Nitrogen adsorption isotherms on carbonaceous materials. Comparison of BETand Langmuir surface areas

This study compares the Brunauer-Emmett-Teller (BET) and Langmuir equations when applied to N-2 adsorption isotherms at 77 K on various carbon blacks and activated carbons. Adsorbent samples varied greatly in the degree of de velopment of their surface area and porosity. The activated carbons were mo re microporous and less mesoporous than the carbon blacks. The equations we re applied up to p/p(0) = 0.20, 0.30, and 0.40. The values of the lineal co rrelation coefficient as a rule were above 0.990. From the derived BET (S-B ET) and Langmuir (S-L) surface areas, the conversion factor (alpha) of S-L to S-BET was calculated. The alpha mean value (<(<alpha>)over bar>) and the standard deviation coefficient (sigma) were also obtained. The N-2 isother ms fit better to the Langmuir equation for the activated carbons and to the BET equation for the carbon blacks. The factor alpha was markedly higher t han the unit for all carbons. It increased significantly with increasing p/ p(0) range. The alpha values were closer for each series of carbons, and la rger for the carbon blacks than for the activated carbons; in spite of the fact that the carbon blacks were more mesoporous. As shown by the a increme nts, the dependence of <(<alpha>)over bar> on the p/p(0) range was less str ong for the activated carbons for the two narrowest p/p(0) ranges. The sigm a values indicated that the influence of the type of carbon on <(<alpha>)ov er bar> was greater for the carbon blacks when fitting up to p/p(0) = 0.30 or 0.40. (C) 2001 Elsevier Science B.V. All rights reserved.