioned earlier. The formation of these carbon-sulfur surface compounds reduces the availability of the carbon surface for any further adsorption, making the regeneration of the sulfurized activated carbon difficult. The proportion of these three forms in which SO2 is adsorbed on the carbon surface depends upon the temperature and composition of the exhaust gases and the nature of the carbon surface. It, therefore, indicates that the experimental conditions for using activated carbon adsorption technique for the recovery of SO2 should be such that there is... 

Nitrogen is usually present in minor amounts, but sulfur can be present in high concentrations, >1%, depending on the precursor that is used to manufacture the carbonaceous material. Besides sulfur that is bonded to carbon, other forms such as elemental sulfur, inorganic sulfate, and organosulfur compounds may be present. The carbon-sulfur surface compounds on carbon blacks are relatively stable, but they desorb as H2S when carbon is heat-treated in H2 between 500 and 1000°C. 

The influence of sulfur surface compounds on the adsorption of polar and nonpolar vapors of varying molecular dimensions was examined by Puri and Hazra. The adsorption of water vapors increased appreciably at relative pressures lower than 0.4 and decreased at higher relative pressures. The effect increased with increase in the amount of sulfur fixed and was attributed to the variation of the pore-size distribution caused by the fixation of sulfur along the pore walls. The adsorption isotherms of methanol and benzene vapors indicated that these larger molecules found smaller and smaller areas as more and more sulfur was being incorporated into the pores. Bansal et prepared carbon molecular sieves by blocking pores of PVDC charcoals by... 

Most important and best known among the surface compounds of carbon are those with oxygen and with sulfur. Other elements, e.g., chlorine and hydrogen, can also serve as end groups. ... 

However, some workers doing research on e.s.r. are convinced that the unpaired electrons are not localized on the carbon surface. This point is not yet decided, as was pointed out by Singer (104). The concentration of unpaired electrons is diminished by formation of surface oxides as was shown by Jackson, Harker, and Wynne-Jones (105). In contrast to these results, Antonowicz (106) found that spin centers originated on formation of surface compounds with oxygen, sulfur, chlorine, etc. Very likely, the type of starting material is decisive for its behavior on surface compound formation. 

The interrupted bonds on carbon surfaces can bind other elements as well as oxygen and sulfur. Not much research has been conducted in this direction, however. The more important of the remaining surface compounds contain hydrogen or chlorine. 

Carbon-sulfur groups are extremely stable surface compounds which cannot be removed by thermal treatment up to 1470 K. Only by a reductive treatment with hydrogen it is possible to clean carbon from sulfur adsorbates. One source of sulfur is the fuel used for the generation of the carbon from which about 90% are covalently bonded to the carbon and 10% are segregated as adsorbate which can be removed by solvent extraction. The abundance of sulfur can amount to several wt%. Removal of the structural sulfur is possible by hydrogen reduction to H2S at about 1000 K. A collection of references on this subject is found in the literature [33]. In activated carbons sulfur can also be present in an oxidized form as sulfate or as C-S-O compounds. 

Activated Carbon Sulfur removal from natural gas by adsorption at ambient temperature on carbon, activated with cupric oxide, is widely used. Carbon physically adsorbs sulfur compounds to its surface and the cupric oxide reacts with hydrogen sulfide. The activated carbon is typically regenerated every 30 days by passing steam through the bed at a temperature of 230°C (450°F) for 8—10 hr while air is injected. Oxygen in the air reacts with the metal sulfide to form the metal oxide and sulfur dioxide. These reactions are ... 

In the first step, the unshared electron pair of the sulfur is attracted to a low-valent nickel atom of the Raney nickel catalyst surface. The compound is adsorbed on the surface through the unshared electron pair of sulfur, which acts as an electron donor. Therefore, the carbon-sulfur bond is weak-... 

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]. 

The introduction of sulfur functionalities to a carbon surface is usually done by heating carbons in the presence of elemental sulfur [183] or other sulfur-containing compounds, such as hydrogen sulfide [122,123], In this way, up to 10% of sulfur can be fixed to the carbon matrix. The temperature of heat treatment varies from 476 to 1273 K. The most common sulfur reactions on the carbon surface are addition to the carbon active sites, substitution of oxygen, and reaction with metals, leading to formation of sulfides. Moreover, a significant amount of CS2 can be deposited on the surface. Generally, it was proposed that at temperatures below 873 K, addition of H2S to the active sites of carbons... 

This chapter provides a comprehensive summary of surface science involved in the application of activated carbon for air cleaning from sulfur containing species such as hydrogen sulfide, sulfur dioxide, and mercaptans. Moreover, the removal of organic sulfur-containing compounds from both gaseous and liquid fuel is addressed. The emphasis is placed on the role of activated carbon surfaces, either unmodified or modified in the processes of adsorption and catalytic oxidation of these pollutants. 

Numerous published results indicate activated carbons as efficient adsorbents for desulfurization either from gas or liquid phase. Complex processes take place on their surface leading to adsorption of sulfur containing compounds, their oxidation and deposition of oxidation products in the pore system. For all of these processes, it is the surface features of activated carbon that govern the removal processes. In the majority of cases it is impossible to separate the role of porosity, pore sizes and pore vohune form tlie role of surface chemistry. Since their coexistence is a must on the surface of activated carbons, the way in which they affect the feasibility of desulfurization is a synergy and its is this unique synei which opens the way for the application of carbon surfaces in separation technology and thus environmental remediation. 

A large number of investigations in acid media have led to the conclusion that the inhibition effect caused by relatively small and simple molecules is due to their adsorption on the metal surface. Compounds of this nature usually contain sulfur and nitrogen, or are of the groups of higher alkyl-alcohols and fatty acids. Typical compounds to be discussed here in more detail are quinoline and thiourea derivatives. Fig. 5 shows a comparison of the effectiveness of several such compounds determined by means of weight loss measurements on carbon steel in 5% sulfuric acid at 40° C. as a function of the inhibitor concentration. A cur-... 

Other atmospheric factors such as gaseous impurities in the air (e.g. sulfur dioxide, ammonia, carbon monoxide, ozone, carbon dioxide, halogen compounds, or formaldehyde) and solid impurities in the atmosphere (airborne dust, sand, and soot) result in an acceleration of aging processes in polymers. Whereas the effects of the gaseous impurities on polymers are mainly chemical in nature, the solid particles mostly cause abrasive damage to the plastic surface. 

The liquid (sulfur-based compounds) and sohd sulfur cathodes (items 6 and 7) do not develop surface chemistry that can be separated from their main electrochemical redox reactions. Hence, when the reduction of sulfur SO2 or SOCI2 produces insoluble species such as LiCl, LijS, and LijO, they precipitate on the current collector [9]. When formed, LijS can be reoxidized, up to elemental sulfur, via various LLS intermediate compounds [10]. Hence, the current collector, which may be aluminum (Al) plus carbon in the case of sulfur cathodes or carbon in the case of SOCI2 cathodes, does not develop intrinsic surface chemistry beyond the precipitation of the reduction products of the active mass. 

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