Effect of the surface oxygen groups on methane adsorption on coals

To investigate the influence of surface oxygen groups on methane adsorption on coals, one bituminous coal was modified with H2O2, (NH4)2S2O8 and HNO3 respectively, to prepare coal samples with different surface properties. The oxygen groups on coal surface were characterized by X-ray photoelectron spectroscopy (XPS). The textures of the coal samples were investigated by N2 adsorption at 77 K. Their surface morphologies were analyzed by scanning electron microscopy (SEM). The methane adsorption behaviors of these coal samples were measured at 303 K in pressure range of 0–5.3 MPa by volumetric method. The adsorption data of methane were fitted to the Langmuir model and Dubinin–Astakhov (D–A) model. The fitting results showed that the D–A model fitted the isotherm data better than the Langmuir model. It was observed that there was, in general, a positive correlation between the methane saturated adsorption capacity and the micropore volume of coals while a negative correlation between methane saturated adsorption capacity and the Ototal/Ctotal. The methane adsorption capacity was determined by the coal surface chemistry when the microporosity parameters of two samples were similar. Coal with a higher amount of oxygen surface groups, and consequently with a less hydrophobic character, had lower methane adsorption capacity.