An Attempt at a More Precise Evaluation of the Approach to Mesopore Size Distribution Calculations Depending on the Degree of Pore Blocking

Abstract

A comparison of literature methods for the calculation of the mesopore size distribution (PSD) is made on the basis of nitrogen adsorption isotherms of aluminum hydroxide, aluminum oxide, and silica gel characterized by well defined H1, H2, H3, and H4 hysteresis loops. New, more detailed models of various types of mesopore structures are proposed. The corresponding hysteresis loops and PSD curves are considered in terms of different degrees of adsorbate pore blocking. The product of the differences between the prevailing pore neck and void sizes and the spontaneously diffusing volume determined from the two branches of the hysteresis loop is proposed as a quantitative measure of the degree of blocking. Thus, as the degree of blocking increases, the hysteresis loops follow the sequence H1 < H3 ⩽ H4 ⪡ H2.On the basis of this sequence it is shown that for PSD calculation, it is better to use the desorption branch, which bears more information about the degree of blocking than does the adsorption branch. Comparison of 13 main formulas for PSD calculation using the desorption branch shows that for structures with a minimum degree of blocking (H1 hysteresis), the best results are achieved with the Barrett-Joyner-Halenda formula, while in the cases of higher degrees of blocking (H3 hysteresis), the Innes formula is most suitable.