A novel method for estimating the C-values of the BET equation in the whole range 0<P/PO<1 using a Scatchard-type treatment of it

The BET equation, rearranged in a Scatchard-type form, V[1-(P/P-o)]/(P/P-o) vs. V[1 - (P/P-o)], was used for the calculation of Specific surface areas of a large number (127) of porous materials. The tested solids had surface areas from 20 Lip to 2200 m(2) g(-1) and they were of very diverse nature, like clays, mesoporous silicas and aluminas, aluminophosphates with mixed microporosity and mesoporosity, materials with only external porosity based on manganese, zeolites and carbon cloths. The Scatchard-type form of BET equation for all the above solids provide inclined-V type plots, i.e. >, with an inversion point, termed I-point. The slope of these plots is not constant and corresponds to (C - 1) from which the values of C can be estimated in the whole range 0 < P/P-o < 1. Those non-constant C quantities obtain appreciable values (1000-1500) for very low (PIP,). As the I-point is approached at higher (P/P-o) the C-values approach the C-BET values (C-constant) calculated from the classical BET-plots. Exactly, at the I-point the C - values suffer a transition to infinitive. Then below the I-point, at even higher (P/P-o) the (C - 1) values become negative. From the corresponding plots it has been possible to calculate C in the whole range of (PIP,) with a notable discontinuity at the I-point. The intercept of the plots V [1 - (P/P-o)]/(P/P-o) vs. V[1 - (P/P-o)] corresponds to CVm and the V-m value can be estimated if C is known from the slope. Since the slope is not generally constant, this method cannot be used. Nevertheless the I-point corresponds exactly to V[1 - (P/P-o)] = V-m, i.e. the volume of nitrogen necessary for the formation of a monolayer. The ssa's calculated using this I-point method, are practically identical to those obtained from the classical BET-plots. This methodology provides the possibility to determine the Values of the non-constant C-term of the BET equation in the whole range 0 < (P/P-o) < 1 as well as the ssa from the I-Point. (C) 2004 Elsevier Inc. All rights reserved.