# A Wide-Ranging NMR Probe for Multiscale Structures
## 5 Applications
### Porous Carbon Materials  
Compared to zeolites and mesoporous silica, the xenon NMR analysis of porous carbonaceous solids is challenging due to their structural features. Nonetheless, this technique has proven extremely useful even in the characterization of this type of materials. Many different carbonaceous solids have been studied [110, 111, 112, 113], and a quantitative model has also been proposed, as outlined before [39, 40]. The porous structure of the mesoporous CMK-3 carbon material has been characterized by variable pressure and 2D EXSY hyperpolarized xenon NMR experiments [114]. Excluding the free gas signal, two different peaks were detected in the spectra, and, owing to their chemical shift values and their pressure dependence, they were attributed to two populations of pores of different sizes: micropores and mesopores. The presence of both micropores and mesopores in CMK-3 could be explained by two reasons: (1) presence of two types of carbon particles due to inhomogeneity in the synthetical process, and (2) presence of micropporosity in the mesoporous structure or, in other words, presence of a hierarchical structure. 2D EXSY confirmed the second hypothesis: only mesopore-gas and mesopore-micropore cross-peaks were detected at a small mixing time (50 ms), indicating that the gas had to go through the mesoporous structure to reach the smaller micropores (Figure 11). Thus, the presence of a hierarchical structure in CMK-3 was demonstrated for the first time. Carbon nanotubes (CNT) are among the most attractive carbon-based materials. These materials have shown remarkable performances in terms of electrical conductivity [115], thermal conductivity [116], mechanical strength [117], and gas sensing [118]. CNTs can be divided into two general categories. Single-wall carbon nanotubes (SWCNTs) are single tubes with diameters in the nanometer range, while multi-wall carbon nanotubes (MWCNTs) are constituted by nested single-wall nanotubes held together by weak intermolecular interactions. In 2005, Xe NMR was applied to study the porous structure of single- and multi-wall carbon nanotubes [119]. Even though the adsorption of xenon was weak, the analysis of variable temperature spectra and spin-lattice relaxation times revealed that, in SWCNTs, xenon adsorbed preferentially on the metal impurities, while the slightly stronger  
Figure 11: Hyperpolarized \({}^{129}\)Xe 2D exchange spectroscopy (EXSY) spectrum of CMK-3 acquired at 293 K and a mixing time of 50 ms. Reprinted with permission from reference [114]. Copyright 2009 Carbon. adsorption in MWCNTs was due to the presence of defects, which acted as preferential adsorption sites for xenon. In the same year, xenon adsorption on MWCNTs was also studied by [PERSON] et al. [120]. Open and closed nanotubes (as determined by TEM images) were analyzed, and this allowed signals arising from xenon on the outer nanotube surface and xenon located in the interior of the nanotubes to be distinguished between. The weak adsorption of xenon was indicated by a weak pressure dependence of the chemical shift values. Moreover, CNTs were also analyzed after a ball milling process. Both open and closed CNTs displayed a single signal after ball milling, suggesting fast exchange between external and internal xenon atoms, indicating that ball milling opened up the interior volume of the nanotubes. Lastly, variable temperature data were fit to Equation (10) to determine the physical parameters of adsorption. In 2014, CMK-3 and a series of micro and mesoporous carbide-derived materials were studied with high pressure \({}^{129}\)Xe NMR [121]. Using a homemade apparatus, variable pressure spectra were acquired up to 18 bar, and the dependence of the adsorbed xenon chemical shift on the relative pressure was studied. The shape of the resulting curves for different materials resembled adsorption isotherms for micro and mesopores, allowing elucidation of the different porous structures.