X-RAY PHOTOELECTRON SPECTROSCOPY INVESTIGATION OF NITROGEN TRANSFORMATION IN CHINESE OIL SHALES DURING PYROLYSIS; pp. 129–145Full article in PDF format | https//doi.org/10.3176/oil.2017.2.03
X-ray photoelectron spectroscopy (XPS) was used to investigate changes in nitrogen functionalities present in Chinese Huadian (HD), Maoming (MM) and Yaojie (YJ) oil shales during pyrolysis. Throughout the process (T ≤ 600 °C), most of the nitrogen contained in raw oil shale samples was retained in their semi-cokes. Five peaks of nitrogen functionalities (N 1s) appeared in the XPS spectra of raw HD, MM and YJ oil shale samples and their semi-cokes: N-6 (pyridine), N-A (amino), N-5 (pyridone), N-Q (quaternary nitrogen) and N-X1 (pyridine N-oxide). To obtain an acceptable fit, an additional peak at 404 (±0.5) eV (N-X2) was required in the N 1s spectra of the samples. N-5 could either represent pyridone or a mixture of pyridone and pyrrolic nitrogen forms, the most abundant ones in all samples. At a relatively low temperature (300 °C) the desorption reaction occurred and the amount of chemisorbed oxygen associated nitrogen (N-X2) decreased significantly. As the pyrolysis temperature increased from 300 to 500 °C, pyridine N-oxide was converted to pyridone, and, simultaneously, the latter was converted to pyridine and pyridine structures associated with oxygen – quaternary nitrogen. In the semi-cokes of Huadian and Maoming oil shale samples at 600 °C, most of the pyridone was converted into pyridine and quaternary nitrogen. At this temperature, especially the condensation reaction of pyridine into quaternary nitrogen occurred in the semi-coke of Yaojie oil shale sample, while quaternary nitrogen represented the nitrogen atoms in the interior of precursors of the graphene layers.
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