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

Abstract

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.