A statistical model for the heterogeneous structure of porous catalyst pellets

Abstract

The complex structures of the void space of porous media are often characterised by parameters such as pore network connectivity and lattice size. This paper presents a comparison of the estimates of these parameters obtained from two previous methods based on nitrogen sorption and mercury porosimetry, and also from a new, completely independent approach based on pulsed-gradient spin-echo nuclear magnetic resonance (PGSE NMR). It was found that the new PGSE NMR technique obtains estimates of connectivity and lattice size in agreement with nitrogen sorption but different to mercury porosimetry. This difference was attributed to the various physical processes involved actually probing different aspects of the pore space geometry. It was further suggested that the representation of the pore structure derived from either nitrogen sorption or PGSE NMR is really a mapping of the real pore space onto an equivalent abstract, random pore bond network. However, it has been shown that this mapping does capture some of the characteristic properties of the pore space that control transport over mesoscopic (<10 μm) length scales. For materials which additionally possessed macroscopic (>10 μm) structural heterogeneity, it was found that the model could also be adapted to predict the macroscopic transport properties of the porous medium.