Abstract
AbstractA large number of functional and catalytic materials exhibit porosity, often on different length scales and with a hierarchical structure. The assessment of pore sizes, pore geometry, and pore interconnectivity is complex and usually not feasible by classical spectroscopic and diffraction techniques. One of the most powerful methods to probe these parameters is nuclear magnetic resonance (NMR) spectroscopy of xenon, which is introduced into the pore system. Adsorbed to the pore walls, it acts as probe nucleus. In this tutorial review, an introduction into the basic principles of 129Xe NMR spectroscopy and the models developed to determine pore sizes in different materials are given. The possibilities and limitations of this method for obtaining insights into hierarchical structures of functional materials are highlighted and a review of recent works is presented.
Highlights
The use of xenon as probe molecule for pore sizes, pore geometry, and pore interconnectivity is complex and usually not feasible by classical spectroscopic and diffraction techniques
One of the most powerful methods to probe these parameters is nuclear magnetic resonance (NMR) spectroscopy of xenon, which is introduced into the pore the NMR investigation of porous materials was first demonstrated in the 1980s by Ito and Fraissard on zeolites,[4] showing that the chemical shift of xenon was influenced by the pore size and the presence of system
In the taking into account the natural abundance, is around 32 times second part, we review the most recent developments and higher than the value for 13C, making 129Xe overall a convenient applications in the field in order to exemplify the benefits and nucleus to study by NMR spectroscopy.[3]
Summary
The description of the chemical shift has previously been introduced as a sum of several contributions, the observed chemical shift in a real system is the weighted average of all contributions the 129Xe nucleus experiences on the time scale of an NMR experiment (on the order of milliseconds). At room temperature, xenon atoms exchange quickly between different adsorption sites and the gas phase, implying that during the NMR experiment, they are able to move from the gas phase to an adsorption site or vice versa. This phenomenon is called chemical exchange (in contrast to nuclear spin exchange). The observed chemical shift is averaged between the gas phase signal and the chemical shift of xenon adsorbed on a specific site, both multiplied by the respective molar fractions. A signal of gaseous xenon in the excess gas phase between the particles is visible at 0 ppm. The free gas is not or only slowly exchanging with adsorbed xenon
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