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

Abstract 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 O2 adsorption or titration, that the chars derived from corncob pyrolysis in the temperature range of 300–750 °C contain 3 types of radicals, instead of the 2 types already reported in the literature, such as the one that reversibly adsorbs O2 and the one that does not adsorb O2. The newly found radicals in fresh corncob chars locate mainly at the methylene moieties linked to aromatic rings, and irreversibly disappear by exposure to O2, 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 °C while the second stage takes place in the pyrolysis temperatures of 600–750 °C. This two stage behavior can be characterized by the structural parameters determined commonly by 13C nuclear magnetic resonance, Raman spectroscopy, in-situ diffused reflectance Fourier Transform infrared spectroscopy, elemental composition, O2 adsorption, as well as ESR.