A study on the new type of radicals in corncob derived biochars

Biochars prepared from pyrolysis of biomass have been studied for a broad range of applications. It was reported that biochars contained radicals measurable by electron spin resonance (ESR) but the radicals have not been quantitatively related with the pyrolysis conditions nor with the chars' structure. This work shows, through O-2 adsorption or titration, that the chars derived from corncob pyrolysis in the temperature range of 300-750 degrees C contain 3 types of radicals, instead of the 2 types already reported in the literature, such as the one that reversibly adsorbs O-2 and the one that does not adsorb O-2. The newly found radicals in fresh corncob chars locate mainly at the methylene moieties linked to aromatic rings, and irreversibly disappear by exposure to O-2, which leads to the formation of C-O and C=O bonds. It is also found that the evolution of corncob char structure shows a two stage behavior, the first stage takes place in the pyrolysis temperatures of 300-600 degrees C while the second stage takes place in the pyrolysis temperatures of 600-750 degrees C. This two stage behavior can be characterized by the structural parameters determined commonly by C-13 nuclear magnetic resonance, Raman spectroscopy, insitu diffused reflectance Fourier Transform infrared spectroscopy, elemental composition, O-2 adsorption, as well as ESR.