Nitriding an Oxygen-Doped Nanocarbonaceous Sorbent Synthesized via Solution Plasma Process for Improving CO₂ Adsorption Capacity

The synthesis of carbon nanoparticles (Cn) and oxygen-doped nanocarbon (OCn) was successfully done through a one-step synthesis by the solution plasma process (SPP). The Cn and OCn were nitrogen-doped by nitridation under an ammonia atmosphere at 800 °C for 2 h to yield NCn and NOCn, respectively, for carbon dioxide (CO₂) adsorption. The NOCn exhibited the highest specific surface area (~570 m² g^-1) and highest CO₂ adsorption capacity (1.63 mmol g^-1 at 25 °C) among the synthesized samples. The primary nitrogen species on the surface of NOCn were pyridinic-N and pyrrolic-N. The synergistic effect of microporosity and nitrogen functionality on the NOCn surface played an essential role in CO₂ adsorption enhancement. From the thermodynamic viewpoint, the CO₂ adsorption on NOCn was physisorption, exothermic, and spontaneous. The NOCn showed a more negative enthalpy of adsorption, indicating its stronger interaction for CO₂ on the surface, and hence, the higher adsorption capacity. The CO₂ adsorption on NOCn over the whole pressure range at 25-55 °C best fitted the Toth model, suggesting monolayer adsorption on the heterogeneous surface. In addition, NOCn expressed a higher selective CO₂ adsorption than Cn and so was a good candidate for multicycle adsorption.