Heats of combustion and electron spin resonance of ultrafine graphite

Abstract

Heats of combustion, ( ΔH c) and ESR measurements on ultrafine graphite, an amorphous “degraphitized” form, depended on prior exposure of the material to air. Air-free samples: ΔH c increased from 18 to 35% over the literature value for polycrystalline graphite; the ESR signal was a symmetrical Lorentzian line centered at g = 2.0034. Air-exposed samples; ΔH c was about 4% higher than the literature value; the ESR signal was reversibly and asymmetrically broadened to 4 times the air free value and was nearly Gaussian. The different reactivities of edge and planar carbon atoms in the finely divided material are invoked to explain the combustion data. The asymmetric broadening of the ESR signal on exposure to air is accounted for by postulating anisotropic dipolar coupling between physisorbed molecular oxygen and the unpaired spins. The linewidth data support a model of thin graphite platelets roughly 10 Å thick and 30 Å in diameter.