Xerogel immobilized N/S/O-doped carbon quantum dots from shrimp shells based chitosan for CO₂ adsorption

Rising CO₂ emissions underscore the need for sustainable and efficient capture materials. This study develops N/S/O-doped carbon quantum dots (CQDs) derived from shrimp-shell chitosan and stabilized within xerogel matrices (N/S/O-CCQD-X) to overcome the common limitations of CQDs such as aggregation, instability, and moisture sensitivity. The aims of the work were to (i) evaluate the role of xerogel immobilization in improving CQD stability, (ii) synthesize N/S/O-doped CQDs using varying chitosan:thiourea:KOH ratios, (iii) assess CO₂ adsorption performance under different process conditions, and (iv) analyse adsorption behaviour through isotherm, kinetic, and thermodynamic models. CQDs were synthesised by hydrothermal treatment with varying chitosan:thiourea:KOH ratios and immobilised in xerogels at different CQD-to-solution ratios. Characterisation included FTIR, XPS, CHNS, HRTEM, BET, FESEM and fluorescence spectroscopy to confirm functionalisation, particle morphology and pore structure. CO₂ adsorption performance was measured in a continuous fixed-bed breakthrough rig. Initial screening used 30 °C, 5 vol% CO₂ and 100 mL·min⁻¹, additional tests varied temperature (30–110 °C), CO₂ concentration (2–10 vol%) and flow rate (60–140 mL·min⁻¹). Adsorption data were analysed by isotherm, kinetic and thermodynamic modelling. Xerogel-immobilised CQDs maintained dispersion and active sites, the 1:50 xerogel formulation was selected after initial screening for detailed doping ratio studies. BET and FESEM indicated a microporous xerogel network supporting gas diffusion and preserving N/S/O functionalities confirmed by FTIR, XPS and CHNS. Synthesis and screening of doping ratios identified CQD-X 133 as the best performing doped sample (breakthrough capacity 234.93 mg·g⁻¹ under initial screening). HRTEM showed well-dispersed CQDs with an average diameter of 4.27 ± 1.06 nm. Under optimised conditions the material reached 358.80 mg·g⁻¹ at 30 °C, 100 mL·min⁻¹, 5 vol% CO₂, and the highest measured uptake was 402.12 mg·g⁻¹ at 30 °C, 100 mL·min⁻¹, 2 vol%CO₂. Temperature, flow rate and CO₂ concentration influenced breakthrough and capacity, with optimal adsorption conditions identified in the parameter study. Equilibrium data fit best to the Redlich–Peterson model (mixed monolayer–multilayer on a heterogeneous surface), kinetics followed a pseudo-second-order model, and thermodynamics gave ΔG < 0, ΔS < 0 and ΔH = –9.49 kJ·mol⁻¹ indicating spontaneous, exothermic adsorption dominated by physisorption with complementary localized chemisorptive sites. Xerogel immobilisation stabilises biomass-derived, heteroatom-doped CQDs and preserves active N/S/O sites that enhance CO₂ affinity. CQD-X 133’s superior performance demonstrates that precursor ratio design can tune both adsorption capacity and thermal resilience. Overall, this study demonstrates the novelty and practical potential of combining waste-derived CQDs with a xerogel matrix to create a stable, functionalised, and scalable adsorbent for low-energy CO₂ capture applications.