The Hydrogen Adsorption Behavior of Mechano-Chemically Activated Carbon from Indonesian Low-rank Coal: Coupled Langmuir and Dubinin-Astakhov Isotherm Model Analysis

摘要

This study aims to produce activated carbon from low-rank coal from East Kalimantan, Indonesia by a mechano-chemical method and to determine its adsorption parameters: hydrogen uptake/capacity, activation energy, the structural heterogeneity parameter, and the isosteric heat of adsorption. The hydrogen uptake/capacity of the coal was determined by a volumetric adsorption test using constant-volume-variable-pressure (CVVP). The characteristic adsorption parameters, such as hydrogen uptake, characteristic energy and heterogeneity structure factor, were determined using the coupled Langmuir and Dubinin-Astakhov (D-A) isotherm models, with the assumption that the hydrogen uptake value would be similar, irrespective of the model used. We found that the mechano-chemical method significantly reduced the particle size of the activated carbon relative to the untreated control, by approximately 60%. In addition, the activation process yielded a higher surface area for the activated carbon (390 m2/g) compared to the untreated control (90 m2/g). We also found that greater surface area led to a greater uptake of hydrogen by the activated carbon (40.17±1.56)×10-3 kg/kg than by the untreated control (7.94±1.56)×10-3 kg/kg. We also found that the heterogeneity factor of the activated carbon was 3.73±0.23, lower than the untreated control 4.65±0.56, which reflects the more heterogeneous pore diameter sizes for the activated carbon compared to the untreated control. Lastly, using the obtained adsorption parameters, we observed that the hydrogen uptake-dependent isosteric heat of adsorption on the activated carbon changed rapidly in the initial and final stages compared to the untreated control due to the adsorption of hydrogen by smaller pores which reside inside larger ones.