Investigating synergistic effect of controlling hierarchical pore structure and chemical modification on CO2 adsorption kinetics of reduced graphene oxide aerogel

The hierarchical pore structure of amine-functionalized reduced graphene oxide aerogels (rGOAs) plays a vital role in CO₂ adsorption. Herein, the self-assembly of rGOAs was investigated by adjusting graphene oxide concentration and reduction time of hydrothermal, and the effect of the hierarchical pore structure on adsorption capacity, kinetics, and rate-limiting models were discussed. The highest CO₂ adsorption (2.7 mmol/g) was achieved under hydrothermal synthesis conditions of 2 mg/mL graphene oxide concentration over 10 h. This high adsorption is attributed to the enhancement in meso and micro surface areas, as indicated by BET results (169 and 152 m²/g, respectively), the presence of adequate macropores (FE-SEM results), the presence of heteroatoms (N and O) according to XPS, FTIR, and EDX results, and a high NH/N_T (-NH- spectral area/total amine spectral areas) ratio of 0.35. Additionally, the highest structural defect (1.17) was observed in RAMAN results. The Elovich model demonstrates good agreement with experimental data, indicating heterogeneous CO₂ adsorption. The high S_m/S_T (micro surface area/micro + meso surface area) ratio (47%) and adequate macropores in the sample prepared with 2 mg/mL graphene oxide and 10 h, led to enhanced CO₂ physisorption. The high surface area increased the accessibility of active sites for chemisorption. The presence of macropores in this sample accelerates the mass transfer of CO₂ molecules, and the initial adsorption rate of the Elovich model (α = 0.032) is the highest. High graphene oxide concentration increased the surface barrier diffusion value due to decreased macropores. Intra-particle diffusion was identified as the rate-limiting kinetic model for CO₂ diffusion.