CO2 Adsorption by Amino-Functionalized Graphene-Silica Gels

Abstract

This work evaluates the CO₂-adsorption relevance and cycling stability of graphene oxide-silica (GO-SiO₂) and reduced graphene oxide-silica (rGO-SiO₂) gels after amine functionalization, demonstrating high-capacity retention under repeated adsorption-desorption cycles: rGO-SiO₂-APTMS retains ≈96.3% of its initial uptake after 50 cycles, while GO-SiO₂-APTMS retains ≈90.0%. The use of surfactants to control the organization of inorganic and organic molecules has enabled the development of ordered mesostructures, such as mesoporous silica and organic/inorganic nanocomposites. Owing to the outstanding properties of graphene and its derivatives, synthesizing mesostructures intercalated between graphene sheets offers nanocomposites with novel morphologies and enhanced functionalities. In this study, GO-SiO₂ and rGO-SiO₂ gels were synthesized and characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TG), mass spectrometry (MS), N₂ adsorption-desorption isotherms, and transmission electron microscopy (TEM). The resulting materials exhibit a laminar architecture, with mesoporous silica domains grown between graphene-based layers; the silica contents are 83.6% and 87.6%, and the specific surface areas reach 446 and 710 m²·g^-1, respectively. The laminar architecture is retained regardless of the surfactant-removal route; however, in GO-SiO₂ obtained by solvent extraction, a fraction of the surfactant remains partially trapped. Together with their high surface area, hierarchical porosity, and amenability to surface functionalization, these features establish amine-grafted graphene-silica gels, particularly rGO-SiO₂-APTMS, as promising CO₂-capture adsorbents.

Keywords: CO2 capture; amine functionalization; graphene; graphene oxide; mesostructure; silica gels; sol–gel; surfactant removal.

Figure 1

XRD patterns of GO (graphene oxide) and GO–CTAB (graphene oxide treated with CTAB). The basal reflection is indexed as (001); d₀₀₁ increases from ~10 Å (GO, 2θ ≈ 8.8°, Cu Kα) to ~33 Å (GO–CTAB, 2θ ≈ 2.7°) upon CTAB intercalation, evidencing gallery expansion.

Figure 2

High-resolution C 1s XPS spectra of GO-SiO₂ (graphene oxide–silica gel) (a) and rGO-SiO₂ (reduced graphene oxide–silica gel) (b).

Figure 3

High-resolution Si 2p XPS spectra of GO-SiO₂ (a) and rGO-SiO₂ (b).

Figure 4

TG curves of GO and GO-CTAB.

Figure 5

GO-SiO₂ sample: (a) TG (black) and DSC (red); (b) TG (black), with MS monitoring at m/z 18 (blue) and 44 (green). rGO-SiO₂ sample: (c) TG (black) and DSC (red); (d) TG (black) with monitoring at m/z 18 (blue) and 44 (green).

Figure 6

(a) BJH pore-size distributions, showing modes at ~23 Å (GO-SiO₂) and ~27 Å (rGO-SiO₂). (b) N₂ adsorption–desorption isotherms (type IV(a), H3).

Figure 7

(a) TEM image of the GO-SiO₂ gel; (b) TEM image of the rGO-SiO₂ gel.

Figure 8

(a,c) TG (black) and DSC (red) curves; (b,d) TG (black) with MS monitoring at m/z 18 (blue) and 44 (green) for GO-SiO₂-APTMS (graphene oxide–silica gel functionalized with APTMS, top) and rGO-SiO₂-APTMS (reduced graphene oxide–silica gel functionalized with APTMS, bottom).

Figure 9

CO₂ adsorption isotherms at 25 °C of GO-SiO₂-APTMS and rGO-SiO₂-APTMS.

Figure 10

Representative isothermal CO₂ capture–regeneration cycles at 25 °C for rGO-SiO₂-APTMS under a 20% CO₂/N₂ stream (adsorption) and N₂ at 110 °C (regeneration). A rapid initial uptake is followed by a slower approach to equilibrium within each cycle; capacity retention over 50 cycles is reported in Table 6.

Figure 11

Synthesis diagram of GO-SiO₂ and rGO-SiO₂ gels.

Figure 12

Diagram of the TG cycle process.