Basal plane site

Fig. 19.6 Comparison of relative affinities of edge vs. basal-plane sites for adsorption of C02 and N2 on graphene. (Adapted from [107].)...

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]. 

In addition to edge(Mo)sites there are basal-plane sites, probably SH groups, on which carbonium ion intermediates may be formed in certain reactions, e.g., isomerization of 2-methylbut-l-ene to 2-methylbut-2-ene. Such isomerizations can proceed in the absence of gaseous hydrogen. 

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,... 

For adsorption on the basal plane sites, the energy of adsorption is substantially lower when the H atoms are occupying adjacent sites. Comparing Model F with... 

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 of electron transfer for basal plane sites has been reported to correspond to 10 cm s for the oxidation of ferrocyanide and is considered to be possibly even zero [4-6]. How does one know that this is actually correct As shown in Fig. 3.9a, a strangely distorted voltammogram would be observed in the limit of very low defect density [23]. Due to the fact that two peaks have never been observed experimentally, it is generally accepted that edge plane electron transfer kinetics are anomalously faster over that of basal plane the latter is sometimes referred to as being inert [5,6,23]. Interested readers are directed to the elegant work of Davies et al. and Ward et al. to further appreciate this work [5,24]. 

Further evidence on the role of edge plane sites versus basal plane sites has been reported [4] by the selective blocking of the basal plane sites of HOPG with a polymer whilst the edge plane sites were left exposed. Identical voltammetric behaviour was observed with this modified surface when compared to that of the initial bare electrode and with numerical simulations, confirming the edge planes to be the sites of electrochemical activity Fig. 3.10 depicts how this was achieved. 

Last, it is important to note that researchers will (and have already done so) dispute the extensive literature reported above. As such it has been reported that under certain (limited) conditions the basal plane sites have measurable electrochemical activity [25-27]. Using elaborate Scanning Electrochemical Cell Microscopy (SECM) it has been reported that the basal plane sites of freshly exposed HOPG display considerable electroactivity which, interestingly, is time dependant, in that exposure to air for less than one hour after cleaving leads to a decrease in the observed electron transfer rates at the basal surface [27]. Such work is highly fascinating and studies into this time-dependent surface effect are, at the... 

Fig. 3.10 Initially a HOPG surface is cleaved to produce a fresh surface (stage 1). In stage 2, M0O2 nanowires are formed exclusively along the edge plane sites. In stage 3, the basal plane sites are covered by the electrochemical reduction of 4-nitrobenzenediazonium cations. Stage 4 then involves exposing the edge plane sites by dissolution of M0O2 in HCl. Reproduced from Ref. [4] with permission from Wiley...

Electrode Material 4 Graphene Basal Plane Sites... 

Edge Plane Sites versus Basal Plane Sites and HOPG as a Model.310... 

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