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

This study compares the Brunauer–Emmett–Teller (BET) and Langmuir equations when applied to N2 adsorption isotherms at 77 K on various carbon blacks and activated carbons. Adsorbent samples varied greatly in the degree of development of their surface area and porosity. The activated carbons were more microporous and less mesoporous than the carbon blacks. The equations were applied up to p/p0=0.20, 0.30, and 0.40. The values of the lineal correlation coefficient as a rule were above 0.990. From the derived BET (SBET) and Langmuir (SL) surface areas, the conversion factor (α) of SL to SBET was calculated. The α mean value (ᾱ) and the standard deviation coefficient (σ) were also obtained. The N2 isotherms fit better to the Langmuir equation for the activated carbons and to the BET equation for the carbon blacks. The factor α was markedly higher than the unit for all carbons. It increased significantly with increasing p/p0 range. The α values were closer for each series of carbons, and larger 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 ᾱ increments, the dependence of ᾱ on the p/p0 range was less strong for the activated carbons for the two narrowest p/p0 ranges. The σ values indicated that the influence of the type of carbon on ᾱ was greater for the carbon blacks when fitting up to p/p0=0.30 or 0.40.