Kinetic and Thermodynamic Analysis of CO 2 Adsorption on Graphitic Carbon Nitride

Abstract

The increasing atmospheric concentration of carbon dioxide (CO2) has intensified the demand of efficient, low-cost, and regenerable adsorbent materials for carbon capture applications. In this study, graphitic carbon nitride (g-C3N4) was synthesized via thermal polycondensation of urea and systematically characterized to investigate its CO2 adsorption behavior. Structural characterization by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) confirmed the successful formation of g-C3N4. CO2 adsorption experiments conducted at 303, 313, and 323 K showed that equilibrium adsorption capacity decreased from 1.04 to 0.67 mmol g -1 with increasing temperature, indicating an exothermic adsorption process. Kinetic analysis using pseudo-first-order, pseudo-second order, and intraparticle diffusion models showed that intraparticle diffusion contributed substantially to the adsorption process, while the non-zero IPD intercepts indicated additional boundary-layer resistance. Thermodynamic analysis showed a low isosteric heat of adsorption (2.61-4.68 kJ mol -1 ), a small negative enthalpy change (ΔH = -1.70 kJ mol -1 ), negative Gibbs free energy (ΔG = -0.23 to -0.32 kJ mol -1 ), indicating weak, exothermic, and thermodynamically favorable adsorption. The novelty of this work lies in the integrated kinetic and thermodynamic analysis of pristine g-C3N4, providing mechanistic insight into CO2 adsorption on a metal-free adsorbent. The results indicate that the weak and reversible adsorption interactions may facilitate low-energy CO2 regeneration, although cyclic adsorption–desorption experiments are required to confirm its long-term regenerability.