SURFACE HETEROGENEITY STUDIED BY A QUASI-EQUILIBRIUM GAS-ADSORPTION PROCEDURE

It is shown here that for the study of the surface heterogeneity and microporosity of mineral surfaces, the necessary enhancement in the resolution of the adsorption isotherms may quite satisfactorily be obtained by using the quasi-equilibrium procedure. The latter is applied in its volumetric version, where a slow, constant, and continuous flow of adsorbate is introduced into the adsorption cell. From the recording of the quasi-equilibrium pressures (here in the range from 10-4 to 500 mbars) vs time, the adsorption isotherm is derived. Various tests were carried out for checking the validity of the results. Surface area values obtained for alumina by use of this setup are identical with those obtained by classical volumetric equipments. This quasi-equilibrium volumetric procedure yields the very first stages of the adsorption process and, thus, provides information about surface heterogeneity. In the case of a microporous sample: sepiolite, nitrogen, and argon adsorption isotherms show the high adsorption energy of the pores and the partial filling of the microporosity while the use of C02 at 0 and 20°C shows the filling of all the structural microporosity, confirming the theoretical value found by Rautureau and Tchoubar.1 This apparatus is suitable for samples with surface areas higher than 3 m2/g. © 1990, American Chemical Society. All rights reserved.