Importance of oxidation reactions in creating pores in physical activation of biomasses

Carbonization plus activation of resulting biochar is a commonly used method for producing activated carbon (AC) from biomass. Nevertheless, carbonization produces biochar of highly aromatic nature, increasing difficulty for creating pores via activation. Herein, O2 (7.7%) was introduced to pre-oxidize lotus root and other biomasses-derived biochar before being activated with CO2 or H2O for tackling chemical inertness of biochar at 800 °C. The results indicated that O2 could enhance mass yield of AC (by 16.5% in CO2 activation and 50.6% in a H2O activation) and bio-oil. Moreover, O2 oxidized organics on AC, forming oxygen-containing aliphatic structures. Such a transformation enhanced reactivity of biochar with CO2 or H2O, facilitating generation of pores. The AC from activation with O2-CO2 was significantly higher than that from CO2 alone (533.7 m2 g−1 of AC from O2-CO2 activation versus 345.2 m2 g−1 of AC from CO2 activation). O2 also could remarkably increase the specific surface area of AC activated with H2O by 30.0%. Such an enhancement in pore structures was confirmed in activation of cellulose, pine needle and spirulina with O2 presence. The pre-oxidation activated biochar destroyed partially rigid structure, facilitating gasification with CO2/H2O, forming dominantly micropores with enlarged diameters. The characterization with in-situ IR indicated that O2 could enhance aromatization via aiding formation of aromatic C = C from olefinic C = C, producing AC of more regular alignments of carbon crystals.