Characteristics of Pore Structure in the Evaporated Stage of Wood-Derived Carbon

Abstract

Major water loss in the first stage of wood-derived carbon preparation had a direct impact on pore structure during the subsequent pyrolysis of biochar materials. To explore the changes in pore space at this stage, poplar was used as the subject of this study. The pore morphology before and after water evaporation was quantitatively characterized by scanning electron microscope, nitrogen adsorption method, and mercury injection method. Both the fractal dimensions of the multistage pores were obtained by the Frenkel-Halsey-Hill model and the thermodynamic relationship model. The results showed as follows: the adsorption isotherms were presented the mixed type of II and IV after supercritical drying, indicating the mesopores and macropores of wood. The adsorption isotherms after conventional drying were characterized a type II, which was inferred to the reduced mesopores compared to supercritical drying. The hysteresis loops of all wood samples were examined for the H3 type, which was deduced from the presence of slit-like pores. After treatment at 103±2 °C, the porosity and its connectivity properties are reduced by the mercury pressure data. For <50 nm the pore size distribution of the samples shows a shoulder peak at around 6.5 nm. Simultaneously, the pore structure is significantly deformed and the pore size distribution produces smaller peaks at 3014 and 12510 nm. The pore fractal dimension increases in the <50 nm stage. The fractal dimension of pores decreases when the pore size is 50 nm~5 μm. Stability of the pore fractal dimension in the pore size >50 nm stage. This indicates that the conventional drying treatment made the micropore complexity increase, and leads to a reduced or invariable pore structure complexity for the >50 nm. The fractal analysis could assist in better analyzing the changes of pore structure and provide a new perspective for studying the pore structure in the carbonation stage.