EVIDENCE FOR THE PROTONATION OF BASAL-PLANE SITES ON CARBON

The nature of the surface basicity of two series of highly pure, chemically and/or thermally pretreated carbons has been investigated by (a) HCl adsorption, (b) stepped temperature-programmed desorption, (c) electrophoresis, and (d) mass titration. Following a comprehensive review of the pertinent literature, it is concluded that two mechanisms are necessary and sufficient to account for the basic properties of carbons. For carbons having low oxygen contents, molar HCl/O adsorption ratios exceeding two indicate that an oxygen-free type of site is involved in the adsorption process. Evidence favoring electron donor-acceptor (EDA) interactions of the type C(pi) + H3O+ double-line arrow pointing left and right C(pi)-H3O+ is presented. The postulated C(pi) site and its interaction with adsorbed H3O+ ions is described in detail. As the oxygen content of the carbon increases, the molar HCl/O ratios decrease sharply and level off at nonzero values, which are small due to the presence of nonbasic surface oxides. When these are removed by heat treatment (at 1073 K), the molar HCl/O adsorption ratio is again found to exceed two at low oxygen coverages, but levels off at ca. 1 for higher oxygen coverages. Of all the basic surface oxides proposed in the literature, only the pyrone-type groups can account for this HCl/O ratio. Therefore, it is proposed that the basicity of carbon surfaces arises from a combination of EDA and pyrone-type interactions. The predominance of either will be dictated by the oxygen content of the carbon in question.