XPS and 15N NMR Study of Nitrogen Forms in Carbonaceous Solids

A combination of XPS and solid-state 15N NMR have been used to characterize the nitrogen forms in a variety of different carbonaceous samples having high natural nitrogen abundance. It is currently difficult to unequivocally interpret and quantify individual 15N NMR and XPS spectra. The advantages of using a multiple technique approach for nitrogen speciation as well as the limitation of XPS nitrogen (1s) and 15N NMR spectroscopy are discussed. XPS and 15N NMR results show that pyridinic and pyrrolic nitrogen are present in quinoline and isoquinoline pyrolysis chars. Pyridinic nitrogen is the most abundant form in quinoline pitch while isoquinoline pitch produced the most pyrrolic nitrogen. A small amount of quaternary nitrogen (∼10 mol %) appeared in the XPS spectrum of the chars, however, no additional nitrogen forms were identified above the noise level in the 15N NMR spectra. The 15N NMR spectrum of Green River kerogen shows a single broad feature centered around −245 ppm in the 15N NMR, consistent with the position of pyrrolic nitrogen forms. XPS spectra of Green River kerogen show a large peak at 400.2 eV and the peak at this position in the curve-resolved nitrogen (1s) spectrum is consistent with pyrrolic nitrogen forms. Other XPS peaks were consistent with and attributed to pyridinic quaternary and amine nitrogen species. Acid treatment of Green River kerogen resulted in an increase in the quaternary nitrogen feature in both the XPS and 15N NMR spectra with a concomitant decline in the XPS pyridinic and amine nitrogen features. Pyrolysis of kerogen resulted in the appearance of a peak near −80 ppm in the 15N NMR spectrum attributed to pyridinic nitrogen forms. XPS spectra also show an increase in pyridinic nitrogen upon pyrolysis. XPS results show that the major peak in the curve-resolved nitrogen (1s) spectrum of both fresh and pyrolyzed peats appears at 400.2 eV. Both amide and pyrrolic nitrogen appear at this energy position and it is impossible to distinguish between these two species based on XPS data alone. For fresh peat, the major peak in the 15N NMR spectrum occurs at chemical shift positions consistent with the presence amide nitrogen forms. The main peak in the 15N NMR spectrum of pyrolyzed peats broadens and shifts toward higher field. This change is associated with the loss of amide forms and the appearance of pyrrolic nitrogen forms