Water adsorption on nitrogen-doped carbons for adsorption heat pump/desiccant cooling: Experimental and density functional theory calculation studies

Nitrogen-doped porous carbons were prepared via chemical activation of chitosan using alkali-metal carbonates (Na2CO3, K2CO3, Rb2CO3, and Cs2CO3), and the water adsorption isotherms of prepared carbons were measured to estimate the performance of the carbons in adsorption heat pump/desiccant cooling. Pre-doping of an alkali carbonate activator into the chitosan precursor resulted in a highly developed micropore structure and the introduction of macropores. The macropore structure clearly depended on the alkali carbonate activator. Na2CO3 resulted in two-dimensional macropores because of the hard-template effect, whereas the other alkali carbonate activators resulted in three-dimensional macropores with different cell sizes. A density functional theory calculation study based on X-ray photoelectron spectroscopy analyses of nitrogen-doped activated carbons indicated that pyridinic (pyridine, pyridone, and pyridine N-oxide), amide, and quaternary N groups might be effective for low-pressure water adsorption. Although the water adsorption capacities of the prepared carbons at P/P-0 > 0.5 increased with increasing micropore volume, those at the low-pressure region (P/P-0 < 0.1) were dependent on the surface density of nitrogen or oxygen functional groups. In addition, the shape of water isotherms preferable for adsorption heat pump/desiccant cooling was observed for the carbons with an appropriate surface density of nitrogen functional groups.