Carbon 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,... 

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. 

The mechanism of poisoning automobile exhaust catalysts has been identified (71). Upon combustion in the cylinder tetraethyllead (TEL) produces lead oxide which would accumulate in the combustion chamber except that ethylene dibromide [106-93-4] or other similar haUde compounds were added to the gasoline along with TEL to form volatile lead haUde compounds. Thus lead deposits in the cylinder and on the spark plugs are minimized. Volatile lead hahdes (bromides or chlorides) would then exit the combustion chamber, and such volatile compounds would diffuse to catalyst surfaces by the same mechanisms as do carbon monoxide compounds. When adsorbed on the precious metal catalyst site, lead haUde renders the catalytic site inactive. 

The physicochemical properties of carbon are highly dependent on its surface structure and chemical composition [66—68], The type and content of surface species, particle shape and size, pore-size distribution, BET surface area and pore-opening are of critical importance in the use of carbons as anode material. These properties have a major influence on (9IR, reversible capacity <2R, and the rate capability and safety of the battery. The surface chemical composition depends on the raw materials (carbon precursors), the production process, and the history of the carbon. Surface groups containing H, O, S, N, P, halogens, and other elements have been identified on carbon blacks [66, 67]. There is also ash on the surface of carbon and this typically contains Ca, Si, Fe, Al, and V. Ash and acidic oxides enhance the adsorption of the more polar compounds and electrolytes [66]. 

Lithium carbonate and hydrocarbon were identified in XPS spectra of graphite electrodes after the first cycle in LiPF6/EC-DMC electrolyte [104]. Electrochemical QCMB experiments in LiAsF6/EC-DEC solution [99] clearly indicated the formation of a surface film at about 1.5 V vs. (Li/Li+). However the values of mass accumulation per mole of electrons transferred (m.p.e), calculated for the surface species, were smaller than those of the expected surface compounds (mainly (CF OCC Li ). This was attributed to the low stability of the SEI and its partial dissolution. 

The specific behavior of surface compounds, being the propagation centers of polymerization catalysts, are mainly determined by two of their features the coordinative insufficiency of the transition metal ion and the presence of the transition metal-carbon bond. 

Subsequent elegant work by Lambert and coworkers61 has shown that, while under UHV conditions the electropumped Na is indistinguishable from Na adsorbed by vacuum deposition, under electrochemical reaction conditions the electrochemically supplied Na can form surface compounds (e.g. Na nitrite/nitrate during NO reduction by CO, carbonate during NO reduction by C2FI4). These compounds (nitrates, carbonates) can be effectively decomposed via positive potential application. Furthermore the large dipole moment of Na ( 5D) dominates the UWr and O behaviour of the catalyst-electrode even when such surface compounds are formed. 

X-Ray studies confirm that platinum crystallites exist on carbon supports at least down to a metal content of about 0.03% (2). On the other hand, it has been claimed that nickel crystallites do not exist in nickel/carbon catalysts (50). This requires verification, but it does draw attention to the fact that carbon is not inert toward many metals which can form carbides or intercalation compounds with graphite. In general, it is only with the noble group VIII metals that one can feel reasonably confident that a substantial amount of the metal will be retained on the carbon surface in its elemental form. Judging from Moss s (35) electron micrographs of a reduced 5% platinum charcoal catalyst, the platinum crystallites appear to be at least as finely dispersed on charcoal as on silica or alumina, or possibly more so, but both platinum and palladium (51) supported on carbon appear to be very sensitive to sintering. 

Fig. 12.4. Vapor-to-water transfer data for saturated hydrocarbons as a function of accessible surface area, from [131]. Standard states are 1M ideal gas and solution phases. Linear alkanes (small dots) are labeled by the number of carbons. Cyclic compounds (large dots) are a = cyclooctane, b = cycloheptane, c = cyclopentane, d = cyclohexane, e = methylcyclopentane, f = methylcyclohexane, g = cA-l,2-dimethylcyclohexane. Branched compounds (circles) are h = isobutane, i = neopentane, j = isopentane, k = neohexane, 1 = isohexane, m = 3-methylpentane, n = 2,4-dimethylpentane, o = isooctane, p = 2,2,5-tri-metbylhexane. Adapted with permission from [74], Copyright 1994, American Chemical Society...

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... 

Adsorption occurs when an organic molecule is brought to the activated carbon surface and held there by physical and/or chemical forces. The quantity of a compound... 

Surface groups consisting of atoms foreign to the structure can be formed on a great variety of substances. It is not intended to discuss all possibilities this would surpass the scope of an article limited in volume. Furthermore, research in this field has but begun surface compounds have been studied only on a selected group of substances. Most of the investigated substances, however, are very important from an industrial viewpoint. Therefore, in this article the chemistry of surface compounds will be described for a few characteristic and well-known examples. Borderline cases, such as the chemisorption of carbon monoxide on metals, will not be considered. 

Only a few examples, chosen also for reasons of the author s familiarity with them, will be discussed surface compounds on carbon, on silica, on titania, and, less extensively, on alumina and silica-alumina. 

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. ... 

Thus, two kinds of surface oxides became known. Basic surface oxides are formed always when a carbon surface is freed from all surface compounds by heating in a vacuum or in an inert atmosphere and comes into contact with oxygen only after cooling to low temperatures. As is now known 24), the irreversible uptake of oxygen starts at ca. —40° there is only reversible, physical adsorption at lower temperatures. Acidic surface oxides are formed when carbon is treated with oxygen at temperatures near its ignition point. King 21) found the maximum... 

Wetting properties. A clean carbon surface is hydrophobic. Surface oxides provide sites of adsorption for water and other polar compounds. The more surface oxides there are, the more distinct is the hydrophilic behavior of the carbon. This was confirmed by Healey et al. (39) for graphitized carbon black, and by Kraus (40) and A. V. Kiselev and his group (41) for carbon black. Beebe and Dell (42) measured the sulfur dioxide adsorption on channel black and found an increase after oxidation at 600°. Further evidence for selective adsorption of polar compounds was provided by Gasser and Kipling (43). 

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. 

Diamond is the prototype of all alijihatic compounds. One would expect on its surface free valences which are capable of surface compound formation. The surface compounds on diamond should differ somewhat in character as compared to the surface compounds on aromatic graphite or microcrystalline carbon. Apart from singly bonded carbon atoms on the edges and corners of diamond crystals... 

Phosphate and phosphonate molecules have a very high affinity for calcium carbonate surfaces, as shown by their influence on the precipitation and growth of calcite [164-166]. Accordingly, organophosphorus compounds such as alkylphosphoric acids [167-169] and phosphonic acid-terminated polyoxyethylene [170, 171] have been used to modify the surface of CaCOs powders. 

These carbonaceous catalysts can be obtained by the sulfonation of incompletely carbonized organic compounds [42]. Note that starch and cellulose can be used as carbon precursor [43, 44]. After the incomplete pyrolysis of the carbon precursor, the SO3H groups have been introduced by sulfonation with sulfuric acid (Scheme 3). After this treatment, the presence of phenolic hydroxyl, carboxylic acid, and sulfonic groups at the surface of these amorphous carbonaceous materials has been demonstrated. 

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