SURFACE HETEROGENEITY FROM LOW TEMPERATURE ISOTHERMS

Low temperature isotherms have been analyzed by the method of Ross and Olivier for adsorption of hydrogen and nitrogen at 78-90 K and benzene at 273-298 K on about 20 solid systems. In most cases the experimental data fit upto a coverage of 0.7-0.8 with the theoretical isotherms drawn on the basis of Hill·de Boer model of local adsorption, but the monolayer volumes derived thus are much greater than the experimental values. There is no fitting of the experimental data with the theoretical isotherms for adsorption of benzene at 293 K on active carbons and hexagonal form of boron nitride, whereas for hydrogen adsoption on alumina-promoted iron catalyst at 77.3 and 90.1 K the fitting is excellent. The disagreement has been explained on the basis of interactions occurring during the process of adsorption and it has been suggested that such curve-fitting may be used as a rapid method to scan the surface heterogeneity or to assess the extent of interaction be­ tween a particular adsorbate and a number of adsorbents prepared by different formulations.