ESR and HRTEM study of carbon-coated nanocrystalline MgO

Carbon-coated nanocrystalline MgO samples were prepared by butadiene pyrolysis at 500degreesC over aerogel-prepared MgO samples. Samples with carbon loadings of 1.2, 3.2, 5.0, 10.0, and 15.9 wt % were prepared. Initial carbon formation rate was about 2 wt % per hour. According to HRTEM, the structure of the mineral component in the carbon-mineral materials was not altered by the carbon deposition. At low loading of carbon, it was found to deposit only inside the MgO aggregates 5-10 nm from their outside surface, forming thin individual bands 1.5-2 nm long. At the highest loading, carbon deposits form three-dimensional graphite-like multilayer structures filling the pore volume of the MgO aggregates and cover the outside surface of the aggregates with a thin monolayer coating. A single symmetric Lorentzian line with g = 2.0029 attributable to carbon deposits has been observed in the ESR spectra of the carbon-mineral materials. It gradually narrows from 5.0 to 1.9 G as the carbon loading increases from 1.2 to 15.9 wt %. Nitroxyl radicals formed after dinitrobenzene adsorption on MgO nanocrystals have been used as a spin probe for estimation of the concentration of strong basic sites present on the surface of the carbon-mineral materials and degree of their coverage with carbon. The sample with the carbon loading of 15.9 wt % has practically all such sites blocked with carbon, while samples with carbon concentration between 5 and 10 wt % seem to be the most promising candidates for practical application as destructive adsorbents.