The I-point method for estimating the surface area of solid catalysts and the variation of C-term of the BET equation

The BET equation rearranged in the form [V(1 − P)]/P = CVm − (C − 1)[V(1 − P)] was used for the calculation of specific surface areas of porous catalysts and the variation of C-term during the progress of adsorption. This novel method employs plots of [V(1 − P)]/P vs. [V(1 − P)] which provide inclined-V type lines with an inversion point termed I-point. This I-point corresponds exactly to V(1 − P) = Vm, i.e., the volume of nitrogen necessary for the formation of a monolayer. The slope of the plots corresponds to (C − 1) from which the values of C can be estimated in the whole range 0 < P < 1. As the I-point is approached, the C values approach the CBET values calculated from the classical linear BET plots. The free energy of adsorption in the whole range 0 < P < 1 can be also estimated from the relationship RT ln C.