# A Wide-Ranging NMR Probe for Multiscale Structures
## 5 Applications
### Porous Carbon Materials
The chemical shift value extrapolated at a relative pressure of 1 was in good agreement with the pore sizes determined by nitrogen adsorption data. In addition to that, the analysis of the line shapes and linewidths of the materials provided information regarding the width of the pore size distributions and exchange phenomena related to the connectivity between pores. Lastly, spectra were also acquired after the adsorption of _n_-nonane, which was thought to selectively block micropores, leaving mesopores unaltered. Thus, lower chemical shift values for exchange peaks were expected. On the contrary, a slightly higher \(\delta\) value was observed, as can be seen in Figure 12. This was explained by a reduction in size for the mesopores due to the adsorption of _n_-nonane combined with a stronger xenon-nonane interaction compared to the xenon-wall interaction. Figure 12: \({}^{129}\)Xe NMR spectra of CMK-3 and an ordered mesoporous carbide-based material, taken before and after adsorption of _n_-nonane. Reprinted with permission from reference [121]. Copyright 2014 American Chemical Society. The conversion of biomasses into activated carbons, called biochar, is a very attractive process which obtains specialty materials with a low environmental impact. These materials have proven useful in wastewater treatment, soil conditioning, catalysis, and as construction materials [122; 123]. The activation of biomasses can be achieved via different methods, including pyrolysis and treatment with acids, bases, steam, carbon dioxide, or ionic liquids. These processes deeply change the structure of the starting materials, creating extended porous structures which are responsible for the emerging properties. The porous structures of a series of pecan shell-based biochar were analyzed with \({}^{129}\)Xe NMR by [PERSON] and co-workers [124]. The spectrum of untreated pecan shells only showed the free gas peak close to 0 ppm, indicating the absence of pores in which confined xenon could be distinguished from the free gas. Biochar prepared with different airflow rates during activation (i.e., different nominal oxygen levels) showed very broad peaks between 100-200 ppm, indicating very broad pore size distributions. A slight upfield shift in the peak apex with increasing airflow rates indicated that higher degrees of oxidation created pores with slightly larger average sizes. 2D EXSY experiments also demonstrated that part of the adsorbed xenon was in exchange with the free gas in a time scale of around 100 ms. More specifically, the detected cross peaks were not centered to the adsorbed xenon peak, but they were shifted toward the rightmost part of the broad signal, indicating that only xenon located in the largest pores was able to exchange with the free gas. In a subsequent work, the same authors studied the porosity of biochar derived from almond shells [125]. Biochar was prepared by treating the almond shells in inert atmosphere at 800 \({}^{\circ}\)C and variable times in the range 60-240 min. Some samples were washed with water to remove the ash originated by the thermal treatment. \({}^{129}\)Xe NMR spectra revealed that the porosity of the materials increased with increasing treatment duration. Moreover, the spectra of washed materials showed larger and much more intense adsorbed xenon peaks (relative to the free gas peaks) compared to unwashed materials with the same treatment duration. This result revealed that the waste originated from thermal treatments partially obstructed the porous structures of the materials, significantly altering their properties. This was in accordance with BET studies, which showed an increase in surface area both with increasing treatment time and with water washing. In addition to that, 2D EXSY experiments showed that exchange between the free gas and the porous structure occurred only for some of the water washed samples, proving that this process also had a significant impact on pore accessibility.