ESR study of nanocrystalline aerogel-prepared magnesium oxide

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Abstract
Two signals have been observed in the ESR spectra of nanocrystalline MgO synthesized by an aerogel technique. These signals have been assigned to F+ centers in the nanocrystals and –O–˙CH2 radicals formed from residual methoxide groups present at the points of contact between the nanocrystals. The signal of F+ centers was found to be thermally stable up to 500°C in oxidizing atmosphere. The second signal was somewhat less stable, and could be removed upon heating to 300°C in the presence of oxygen. Similar organic radicals could be generated by UV irradiation of aerogel prepared (AP) samples, while their concentration appeared to follow the concentration of residual organics in the materials. Irradiation of the samples in the presence of oxygen resulted in the formation of 4-coordinated surface [O−˙O2] radical anions, while no signal typical for 3-coordinated radical anions was observed. Strong Lewis acid and basic sites on the surface of AP-MgO were characterized using the adsorption of 2,2,6,6-tetramethyl-4-oxo-1-oxyl-pyperidine (TEMPONE) and dinitrobenzene, respectively.

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