Improved calculations of pore size distribution for relatively large, irregular slit-shaped mesopore structure

Abstract

Present methods for determining pore volume (PV), pore size distribution (PSD), and average pore width (APW) of irregular mesoporous materials from N2 adsorption, including the widely used model developed by Barrett, Joyner and Halenda (BJH), are likely to produce inaccurate results due to simplistic assumptions and approximations. For example, in assuming a simple cylindrical pore geometry and due to various empirical approximations, the BJH method typically underestimates APW for aluminas by 10–30% using the desorption branch of the N2 adsorption isotherm. Here we report improved calculations of PSD based on the Kelvin equation and a proposed Slit Pore Geometry (SPG) model for slit-shaped mesopores of relatively large pore size (>10nm). Two structural factors, α and β, are introduced to correct for non-ideal pore geometries. The volume density function for a log normal distribution is used to calculate the geometric mean pore diameter and standard deviation of the PSD. The Comparative Adsorption (αs) Method (CAM) is also employed to independently assess mesopore surface area and volume. Values of APW calculated by the SPG method are typically 6–20% greater than values predicted by BJH model using the desorption branch of N2 adsorption isotherm. Pore areas and pore volumes calculated from the SPG model are also in excellent agreement with those determined independently by CAM. The SPG method is demonstrated to be especially applicable to determination of PV, PSD, and APW for aluminas and other medium to large pore diameter mesoporous solids consisting of slab-like nano-crystals.