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Analysis of nitrogen adsorption isotherms on porous and nonporous silicas by the BET and αs methods

1972; Elsevier BV; Volume: 38; Issue: 1 Linguagem: Inglês

10.1016/0021-9797(72)90226-3

1095-7103

M.R Bhambhani, P. A. Cutting, K. S. W. Sing, D.H Turk,

Heat and Mass Transfer in Porous Media

Nitrogen adsorption isotherms were determined on a wide range of porous and nonporous silicas. Isosteric heats of adsorption were calculated from the isotherms (over the temperature range −192° to −178°C) on representative materials. Standard data for nitrogen adsorption at −196°C on nonporous hydroxylated silica are tabulated for the ppo range 0.001–0.90. The results indicate that certain high-area silicas are truly nonporous, but some grades of commercial Aerosil are porous. Surface areas are calculated from the isotherms by means of the BET method and the new αs-method. The latter is a graphical procedure in which the amount adsorbed is plotted against αs for the standard adsorption data, where αs is the ratio of the amount adsorbed (at the given pp0) to the amount adsorbed at pp0 = 0.4. Deviations of the a,-plots from linearity are explained in terms of micropore filling and capillary condensation. In the absence of micropore filling, the surface areas calculated from the slope of the αs-plots are in excellent agreement with the BET-areas. Enhanced isosteric heats and C values are associated with micropore filling; the isotherm is therefore distorted in the BET range and the BET-area is not valid. In certain cases, when micropore filling and monolayer coverage at low pp0 are followed by multilayer formation and capillary condensation at higher pp0, a nearly linear αs-plot results, but again neither the BET-area nor the αs-area can provide a meaningful value of the internal surface area.