From chitosan to urea-modified carbons: Tailoring the ultra-microporosity for enhanced CO₂ adsorption

In the past few years, nitrogen-enriched microporous carbons have garnered much attention as emerging CO₂ adsorbents; however, the roles of ultra-microporosity and nitrogen content have rarely been studied. The present work focuses on designing a series of microporous carbons with different concentrations of nitrogen ranging from 0 to 11 at.%. With variation of the concentration of urea and KOH, carbons with a high surface area (368–2150 m² g⁻¹) and high micropore volume (0.2255–1.3020 cm³ g⁻¹) were attained. Tailoring the ultra-microporosity resulted in effective CO₂ adsorption. The optimized material, CUK-112, exhibited the highest micropore volume (<1 nm) among the investigated materials and a maximum adsorption of 280 mg g⁻¹ of CO₂ at 273 K and 1 bar. A slight decrease in the ultra-micropore volume for pores smaller than 1 nm decreased the adsorption capacity to 259.2 mg g⁻¹ of CO₂. The comprehensive study of the CO₂ isotherms within the framework of Dubinin's theory further evidenced the role played by narrow micropores in CO₂ adsorption. However, isosteric heats of adsorption revealed the importance of nitrogen content during the initial adsorption onto heteroatom-containing sites via quadrupole interaction with CO₂.