Chemisorption of CO2 on Nitrogen-Doped Graphitic Carbons

The adsorption of CO₂ on nitrogen-doped graphitic carbon materials, such as graphene nanosheet (GNS) powder and highly oriented pyrolytic graphite (HOPG), was comparatively studied using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Desorption of CO₂ was observed at approximately 380 K for both pyridinic-nitrogen (pyri-N)-doped GNS and pyri-N-doped HOPG samples in the TPD experiments, whereas no CO₂ desorption was observed for graphitic nitrogen-doped HOPG. This indicated that only pyri-N species create identical CO₂ adsorption sites on any graphitic carbon surface. The adsorption energies of CO₂ on pyri-N-doped carbons were estimated between 101 and 108 kJ mol^-1, indicating that chemisorption, rather than physisorption, took place. The CO₂ adsorption/desorption process was reproducible in repeated measurements, and no CO₂ dissociation occurred during the process, suggesting that it is a promising CO₂ capturing material. The O 1s peak of the adsorbed CO₂ clearly appeared at 531.5-532 eV in the XPS measurements. The N 1s peak of pyri-N did not change with CO₂ adsorption, indicating that CO₂ is not directly bound to pyri-N but is adsorbed on a carbon atom near the pyridinic nitrogen via the nonbonding p_z orbital of the carbon atom.