|
|
|
|
K. Hayamizu, S. Hayashi, K. Kamiyama, and M. Kawamura
[Magnetic Resonance of Carbonaceous Solids; Botto, R. E. and Sanada, Y. Ed.; Advances in Chemistry Series 229 ; American Chemical Society:Washington, DC, 1993; pp.295-310]
Solid-state ^{1}H NMR spectra and spin-lattice relaxation times (T_{1}) are reported for eight Argonne Premium coals that were prepared under several different conditions. Measurements were performed on pristine coals, degassed and dried samples, dried coal samples stored in capped vials for about 1 year, and coals treated with two different acids, HCl and HF HCl. In general, nonexponential relaxation curves were analyzed by fitting the data to two exponential decay constants. With acid treatment, an increase in T_{1} was observed for the coals with carbon contents of less than 77%, and a decrease was observed for the higher rank coals. Electron spin resonance (ESR) measurements were performed on coals with and without acid treatment to assess the ^{1}H spin-lattice relaxation mechanisms. The important ^{1}H NMR relaxation mechanism in lignite and subbituminous coals is due to the presence of paramagnetic metal ions, whereas that in bituminous coals due to organic free radicals. The effects of various treatments on the ^{1}H NMR spectra and the T_{1} values of coals are discussed.
