How to characterise smallest ultramicropores?
Quite often there is a request to decide for a new instrument or a measuring routine to characterize pores larger than 0.35 nm. Independent of the fact that 0.3 nm is the critical and 0.35 nm the so-called kinetic molecule diameter of N2, there is an illusion that micropores in the range of 0.4 nm might be characterized by the use of N2 at 78 K. Over the last few years, we have measured numerous ultramicroporous materials. These materials always showed the same characteristics, namely that N2 is adsorbed at 78 K only by pore sizes larger than 0.5 nm. We employed long-term sorption measurements on a narrow pore Zeolite 4A and explain the effect and possible solutions for the characterisation of ultramicropores.
