Hydrogen-treated graphitized carbon blacks. Limiting isosteric heats and entropy changes upon adsorption of hydrocarbons

The study of the effects of hydrogen treatment at 1000° on graphitized carbon blacks was extended to the adsorption of nonpolar molecules. Before and after the hydrogen treatment on the Graphon and graphitized Sterling FT (Sterling FT-G) surfaces, the isosteric heats and entropies of adsorption at extremely low surface coverages of ethane, propane, isobutane, butane, pentane, and hexane were determined by chromatography using the eluted pulse technique. Our results were compared with some earlier determinations obtained by chromatography or by static methods. The abnormally high limiting heats and entropy changes of adsorption on untreated Graphon indicated the presence of a small fraction of surface geometric irregularities, where the interaction of an individual adsorbed molecule occurs with more than one plane of'the adsorbing surface. Some experimental data suggest that surface heterogeneities can be identified as cavities with specific dimensions. These irregularities are not found on the Sterling FT-G surface, which we observed as almost completely free of strong adsorption sites. After the hydrogen treatment, the heats and entropy changes of adsorption on Graphon sharply decreased and their values are almost equal to those measured upon adsorption on Sterling FT-G. This agreement shows that the hydrogen treatment is effective in removing not only traces of surface oxides but also topographical irregularities on the carbon surface. The standard integral adsorption entropy values were calculated from retention volume measurements and by using a relation we derived. The close agreement between our values and those derived by a static method makes it evident that the gas chromatography technique can definitely also be of great use in the determination of adsorption entropies at very low surface coverages.