Mercury Porosimetry

Porous or powdered materials are present in nature, infrastructure and industrial processes. Soil, concrete and plaster are porous structures, while porous materials in the form of spheres, extrusions or membranes are used for gas separation, liquid purification, chromatographic analysis or catalysis.

In all these fields, where adsorption is often at the heart of the process, it is important to quantify both the extent of the reactive surface, and the pore size distribution, and, if possible, also to have an idea of the morphological and topological organization of the pore structure. Indeed, the efficiency of a process depends on both favorable thermodynamics and easy transport conditions. For example, affinity and adsorption capacity are favored by a large specific surface area, which is generally obtained by reducing pore size, whereas this reduction is unfavorable to transport.

Process optimization therefore requires in-depth knowledge of the texture of the materials used, in terms of specific surface area, pore size distribution and pore network organization. In this context, the main methods for characterizing divided matter are based on gas adsorption, usually nitrogen or argon at low temperature. These methods provide access to the specific surface area of most divided materials, but only give access to a reliable pore size distribution in the microporous (< 2 nm) and mesoporous (2-50 nm) domains.

It's clear that most of the active sites for many high surface area materials lie in this micro-mesoporous domain, but using these materials in a process requires shaping (spheres, granules, monoliths) in which macropores (sizes greater than 50 nm) are present, and particularly important for transport properties. We therefore need methods capable of performing this type of characterization: assessing pore size from the nano to the macro scale.

This is the case with mercury porosimetry, the subject of this article, which is based on capillary phenomena such as nitrogen adsorption, and enables pore size analysis from a few nanometers to a few hundred micrometers. The principle is simple: mercury is a non-wetting liquid on the surface of most materials, so pressure must be exerted on it to force it into a pore. This article presents the simple relationship between the intrusion pressure exerted on mercury and the size of the pore under consideration: the measurement of the intrusion volume of mercury as a function of the pressure exerted is directly a cumulative pore size distribution. However, as with any experimental method, there are a number of calculation assumptions and technical limitations that must be rigorously taken into account to optimize the use of the data obtained. This is the purpose of this article,...