Nitrogen in aramid-based activated carbon fibers by TPD, XPS and XANES

Activated carbon fibers were prepared from Nomex® [poly(m-phenylene isophthalamide)] by either H3PO4 activation, H3PO4–CO2 activation, or simply CO2 or steam activation. These treatments converted amide groups from the polymer precursor into complex and heterogeneously distributed nitrogen functionalities. TPD, XPS and XANES were used to study the effects of these treatments on the local bonding environment around nitrogen in the resulting carbons. These analytical techniques showed that nitrogen atoms are present in the 6-membered rings located at the edges of condensed polyaromatic systems as pyridine-like sp2 nitrogen (N1 or N2) or in the interior, where nitrogen replaces one carbon atom and is bonded to three carbon neighbors (N3). The occurrence of a species (N2) hypothetically related to a pyridinic cycle bearing oxygen substituents or intracyclic oxygen atoms could be correlated with the degree of oxidation of the carbon surface. Assuming that a relative N3 increase is indicative of aromatization and that the reverse, correlated with a N2 increase, is indicative of surface oxidative denitrogenation, the ratio between these nitrogen species revealed that aromatization and oxidative denitrogenation processes occur sequentially or simultaneously to different extents according to the type of carbon activation and to the burn-off degree. Physical activation involves thermal aromatization reactions during the carbonization stage and the subsequent isothermal activation one. In this second activation stage, co-occurring thermal oxidation reactions lead to a less intense denitrogenation during CO2 activation than during steam activation. H3PO4 activation induces the largest nitrogen retention in the final product in a double process of aromatization and “auto-activation” producing a moderate oxidative attack of nitrogen. However, an increase of the H3PO4 ratio fostered the oxidation of the carbon surface and consequently enhanced nitrogen gasification during the subsequent activation.