129Xe NMR study of carbonaceous materials:: effects of surface chemistry and nanotexture

Some aspects of the relationship between the physicochemical state of microcrystalline carbons and the Xe-129 NMR parameters of adsorbed xenon have been investigated. No correlation between the values of the Xe-129 chemical shift (delta(o)) and the substructural characteristics of the carbons (interlayer spacing, crystallite size, content of ordered or amorphous carbon of the samples), micropore volume and specific surface area was revealed. The delta(o) values increase linearly with the surface concentrations of 'strong' adsorption sites for H2PdCl4, and chemisorbed heteroatoms (O, Cl) as well. HREM studies of carbon surface microstructure revealed delta(o) to be nanotexture-dependent. The lowest delta(o) value (similar to 14 ppm) was detected for a graphitized carbon black, the surface of which consists primarily of exposed basal planes of carbon crystallites, and the highest delta(o) value (similar to 100 ppm) for a carbon sample with exposed edge faces. Medium delta(o) values (50+/-10 ppm) were found for carbons the surface of which was roughened by steps formed by carbon layers of different size. The Xe-129 NMR technique, in combination with the common procedures for the determination of the surface area of solids, may open a way to estimate the contribution of the edge faces of carbon crystallites to the total surface area of unoxidized carbons. (C) 1999 Elsevier Science Ltd. All rights reserved.