Confirmation of pore formation mechanisms in biochars and activated carbons by dual isotherm analysis

Abstract

In this study biochars and activated carbons were synthesized either directly via the pyrolysis of sodium carboxymethyl cellulose (NC) or via hydrothermal carbonization of sawdust (SD) in an aqueous solution of KOH. The amount of porogen was varied by modulating the degree of sodium carboxymethyl substitution on NC or the amount of KOH mixed in solution with SD. Pore size distributions (PSDs) of these carbons were determined from the dual fit of kernels based on the two-dimensional version of the nonlocal density functional theory (2D-NLDFT) heterogeneous surface models to either N₂ and H₂ or O₂ and H₂ isotherms measured at −196 °C. By comparing PSDs of carbons from the same starting material at increasing degrees of activation, we show that those derived using O₂ and H₂ isotherms not only give more detail of variations in pore size but that the results also fit better with current understandings of porosity development in carbons derived through oxidative activation. This is likely a result of superior diffusion of O₂ into ultramicropores at low pressure relative to N₂.