Chemisorption of CO₂on Nitrogen-Doped Graphitic Carbons

The adsorption of CO₂on 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 CO₂was 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 CO₂desorption was observed for graphitic nitrogen-doped HOPG. This indicated that only pyri-N species create identical CO₂adsorption sites on any graphitic carbon surface. The adsorption energies of CO₂on pyri-N-doped carbons were estimated between 101 and 108 kJ mol^-1, indicating that chemisorption, rather than physisorption, took place. The CO₂adsorption/desorption process was reproducible in repeated measurements, and no CO₂dissociation occurred during the process, suggesting that it is a promising CO₂capturing material. The O 1s peak of the adsorbed CO₂clearly appeared at 531.5-532 eV in the XPS measurements. The N 1s peak of pyri-N did not change with CO₂adsorption, indicating that CO₂is not directly bound to pyri-N but is adsorbed on a carbon atom near the pyridinic nitrogen via the nonbonding p_zorbital of the carbon atom.