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 pz orbital of the carbon atom.

Lewis Basicity of Nitrogen-Doped Graphite Observed by CO2 Chemisorption.

The characteristics of CO2 adsorption sites on a nitrogen-doped graphite model system (N-HOPG) were investigated by X-ray photoelectron and absorption spectroscopy and infrared reflection absorption spectroscopy. Adsorbed CO2 was observed lying flat on N-HOPG, stabilized by a charge transfer from the substrate. This demonstrated that Lewis base sites were formed by the incorporation of nitrogen via low-energy nitrogen-ion sputtering. The possible roles of twofold coordinated pyridinic N and threefold coordinated valley N (graphitic N) sites in Lewis base site formation on N-HOPG are discussed. The presence of these nitrogen species focused on the appropriate interaction strength of CO2 indicates the potential to fine-tune the Lewis basicity of carbon-based catalysts.

Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts.

Nitrogen (N)-doped carbon materials exhibit high electrocatalytic activity for the oxygen reduction reaction (ORR), which is essential for several renewable energy systems. However, the ORR active site (or sites) is unclear, which retards further developments of high-performance catalysts. Here, we characterized the ORR active site by using newly designed graphite (highly oriented pyrolitic graphite) model catalysts with well-defined π conjugation and well-controlled doping of N species. The ORR active site is created by pyridinic N. Carbon dioxide adsorption experiments indicated that pyridinic N also creates Lewis basic sites. The specific activities per pyridinic N in the HOPG model catalysts are comparable with those of N-doped graphene powder catalysts. Thus, the ORR active sites in N-doped carbon materials are carbon atoms with Lewis basicity next to pyridinic N.