Texture of divided materials Pore size of nanoporous materials from nitrogen adsorption

This article reviews the most widely used methods for characterizing the pore size of nanoporous materials by nitrogen adsorption, in the width range from 0.1 to 50 nm.

Even when they are of natural origin (activated carbons, activated clays), these materials are most often treated to "adjust" them (in terms of pore size, specific surface area, surface chemical functions, etc.) with a view to their applications, which are numerous and include the following:

• lowering natural gas storage pressure (to lighten cylinders and enable their use on natural gas-powered vehicles);

• purification and recycling of aircraft atmosphere;

• retention and reuse of petrol vapours from car fuel tanks;

• retention and reuse of solvent vapours from paint tunnels;

• remediation of soils contaminated by heavy metals;

• separation of gases from air at ambient temperature, without the need for energy-intensive liquefaction and distillation;

• the storage and gradual release (or "sustained release") of active drug ingredients, to ensure constant concentration in the body despite widely spaced drug doses;

• the development of solar refrigeration machines exploiting the highly endothermic nature of water or alcohol vapour desorption for vaccine storage in desert environments;

• recovery of hydrogen from refinery off-gases (where it costs nothing) for use as a clean fuel;

• carbon dioxide sequestration to limit global warming.

It's easy to see why pore width plays such an important role in these applications. It is this width that enables us to develop both "molecular sieve" properties, allowing only a certain size of molecule to pass through, and physical adsorption energy (the narrower the pore, the higher the energy), enabling us to exploit the corresponding thermal effect, or a permeability to gases and liquids (thanks to several "hierarchical" pore sizes) capable of accelerating chemical engineering operations, or even the "useful" storage capacity of gases with sufficient adsorption energy to allow good retention but low enough to allow easy and as complete as possible recovery of the gas...

To characterize the size of nanopores, this article uses nitrogen adsorption, which is the most widely used approach, especially when the material itself is intended for an application involving adsorption.

Today's automated equipment makes it possible to carry out routine measurements, which is a definite advantage,...