Finally, another factor that may contribute to the large EEO2/EEN2O ratio is the dissociative sticking probabilities for O2 vs. N2O. As noted, the O2 bond is stronger than the N2-O bond, but O2 can form two C-O bonds, while N2O dissociates to form one C-O bond, losing N2. That means that O2 dissociation is likely only at sites where two under-coordinated -C * centers are available, while N2O can presumably also dissociate at isolated -C * sites. That might tend to favor N2O etching, particularly for NPs with few under-coordinated sites, however, there are indications in the literature that N2O sticking is less favorable than O2 sticking. For example, it is found that the presence of O2 inhibits the high temperature reaction of N2O with carbon surfaces, leading to the suggestion of weaker adsorption for N2O vs. O2 on carbon active sites. Another factor may be steric. Theoretical studies examined how the N2O collision orientation affected the energetics for N2O chemisorption. On a clean graphite surface, where reactive sites (-C * ) were mostly under-coordinated centers on zigzag and armchair edges, showed that EEN2O increased substantially during the first few hours of 1800 K etching, although that NP later passivated against N2O attack after ~7 hours of etching. Surprisingly, the sublimation rates measured back-to-back during the same experiments decreased substantially over time, i.e., sublimation and etching rates evolved in opposite directions (Fig. ). In contrast, in O2-graphite NP experiments measuring back-to-back etching and sublimation rates as a function of reaction time and TNP, etching and sublimation evolved together, both slowing over reaction time. For O2, the similar evolution of etching and sublimation rates was taken as evidence that both processes occur primarily at similar types of surface sites. O2 dissociative adsorption leading to etching of bulk graphite has been shown to occur almost exclusively at under-coordinated surface sites, such as exposed basal plane edges or at basal plane defects. Graphite sublimation occurs by desorption of small carbon clusters, and the sublimation rates are weighed averages over the desorption rates from different types of surface sites. Desorption processes with low activation energies (Eas), i.e., those that require breaking the fewest and weakest C-C bonds, have faster kinetics and therefore make outsized contributions to the overall sublimation rate. Under-coordinated basal plane edge and defect sites should have lower Eas than perfect basal plane sites, and therefore are expected to dominate the sublimation rate. Since these sites are also likely to be active for O2 adsorption, it is not surprising that sublimation and O2 etching rates should vary similarly as the NPs evolve.