Activation of lignin-derived biochar with mixed H2O and CO2: Characterization of reaction intermediates and investigation their potential synergistic effects

Physical activation of biomass with H2O or CO2 is an important method for producing activated carbon (AC). Activation of biomass with CO2 or H2O follows varied mechanisms and the use of mixed CO2/H2O might show potential synergistic effects for creating pores. This was investigated by activation of lignin-derived biochar with H2O, CO2 and their mixture. The results demonstrated that H2O as an activator was much more effective than CO2 for generation of pores (804.1 m2g  1 versus 516.1 m2g  1in CO2), especially mesopores (40% versus 14% in CO2). This was achieved via enhancing cracking/gasification reactions of the biochar with H2O. The synergistic effect of CO2 and H2O for activation of the lignin-derived biochar was insignificant as cracking/gasification reactions were not enhanced, leading to remarkably lower specific surface area of the AC (674.8 m2g  1) than that with H2O as an activator alone. Competitive adsorption of CO2 and H2O existed and preferable adsorption of CO2 hindered deoxygenation of the biochar with H2O. The in-situ IR characterization indicated that CO2 presence suppressed decarbonylation reactions, retaining more oxygen species in form of C--O, while it was the opposite in H2O. Additionally, activation with H2O formed numerous round-shape dents on surface of activated carbon, while the activation with CO2 formed the characteristic round edges. The AC activated with H2O showed superior capability for adsorption of phenol than that activated with CO2 or mixed H2O/CO2.