Comparison of textural information from argon(87 K) and nitrogen(77 K) physisorption

Abstract

Argon(87K) and nitrogen(77K) adsorption of three types of porous samples (microporous, microporous-mesoporous and mesoporous)
was evaluated. The shapes of isotherms, specific surface areas, mesopore-, micropore- and net pore volumes, pore-size distributions
(PSD) and positions of micropore and mesopore PSD’s maxima were compared to ascertain the credibility of individual adsorbates
for the texture information estimation. The shapes of adsorption isotherms for Ar and N2 of all samples are similar and the adsorbed amounts at relative pressure x=0.975 differ slightly. For mesoporous samples some differences are observed between specific mesopore surface areas derived
from nitrogen and argon isotherms. Radii of pore size maxima from Ar(87K) PSD’s are on average systematically lower than
from N2(77K) for all samples. The Saito-Foley approach for ZSM-5 samples gives consistently lower mean pore radius from N2(77K) adsorbate than from Ar(87K). This difference probably arises from the multiplication factor, Ω, in the Saito-Foley
equation which includes physical properties (magnetic susceptibility, polarizability etc. at 77 and 87K) and is not easy to obtain with sufficient precision. The use of both adsorbates Ar(87K) and N2(77K) possesses some advantages as well some disadvantages and the comparison of textural properties of individual samples
must be evaluated with respect to adsorbate.

KeywordsNitrogen adsorption isotherm–Argon adsorption isotherm–ZSM-5 zeolite–TiO2
–Al-SBA-15