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

Some aspects of the relationship between the physicochemical state of micro crystalline carbons and the Xe-129 NMR parameters of adsorbed xenon have be en investigated. No correlation between the values of the Xe-129 chemical s hift (delta(o)) and the substructural characteristics of the carbons (inter layer 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 'stron g' adsorption sites for H2PdCl4, and chemisorbed heteroatoms (O, Cl) as wel l. HREM studies of carbon surface microstructure revealed delta(o) to be na notexture-dependent. The lowest delta(o) value (similar to 14 ppm) was dete cted for a graphitized carbon black, the surface of which consists primaril y 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. Med ium delta(o) values (50+/-10 ppm) were found for carbons the surface of whi ch was roughened by steps formed by carbon layers of different size. The Xe -129 NMR technique, in combination with the common procedures for the deter mination of the surface area of solids, may open a way to estimate the cont ribution of the edge faces of carbon crystallites to the total surface area of unoxidized carbons. (C) 1999 Elsevier Science Ltd. All rights reserved.