Carbon basal planes

Much work has been done on the effect of the addition of impurities (salts and metals, chiefly) on the reactivity of carbon. Quantitatively, the effects are difficult to understand, since they are functions of the location of the impurity in the carbon matrix and the extent of interaction of the impurity with the matrix. Long and Sykes (94) suggest that impurities affect carbon reactivity by interaction with the 7r-electrons of the carbon basal plane. This interaction is thought to change the bond order of surface carbon atoms, which affects the ease with which they can leave the surface with a chemisorbed species. Since the 7r-electrons in carbon are known to have high mobility in the basal plane, it is not necessary that the impurity be adjacent to the reacting carbon atom. Indeed, it is thought that the presence of the impurity at any location on the basal plane is sufficient for it to affect the reaction. 

Figure 2.1 Acidic and basic oxygen-containing functionalities of carbon surface (a) carboxyl groups, (b) lactone, (c) hydroxyl, (d) carbonyl, (e) quinone, (f) ether, (g) pyrone, (h) carboxylic anhydride, (i) chromene, (j) lactol, and (k) % electron density on carbon basal planes.

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 ... 

Fig. XVII-18. Contours of constant adsorption energy for a krypton atom over the basal plane of graphite. The carbon atoms are at the centers of the dotted triangular regions. The rhombuses show the unit cells for the graphite lattice and for the commensurate adatom lattice. (From Ref. 8. Reprinted with permission from American Chemical Society, copyright 1993.)...

Fig. 2. Schematic representation of basal plane orientation in several types of carbon fibers. (A) Single crystal graphite. (B) ex-pitch carbon fiber. (C) ex-PAN carbon fiber, (D) VGCF.

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... 

The SEI is formed by parallel and competing reduction reactions and its composition thus depends on i0, t], and the concentrations of each of the electroactive materials. For carbon anodes, (0 also depends on the surface properties of the electrode (ash content, surface chemistry, and surface morphology). Thus, SEI composition on the basal plane is different from that on the cross—section planes. 

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