Effect of nitrogen-containing groups on methane adsorption behaviors of carbon spheres

• The contents of nitrogen groups over the modified CS surface were increased.
• The D–A equation fitted well with the experimental data.
• Effect of surface properties on methane adsorption of CS was studied.
• Pore structure of CS affected the methane adsorption behavior.

Methane adsorption behaviors of surface treated carbon spheres were studied in this work. Nitrogen-containing groups containing carbon spheres were prepared by urea, ammonia aqueous solution and NH3 treatments, respectively. The textural properties were characterized by N2 adsorption/desorption isotherm at 77 K. Their surface morphologies were analyzed by scanning electron microscopy (SEM). The surface properties of carbon spheres were confirmed by ultimate analysis and X-ray photoelectron spectroscopy (XPS). The adsorption characteristics of methane onto carbon spheres were measured at 298 K and pressures up to 5.0 MPa by a volumetric method. The D–A model fitted the experimental data well. As a result, the functionalized carbon spheres led to a decrease in adsorption capacity as compared with pristine CS, and there was a positive correlation between the saturated adsorption capacity and the BET SSA and total PV of samples. When the BET SSA and total PV were considerable, the surface properties, such as nitrogen-containing groups, were the more sensitive factors. Meanwhile, a higher nitrogen content, especially a higher pyridine nitrogen content, provided more positive adsorptive sites for methane adsorption. Therefore, the more appropriate the textural structure and the surface properties, the better the methane adsorption capacity.