Reconciliation of carbon oxidation rates and activation energies based on changing nanostructure

Abstract

HRTEM reveals the nanostructure of the model carbons and changes therein due to oxidation. The variation in oxidation rates during the oxidation process highlights the dependence of oxidation rates on carbon nanostructure and in particular, the carbon atom site types. Changes in nanostructure necessarily occur with change in the relative proportion of site types. In turn, the oxidation rate changes accordingly. An inside-out burning mode, onion peeling, graphitization and edge site creation are behaviors observed during the course of oxidation of the model carbons. The rate for arc-generated soot falls off exponentially while that for M1300, a carbon black exhibits a parabolic increase. R250, another carbon black maintains a constant rate, lower than that for graphitized multi-walled carbon nanotubes (MWCNTs) or graphitic carbon onions. The former exhibit exclusive interior oxidation while the latter graphitic materials exhibit only edge site oxidation. XPS confirmed the interpretation of graphitic lamella that was restricting further oxidation for the arc soot, by showing an increase in the sp2/sp3 content. Raman spectra are related to oxidation induced defects in the MWCNTs and onions.