Preparation and evaluation of nitrogen-tailored hierarchical meso-/micro-porous activated carbon for CO₂ adsorption

In this study, nitrogen-tailored hierarchical meso-/micro-porous activated carbons were successfully fabricated from cypress sawdust by H₃PO₄ activation with further nitrogen modification using three kinds of nitrogen source (i.e. nitic acid, urea and melamine). The produced carbons were used as adsorbents for CO₂ capture. The physic-chemical properties of the produced carbons were characterized by N₂ adsorption-desorption, fourier-transform infrared spectroscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. The effects of pore structure and nitrogen content on CO₂ adsorption were investigated. It was found that H₃PO₄ activation would turn cypress sawdust into mesoporous carbon (AC), nitrogen doping could induce the development of microporosity and also increase the basicity of the carbon framework, which favoured for CO₂ adsorption. Among the nitrogen-tailed carbons, HNO₃-treated activated carbon (AC-N) showed the highest V _mic (0.127 cm³/g), the largest CO₂ adsorption capacity (2.8 mmol/g at 273 K, 1 bar) and the best CO₂/N₂ selectivity as compared to urea and melamine treated ones. The adsorption experiments showed that the presence of microporosity and pyrrolic-N on the carbons were responsible for CO₂ adsorption, the oxygen functional groups on AC-N might also contribute to higher CO₂ uptake, and the mesoporous structure could favour for the fast mass transfer of CO₂. The results of CO₂ adsorption heat confirmed the high affinity of the prepared carbons to CO₂. This study provides a strategy to produce hierarchical meso-/micro-porous activated carbons with enriched nitrogen functional groups, which favoured for CO₂ adsorption.