Carbon basal plane site

The oxidation of carbon can also be catalyzed. Two fundamentally different cases should be discriminated. Transition metal oxides and carbides were found to be efficient local sources of atomic oxygen increasing its abundance much above the uncatalyzed case. Streams of diffusing oxygen atoms created decorated pathways of nonselective oxidation of basal plane sites as detected by transmission electron microscopy [117, 118]. 

Weak basicity is exhibited by tt electrons in C = C double bonds and aromatic systems. Evidence for the protonation of basal plane sites on carbons was presented by Leon y Leon et al. [56]. In Fig. 13.5 of Ref [61] a ratio of 2.55 of adsorbed HCl (in mol) per mol of chemisorbed O atoms was given for a carbon black with a low oxygen content. The oxygen content before immersion was taken as a basis. This is much higher than the ratio of 1 expected for pyrone-type structures. However, from the data in Table 2 of this reference, a HCl/O ratio of 1.28 can be calculated, which is much closer to 1. 

C is a basal plane site characterized by the presence of delocalized n electrons, which thus acts as a Lewis base center. Such sites are probably located in n-electron-rich regions within the basal planes of carbon crystallites (graphenes), away from the edges [63]. Evidence favoring their electron donor-acceptor (EDA) interactions, such as Reaction (3.1), was presented [63] by investigating a polymer-derived microporous carbon and a furnace carbon black, using HCl adsorption isotherms,... 

Oxygen surface groups are not the only centers conditioning the catalytic behavior of carbon-supported catalysts. Thus, when a high surface area carbon black is subjected to heat treatment in an inert atmosphere at temperatures ranging from 1600 to 2200 °C there is not only a decrease in surface area, but also an increase in crystalline ordering, associated with an increase in the basicity of the carbon, which cannot be explained by basic groups. The basicity of the carbon surface is explained in terms of the (ir) sites of the carbon basal plane, which upon interaction with water lead to the equation ... 

Leon y Leon CA, Solar JM, Calemma V, Radovic LR. Evidence for the protonation of basal plane sites on carbon. Carbon 1992 30(5) 797-811. 

The rate and mechanism are different on the basal plane and edge sites of carbon. The reactions involving oxygen are two to three orders of magnitude slower on the basal plane than on the edge sites, because of the weak adsorption of oxygen molecules on the basal plane surface [34]. 

The basic building block of carbon is a planar sheet of carbon atoms arranged in a honeycomb structure (called graphene or basal plane). These carbon sheets are stacked in an ordered or disordered manner to form crystallites. Each crystallite has two different edge sites (Fig. 2) the armchair and zig-zag sites. In graphite and other ordered carbons, these edge sites are actually the crystallite planes, while in disordered soft and hard carbons these sites, as a result of turbostratic disorder, may not... 

In this paper, we presented new information, which should help in optimising disordered carbon materials for anodes of lithium-ion batteries. We clearly proved that the irreversible capacity is essentially due to the presence of active sites at the surface of carbon, which cause the electrolyte decomposition. A perfect linear relationship was shown between the irreversible capacity and the active surface area, i.e. the area corresponding to the sites located at the edge planes. It definitely proves that the BET specific surface area, which represents the surface area of the basal planes, is not a relevant parameter to explain the irreversible capacity, even if some papers showed some correlation with this parameter for rather low BET surface area carbons. The electrolyte may be decomposed by surface functional groups or by dangling bonds. Coating by a thin layer of pyrolytic carbon allows these sites to be efficiently blocked, without reducing the value of reversible capacity. 

Fig. 19.1 Creation of three internal edge sites, or a (mono)vacancy, by removal of one basal-plane carbon atom in graphene.

Site of the. acidic surface oxides. The question whether the acidic surface oxides are bound to the periphery of the carbon layei-s or to the basal planes of the crystallites could be resolved by oxidation of a graphitized carbon black (46). The particles of carbon black are, at first approximation, spherical. The graphite-like crystallites show such preferential orientation that their c axis are aligned in a radial direction (64, 65). A schematic representation of this secondary structure is given in Fig. 1. On recrystallization between 2000 and 3000°, many small... 

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